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Re-Os isotope systematics in black shales from the Lesser Himalaya: Their chronology and role in the Os-187/Os-188 evolution of seawater
Abstract
Re and Os abundances and Os isotope systematics have been measured in a number of black shales sampled from outcrops and two underground phosphorite mines, Maldeota and Durmala, in the Lesser Himalaya. The Re and Os concentrations in these samples exhibit wide range, 0.2 to 264 ng/g and 0.02 to 13.5 ng/g, respectively, with a mean Re/Os of ∼25. The 187Os/188Os of these samples also show a broad range 1.02 to 11.6, with many of them far more radiogenic than the currently eroding continental crust. The black shales from the Maldeota and Durmala mines, collected a few meters above the Krol-Tal (Pc-C) boundary and occurring in the same stratigraphic horizon, yield 187Re–187Os isochron ages of 554 ± 16 and 552 ± 22 Ma, respectively, consistent with fossil evidences and those assigned for the Pc-C boundary at various other locations. These results indicate closed system behaviour of Re and Os in these mine samples and reaffirm the validity of 187Re–187Os pair to date organic-rich sediments. The age of these mine samples from the outer belt seems to be a few hundred millions of years younger than the preliminary Re-Os age of 839 ± 138 Ma for black shales from Theog, collected from the inner belt. These initial results lean toward the hypothesis that the inner belt sediments were deposited earlier than the outer belt; however, more analyses of black shales are needed to confirm this. The role of weathering black shales from the Himalaya in contributing to the Os isotope evolution of seawater over the past ∼25 Ma was assessed using a simple budget model for seawater Os and assuming values for Os fluxes and 187Os/188Os based on the available limited data on Os concentration and its 187Os/188Os in rivers and oceans. These calculations indicate that the required contributions from black shale weathering to account for the entire increase in oceanic 187Os/188Os over the past ∼16 Ma is difficult to satisfy by varying only the Os flux from HTP rivers (187Os/188Os kept constant at 1.324) or their 187Os/188Os (188Os flux constant at today’s value, ∼131 moles/yr). The requirements to balance Os isotope budget in ocean through weathering of black shales, however, is easier to accommodate if both the Os flux from HTP (Himalayan-Tibetan Plateau) rivers and their 187Os/188Os are allowed to vary. Measurements of Os abundance in HTP rivers and its isotopic composition are needed to test the validity of the model results.
... The Krol-Tal succession of the Lesser Himalaya archives fossils of small shelly fauna, acritarchs, and trilobites (e.g., Hughes et al., 2005;Tiwari, 1999). The presence of the Pc-C boundary in this succession has been established on the basis of chemo-, chrono-, and bio-stratigraphic records (Aharon et al., 1987;Bhargava et al., 1998;Singh et al., 1999). Further, sequence and carbon isotopic stratigraphy of the Krol Group has also been correlated with the Dengying and Doushantuo Formations in South China (Aharon et al., 1987;Jiang et al., 2003;Kaufman et al., 2006). ...
... This succession is characterized by assemblages of small shelly faunal genera, such as Anabarites, Protohertzina, and Maldeotaia (Bhatt et al., 1985;Brasier & Singh, 1987). Chemo-(carbon and oxygen isotopes), bio-(acritarchs, small shelly fauna, trilobites, and brachiopods) and chrono-(535 ± 11 Myr; Singh et al., 1999) stratigraphic evidence mark the Pc-C boundary near the Krol-Tal transition (Aharon et al., 1987;Azmi, 1983;Jiang et al., 2003). An older Rb-Sr age of 626 ± 13 Myr (Sharma et al., 1991) for the shales has also been reported for the Lower Tal, which may have been influenced by sediment provenance age. ...
... The proposed Pc-C boundary at the base of the Tal Formation (Bhargava et al., 1998;Tarhan et al., 2014) and tentative age information for the succession (Etienne et al., 2011;Richards et al., 2005) are also shown. (c) Lithological, carbon isotopic (Mazumdar & Banerjee, 2001), and chronological (* Singh et al., 1999;# Sharma et al., 1991;and $ Tripathy, 2011) (1) where, the subscripts, sample and PAAS, stand for the concentrations in the sample and Post-Archean Australian shales, respectively. ...
The Precambrian‐Cambrian (Pc‐C) boundary marks significant biological, atmospheric, and oceanic changes. These changes include extinction of the Ediacaran fauna, initiation of complex lifeforms, and oxygenation of the atmosphere and oceans. In this contribution, elemental and Mo‐S isotopic compositions of organic‐rich shales overlying the Pc‐C boundary from the Tal Formation, Lesser Himalaya, have been investigated. These datasets are used to reconstruct past oceanic redox state and sulfate concentrations. The principal component analysis of the elemental dataset identifies six major factors, with factors associated with organic matter and sulfide phases accounting for about half of the total variance. Iron speciation and Mo/U data suggest that the shales were deposited in anoxic and ferruginous deep water conditions. The δ⁹⁸Mo data (1.5 ± 0.2‰) and their mass balance calculations indicate that the areal extent of sulfidic waters and pyrite burial rates were about 2–4 times higher during the Pc‐C transition than in the modern ocean. The pyrite‐δ³⁴S values (3.6–8.3‰) for the Tal shales are isotopically heavier compared to modern‐day sedimentary pyrites (∼−21‰). Calculations involving earlier‐reported δ³⁴S values for early Cambrian seawater and our measured pyrite‐δ³⁴S data estimate the seawater sulfate concentration (8 ± 3 mM) during their deposition. This sulfate value for the Tal basin is higher than that reported for the late Neoproterozoic ocean (<5 mM), attributable to increasing oxygen availability and continental supply during this time. The observed basinal conditions and high terrestrial input may have influenced metazoan diversification.
... Sediments rich in organic matter are efficient scavengers of redox-sensitive elements, including Re and Os; thus, both Re and Os are sequestered by ORS at the time of deposition from the overlying water column or from pore fluids in uncompact sediments (Koide et al., 1991;Ravizza and Turekian, 1992;Yamashita et al., 2007). Distinctively high Re/Os ratios in both marine and lacustrine ORS and the long half-life of 187 Re radiometric decay to 187 Os work in tandem to provide depositional ages of ORS (Cumming et al., 2012;Georgiev et al., 2011;Kendall et al., 2009;Singh et al., 1999;Tripathy et al., 2014;Xu et al., 2009;Yang et al., 2009). ...
... As a result, Re may have been less efficiently sequestered than Os, resulting in the observed increase in Os* without a commensurate increase in Re concentration. Re-Os isotopic systematics in ORS have been shown to effectively constrain their depositional ages (Georgiev et al., 2011;Kendall et al., 2009;Ravizza and Turekian, 1989;Singh et al., 1999;Tripathy et al., 2014). However, the success of Re-Os systematics as a geochronometer for ORS relies on the fulfilment of the basic assumptions that (i) 187 Os/ 188 Os of the sample suite is homogenous at the time of deposition, (ii) 187 Re/ 188 Os of the sample suite is heterogeneous at the time of deposition, and (iii) the system remains closed (i.e., no gain or loss of Re and Os since the deposition of the ORS). ...
... Thus, a key factor that can severely affect the applicability of the Re-Os geochronometer is the heterogeneity in the Os i of sediments at the time of deposition. Marine ORS with authigenic Re and Os drawn from a homogenous seawater pool generally satisfy this assumption (Georgiev et al., 2011;Singh et al., 1999;Tripathy et al., 2014;Xu et al., 2009). With rapid accumulation of heterogeneous woody debris and other organic materials in a coal mire, however, it is plausible that post-depositional isotopic equilibration is incomplete. ...
Coal horizons in terrestrial sedimentary sections are key to understanding the structural and thermal evolution of sedimentary basins. Precise and accurate age information for coal horizons can define correlations and the temporal evolution of sedimentary successions. While the biostratigraphy for coal horizons yields relative ages, tied to radiometric ages through the continuously refined geological time scale, direct radiometric ages of coal horizons have been derived only from tonsteins, lithified ash beds. The Re-Os geochronometer has been proven to record accurate and precise depositional ages for sedimentary rocks rich in organic matter. However, the reliability of the Re-Os geochronometer has not yet been assessed for coals deposited in strictly terrestrial environments. Here, in order to test the Re-Os geochronometer, we measure the Re, Os concentrations and isotopic composition of terrestrial coals from two distinct environments: the Finnmark Platform, southern Barents Sea and the Central Appalachian Basin (CAB). Samples from the Finnmark platform (Soldogg Formation) also include an adjacent organic-rich shale horizon. The shale and two coal horizons were also analysed for total organic carbon (TOC) and Rock-Eval data. Results show that the Soldogg coals and shale reached the oil generation widow (Tmax = 448 ± 2 °C), with Type III or mixed Type II/III organic matter, supporting the model for deposition in overbank deposits in a fluvial setting. In contrast, the Fire Clay coals in the CAB were deposited in a broad expanse of mires. Average Re (0.5 ± 0.4 ng/g) and Os (23 ± 26 pg/g) concentrations of terrestrial Soldogg and Fire Clay coals are similar to those of terrestrial Maghara coal and a few orders of magnitude lower than the marine-influenced Matewan coal. Elevated Re/Os* ratio (Os* = Common Os; Os corrected for ingrowth of ¹⁸⁷Os since deposition of the sample) for the Fire Clay coals suggests highly reducing and/or euxinic conditions prevailing in the water column of the extensive Carboniferous peat-forming mire in which these coals were deposited. In contrast, lower Re/Os* for Soldogg coals highlight less reducing conditions, likely in a short-lived mire in a dynamic braided stream system. Re and Os* concentrations of terrestrial coals are on the order of Re and Os of plant materials and river sediments, suggesting that debris from plant materials and weathered continental crust are significant contributors of Re and Os to terrestrial coals, with some contribution from water column of the coal mire.
This study is the first attempt to apply the Re-Os geochronometer on terrestrial coals. In case of the Soldogg coals and shale, the ¹⁸⁷Re/¹⁸⁸Os vs. ¹⁸⁷Os/¹⁸⁸Os relationship displays scatter with no discernible linear trend. The scatter is ascribed to heterogeneity in the initial ¹⁸⁷Os/¹⁸⁸Os ratios (Osi) and lack of post-depositional isotopic equilibration during subsequent diagenesis. For the Fire Clay coals, ¹⁸⁷Os/¹⁸⁸Os and ¹⁸⁷Re/¹⁸⁸Os are well correlated; linear regression of the data, however, does not yield a geologically reasonable age. Rather, the linear relationship results from binary mixing between two end-members: The Fire Clay coal and tonstein, suggesting that Re-Os systematics of terrestrial coals depositing very close to tonstein beds can be significantly disturbed. We conclude that the post-depositional equilibration with a large Re-Os reservoir, as with a marine incursion, is necessary to record a meaningful Re-Os radiometric age for the coals.
... The Mussoorie syncline of northwest India preserves one the most complete stratigraphic records of oLH rocks and includes, from base to top: mixed siliciclastic rocks of the Shimla Group, the diamictite-bearing Blaini Formation, Krol Group carbonate, and fossiliferous mixed siliciclastic strata of the Tal Group ( Figure 3) (Hughes et al., 2005;Jiang et al., 2002;Singh et al., 1999;Valdiya, 1980). Near Mussoorie and Lansdowne, shallow marine deposits of the Cretaceous to Paleogene Subathu-Singtali Formations unconformably overlie the Tal Group, where they are preserved structurally below higher grade MCT hanging wall klippen. ...
... Nanital marks the easternmost known extent of exposed oLH Krol-Tal Group rocks (Figure 1). Preservation of the Krol-Tal belt of the upper oLH in northwest India is particularly significant as it contains black shale units enriched in radiogenic 187 Os (Figures 2 and 3) (Singh et al., 1999). The exhumation and weathering of these shale deposits are proposed to be the initial contributors to the 16 Ma Neogene increase in 187 Os/ 188 Os (sw) (Myrow et al., 2015). ...
... Isotopic compositions of Himalayan rocks vary substantially between different tectonostratigraphic zones and rock types across the orogen. Lesser Himalayan black shale units, for example, are markedly enriched with high concentrations of radiogenic Os and yield anomalously high 187 Os/ 188 Os values ranging from 12 to exceptionally high values of 114 compared to surrounding rocks of the GH and TH, with typical bedrock values ranging from 0.5-1.5 ( Figure 2) (Pierson-Wickmann et al., 2000;Singh et al., 1999). Isotopically enriched black shale and enriched fine-grained clastic rocks occur as centimeter to tens of meters thick beds, which are persistent throughout oLH sequences ( Figure 3) and low-grade metasedimentary rocks of the iLH. ...
Shifts in global seawater 187Os/188Os and 87Sr/86Sr are often utilized as proxies to track global weathering processes responsible CO2 fluctuations in Earth history, particularly climatic cooling during the Cenozoic. It has been proposed, however, that these isotopic records instead reflect the weathering of chemically distinctive Himalayan lithologies exposed at the surface. We present new zircon (U-Th)/He thermochronometric and detrital zircon U-Pb geochronologic evidence from the Himalaya of northwest India to explore these contrasting interpretations concerning the driving mechanisms responsible for these seawater records. Our data demonstrate in-sequence southward thrust propagation with rapid exhumation of Lesser Himalayan strata enriched in labile 187Os and relatively less in radiogenic 87Sr at ∼16 Ma, which directly corresponds with coeval shifts in seawater 187Os/188Os and 87Sr/86Sr. Results presented here provide substantial evidence that the onset of exhumation of 187Os-enriched Lesser Himalayan strata could have significantly impacted the marine 187Os/188Os record at 16 Ma. These results support the hypothesis that regional weathering of isotopically unique source rocks can drive seawater records independently from shifts in global-scale weathering rates, hindering the utility of these records as reliable proxies to track global weathering processes and climate in deep geologic time.
... In this context, occurrence of a black shale (Bijaigarh shale) sequence in the Kaimur Group (Singh, 1980) provided an opportunity to employ the Re-Os chronometer to determine the depositional age for the group. Re and Os in black shale are mainly hydrogenous/authigenic in nature and hence radiogenic growth of 187 Os through β-decay of 187 Re provides a reliable measure of depositional age for the shales (Ravizza and Turekian, 1989;Cohen et al., 1999;Singh et al., 1999;Kendall et al., 2009a;Yang et al., 2009;Rooney et al., 2010;Georgiev et al., 2011;Tripathy et al., 2014a). ...
... Re concentrations of the samples were estimated using isotope dilution techniques. Isotopic compositions of in-house Re and Os standards were regularly measured during this study; the Os standard used in this study is the same used by Martin (1990) and Singh et al. (1999), whereas the Re standard is from Rahaman and Singh (2010). The average 187 Os/ 188 Os ratio of the standard is found to be 0.10687 ± 0.0009 (1σ; n = 20), whereas 185 Re/ 187 Re ratio of the Re standard is found to be 0.5964 ± 0.0064 (1σ; n = 7). ...
... The oxidative delivery of continental osmium to the seawater must have varied over time in response to the change in atmospheric oxygen concentration, resulting in a temporal change in seawater 187 Os/ 188 Os ratios. Compilation of earlier reported Os i ratios of marine Proterozoic shales (Singh et al., 1999;Hannah et al., 2004Hannah et al., , 2008Kendall et al., 2004Kendall et al., , 2006Kendall et al., , 2009aKendall et al., , 2009cCreaser and Stasiuk, 2007;Azmy et al., 2008;Rooney et al., 2010Rooney et al., , 2011van Acken et al., 2013;Zhu et al., 2013;Sperling et al., 2014) shows significant temporal variations (Fig. 6). The Os i ratios of marine shales deposited during the early Paleoproterozoic were unradiogenic (about 0.1; Fig. 6). ...
... associated with Himalayan deformation. Such eroded rocks may include the late Neoproterozoic-Cambrian strata of the Lesser Himalaya, some of which are enriched in radiogenic 187 Os, particularly a shale unit in the Tal Group (Singh et al., 1999;Pierson-Wickmann et al., 2000). The spatial and temporal pattern of erosion and chemical weathering of these strata may have been an important driving factor for secular changes in Neogene seawater 187 Os/ 188 Os and 87 Sr/ 86 Sr. ...
... Multiple workers have suggested that erosion of organic-rich sedimentary rock in the Himalaya led to the dramatic rise in Os isotopic values (e.g., Pegram et al., 1992;Ravizza, 1993). Links were specifically made to black shale in the early Cambrian Tal Group of the upper Lesser Himalayan succession, which is greatly enriched in radiogenic Os (Singh et al., 1999;Pierson-Wickmann et al., 2000), but the putative discrepancy between the timing of exposure of Tal Group strata and the increase in 187 Os/ 188 Os of seawater led some workers to question the relationship between radiogenic sources and changes in marine Os isotope ratios. Based on discussions The strontium isotope data can similarly be fit with two linear segments, from 22.2 to 15.8 (dR/dt = 6.86 × 10 −5 Ma −1 ) and from 15.5 to 9.2 Ma (dR/dt = 2.67 × 10 −5 Ma −1 ). ...
... above, we postulate that initial uplift and exhumation of upper Lesser Himalayan strata (possibly the oLH) took place at ∼16 Ma, and that it is recorded in marked inflections in the marine Os and Sr records at that time. Decreased erosion of the GH and increased erosion of younger oLH strata could have plausibly driven the observed changes in the Os and Sr paleoseawater curves beginning near 16 Ma, because radiogenic Os is concentrated in organic rich shale that accumulated in proximal parts of the Neoproterozoic and Cambrian margin (e.g., Singh et al., 1999), and radiogenic Sr is concentrated in the highgrade rocks of the GH (France-Lanord et al., 1993;Ahmad et al., 2000). ...
... globally (Peucker-Ehrenbrink and . Discharge that is more radiogenic than average continental crust ( 187 Os/ 188 Os > 1.4) suggests preferential dissolution of radiogenic Os from crustal sulfides, Precambrian crystalline bedrock, and/or organic-rich sediments (Peucker-Ehrenbrink and Ravizza, 1996;Singh et al., 1999Singh et al., , 2003Lu et al., 2017). The input from groundwater is extremely poorly constrained, with only a limited dataset from shallow (<74 m) groundwater aquifers (Paul et al., 2010). ...
... Due to siderophile and chalcophile characteristics, Os predominantly resides in sulfide minerals in the continental crust. An additional source of continental Os is the weathering of organic-rich shales (Ravizza and Esser, 1993;Singh et al., 1999;Dubin and Peucker-Ehrenbrink, 2015), which are enriched in Os through previous cycles of oxidative weathering and deposition. Because Os predominantly resides in easily weathered phases (e.g., Greber et al., 2015), only minor surface oxygenation is required to mobilize Os in the hydrologic cycle. ...
The rhenium-osmium (187Re-187Os) system is a highly versatile chronometer that is regularly applied to a wide range of geological and extraterrestrial materials. In addition to providing geo- or cosmo-chronological information, the Re-Os system can also be used as a tracer of processes across a range of temporal (millennial to gigayear) and spatial scales (lower mantle to cryosphere). An increasing number of sulfide minerals are now routinely dated, which further expands the ability of this system to refine mineral exploration models as society moves toward a new, green economy with related technological needs. An expanding range of natural materials amenable to Re-Os geochronology brings additional complexities in data interpretation and the resultant translation of measured isotopic ratios to a properly contextualized age. Herein, we provide an overview of the 187Re-187Os system as applied to sedimentary rocks, sulfides, and other crustal materials and highlight further innovations on the horizon. Additionally, we outline next steps and best practices required to improve the precision of the chronometer and establish community-wide data reduction procedures, such as the decay constant, regression technique, and software packages to use. These best practices will expand the utility and viability of published results and essential metadata to ensure that such data conform to evolving standards of being findable, accessible, interoperable, and reusable (FAIR).
... Carbonaceous shales are good candidates for Re-Os geochronology because they are likely to be hydrologically sealed and because they provide a reducing environment that immobilizes Re and Os. Whole rock Re-Os dating of carbonaceous shales has seen modest success, though uncertainties are, with one exception [4], relatively large (F3% or more; [5][6][7]). Scatter of isotopic data may result from either spatial variability in initial 187 Os/ 188 Os ratios [5,[7][8][9][10], or variable time of isotopic closure in hydrologically open systems [9]. ...
... Several studies document spatial or temporal excursions in seawater 187 Os/ 188 Os resulting from local input of low 187 Os/ 188 Os hydrothermal fluid [13,16,17], large impacts of extraterrestrial material [18], and variable rates of chemical weathering [5,[19][20][21][22]. Most important is oxidative weathering of sulfidic, carbonaceous sedimentary rocks, a key reservoir for crustal Re, and therefore of radiogenic 187 Os [6,18,23,24]. Given the short residence time of Os in seawater (probably less than 12 ky), excursions caused by impacts or local hydrothermal events are unlikely to persist for more than a few 100 ky [18]. ...
Re-Os dating of synsedimentary to early diagenetic pyrite from carbonaceous shale that straddles the boundary between the Rooihoogte and Timeball Hill formations, Transvaal Supergroup, South Africa, provides a precise isochron age of 2316 +/- 7 Ma ( +/-4 statistical uncertainty if error on decay constant is excluded) and a chondritic initial (187)Os/(188)Os ratio of 0.1121 +/- 0.0012. These units were deposited between what are interpreted as the second and third of three Paleoproterozoic global glacial events, and thus provide minimum and maximum ages, respectively, for these events. The Rooihoogte Formation is correlative with the Duitschland Formation, which records previously undated carbon isotope excursions. Because the pyrite samples show no evidence of mass independent fractionation of sulfur and have highly negative delta(34)S values, the rise of atmospheric oxygen most likely began prior to 2.32 Ga. The chondritic initial (187)OS/(188)Os ratio requires that primitive hydrothermal sources of Os dominated the Os budget of seawater at 2.32 Ga. Os is mobile only under oxidizing conditions. Therefore, the chondritic initial (187)Os/(188)Os ratio may reflect atmospheric oxygen levels too low to introduce sufficient riverine flux of dissolved radiogenic Os to offset the primitive hydrothermal/magmatic flux. Even if atmospheric oxygen levels had increased significantly by 2.32 Ga, anoxic conditions throughout the Archean most likely limited Os enrichment in Archean marine shales, so that their subsequent exposure and weathering was unable to provide a significant amount of radiogenic Os to Paleoproterozoic seawater.
... Sarkar et al. (2000) assigned an age of 1700–1550 Ma for Deoban dolomite based on the model Pb/Pb ages of galena. However, an age of 839 + 139 Ma was assigned to the Deoban equivalent in Himachal Pradesh, called the Shali Formation, based on Re– Os dating of black shale (Singh et al. 1999). Calcsilicate rocks within the Shali-Deoban Formation possess an inner LH Nd isotopic signature (Ahmad et al. 2000). ...
... The Mandhali Formation is a c. 2500 m-thick succession of carbonaceous pyritic phyllite, limestone and pebble conglomerate exposed north of the Almora klippe. Webb et al. (2011) state the age of the Mandhali Formation as 839 + 139 Ma, citing the work of Singh et al. (1999); however, the latter workers dated the Shali–Deoban Formation, not the Mandhali Formation. The Mandhali Formation is correlated with the Basantpur Formation of the lower Simla Group (Kumar & Brookfield 1987; Webb et al. 2011). ...
... Sarkar et al. (2000) assigned an age of 1700-1550 Ma for Deoban dolomite based on the model Pb/Pb ages of galena. However, an age of 839 + 139 Ma was assigned to the Deoban equivalent in Himachal Pradesh, called the Shali Formation, based on Re-Os dating of black shale (Singh et al. 1999). Calcsilicate rocks within the Shali-Deoban Formation possess an inner LH Nd isotopic signature (Ahmad et al. 2000). ...
... The Mandhali Formation is a c. 2500 m-thick succession of carbonaceous pyritic phyllite, limestone and pebble conglomerate exposed north of the Almora klippe. Webb et al. (2011) state the age of the Mandhali Formation as 839 + 139 Ma, citing the work of Singh et al. (1999); however, the latter workers dated the Shali-Deoban Formation, not the Mandhali Formation. The Mandhali Formation is correlated with the Basantpur Formation of the lower Simla Group (Kumar & Brookfield 1987;Webb et al. 2011). ...
We integrate U–Pb zircon geochronology and 1Nd(0) values with field mapping to
determine which tectonostratigraphic units are represented to the north, south and within the
Almora klippe in Kumaun, NW India. Rock in the Almora klippe and north of the Main Central
thrust (MCT) have Neoproterozoic (c. 900 Ma) detrital zircon ages, coupled with similar 1Nd(0)
(27.6 to 211.8) values, suggesting that these two units are the same tectonostratigraphic unit,
and that the Almora klippe is the southern continuation of the MCT or another thrust in the
Greater Himalayan thrust system. However, north of the Almora klippe, detrital zircon age populations
establish the presence of Palaeoproterozoic rock instead of the previous interpretation of
Neoproterozoic rocks. These Palaeoproterozoic Lesser Himalayan (LH) rocks are carried by the
Ramgarh–Munsiari thrust (RMT) dipping south and folded underneath the klippe. South of the
klippe, the RMT carries both the less metamorphosed Palaeoproterozoic and Neoproterozoic LH
rocks, in disagreement with the idea that only Neoproterozoic–Cambrian LH rocks are present
south of Almora klippe. These results suggest that previous cross-sections in Kumaun are incorrect
and must be re-evaluated.
... The dashed vertical line marks the limit of the measured unradiogenic Os that can be directly contributed by vehicular emission in the atmosphere. Moraine Os isotopic ratios lie within the crustal range of both eroding, that is, global river (Levasseur et al., 1999) and noneroding crust: Higher Himalayan Crystalline Sequence (Pierson-Wickmann et al., 2000), black shale (Ackerman et al., 2019;Selby & Creaser, 2003;Singh et al., 1999;van Acken et al., 2019). Cryoconite, in contrast, exhibits a range in 187 Os/ 188 Os ratios that encompass crustal to unradiogenic signature similar to that of recycled catalytic converters (Poirier & Gariépy, 2005), volcanic aerosol (Krähenbühl et al., 1992;Yudovskaya et al., 2008), and municipal waste (Funari et al., 2016). ...
Himalayan glaciers are invariably covered by supra‐glacial debris. Of these glaciers, the Chhota Shigri Glacier (CSG) in the western Himalaya has minimal debris cover (3.4%), yet has a comparable melt rate to other Himalayan glaciers. Utilizing osmium isotopic composition, and major and trace element geochemistry of cryoconite, a dark colored aggregate of mineral and organic materials on the surface of the ablation zone of the CSG, we show that the surface of CSG is essentially free of anthropogenically emitted particles, contrary to many previous findings. Given this and the overall lack of debris, we conclude that the high melt rate of CSG is primarily related to the increase of the Earth's near‐surface temperature linked directly to global warming. Therefore, the future meltwater supply for glacial‐fed rivers originating from Lahaul and Spiti region would be most vulnerable for >50 million population living downstream and requires immediate attention.
... Based on the order of superposition, the Krol Group sediments, underlying the Tal Group, are older and are considered Ediacaran-Early Cambrian in age. Black Shales occurring in the Deo-Ka-Tibba Formation in Maldeota and Durmala areas of the Mussoorie Syncline were subjected to Re-Os dating, which yielded a geochronological age of 535 ± 11 Ma (Singh et al., 1999). ...
An assemblage of microfossils of moderate diversity, with the remarkable occurrence of ECAP acritarchs, is reported from the Edi-acaran Krol 'A' (= the Mahi Formation) succession of Lesser Himalaya, India. Microfossils occur in the chert nodules exposed in Solan district, Himachal Pradesh. Two microfossils, Barogophycus symmetricus n. gen. n. sp. and Botominella lineata are new to the well-established Krol assemblage. The paper concentrates predominantly on fossil eukaryotic filamentous and coccoidal microorganism of simple morphology. The assemblage is dominated by remains of prokaryotic cyanobacteria and demonstrates the diversity achieved by microorganisms at the beginning of the Ediacaran Period in the aftermath of the Marinoan glaciation. Filamentous and coccoidal microorganisms differ in taxonomic composition from the Mesoproterozoic microbiotas. The Krol microfossils assemblage has biostrati-graphic potential and usefulness in broadly demarcating the different levels of the Ediacaran strata. Because of the presence of ECAP acritarchs, along with the appearance of other eukaryotic microorganisms of filamentous and coccoidal morphology, the general level of the Krol 'A' assemblage is considered as Ediacaran.
... The Re-Os isotopic system can be used as an effective geochronometer for constraining the formation/deposition age of various geological systems and archives that include mantle mafic-ultramafic rocks (Esperança et al., 1997;Rudnick and Walker, 2009;Yang et al., 2014), meteorites (Luck and Allègre, 1983;Smoliar et al., 1996;Walker and Morgan, 1989), organic matter rich sedimentary rocks (ORS; shales; Kendall et al., 2009;Ravizza and Turekian, 1989;Tripathy et al., 2014), coals (Goswami et al., 2018;Tripathy et al., 2015), and ore sulfide deposits (Han et al., 2007;Stein, 2014;Stein et al., 2004;2001;Zhang et al., 2008). The disintegration half-life of 187 Re is an order of magnitude higher than the Earth's age, making the Re-Os geochronometer a useful dating tool for covering a wide timescale, from Archean to Cenozoic (Poirier and Hillaire-Marcel, 2011;Singh et al., 1999;Xu et al., 2009). In addition to establishing the chronology, Re-Os isotopic compositions can be applied as a geochemical tracer for identifying the crustal input to the ocean. ...
The discovery of radioactivity in the early 20th century led to the development of several radiometric dating methods (e.g., Rb-Sr, Sm-Nd, Re-Os, U-Pb, etc.). These radiometric dating methods are frequently used in earth science studies to constrain the deposition/formation timing of various natural archives (e.g., bulk rocks, minerals, carbonaceous materials, detrital clastic sedimentary materials, ore deposits, hydrocarbon deposits). The last few decades have witnessed significant improvements in overall accuracy and precision of these absolute radiometric dating methods due to continuous developments and refinements in sample processing and analytical techniques. In this contribution, we discuss some of the frequently used radiometric dating techniques for obtaining absolute ages in various natural archives and associated advancements in the instrumentation. The present attempt emphasizes on a multi-mineral and multi-isotopic approach with continuous developments in obtaining better precision and accuracy in the ages through improved analytical and measurement protocols that are the pre-requisite in absolute dating.
... Moreover, the sparsity of the record is amplified by exceptionally poor age constraints on many of these critical formations-in particular, those frequently invoked as evidence of the first major shifts in a given proxy record. Examples include the Upper Carbonate Member of the Wynniatt Formation, which has been dated at ca. 761 Ma ± 41 Ma (van Acken, Thomson, Rainbird, & Creaser, 2013) or the Simla Group of Northern India, which has been dated at ca. 839 ± 139 (Singh, Trivedi, & Krishnaswami, 1999). As a result, it is nearly impossible to claim a robust geochemical shift near 800 Ma. ...
Few topics in geobiology have been as extensively debated as the role of Earth's oxygenation in controlling when and why animals emerged and diversified. All currently described animals require oxygen for at least a portion of their life cycle. Therefore, the transition to an oxygenated planet was a prerequisite for the emergence of animals. Yet, our understanding of Earth's oxygenation and the environmental requirements of animal habitability and ecological success is currently limited; estimates for the timing of the appearance of environments sufficiently oxygenated to support ecologically stable populations of animals span a wide range, from billions of years to only a few million years before animals appear in the fossil record. In this light, the extent to which oxygen played an important role in controlling when animals appeared remains a topic of debate. When animals originated and when they diversified are separate questions, meaning either one or both of these phenomena could have been decoupled from oxygenation. Here, we present views from across this interpretive spectrum—in a point–counterpoint format—regarding crucial aspects of the potential links between animals and surface oxygen levels. We highlight areas where the standard discourse on this topic requires a change of course and note that several traditional arguments in this “life versus environment” debate are poorly founded. We also identify a clear need for basic research across a range of fields to disentangle the relationships between oxygen availability and emergence and diversification of animal life.
... Based on the order of superposition, the Krol Group sediments, underlying the Tal Group, are older and are considered Ediacaran-Early Cambrian in age. Black Shales occurring in the Deo-Ka-Tibba Formation in Maldeota and Durmala areas of the Mussoorie Syncline were subjected to Re-Os dating, which yielded a geochronological age of 535 ± 11 Ma (Singh et al., 1999). ...
... Additionally, the 187 Re-187 Os isotope system has been used to constrain the depositional age of the Niutitang Formation in SE Chongqin area. The 187 Re-187 Os isotope system has been demonstrated to be a precise and accurate depositional geochronometer for black shales (Ravizza and Turekian, 1989;Singh et al., 1999;Creaser et al., 2002;Schaefer and Burgess, 2003;Selby and Creaser, 2003;Cohen, 2004;Cumming et al., 2012). Precise Re-Os isochron ages indicate that post-depositional processes have not affected the dated rock units (Jaffe et al., 2002;Kendall et al., 2009b;Rooney et al., 2012), allowing the confident application of tracers such as the Mo isotope proxy and RSEs. ...
The Early Cambrian was a key period in Earth's history, and the reconstruction of ancient ocean chemistry on spatial and temporal scales can contribute to a better understanding of events during that period. New Early Cambrian data for Re–Os and Mo isotopic compositions and redox-sensitive elements (RSE) are reported here for the Niutitang Formation, SE Chongqin, SW China. A Re–Os age of 520 ± 30 Myr (Model 3, 2σ, n = 21, MSWD = 62) and a (¹⁸⁷Os/¹⁸⁸Os)i ratio of 0.79 ± 0.11 were obtained, consistent with previously reported data for equivalent strata from other areas. Separate regression of samples with (¹⁸⁷Os/¹⁸⁸Os)i of 0.72–0.79 and 0.81–0.89 produce more precise ages of 520.1 ± 9.5 Myr (Model1, 2σ, n = 7, MSWD = 1.0) and 513 ± 10 Myr (Model 1, 2σ, n = 8 MSWD = 0.96) respectively, suggesting heterogeneity in the primary source of Os as the dominant cause of scatter and uncertainty. High initial ¹⁸⁷Os/¹⁸⁸Os of 0.79 ± 0.11 may indicate a high Os input from oxidative weathering of upper continental crust for Early Cambrian ocean. Based on RSE data such as Mo, U, V enrichment and Ni/Co, V/Cr, V/(V + Ni), MoEF/UEF, Re/Mo ratios, two geochemically distinct zones (upper and lower) can be delineated in the sedimentary sequence, as reflected in drillcore sections. The upper section (n = 9) is characterized by low total organic carbon (TOC) contents (mean 1.8 ± 1.5 wt%), low RSE contents (Mo (0.008 ± 0.006) × 10⁻³ g/g; V (0.22 ± 0.15) × 10⁻³ g/g; U (0.008 ± 0.004) × 10⁻³ g/g), and low V/Cr (2.0 ± 1.7), Ni/Co (5.9 ± 3.5), MoEF/UEF (2.6 ± 1.8), and Mo/TOC (6.0 ± 3.9) ratios; in contrast, the lower core section (n = 13) is characterized by high TOC contents (4.4 ± 2.2 wt%), high RSE contents (Mo (0.10 ± 0.08) × 10⁻³ g/g; V (1.0 ± 1.0) × 10⁻³ g/g; U (0.05 ± 0.04) × 10⁻³ g/g), high V/Cr (6 ± 5), Ni/Co (24 ± 14), MoEF/UEF (6.3 ± 2.2), and Mo/TOC (21 ± 9) ratios (uncertainties are ±1SD here). Significantly decreasing trends of those redox proxies from lower to upper sections suggest that the depositional conditions evolved from anoxic/euxinic to oxic condition. All samples display extremely low Re/Mo ratios of 0.2 × 10⁻³–1.9 × 10⁻³, indicating the absence of intermediate reducing conditions, possibly reflecting rapid ocean oxygenation. The Mo isotopic composition in the sedimentary succession of the YC9 core exhibits a large variation, with δ98/95Mo values of 0.04–2.00‰. Upper section δ98/95Mo values display an opposite trend to Mn content and negative correlation with Mo/TOC ratios, indicating Mo fractionation associated with adsorption on Mn oxides or oxyhydroxides. Lower section δ98/95Mo values are positively correlated with Mo/TOC ratios and negatively with Re/Mo ratios, suggesting incomplete conversion of molybdates to tetrathiomolybdates under weakly euxinic conditions. High δ98/95Mo of 2.00‰ and high average Mo/TOC ratios of 21 (10⁻⁶ g/g)/wt% were observed in euxinic shales in the Niutitang Formation, suggesting extensive ocean oxygenation may have taken place at ca. 520 Ma.
... La comparaison entre les valeurs des rapports isotopiques 187 0/ 88 0s conigés et non corrigés de la contribution du blanc montre qu'elles sont semblables pour la plupart des échantillons, donc ce choix ne changera pas les conclusions de l'étude Re/Os. En outre, les échantillons IM99-51 (Singh et al. , 1999) Analyse isotopique Re/Os impmiante du blanc pour les échantillons pauvres en Os, soit par le fonctionnement du système isotopique en système ouvert (Foster et al., 1996) ...
L'analyse structurale de la zone de faille du gisement Ag-Hg d'Imiter confirme la superposition de deux régimes tectoniques, synchrones de l'épisode argentifère, successivement extensif N-S à gangue siliceuse puis transtensif sénestre. Un réseau filonien rhyolitique caractérisé par des injections dans les plans d'ouverture E-W du premier régime extensif, est associé au dépôt des minéralisations. Les mesures in situ U/Pb sur mono zircon à microsonde ionique CAMECA IMS 1270 ont permis de dater deux événements magmatiques distincts à 572 ± 5 Ma (granodiorite de Taouzzakt) et 550 ± 3 Ma (rhyolite de Takhatert). Les analyses 40Ar/39Ar ont fourni des résultats qui confirment l'existence des deux événements magmatiques et hydrothermaux à Imiter associés à un épisode de minéralisation à métaux de base (563 à 577 Ma) puis épithermal argentifère. Les isotopes du soufre suggèrent deux réservoirs pour cet élément, un réservoir magmatique et un autre métasédimentaire. Les données Re/Os indiquent sans ambiguïté une origine mantellique du stock métal. Les minéralisations Ag-Hg d?Imiter sont interprétées dans le cadre d'un modèle épithermal neutre dont les deux paramètres essentiels sont la présence d'une extension de direction N-S permettant le piégeage des minéralisations et l'injection de magmas rhyolitiques. Ceux-ci fournissent également l'énergie nécessaire au développement des circulations hydrothermales. La datation de la minéralisation des gisements de Bou Madine (553 Ma, U/Pb sur zircon) et de Bou Azzer (533 Ma U/Pb sur zircon du trachyte d'Aghbar et 218 Ma 39Ar/40Ar sur adulaire) montre l'importance de l'événement métallogénique lors de la transition Précambrien/Cambrien et l'existence probable de minéralisations post-cambriennes. L'intégration de l'ensemble de ces résultats dans un modèle géodynamique met en évidence, lors de la transition Précambrien/Cambrien l'évolution de l'Anti-Atlas d'un contexte de marge continentale active à celui d'une marge continentale passive.
... As radiogenic Os is efficiently mobilized during weathering of organic-rich shale and can lose 45-90% of its initial Os (Peucker-Ehrenbrink & Blum, 1998;Peucker-Ehrenbrink & Ravizza, 2000;Ravizza & Esser, 1993), such contributions can be even more radiogenic than average continental runoff ( 187 Os/ 188 Os 1.4). Rivers draining shale deposits frequently have high 187 Os/ 188 Os, and it has been proposed that black shale can provide a more radiogenic flux of Os than other rock types (Dalai & Ravizza, 2010;Ravizza & Esser, 1993;Singh et al., 1999). However, Os released from shale can be incorporated into secondary iron oxides and clay minerals during weathering and in rivers, which may restrict the flux of labile radiogenic Os (Dalai & Ravizza, 2010;Pierson-Wickmann et al., 2002). ...
The sources of terrestrial material delivered to the California margin over the past 7 Myr were assessed using 187Os/188Os, Nd, and Pb isotopes in hydrogenetic ferromanganese crusts from three seamounts along the central and southern California margin. From 6.8 to 4.5 (± 0.5) Ma, all three isotope systems show more radiogenic values at Davidson Seamount, located near the base of the Monterey Canyon System, than in Fe-Mn crusts from the more remote Taney and Hoss seamounts. At the Taney seamounts, approximately 225 km farther offshore from Davidson Seamount, 187Os/188Os values, but not Pb and Nd isotope ratios, also deviate from the Cenozoic seawater curve towards more radiogenic values from 6.8 to 4.5 (± 0.5) Ma. However, none of the isotope systems in Fe-Mn crusts deviate from seawater at Hoss Seamount located approximately 450 km to the south. The regional gradients in isotope ratios indicate that substantial input of dissolved and particulate terrestrial material into the Monterey Canyon System is responsible for the local deviations in the seawater Nd, Pb, and Os isotope compositions from 6.8 to 4.5 (± 0.5) Ma. The isotope ratios recorded in Fe-Mn crusts are consistent with a southern Sierra Nevada or western Basin and Range provenance of the terrestrial material which was delivered by rivers to the canyon. The exhumation of the modern Monterey Canyon must have begun between 10 and 6.8 ± 0.5 Ma, as indicated by our data, the age of incised strata, and paleo-location of the Monterey Canyon relative to the paleo-coastline.
... Isochron-based initial Os isotope ratios for black shales and other carbonaceous sediments through time with data taken fromCohen et al. (1999),Singh et al. (1999),Mao et al. (2002), ...
This study reviews the geology of the early Proterozoic Pechenga Series, which comprises four sedimentary-volcanic cycles attaining a total thickness of 10 km, and presents new petrographical, mineralogical, geochemical, and Sm-Nd, Pb-Pb, Re-Os and Rb-Sr isotopic data, with particular emphasis on ferropicritic rocks. The ferropicritic magmatism led to the formation of gabbro-wehrlite intrusions and related Ni-Cu deposits in the so-called productive pile, the sedimentary part of the uppermost cycle (Pilgujarvi Suite). An extrusive facies is represented by ferropicritic tuffs, massive and pillowed lavas, and thick, differentiated lava flows, which occur mainly in the upper part of the productive pile and at three levels in the overlying volcanic unit. Nd, Sr, and Os isotopic and geochemical data for the ferropicrites are inconsistent with substantial degrees of contamination with older crustal material and favor an intraplate geotectonic setting. -from Author
... In previous studies, the hydrothermal origin of Ni-Mo polymetallic layers of the basal Niutitang Formation was proposed, in terms of their lower initial 187 Os/ 188 Os ratio than that of the Early Cambrian black shale (1.10-1.24) from Lesser Himalaya (Jiang et al., 2007;Singh et al., 1999). However, the only slightly lower 187 Os/ 188 Os ratios of these metalliferous and non-metalliferous layers imply a major contribution of normal seawater, likely with minor inputs of hydrothermal fluids through which the initial 187 Os/ 188 Os ratio was drew down slightly in comparison with modern seawater. ...
The Terreneuvian Epoch (541–521 Ma)is also an important period for metallogenesis in South China, as was represented by the widespread occurrences of Ni-Mo polymetallic layers on the antecedent shallow platform margin and the V-enriched black shales in deeper slope-basin settings. In this study, we have measured Re-Os isochron ages of Ni-Mo polymetallic layers (Songlin, Niuchang, Sancha, Chuanpengwan), V-rich black shales (Bahuang), and non-metalliferous black shales (Shuidong) in the basal Niutitang Formation in Guizhou and Hunan province, South China. The Ni-Mo polymetallic layers and V-enriched black shales have similar Re-Os isochron ages, suggesting concurrent deposition of these two types of metalliferous ores. This suggestion is consistent with the traditional stratigraphic correlation by using the nodular phosphorite bed directly underlying these metalliferous layers as a stratigraphic marker. Furthermore, the metalliferous ores and non-metalliferous black shales have similar initial 187Os/188Os ratios of 0.8–0.9, arguing for a dominant seawater origin with minor contribution of hydrothermal activity. Furthermore, Re-Os isotopic data also imply that Ni-Mo and V ore might have derived from the same source. We suggest that the spatial distribution of metalliferous ores can be explained by the development of non-sulfidic anoxic-suboxic wedge (NSASW) in the slope-basin and sulfidic wedge in the previous platform margin. Upwelling of deep water first transects the mildly reduced, organic rich NSASW, in which V (V) is reduced to V (IV), and is preferentially removed from seawater by organometallic complex formation. As a result, V-rich black shale deposits in the slope-basin of Yangtze Platform. Further movement of deep water into the sulfidic platform margin results in Ni-Mo polymetallic layer formation. © 2016, China University of Geosciences and Springer-Verlag Berlin Heidelberg.
... The graphitic and sulfidic country rocks are strongly radiogenic with high Re/ Os ratios. These are not unexpected characteristics of organicrich shales (Ravizza and Turekian 1992;Horan et al. 1994;Singh et al. 1999;Ripley et al. 2001;Creaser et al. 2002;Selby and Creaser 2003;Georgiev et al. 2011;Zhu et al. 2013), suggesting that the interaction with mantle-derived magma could lead to strongly anomalous Os isotope ratios of the igneous material. We graphically illustrate how mixing at 110 Ma between Os derived from the sulfidic/carbonaceous sedimentary material and a mantle-derived magma could lead to the anomalously high Os isotope ratios of contaminated magmas and the sulfides derived from them. ...
The Duke Island Complex is one of the several “Ural-Alaskan” intrusions of Cretaceous age that occur along the coast of SE Alaska. Significant quantities of magmatic Ni–Cu–PGE sulfide mineralization are locally found in the complex, primarily within olivine clinopyroxenites. Sulfide mineralization is Ni-poor, consistent with petrologic evidence which indicates that sulfide saturation was reached after extensive olivine crystallization. Olivine clinopyroxenites were intruded by magmas that produced sulfide-poor, adcumulate dunites. As part of a study to investigate the potential for Ni-rich sulfide mineralization in association with the dunites, a Re–Os and S isotope study of the dunites, as well as sulfide mineralization in the olivine clinopyroxenites, was initiated. Importantly, recent drilling in the complex identified the presence of sulfidic and carbonaceous country rocks that may have been involved in the contamination of magmas and generation of sulfide mineralization. γOs (110 Ma) values of two sulfidic country rocks are 1022 and 2011. δ34S values of the country rocks range from −2.6 to −16.1 ‰. 187Os/188Os ratios of sulfide minerals in the mineralization hosted by olivine clinopyroxenites are variable and high, with γOs (110 Ma) values between 151 and 2059. Extensive interaction with Re-rich sedimentary country rocks is indicated. In contrast, γOs (110 Ma) values of the dunites are significantly lower, ranging between 2 and 16. 187Os/188Os ratios increase with decreasing Os concentration. This inverse relation is similar to that shown by ultramafic rocks from several arc settings, as well as altered abyssal dunites and peridotites. The relation may be indicative of magma derivation from a sub-arc mantle that had experienced metasomatism via slab-derived fluids. Alternatively, the relation may be indicative of minor contamination of magma by crustal rocks with low Os concentrations but high 187Os/188Os ratios. A third alternative is that the low Os concentrations and elevated 187Os/188Os ratios denote subsolidus interaction with seawater or meteoric water. δ34S values of the dunites range between −6.4 and 6.6 ‰, and are consistent with the addition of S during fluid–rock interaction and serpentinization. The sharp contrast between the Os isotope ratios of the dunites and those of the sulfide mineralization illustrate that magmas that were spatially part of the same intrusive system may have experienced very different histories of interaction with country rocks. An important corollary is that because of the concentrations of Os and S, elevated Os isotope ratios (a function of high Re concentrations) and variable sulfur isotope ratios of sulfidic and carbonaceous country rocks, both S and Os isotope data from the olivine clinopyroxenite-hosted sulfide mineralization, are consistent with less than ∼2 % of bulk rock contamination. Even lower fractional abundance values may be indicated if the contaminant was a S–C–Os-rich fluid or partial melt derived from the sulfidic–carbonaceous metasedimentary country rocks. Despite the low degrees of contamination, the amounts of Os and S in the sulfide mineralization that may have been derived from country rocks often exceed 50 %.
... Morelli et al., 2004;Morelli et al., 2005;Kerr and Selby, 2010). The Re-Os dating is also an important tool in dating of the organic material-rich black-shales (e.g., Ravizza and Turekian, 1989;Ravizza et al., 1991;Cohen et al., 1999Cohen et al., , 2004Singh et al., 1999;Creaser et al., 2002;Selby and Creaser, 2003;Hannah et al., 2004;Fu et al., 2008). The method is especially Fig. 2. (a) Geological map of the Küre VMS deposit and its surroundings (after Güner, 1980;Pehlivanoğlu, 1985;JICA and MMAJ, 1992;Çakır, 1995 andÇakır et al., 2006). ...
The Re–Os isotopic system is applied for the first time to the sulfide ores and the overlying black-shales at the Küre volcanogenic massive sulfide deposit of the Central Pontides, Northern Turkey. The ore samples collected include predominantly pyrite, accompanied by chalcopyrite, sphalerite and other species. Massive ore is almost free of gangues, whereas the stockwork ore includes quartz and calcite gangue. The composition of sphalerite is similar to ancient and modern massive sulfide mineralizations globally. Microthermometric studies from quartz from the stockwork ore has shown two populations of two-phase fluid inclusions with vapor/liquid ratios between 4 to 28%, low to intermediate Th (161.5–317.0 °C) and low salinities (0.9–5 wt.% NaCl equiv.) which are mostly in good agreement with the ranges for volcanogenic massive sulfide mineralizations. These studies also suggest a H2O–CaCl2–KCl–MgCl2 ore-forming fluid system in a shallower subsurface near the seafloor vents. The Re–Os dating of the LLHR sulfides yield a nominal depositional age of upper Toarcian for the massive sulfide mineralization. Two largely different model ages obtained are attributed to other pyrite crystallization events prior to and postdating the main sulfide deposition. Some lower homogenization temperatures (< 200 °C) from the quartz of the stockwork may also similarly be related to the post-VMS events. It is concluded that a submarine volcanic extrusion episode has continued until upper Toarcian in the Küre Basin, when it has entered a stagnation period that allowed the discharge of hydrothermally circulated sulfide-laden fluids from the seafloor vents. This age data promotes the palaeotectonic models interpreting the Küre Basin as a Permian–early Jurassic marginal back-arc basin of the Devonian-Triassic Karakaya Ocean. The Re–Os data from the overlying black-shale provide a glimpse to the initial Os isotope ratio of the water column at the time of the sedimentation (0.45–0.46 for 180 Ma). The lack of common Os from the sulfides does not let us to infer a source of Os and initial 187Os/188Os ratios from the black shale are not statistically robust to make a significant deduction. A further detailed study on the isotopic composition of the black shale strata may help us to make an apporach to the Os source(s) in the deposition environment of the Küre VMS deposit.
... Fossils are extremely rare throughout most of the Lesser Himalayan succession, although some rich and paleoecologically important assemblages represent the Neoproterozoic-Cambrian transition (Tewari, 1993(Tewari, , 1996Mathur et al., 1997;Gautam and Rai, 1998). Many purported finds of Paleozoic fossils have been proven fraudulent (Jayaraman, 1994), however, and it now appears that much of the sequence consists of rocks of Mesoproterozoic to Early Cambrian age (Brasier and Singh, 1987;Brookfield, 1993;Frank et al., 1995;Parrish and Hodges, 1996;Singh et al., 1999). Thus, the Lesser Himalayan rocks represent a part of the north Indian marginal sequence that is both older and more proximal than that represented by Tibetan zone strata. ...
The Himalaya and Tibet provide an unparalleled opportunity to examine the complex ways in which continents respond to collisional orogenesis. This paper is an attempt to synthesize the known geology of this orogenic system, with special attention paid to the tectonic evolution of the Himalaya and southernmost Tibet since India-Eurasia collision at ca. 50 Ma. Two alternative perspectives are developed. The first is largely historical. It includes brief (and necessarily subjective) reviews of the tectonic stratigraphy, the structural geology, and metamorphic geology of the Himalaya. The second focuses on the processes that dictate the behavior of the orogenic system today. It is argued that these processes have not changed substantially over the Miocene–Holocene interval, which suggests that the orogen has achieved a quasi–steady state. This condition implies a rough balance between plate-tectonic processes that lead to the accumulation of energy in the orogen and many other processes (e.g., erosion of the Himalayan front and the lateral flow of the middle and lower crust of Tibet) that lead to the dissipation of energy. The tectonics of the Himalaya and Tibet are thus intimately related; the Himalaya might have evolved very differently had the Tibetan Plateau never have formed.
... Os/ 188 Os ratio of 1.18 ± 0.02 recently obtained on Lower Cambrian black shale samples in the Lesser Himalaya, India (Singh et al., 1999). 1056Smoliar et al., 1996) The new Re-Os isochron age refines the earlier estimate of Horan et al. (1994) who found a scatter distribution along a reference line of 560 Ma, assuming a minimum initial 187 ...
... The applications of rhenium–osmium (Re–Os) isotopic systematics have been rapidly expanding to various fields in Earth sciences during the last two decades such as cosmochemistry (Morgan et al., 1996; Shen et al., 1996), marine geochemistry (Ehrenbrink and Ravizza, 2000), and environmental fields including continental erosion and climate change (Chesley et al., 2000; Cohen et al., 2004; Pegram et al., 1992), soils (Singh et al., 1999), desert sands (Hattori et al., 2003), loess and dust (Ehrenbrink and Jahn, 2001). This is mostly due to the Contents lists available at ScienceDirect Applied Geochemistry j o u r n a l h o m e p a g e : w w w . ...
187Os/188Os ratios and osmium concentrations were analyzed in sedimentary reference materials JSd-3, JMS-1, and JMS-2, issued by the Geological Survey of Japan, using the isotope dilution method with negative thermal ionization mass spectrometry (ID-N-TIMS). The chemical procedures used in this study are based on Carius tube digestion combined with carbon tetrachloride extraction to separate Os. The obtained 187Os/188Os ratios of JSd-3, JMS-1, and JMS-2 were 0.941-1.149, 0.3894-0.465, and 0.747-0.848, respectively, and their Os abundances were 27.07-39.32 ppt, 80.8-106.6 ppt, and 281-317.2 ppt, respectively. Variations among the samples presumably reflect differences in sources and depositional environments. The Os isotopic ratios and abundances were variable within individual samples. The relative 95% confidence intervals of Os concentrations and isotopic ratios were respectively 13% and 6.2% for JSd-3, 15% and 9.8% for JMS-1, and 4.3% and 4.2% for JMS-2. These variations probably reflect the heterogeneity of the original rock of each reference material. Low concentration and high isotope ratio of JSd-3 probably results from the derivation of the original rocks from continental components with low Os abundance and elevated 187Os/188Os. Inputs of anthropogenic Os were inferred in JMS-1, a nearshore sediment from an urbanized locality, which had high Os contents and relatively low Os isotopic ratios. Greatly elevated Os concentrations and high 187Os/188Os in JMS-2, a pelagic clay, may reflect the scavenging of Os in seawater into Mn-Fe oxyhydroxides deposited on the seafloor.
... This fluid would possibly acquire an Os isotope signature close to that of the penetrated sedimentary rocks. Shales, mudstones, and limestones have radiogenic signatures and typically have Os concentrations that exceed 100 ppt (Raivizza and Turekian, 1989; Esser and Turekian, 1993; Ravizza, 1993; Sign et al., 1999, Chesley et al., 2000 ). This value is a conservative estimate , because saline fluids can contain more than 1 ppb Os (Xiong and Wood, 2000). ...
Re-Os isotopes are used to determine the age of iron oxide Cu-Au mineralization at Candelaria, Chile, and to explore possible genetic links with the batholithic intrusions in the area. Re-Os ages calculated from molybdenite are 114.2 +/- 0.6 Ma and 115.2 +/- 0.6 Ma, and they are interpreted to represent the age of mineralization. These ages are consistent with previously reported ages for biotite alteration that range from 114 to 116 Ma. An isochron calculated by Re/Os ratios from hydrothermal magnetite and sulfides constrains an initial Os-187/Os-188 ratio of 0.36 +/- 0.10. The initial Os-187/Os-188 ratio for sulfide from Bronce mine, a small satellite of the Candelaria orebody, is 0.33 +/- 0.01. These values are broadly similar to the calculated initial Os-187/Os-188 ratio for magmatic magnetite in nearby batholithic rocks that range from 0.20 to 0.41. The relatively radiogenic initial Os-187/Os-188 ratio represents a mixture of mantle and crustal components in the ores and batholitic rocks. The similarity in initial Os-187/Os-188 ratio of the ore and magmatic oxides suggest that the granitic plutons could be the source of metals in the Candelaria district. These data are consistent with the existence of a significant magmatic fluid component in the hydrothermal system as suggested by previously published work. In order to establish a regional perspective, see analyzed ore minerals from other Chilean deposits of the iron oxide (Cu-U-Au-rare earth elements [REE]) class. Magnetite from the Manto Verde iron oxide Cu-Au deposit has Os and Re concentrations of 11 to 17 parts per trillion (ppt) and 4 to 6 parts per billion (ppb), respectively. The initial Os-187/Os-188 ratio is approximately 0.20 and is similar to that of ore minerals from Candelaria and of the Early Cretaceous batholithic intrusions. These data indicate a similar metal source for the mineralization at Manto Verde and Candelaria. Magnetite from three magnetite-apatite deposits of the Chilean iron belt have Re concentrations between 0.8 and 3 ppb and Os concentrations between 11 and 76 ppt. Calculated initial Os-187/Os-188 ratios of these magnetites range from 1.2 to 8.4 and are distinctly different from those of the iron oxide Cu-Au deposits. The cause of the comparatively high radiogenic signatures in the magnetite-apatite ores is probably related to fluid interactions with (i.e., leaching) the surrounding sedimentary rocks during their genesis, in contrast to the iron oxide Cu-Au systems, the Os in magnetite-apatite ores could be derived from sedimentary rocks. This evidence suggests that predominantly basin-derived, nonmagmatic brines formed these magnetite-apatite deposits.
... We have used the values reported by Martin (1991). The potential contaminant is modeled as sulfide (Esser and Turekian, 1993), UM = Cyprus umbers, OC = Cyprus ochres, CP = Cyprus pyrite (Ravizza et al., 2001); BS = black shales (Horan et al., 1994;Cohen et al., 1999;Singh et al., 1999); MLO = organic material from Mariano Lake, MLS = sulfide from Mariano Lake uranium deposit, NHS = Norwood Hill diagenetic sulfides (all from Jurassic Morrison Formation; Hannah and Scherstén, 2001, and unpub. data); PG = Precambrian gneisses and schists Ripley et al., 1998); SHV = submarine hydrothermal vents (McCandless et al., 1999). ...
Economic sulfide deposits have been identified in only a handful of Proterozoic massif-type anorthosite complexes. Yet exploration interest has expanded greatly in the last decade since discovery of the Voisey's Bay deposits. The paucity of new discoveries may reflect the unique confluence of geologic conditions required to concentrate sulfides in this environment. Sulfide minerals in anorthosite complexes are generated when the silicate magma reaches sulfide saturation and exsolves an immiscible sulfide melt. If this occurs early in magma emplacement. the dense sulfide melt will tend to stall in feeders or near the base of the magma body. Thus, sulfide concentrations will be spatially confined in large anorthosite complexes, with sulfide exposure dependent entirely on erosional level. Generation of economic sulfide concentrations may require assimilation of crustal sulfide, either from sulfidic metasedimentary rocks or from volcanogenic or sediment-hosted massive sulfides. These sulfide sources are prevalent in lift settings and hence in the suture zones that commonly characterize the country rocks of major anorthosite-associated intrusive complexes. Nevertheless, development of sulfide concentrations may depend on the fortuitous intersection of magma with sulfidic country rocks. Rhenium-osmuim (Re-Os) isotope data are available for parts of three Proterozoic anorthosite complexes in each case, high initial Os-187/Os-188 requires a predominantly crustal source for the Os in the sulfide minerals, Modeling of Re-Os isotope data from the 1559 Ma Suwalki anorthosite massif leads to the hypothesis that sulfide saturation results from direct assimilation of crustal sulfide by a mantle-derived magma. Subsequent interaction of the sulfide phase with additional, uncontaminated silicate melt increases the concentrations of Re, os, and other chalcophile elements in the sulfides and shifts the Os-187/Os-188 of the crustally derived sulfide toward that of the mantle-derived silicate melt. Whereas we cannot rule out a crustal source for the parental magma at Suwalki, Re-os data also permit a mantle-derived melt modified by crustal contamination, We model two end-member processes that can form sulfide concentrations in mantle-derived melts by assimilation of crustal material. The first process assumes that upwelling magmas are contaminated by bull, silicate crust, bringing the magma to sulfide saturation and producing all immiscible sulfide melt. The magma continuously assimilates crustal material so that the silicate melt maintains high Os-187/Os-188. With increasing R (the mass ratio of silicate to sulfide liquid), concentrations of chalcophile elements, including Re and Os, increase in the sulfide melt but Os-187/Os-188 remains roughly constant. The second process assumes that upwelling magmas are contaminated by Re-Os-rich crustal sulfide, generating copious immiscible sulfide melt with Re and Os concentrations and Os-187/Os-188 not far removed from those of the contaminant. Assimilation of crustal material declines as the magma establishes armored conduits and/or chemical and thermal gradients between the magma and pristine country rock. With increasing R, continued flux of relatively uncontaminated magma increases Re and Os concentrations ill the sulfide but draws Os-187/Os-188 downward toward lower mantle values, if this second process dominates at Suwalki. the high gamma(Os) limits R to very low values, probably <50, affirming substantial input of externally derived sulfur without subsequent interaction with silicate melt. Whereas reality no doubt lies between these end-member processes, a model based on the second process, using geologically reasonable model parameters and physical processes, generates Re-Os data that agree well with measured Os-187/Os-188 values at Suwalki.
... The top-most argillaceous dolomitic formation of the Krol Group is overlain by bedded cherts, black shales, and pyrite-bearing phosphorites of the Lower Tal Formation (Chert-Phosphorite Member; Banerjee et al. 1997;Mazumdar and Banerjee 2001). Black shales within this unit with well-defined light carbon isotopes (Banerjee et al. 1992) gave Re-Os isochron ages for the Precambrian-Cambrian (554 ± 16 and 552 ± 22 Ma; Singh et al. 1999), and paleontological evidence (Brasier and Singh 1987;Mazumdar and Banerjee 1998) suggests Nemakit-Daldynian to Early Tommotian age. The Chert-Phosphorite Member (Mb.) is followed upward in the succession by marly to arenaceous members of arenites and microbial laminites of the Upper Tal Formation. ...
The occurrence of a trioctahedral analog of illite, the dioctahedral interlayer-deficient K-mica, has long been debated. Due to the inherent difficulties of determining structure and chemical composition of the extremely fine-grained material, earlier descriptions based on separated material are equivocal. Here we describe low-temperature (diagenetic) formation of fluorophlogopite, which is interlayer-deficient and therefore analogous to illite, using high-resolution in situ methods (transmission electron microscopy, TEM, with preparation by focused ion beam milling, combined with wavelength-dispersive analysis by field-emission gun electron microprobe). The average composition is K0.5Mg2.8V0.01Fe0.005 [Si3.15Al0.85O10(OH)0.65F1.35], including minor amounts of NH4 for charge compensation as determined by electron energy loss spectroscopy. The K-deficient Mg-mica occurs in layer packages of ~10 layers, and no indications for interlayering with other sheet silicate layers such as chlorite or vermiculite could be identified with TEM. X-ray powder diffraction patterns of separated material confirm the absence of smectite components.The mineral was identified in phosphorites from the lowermost Cambrian Tal Group, Mussoori Syncline, Lesser Himalayas, India. The rocks are alternating phosphatic mudstones and phosphatic dolostones, at times interbedded with phosphate-poor carbonate layers, which are rich in organic matter. Sedimentary fluorophlogopite occurs in both rock types and in two textural associations; one in vesicles filled with amorphic organic matter, the other as reaction rims around illite, which contains up to 5 wt% V2O3 in its rims. Textural arguments favor an early diagenetic formation of both, V-bearing illite and fluorophlogopite, closely associated with organic matter and linked to dolomitization. The high-F content stabilizes phlogopite to low temperatures. Our findings confirm that the stability field of fluorophlogopite extends from magmatic to metamorphic and sedimentary conditions.
... Rhenium -osmium dating of black shales provides another example of innovation in the dating of sediments. When used as a chronometer (Ravizza and Turekian, 1989), the pair of isotopes 187 Re -187 Os can indicate closed-system behaviour for Re and Os and provide isochron ages consistent with fossil evidence (Cohen et al., 1999;Singh et al., 1999). ...
The dating of land surfaces has long posed problems for geomorphologists. Relative methods (stratigraphic, geomorphic, topographic) are sound and convincing. Exhumed forms may complicate identification and relationships, for both epigene and etch forms have been buried, and exhumed, but in tectonically undisturbed areas, the higher surfaces are older than those preserved at lower levels. Also, surfaces have an age range. The relationship of surfaces with volcanic deposits, old shorelines, and genetically related sedimentary sequences provides sound ages, and correlation with dated duricrusts and faults is also useful. There are no temporal limits to relative dating, for the methods are equally applicable to the dating, say, of Proterozoic surfaces as of those of Pleistocene age. The disadvantage of such methods is that the necessary evidence is frequently either not preserved or not exposed.The so-called ‘absolute’ (physical, numerical) methods, and especially those based on exposure age dating with in situ cosmogenic radionuclides, are appealing because they produce direct numerical ages, and appear to be widely applicable, but there are severe temporal limitations, and sampling problems complicate, and may invalidate, interpretation. Absolute age determinations must be consistent with the stratigraphic and geomorphologic settings. The best results are obtained when physicists and earth scientists pool their knowledge and experience. A background in local and regional geology is especially important.
Ediacaran to Early Cambrian sequences are well preserved in several synclines within the Lesser Himalaya. We have conducted multi-proxy geochemical and stable isotopic studies on carbonaceous sediments of the basal Tal Group to decipher redox conditions of Earliest Cambrian Ocean. Organic matter is immature, predominantly aliphatic and likely derived from marine phytoplankton. The S/C(organic) ratios above 0.4; strong enrichment of redox sensitive trace elements V, Mo, U and Cd; higher DOPT, U/Th, V/Sc, Mo/Sc and V/(V+Ni) ratios; V/Al-U/Al-Mo/Al systematics and U-Mo enrichment factors indicate anoxic to euxinic conditions of deposition. Large variations (5-7 ‰) in 13C likely reflect spatial heterogeneity or vertical gradient in DIC compositions of the Early Cambrian Ocean, and possibly captures the part of Basal Cambrian negative Carbon isotope Excursion (BACE). Finely disseminated texture of pyrite framboids with sizes mostly below 10 µm suggest sulfide reduction within the water column in an open system. Isotopically heavy 34S values (+4.5 – +22.8 ‰) overlap with the 34Ssulfate of contemporary Fortunian ocean (+10–+40‰) suggesting sulfidic conditions and small size of the sulfate reservoir. Whereas, the REE+Y proxies of the same set of samples show oxidised seawater like patterns, with strong HREE enrichment, positive Y and La anomalies and negative Ce anomaly (avg. Ce/Ce* = 0.78), indicating REE uptake by phytoplankton from an oxidised euphotic surface zone. The combined geochemical and isotopic data suggest strongly stratified ocean with sulfidic bottom water and oxidised surface water. Episodic shoaling of anoxic deep water onto the surface possibly caused the extinction of benthic Ediacaran fauna, and the surface oxidised zone likely provided the niche for further metazoan radiation.
The gneisses and associated metasediments and intrusive ca. 1860 Ma Wangtu Granite, designated as the JeoriWangtu-Bandal Gneissic Complex (JWBC), formed the basement
for the oldest Lesser Himalayan rocks, when India was part of the Columbia supercontinent. A mega rift extending over the entire length of the present Himalaya
developed around ca.1800 Ma. It was accompanied with outpouring of the tholeiitic volcanics, the event was diachronous that younged towards the east. In this shallow marine basin, the Rampur/ Berinag/ Kuncha/Kushma/Ranimata Daling/ Shumar groups
accumulated. The lowest thrust sheet, labelled as the Kulu/ Munsiari/Ramgarh Porphery/ Daling/ Jashidanda thrust sheets, were nucleated a bit later or more or less coinciding with the rifting. The thrusting involved the basement rocks, which occur as tectonic slices in all the sectors of the Himalaya. Sedimentation of the Rampur Group and equivalent sequences ceased and after a pronounced hiatus, a carbonate shelf developed during the Mesoproterozoic-Early Neoproterozoic, in which sequences, variously designated as the
Shali/Larji/Deoban/Calc “Series”/Dhadang/Baxa/Dedza groups were deposited over the Rampur and equivalent sequences. At that time India formed part of the Rodina.
Rifting ca. 830 Ma, demolished the Shali-Baxa carbonate shelf. The succeeding rifted basin, located over the disrupted Mesoproterozoic-Early Neoproterozoic sequences, was also extensive and covered the nascent Kulu and equivalent thrust sheets. More or less simultaneously or with some time lag, the Chaur Granite and the embryonic Jutogh Thrust Sheet were emplaced. In this rifted basin, the Simla/ Jaunsar/ Nathuakhan and allied groups were deposited. The sedimentation in this basin terminated ~730 Ma, its sediments were raised to form an upland, which became abode of the Blaini valley glaciers. The glaciers descended in a marine basin with the Simla (which constituted the Outer Krol Belt) and the Jaunsar (Inner Krol Belt) forming the floors. A post-Blaini marine regression restricted the sea in which the Ediacaran Infra Krol and Krol sequences conformably succeeded the Blaini Formation. At the termination of the Krol sedimentation, the Outer Krol Belt was uplifted, the event coincided with the rifting reported in the Tethyan Bhutan. The sedimentation of the Tal Group continued in the Inner Krol Basin sited over the Jaunsar Group. Another basin (Chilar) was carved towards the SE of the Outer Krol basin, which possibly lasted up to the end of the Ediacaran. The late Cambrian-Early Ordovician Kurgiakh Orogeny terminated the Tal basin. As a component of the Kurgiakh orogeny in its early phase, the Cambrian Tethyan basin together with the underlying Vaikrita rocks was accredited to the Lesser Himalaya Tectogen; the event was associated with Early Palaeozoic granite emplacement and metamorphism. The Early Ordovician witnessed a rather extensive marine transgression, which possibly lasted till the Middle Silurian. Post Middle Silurian, in short intervals during the Permian and Cretaceous, no sedimentation took place in the Lesser Himalayan terrain which remained a positive area; its erosion contributed sediments to the Tethyan basin. The rifting in the
Gondwanaland/ Pangea, opened old lineaments, mainly close to the present foothill region to provide pathways to the Permian and Cretaceous seas. The subduction of the Indian Plate commenced during the Thanetian, consequently, the foreland basin was formed at the present foothill region; its embayments extended in the Lesser Himalaya over the folded and deeply eroded pre-Paleocene sequences. The continued subduction of the Indian Plate caused the earlier nucleated thrust sheets to advance southward and as a consequence of which the foreland basin shifted farther south. The Lesser Himalaya formed the provenance of this basin in which the Siwalik succession was deposited.
Due to impact of advancing crystalline thrust sheets, the Shali and Simla Group rocks moved partly over the Paleogene of the foreland basin. The Outer and Inner Krol Belts slid on their floors as superficial thrust sheets. The Late Cambrian folds, which are co-axial with the Himalayan folds, got accentuated and also selectively overturned. The Lesser Himalaya part was elevated. Weathering led to extensive erosion and release of the stresses, which periodically activated various faults causing earthquakes.
Recent decades have witnessed an increasing number of studies investigating petroleum systems with the application of rhenium-osmium (Re-Os) isotopic geochemistry. Here, we review the use of the 187Re-187Os geochronometer with respect to the geochemical behaviour of rhenium and osmium in hydrocarbon-related geological processes. The Re-Os budget in hydrocarbon source rock predominantly originates from natural water columns during its deposition. Open seawater tends to have a homogeneous Os isotopic composition because its residence time in seawater is longer than the time taken for ocean mixing. On the contrary, restricted water bodies (e.g., lakes) may have heterogeneous Os isotopic compositions due to the greater amount of terrigenous input. Hydrogenous Re and Os atoms are sequestered from the water body into sedimentary organic matter and transferred into crude oil through thermal maturation of organic matter. Thermal maturation likely does not significantly alter the Re-Os isotopic systematics of the source rock as a Re-Os isochron age of 442±21 Ma (2σ) is yielded in this study for over matured source rocks within the Silurian Longmaxi Formation from the Sichuan Basin. Re-Os atoms are mainly hosted by the highly polar/aggregating/aromatic asphaltenes in hydrocarbons, possibly chelating with organic complexes or occurring as metalloporphyrins. Resin and aromatic hydrocarbons also contribute to the Re-Os budget, but are 2 to 3 orders of magnitude lower than that of asphaltenes, whereas saturates do not contain appreciable Re-Os contents. The distribution of Re-Os atoms in hydrocarbons is heterogeneous because the duplicate analysis of pure single bitumen samples yields similar 187Os/188Os ratios whereas variable 187Re/188Os ratios. The Re-Os system in crude oils can be reset during transport away from the source rocks, with Os-rich organic fractions more readily expelled than Re-rich fractions. Contact with metal-rich fluids (e.g., hydrothermal fluid) or compositional changes related to asphaltene contents (e.g., deasphalting, biodegradation, thermal cracking and thermochemical sulphate reduction) are also likely to alter the Re-Os systematics in hydrocarbons. These geochemical features enable the 187Re-187Os isotopic system to have robust applicability for petroleum system investigations, which may use the Re-Os radiometric tool for: (1) stratigraphic correlation of source rocks; (2) dating geological events altering the asphaltene content in hydrocarbon such as hydrocarbon generation, thermochemical sulphate reduction, etc., and; (3) fingerprinting hydrocarbons. Regardless of the robustness of the 187Re-187Os geochronometer for petroleum system investigations, there are several pending questions such as partitioning between solid organic species or between organic matter and sulphide, chelating sites in hydrocarbons and Os isotopic equilibration between hydrocarbon subfractions. To improve the understanding of the Re-Os behaviour in petroleum systems, we underscore multi-proxies-based geochemistry (e.g., inorganic-organic geochemistry) and experimental studies (e.g., hydrous pyrolysis).
This chapter presents the currently available ¹⁸⁷Os/¹⁸⁸Os seawater data and emphasizes some of the challenges involved in reconstructing the seawater record from the present to the Precambrian. Radioactive decay of long-lived ¹⁸⁷Re to ¹⁸⁷Os, large differences in the compatibilities of rhenium (mildly incompatible) and osmium (very compatible) during magmatic differentiation, and various processes that mobilize and transport Os into the ocean shape this seawater record. Differences in magmatic compatibilities of Re and Os have created large isotope contrasts between osmium delivered to seawater from the radiogenic Earth’s crust (e.g. rivers, submarine groundwater discharge, aeolian dust) compared to osmium from the unradiogenic mantle (submarine hydrothermal vents) and undifferentiated extraterrestrial matter. Together with the relatively short (10³–10⁴ yr) residence time of osmium in seawater these factors are responsible for the very dynamic isotope evolution of seawater that offers great potential for chemostratigraphic reconstructions. A full assessment of the chemostratigraphic utility of the marine ¹⁸⁷Os/¹⁸⁸Os record requires that it be reconstructed in multiple ocean basins at a temporal resolution approaching the marine residence time of Os. Presently, large time intervals are undersampled. Where high temporal resolution studies have been conducted at widely separated sites, observed ¹⁸⁷Os/¹⁸⁸Os offsets between sites exceed analytical uncertain. It is therefore a possibility that subtle offsets between the ¹⁸⁷Os/¹⁸⁸Os of seawater in different ocean basins could have existed in the past.
Discs/discoidal body fossils, an important constituent of Ediacaran biota, are reported from the sandstone unit of the Hotpet Formation of the Bhima Group, South India. Nine specimens preserved as circular to ovate structures are described and attributed to Ediacaran discs and are akin to Fermeuse-style preservation of Ediacaran fossils. These discs, occur as impressions on the bedding planes, are referred as Charniodiscus-like fossils. This is the first report of Ediacaran discs from the Bhima Group, south India.
The rhenium-osmium (Re-Os) isotopic system preserved in ancient seafloor hydrothermal sulfide deposits can be used to gain insight into the Os composition of ancient seawater. In particular, the age of sedimentary exhalative (SEDEX) deposits can be determined by measuring Re-Os isotopes in syn-depositional pyrite, a common constituent of SEDEX deposits. Here we report on the Gaobanhe sediment-hosted, polymetallic pyrite deposit which is hosted within the Gaoyuzhuang Formation in the central part of the northern margin of the North China Craton (NCC). Re-Os isotopic data from syn-depositional pyrite yield an isochron age of 1505 ± 55 Ma, constraining the timing of primary mineralization, as well as the minimum age of the host sediments to the Mesoproterozoic. Analyzed pyrite grains indicate an initial ¹⁸⁷Os/¹⁸⁸Os (Osi) value of 0.181 ± 0.066, suggesting that the mineralizing fluids were an admixture of seawater and submarine hydrothermal fluids, with the Os being primarily derived from seawater. The Osi value is slightly higher than that of Paleoproterozoic seawater (~0.13), suggesting that there was a radiogenic riverine flux to the oceans and that atmospheric oxygen must have been sufficiently high at that time to drive this increased, continental radiogenic Os input to the oceans. We propose that the Re-Os isotopic system, as recorded by these SEDEX deposits, may provide key insights when examining changes in the Os isotopic composition of seawater throughout the Precambrian.
A new technique is developed for in situ measurement of 187Os /188Os in sulfide using LA-MC-ICP-MS, with the improvement of the spatial resolution and quantification limits. In this study, an all-ion counters (all-IC) configuration is adopted for static concurrent measurement of 187Os+187Re and 188Os along with 185Re. To cross-calibrate the detection efficiencies of the different ion counters, an Au standard solution is introduced to the instrument and its signal is measured in each ion counter by a dynamic peak jumping mode. Then, the signal intensities of IC1, IC2 and IC3 are all normalized to that of IC0. The sample-standard bracketing (SSB) method is adopted not only for the Re interference and Os mass fractionation correction but also for the monitoring of detection efficiency during continuous measurement. In detail, the in-house sulfide standard CR-1P (only spiking Re) is specifically employed for correcting interference 187Re on 187Os and monitoring detection efficiency variation in IC0 and IC1. The mass fractionation of Os and detection efficiency variation of IC1 and IC2 are calibrated against another in-house sulfide standard CO-1P (only spiking Os). Even though the 188Os intensity decreases approximately 20 times, our improved method still provides accurate and precise results of 187Os/188Os ratio, whereas published methods that employ Faraday detectors for the signal collection do not work. Furthermore, the threshold of reliable interference correction of 187Re on 187Os is estimated by analysing synthetic sulfide with different Re/Os ratios. The analytical results demonstrate that the relative deviations (RD) of the calculated 187Os/188Os ratio will be limited to within 0.8%, 1%, 3% and 5%, when 187Re/188Os is lower than 0.055, 0.09, 0.42 and 0.75 (i.e., Re/Os: 0.012, 0.019, 0.089 and 0.159), respectively. In general, this new method significantly improves the spatial resolution and quantification limits of in situ measurements, which is beneficial for analysing sulfide in the depleted subcontinental lithospheric mantle (SCLM).
Reconstructing the stratigraphic architecture of deposits prior to Cenozoic Himalayan uplift is critical for unravelling the structural, metamorphic, depositional and erosional history of the orogen. The nature and distribution of Proterozoic and lower Paleozoic strata have helped elucidate the relationship between lithotectonic zones, as well as the geometries of major bounding faults. Stratigraphic and geochronological work has revealed a uniform and widespread pattern of Paleoproterozoic strata >1.6 Ga that are unconformably overlain by <1.1 Ga rocks. The overlying Neoproterozoic strata record marine sedimentation, including a Cryogenian diamictite, a well-developed carbonate platform succession and condensed fossiliferous Precambrian–Cambrian boundary strata. Palaeontological study of Cambrian units permits correlation from the Indian craton through three Himalayan lithotectonic zones to a precision of within a few million years. Detailed sedimentological and stratigraphic analysis shows the differentiation of a proximal realm of relatively condensed, nearshore, evaporite-rich units to the south and a distal realm of thick, deltaic deposits to the north. Thus, Neoproterozoic and Cambrian strata blanketed the northern Indian craton with an extensive, northward-deepening, succession. Today, these rocks are absent from parts of the inner Lesser Himalaya, and the uplift and erosion of these proximal facies explains a marked change in global seawater isotopic chemistry at 16 Ma.
The Middle Eocene Climatic Optimum (MECO) represents a ~500-kyr period of global warming ~40 million years ago and is associated with a rise in atmospheric CO2 concentrations , but the cause of this CO2 rise remains enigmatic. Here we show, based on osmium isotope ratios (187Os/ 188Os) of marine sediments and published records of the carbonate compensation depth (CCD), that the continental silicate weathering response to the inferred CO2 rise and warming was strongly diminished during the MECO-in contrast to expectations from the silicate weathering thermostat hypothesis. We surmise that global early and middle Eocene warmth gradually diminished the weatherability of continental rocks and hence the strength of the silicate weathering feedback, allowing for the prolonged accumulation of volcanic CO2 in the oceans and atmosphere during the MECO. These results are supported by carbon cycle modeling simulations, which highlight the fundamental importance of a variable weathering feedback strength in climate and carbon cycle interactions in Earth's history.
A travers l'exemple himalayen, nous étudions les mécanismes influençant la composition en osmium des sédiments durant l'érosion, le transport sédimentaire et le dépôt dans les zones estuariennes. Au niveau du bassin himalayen de la Kali Gandaki, les relations observées entre les concentrations en osmium et les quantités de carbone organique confirment le rôle important des schistes noirs appartenant aux TSS (non radiogéniques) et au LH (radiogénique) malgré leurs faibles répartitions géographiques. Cependant, la composition fortement radiogénique mesurée dans les sédiments du Gange n'est pas couplée a un enrichissement en 187Os dont la quantité moyenne est comparable à celle mesurée dans la croûte continentale. La signature isotopique du Gange résulterait d'un appauvrissement en Os non-radiogénique, témoignant d'une forte dilution par des sédiments dérivés de l'érosion des roches cristallines de l'HHC. Nous montrons à travers l'étude de sédiments estuariens que le comportement de l'osmium en contact avec l'eau de mer est complexe et des échanges sont possibles à l'interface. Le développement analytique réalisé en parallèle des études sur les sédiments a permis de documenter pour la première fois la composition en osmium des eaux souterraines. L'étude des aquifères de la plaine du Bengale montre que les eaux souterraines possèdent des concentrations en osmium significativement plus élevées que les eaux de rivière ou l'eau de mer. Si ce résultat est généralisable aux aquifères mondiaux, un flux global de l'ordre de 170 kg d'osmium par an pourrait être apporté à l'océan. Cet apport est significatif et impliquerait la réévaluation du bilan océanique mondial et une diminution significative du temps de résidence de l'osmium dans les océans. Ce résultat n'est pas anodin puisqu'il pourrait en partie réconcilier le temps de résidence estimé par bilan de masse océanique et les variations glaciaires-interglaciaires observées par le rapport 187Os/188Os dans de nombreux enregistrements marins
Cette étude a permis de déterminer la composition isotopique d'Os des trois principales formations himalayennes (TSS, HHC et LH) à partir de roches sources, ainsi que celle de sédiments de rivières qui représentent le produit d'érosion de la chaîne. Les formations du Haut Himalaya ont un rapport 187 0s/188 0s comparable à celui de la croûte continentale moyenne. Par contre, le LH se distingue par des rapports radio géniques rencontrés dans deux lithologies distinctes: les schistes noirs et les marbres impurs du LH. Un mélange de trois pôles (silicates du Haut Himalaya + schistes noirs du LH + carbonates impurs du LH) permet d'expliquer les signatures des sédiments de rivières à la sortie de la chaîne ou dans le Gange. Les sédiments du Brahmapoutre montrent un rapport moins radio génique que ceux du Gange, expliqué d'une part par la présence d'affleurement d'ophiolites dans le bassin versant du Brahmapoutre et d'autre part par l'absence de carbonates résiduels. L'origine des carbonates radiogéniques dans le LH est à relier au métamorphisme himalayen qui a permis la mobilisation d'Os et de Re à partir des schistes noirs radio géniques du LH. L'altération à basse température étudiée à partir de sols a montré que les compositions isotopiques et les concentrations d'Os et de Re ne varient pas avec la profondeur. Cependant une perte importante d'Os et de Re a probablement lieu dans les premiers stades de l'altération, lors du passage de la roche saine au saprolite. Les sédiments récents du Cône du Bengale montrent que l'érosion de la chaîne himalayenne est la source majeure des sédiments, surtout dans le cône actif. Cependant, dans le reste de la Baie du Bengale, cette source himalayenne peut être masquée par des sources secondaires, comme l'érosion de la chaîne indo-birmane ou l'érosion du Sri Lanka. Il n'y a aucune preuve qu'une perte significative d'Os lors du transport ou après le dépôt dans le Cône se produise. Ceci implique que la contribution de l'érosion himalayenne au budget de l'Os marin est limitée à la phase dissoute, et donc insuffisante pour expliquer l'augmentation du rapport 187 0s/188 0s marin durant les 16 derniers Ma.
The Indian subcontinent’s biota and biostratigraphy is amongst the least well known internationally of any Cambrian succession worldwide. Recent revision of previously described type material and a substantial number of new finds reveal a typical Cambrian skeletonized fauna and an organic walled biota, as well as various trace fossils. This biota, reviewed here synoptically, currently contains 51 non-agnostoid trilobite genera belonging to 50 species; 15 genera and species of agnostoids; one species of bradoriid arthropod; 18 brachiopod genera containing 20 species; echinoderm thecal and columnal plates; a soft-bodied eldoniid; representatives of three hyolith genera; other small shelly fossils some of which are identified to species level; acritarchs; and a variety of ichnotaxa. Regional biostratigraphic zonations for trilobites, brachiopods, small shelly fossils, and trace fossils are herein combined into an integrated Cambrian biostratigraphical scheme that permits correlation along and across the lithotectonic zones of the Himalayan margin and southward onto cratonic India. These consist of 13 named biostratigraphic units for trilobites, seven for brachiopods, three for small shelly and organic-walled fossils, and one for trace fossils. The basal boundary of the Cambrian (~ 541 Ma) is biostratigraphically localised in the Lesser Himalaya between Ediacaran carbonate-rich beds bearing the organic-walled tubular Shaanxilithes ningqiangensis, and dark, highly stratigraphically condensed, phosphate-rich shale bearing Fortunian (~ 535 Ma) and Cambrian Stage 2 age (~ 524 Ma) small shelly fossil assemblages. The oldest macrofossils presently known are somewhat younger, and represent early Cambrian Stage 4. The Redlichia noetlingi trilobite Zone/Botsfordia granulata brachiopod Zone (~ 512 Ma), is widely represented in siliciclastic rocks across and along the Himalaya, spanning the Tethyan, Lesser, and sub-Himalayan lithotectonic zones, and possibly also extending onto the craton. Stage 5 Cambrian fauna are the most diverse and best biostratigraphically characterised, much of which is relatively well preserved in limestone, but presently known only in the Tethyan Himalaya where species-level correlation between the Zanskar and Parahio valleys has been established. There, rates of sediment accumulation were notably high, with some 2000 m of compacted rock deposited within approximately 13 Myr. During this interval, the ranges of some trilobite and brachiopod species (and their zones) were apparently less than one million years long. The indicus trilobite zone has recently been localized, shortly below the prachina zone. Furongian (late Cambrian, ~ 493 Ma) fossils are known only in the Bhutanese Himalaya. Except where faulted, the Cambrian succession is capped by an unconformity throughout the Himalaya. In the Salt Range and Lesser Himalaya this unconformity is sub-Permian, but in the Tethyan Himalaya it is sub-Ordovician. The regional Cambrian can now be correlated globally with reasonable precision, and all parts of the Himalayan margin south of the Yarlung-Tsangpo suture have a core equatorial Gondwanan biota most similar to North China, and particularly, to South China. Links with Australia are suggested by non-cosmopolitan species but Indian biotas share less in common with Australia than with parts of China. Knowledge of the Himalayan Cambrian biota serves a critical role in constraining the Caenozoic uplift and erosional history of the orogen.
As Re and Os are organophilic and redox-sensitive, Re and Os are concentrated from seawater in organic-rich sediments. Re-Os geochronology of organic-rich sediments has the potential to provide direct depositional ages and vital information on the Os isotope. The successful application of Re-Os in organic-rich sedimentary rocks making it possible to obtain absolute ages for the deposition of sedimentary successions directly, and hence the age of some sedimentary deposits and glacial can be dated and constrained. From the Re-Os isotope characteristics obtained for organic-rich sediments, the Palaeoenvironment can be reconstructed, and this helps to know and understand the paleoenvironmental evolution and to date and trace the metallic deposits and oil/gas reservoir, thus playing an important role in studying numerous major issues, such as the mass extinction and the impact events. Based on the Re-Os isotope data available, the authors obtained Os isotope curve for seawater from 2.7 Ga to the present day. In addition, the variations in the Os isotope compositions of seawater since 2.7 Ga are helpful to the study of the paleoenvironmental evolution. In this paper, some factors affecting the precision of Re-Os isochron age for organic-rich sediments are investigated. The Re-Os isotope system will play a unique role in the dating of organic-rich sedimentary rocks and the reconstruction of palaeoenvironment.
The Early Cambrian Niutitang Formation in South China consists of a black shale sequence with a synsedimentary organic carbon-rich polymetallic sulfide layer with extreme metal enrichment locally mined as Ni- Mo-PGE-Au ore. We report a new composite Re-Os isochron age of the sulfide ore layer of 521 ± 5 Ma (n = 14) from three mine sites (Dazhuliushui and Maluhe in the Guizhou province and Sancha in the Hunan province) several hundred kilometers apart. This age corresponds to the biostratigraphic Tommotian age, and is in agreement with recent interpolated Pb-Pb and SHRIMP zircon U-Pb ages from an underlying tuff unit. The initial 187Os/188Os value of the sulfide ore is 0.87 ± 0.07, which is equal to the initial 187Os/188Os ratio of ̃0.80 of the black shale host rock, consistent with the seawater origin of the metals under depositional conditions of very low clastic input.
The Mesoproterozoic Bayan Obo Group in central Inner Mongolia, China, represents a sedimentary sequence deposited in the Bayan Obo Rift Basin. New Re-Os dates are reported here for the black shale (schist) from the group. Re-Os geochronology of the black schist provides an age of 1447±42Ma, which is consistent with the previous work from the literature and provides a direct and new constraint for the age of the group. The dating indicates that the Re-Os isotopes for the black schist have remained undisturbed during lower amphibolite facies metamorphism. The ca. 250Ma age for gold-bearing sulfides also indicates that the Re-Os isotopes for the schist have not been affected by hydrothermal overprinting of reduced fluid. The initial 187Os/188Os value of 0.93±0.14 for the schist is interpreted to represent the Os isotopic composition of the seawater during deposition of the group, which indicates that the Os composition of Mesoprotozoic seawater fluctuated during the Mesoproterozoic, and was temporary very radiogenic similar to present day seawater. The Re-Os data suggests that rifting of the Bayan Obo Basin peaked during ca. 1447Ma, and the Bilute Formation in the Bayan Obo Group may have been deposited in the Bayan Obo Sea in a restricted or semi-restricted depositional environment.
The role that marine organic-rich sediments play in the surficial osmium and rhenium cycles has been assumed to be important due to the high Re/Os ratios and high concentrations of these elements in such reducing sediments compared to the average continental crust. The high Re/Os leads to rapidly increasing 187Os/188Os ratios. We determine how large a role these sediments play in the surficial osmium and rhenium budgets by compiling a geochemical database to quantify the mean 187Re/188Os and 187Os/188Os ratios of organic-rich sediments. We then utilize estimates of the abundance of organic-rich sediments in the geologic record of different ages to estimate an average 187Os/188Os for these sediments. We find that the calculated 187Os/188Os value of organic-rich sediments varies between 2.22 and 2.27 depending on the volume-age model used, with error estimates of 2.07 to 2.42 and 2.12 to 2.48, respectively. Despite the fact that these sediments constitute a relatively small proportion of the upper continental crust, they contribute a disproportionately large amount (> 50%) to the upper continental crustal rhenium and, to a lesser extent (5-10%), osmium inventories owing to their high concentrations of these elements.
New Re–Os isotopic data were obtained from chlorite-grade black shales from the upper Old Fort Point Formation (Windermere Supergroup), a post-glacial Neoproterozoic marker horizon in western Canada. A Re–Os isochron date of 634±57 Ma (MSWD=65, n=5) was determined using the conventional inverse aqua regia digestion medium. However, dissolution of the same samples with a new CrO3–H2SO4 dissolution technique [Chem. Geol. 200 (2003) 225] yielded a much more precise date of 607.8±4.7 Ma (MSWD=1.2). Both dates are in agreement with existing U–Pb age constraints that bracket the Old Fort Point Formation between ∼685 and ∼570 Ma. The distinctive Re–Os systematics recorded by the two analytical protocols is explained by dissolution of a variably radiogenic, detrital Os component by the aqua regia method. In contrast, the CrO3–H2SO4 technique minimizes this detrital component by selectively dissolving organic matter that is dominated by hydrogenous (seawater) Re and Os. The date of 607.8±4.7 Ma is thus interpreted as the depositional age for the upper Old Fort Point Formation providing a minimum age constraint for the timing of the second Windermere glaciation in western Canada. This ice age is correlative with the Marinoan (∼620–600 Ma) ice age and older than the ∼580-Ma Gaskiers glaciation of northeastern North America. The new Re–Os age determined from the CrO3–H2SO4 digestion technique thus provides further support to a growing body of evidence for a global Marinoan glacial episode. Such an interpretation would not be discernable from the imprecise Re–Os date obtained with the aqua regia protocol. These results also indicate the potential for Re–Os radiometric dating of black shales that was not previously recognized. Importantly, neither chlorite-grade metamorphism nor the low organic content (TOC <1%) of the Old Fort Point Formation precluded the determination of a precise Re–Os depositional age using the CrO3–H2SO4 analytical protocol.
The applications of the187Re-187Os isotope pair as a petrogenetic and geologic tracer are increasing in recent years due to several advances in the chemical
extraction and purification of Re and Os, occurring at ppb levels in environmental samples, and in the precise determination
of the Os isotope composition. We have established in our laboratory; based on available methods, chemical procedures and
Negative Thermal Ionisation Mass Spectrometric techniques for the measurement of Re-Os concentrations in environmental samples
and the Os isotope composition in them. Using these techniques, we are able to determine187Os/186Os ratios with a precision of ∼ 1% (±2σμ; twice the standard error of the mean) in several tens of picogram of Os. Preliminary analysis of black shales from the Lower
Tal section of the Maldeota phosphorite mine yields a mean187Re-187Os model age of 597 ± 30 Ma. The187Os/186Os and Os concentration in black shales of the Lesser Himalaya range from 8 to 96 and 0.02 to 13 ng g-1 respectively. The mean187Os/186Os in these samples is ∼ 25, significantly higher than the crustal value of ∼ 10.5, suggesting that these black shales could
be an important source of radiogenic Os to the rivers draining the Himalaya and to the steady increase in187Os/186Os of the oceans through the Cenozoic.
Outlines the salient features and inferred depositional regimes for black shales in the stratigraphic record and provide guidelines for further study. Includes a discussion of OC preservation in modern marine environments as a prelude to interpretation of the ancient record, and of proposed biotic and geochemical indicators of degree of oxygenation in the water column. -from Authors
A systematic study of the major ion chemistry of the Ganga source waters—the Bhagirathi, Alaknanda and their tributaries—has
been carried out to assess the chemical weathering processes in the high altitude Himalaya. Among major ions, Ca, Mg, HCO3 and SO4 are the most abundant in these river waters. These results suggest that weathering of carbonate rocks by carbonic and sulphuric
acids dominates in these drainage basins. On an average, silicate weathering can contribute up to ∼ 30% of the total cations.
The concentration of total dissolved salts in the Bhagirathi and the Alaknanda is 104 and 115mg/l, respectively. The chemical
denudation rate in the drainage basins of the Bhagirathi and the Alaknanda is, respectively, 110 and 137 tons/km2/yr, significantly higher than that derived for the entire Ganga basin, indicating intense chemical erosion of the Himalaya.
Osmium isotopic compositions and concentrations have been determined for deltaic and continental shelf sediments from three major rivers (the Amazon, Changjiang, and Mississippi) and for two loesses from the upper Mississippi River Valley. The loesses have heavy mineral and clay compositions characteristic of Mississippi river and delta sediment. Concentrations range from 15 to 120 pg Os/g, and ¹⁸⁷Os/¹⁸⁶Os ratios range from 8.2 to 10.9. Sediments which are polluted (surface sediment from the Changjiang Estuary) or which have scavenged metals from seawater (Gulf of Mexico slope surface sediment and distal Amazon Shelf sediment) have high osmium concentrations and less radiogenic isotopic compositions. Purely terrigenous sediments have 15–90 pg Os/g, and ¹⁸⁷Os/¹⁸⁶Os= 10.0–10.9. North American loess and Mississippi Delta sediment osmium isotopic compositions are indistinguishable. We believe that ¹⁸⁷Os/¹⁸⁶Os= 10–11 is a good average for currently eroding upper continental crust—these sediments have neodymium isotopic compositions close to the global mean for river sediment. Terrigenous ¹⁸⁷Os/¹⁸⁶Os is significantly more radiogenic than seawater ¹⁸⁷Os/¹⁸⁶Os(8.6), requiring a nonriverine source of osmium to seawater. Terrigenous osmium is also significantly more radiogenic than bulk pelagic clay osmium. We use the improved estimate of terrigenous ¹⁸⁷Os/¹⁸⁶Os to recalculate the flux of extraterrestrial osmium to Pacific marine sediment.
Rhenium and osmium abundances and osmium isotopic compositions were determined by negative thermal ionization mass spectrometry for samples of Devonian black shale and an associated Ni-enriched sulfide layer from the Yukon Territory, Canada. The same composition information was also obtained for samples of early Cambrian Ni-Mo-rich sulfide layers hosted in black shale in Guizhou and Hunan provinces, China. This study was undertaken to constrain the origin of the PGE enrichment in the sulfide layers.Samples of the Ni sulfide layer from the Yukon Territory are highly enriched in Re, Os, and other PGE, with distinctly higher Re/192Os but similar Pt/Re, compared to the black shale host. Re-Os isotopic data of the black shale and the sulfide layer are approximately isochronous, and the data plot close to reference isochrons which bracket the depositional age of the enclosing shales. Samples of the Chinese sulfide layers are also highly enriched in Re, Os, and the other PGE. Re/192Os are lower than in the Yukon sulfide layer. Re-Os isotopic data for the sulfide layers lie near a reference isochron with an age of 560 Ma, similar to the depositional age of the black shale host. The osmium isotopic data suggest that Re and PGE enrichment of the brecciated sulfide layers in both the Yukon Territory and in southern China may have occurred near the time of sediment deposition or during early diagenesis, during the middle to late Devonian and early Cambrian, respectively.
Detrital sediment is carried from land to the sea by three agents, rivers, glaciers, and winds. The shoreline is an arbitrary boundary within the detrital sediment transport system, which extends from a site of origin across areas of temporary storage to a site of long-term deposition. The most important of the agents moving sediment across the land is river transport, estimated to be in the order of 20×1012 kg of sediment annually at present. Analysis of drainage basins indicates that relief and runoff are the most important factors in determining the sediment load of rivers. The competence of rivers to transport sediment is governed by the volume flow, gradient, and the sediment load itself. Today, most large rivers are fed by snowmelt in highland areas, runoff from rainfall in the drainage basin, and groundwater inflow. Along the river course, water is lost to evaporation and groundwater infiltration. River courses can often be divided into two segments, a degradational section in which the gradient is relatively steep and little temporary storage of sediment takes place, and an aggradational section where the gradient is sharply reduced through meandering, and where large-scale temporary sediment storage forms a flood plain. Lakes trap sediment inland and prevent its transport to the sea. Today, many high and mid-latitude rivers are interrupted by lakes of glacial origin. There are also some large areas of internal drainage that deliver no sediment to the sea. The load carried by rivers has been markedly altered by human activity, and may have doubled over the past few thousand years, only to be reduced in the past century by the widespread construction of dams. The ancient use of fire in hunting and its subsequent use in clearing land has increased erosion. Extensive deforestation and cultivation processes have also increased the sediment supply. Dam construction is a relatively new factor and affects the sediment transport system by trapping sediment before it can reach the sea. The resulting lower sediment supply from rivers is, at least in part, compensated by increased coastal erosion. Glacial erosion is difficult to estimate. There is an ongoing controversy whether ice sheets are effective erosive agents or not. Estimates of the present global flux of glacial detritus range from 0.8–50×1012 kg annually, with the lower value most probable. The dust flux is in the order of 0.5 to 0.9×1012 kg annually, but may vary greatly with time.
Rhenium (Re) is one of a suite of elements (including uranium and molybdenum) that display conservative behavior in seawater and are enriched in anoxic sediments. The decay of187Re to187Os provides a geochronometer in ancient sedimentary rocks and gives rise to Os-isotopic variations in nature. In order to better characterize its sources to seawater, Re was measured in three major rivers (Amazon, Orinoco, Ganges-Brahmaputra) and some of their tributaries. Re concentrations span four orders of magnitude (from < 0.02 to 400 pmol/kg), with the highest concentrations found in rivers draining black shales in the Venezuelan Andes. Mainstream Re levels in the three rivers are between 1 and 10 pmol/kg, with a flux weighted average of 2.3 pmol/kg. The residence time for Re in the oceans is estimated to be 750,000 yr with respect to river inputs.Re profiles from the Atlantic and Pacific Oceans confirm that Re behaves conservatively in seawater, with no significant uptake onto particles and/or recycling within the water column. This is also true in the anoxic water column of the Black Sea. Re removal into anoxic sediments occurs at or below the sediment water interface, as demonstrated in sediment pore waters from Chesapeake Bay. In oxic sediments, Re is not cycled with manganese oxides, and it is not enriched in very slowly accumulating pelagic sediments with a large hydrogenous iron and manganese component, or in manganese nodules. Burial of Re in anoxic sediments, which accumulate on 0.3% of the ocean floor, removes approximately 50% of the riverine Re flux to the oceans. Hence, oceanic Re concentrations may be very sensitive to changes in the area of anoxic sedimentation.
Early investigators1,2 noticed that the isotope age curve for sedimentary carbon contains a negative displacement in δ13C at the Precambrian/Cambrian (PC/C) boundary, about 570 million years ago 3, which may have accompanied the profound biotic changes4 from non-skeletal organisms to those with mineralized skeletons. Whereas the certainty of the δ13C shift across the elusive boundary has recently been confirmed from a number of localities5-8, its interpretation remains largely controversial. It has been attributed to variations in oceanic biomass fertility and concomitant fluxes of organic matter burial6-8, changing rates of ocean ventilation9, onset of biomineralization10 and extraterrestrial causes5. Here we report the results of a carbon isotope study from a hitherto unexplored section of marine sediments in the Lesser Himalaya, India, which documents significant isotope variations across the PC/C boundary. These results also provide important details towards resolving the relative timing of the δ 13C shifts and the biotic changes. Evaluation of our data rules out post-depositional isotope exchange as a major cause of the recorded δ13C variations and favours changes in the isotope chemistry of the ocean surface at the close of the Precambrian.
The lithostratigraphic classification of the Blaini-Infra Krol-Krol-Tal succession of the Lesser Himalaya, ranging in age from Terminal Proterozoic to Early Cambrian, has been standardised. The succession rests unconformably over Jaunsar/Simla Group and in turn is unconformably overlain by younger sequences belonging either to Early Permian (Boulder Slate Formation) or (as generally is the case) by Late Cretaceous-Eocene sediments represented by Shell Limestone (Manikot) and Subathu formations, respectively. Reviewing the lithostratigraphy, the status of each formation has either been raised to that of a group or redefined. -from Authors
The oceanic variation of 87Sr/86Sr over at least the last 100 million years has been established through the study of calcareous biogenic deposits found principally in deep-sea sediments. The large picture is that in sediments from about 100 million years ago until about 38 million years ago 87Sr/86Sr shows only small variations but from that time forward there has been an increase. This variation has been ascribed to the changing relative importance with time of two mixing components: Sr with relatively low 87Sr/86Sr derived primarily by the weathering of ridge basalts and added to the oceans at hydrothermal vents and more radiogenic Sr derived from the weathering of continental crust. The approximately 4-million-year residence time of Sr is about a thousand times the mixing time of the oceans, so a marine 87Sr/86Sr record anywhere in the deep ocean basins speaks to global changes. The fine details of the sources and types of the actual continental rocks undergoing weathering continue to be the subject of active discussion.
There is a large uncertainty in our understanding of the behavior of osmium during weathering and transport into deep oceans and the osmium budget of the oceans. The problem stems chiefly from the lack of osmium data on the dissolved load in the rivers and in the estuaries. In this study, the concentration and isotopic composition of osmium have been determined in three North American rivers (the Mississippi, the Columbia, and the Connecticut) and one river draining central Europe and flowing into the Baltic Sea (the Vistula). Osmium concentration in the Mississippi and the Vistula is about 45 femto mol kg^(−1); it is about 14 and 15 femto mol kg^(−1) for the Connecticut and the Columbia, respectively. The ^(187)Os/^(186)Os ratios estimated for the Mississippi and the Vistula are 10.4 and 10.7, respectively. For the Connecticut and the Columbia ^(187)Os/^(186)Os = 8.8 and 14.4, respectively. Of all the rivers examined, the Mississippi is by far the largest, supplying ∼1.6% of the total annual world river flow. Its osmium isotopic composition is identical to the upper Mississippi valley loesses (Esser and Turekian, 1993a) indicating (1) congruent dissolution of the bedrock and (2) little or no impact of anthropogenic sources on the osmium isotopic composition of the dissolved load. The latter observation indicates that the upper limit of the anthropogenic input in the dissolved osmium load of the Mississippi outflow is about 250 g yr^(−1). While the osmium concentration of the Vistula is high the isotopic composition does not appear to have been affected by substantial pollution.
The river data can be used to put limits on the mean residence time of osmium in the oceans (T_(Os) and on the osmium budget of the oceans. If the bulk river influx of dissolved osmium into the oceans is similar to that of the Mississippi, we get a value for the net riverine inflow of osmium of 1680 mol yr^(−1). If there were no sequestering of osmium in the estuaries, this would give a value of T_(Os) of = 1.3 × 10^4 years. As the estuaries may trap a significant amount of river osmium, this is a lower limit so that T_(Os) » 1.3 x 10^4 years. Using the iridium data by Anbar et al. (1996), the Os/Ir ratio for the Vistula is 2.8. We infer that there is no large fractionation between osmium and iridium during the transport of these elements from the continents into the oceans. It follows that the high Os/Ir ratio (∼22) observed in seawater must be related to the different rain out mechanisms of these elements in the estuaries or in the oceans.
Seawater 187 Os/ 188 Os ratios for the Middle Miocene were reconstructed by measuring the 187 Os/ 188 Os ratios of metalliferous carbonates from the Pacific (DSDP 598) and Atlantic (DSDP 521) oceans. Atlantic and Pacific 187 Os/ 188 Os measurements are nearly indistinguishable and are consistent with previously published Os isotope records from Pacific cores. The Atlantic data reported here provide the first direct evidence that the long-term sedimentary 187 Os/ 188 Os record reflects whole-ocean changes in the Os isotopic composition of seawater. The Pacific and the Atlantic Os measurements confirm a long-term 0.01/Myr increase in marine 187 Os/ 188 Os ratios that began no later than 16 Ma. The beginning of the Os isotopic increase coincided with a decrease in the rate of increase of marine 87 Sr/ 86 Sr ratios at 16 Ma. A large increase of 1 in benthic foraminiferal 18 O values, interpreted to reflect global cooling and ice sheet growth, began approximately 1 million years later at 14.8 Ma, and the long-term shift toward lower bulk carbonate 13 C values began more than 2 Myr later around 13.6 Ma. The post-16 Ma increase in marine 187 Os/ 188 Os ratios was most likely forced by weathering of radiogenic materials, either old sediments or sialic crust with a sedimentary protolith. We consider two possible Miocene-specific geologic events that can account for both this increase in marine 187 Os/ 188 Os ratios and also nearly constant 87 Sr/ 86 Sr ratios: (1) the first glacial erosion of sediment-covered cratons in the Northern Hemisphere; (2) the exhumation of the Australian passive margin-New Guinea arc system. The latter event offers a mechanism, via enhanced availability of soluble Ca and Mg silicates in the arc terrane, for the maintenance of assumed low CO 2 levels after 15 Ma. The temporal resolution (three samples/Myr) of the 187 Os/ 188 Os record from Site 598, for which a stable isotope stratigraphy was also constructed, is significantly higher than that of previously published records. These high resolution data suggest oscillations with amplitudes of 0.01 to 0.02 and periods of around 1 Myr. Although variations in the 187 Os/ 188 Os record of this magnitude can be easily resolved analytically, this higher frequency signal must be verified at other sites before it can be safely interpreted as global in extent. However, the short-term 187 Os/ 188 Os variations may correlate inversely with short-term benthic foraminiferal 18 O and bulk carbonate 13 C variations that reflect glacioeustatic events.
The 87 Sr / 86 Sr ratio in sea water has varied over geologic time due to the addition of strontium to the sea from rocks with a variety of 87 Sr / 86 Sr ratios. The measurements by et al. (Geochim. Cosmochim. Acta 34 , 105-120, 1970) of the value of the marine 87 Sr / 86 Sr ratio have been confirmed by several other workers and by some new measurements on JOIDES samples. They form the basis of a model calculation of the relative proportions of `basaltic' ( 87 Sr / 86 Sr = 0.704) and `granitic' ( 87 Sr / 86 Sr = 0.718) strontium being supplied to the sea. For the last 200 million years, the proportions of these two sources appear to reflect the history of global tectonics; `basaltic' during rifting and increasingly `granitic' during the present episodes of uplift and continental collision
In this paper the author describes the results of the Rb-Sr isotopic dating of black shale at the Sinian (Precambrian)–Cambrian boundary at the Meishucun section, Jinning County, Yunnan Province, China. The Sinian and Cambrian strata in eastern Yunnan are well developed, the Sinian–Cambrian boundary section in the vicinity of Meishucun Village in Jinning County being representative. These well developed strata provide excellent conditions for research on the radiometric age of the Precambrian–Cambrian boundary. An Rb-Sr isochron age of 579.7 ±8.2 Ma has been obtained on black shale from the Badaowan Member of the Early Cambrian Qiongzhusi Formation, which occurs at 20 m above the Sinian–Cambrian boundary. So an inferred age of 610±10 Ma is given for the Sinian (Precambrian)–Cambrian boundary.
We have determined the beta-decay half-life of 187 Re to be (4.23 ± 0.13) · 10 10 y, stated at the 95% confidence level. The method consisted of separating the daughter 187 Os atoms from large sources of initially osmium-free rhenium. Yields were measured by the isotope dilution method using inductively coupled plasma mass spectrometry. Precision was gained through the use of two independent spike isotopes on each of five separate, large sources of rhenium. Because of the present lack of knowledge of other unrelated but equally important parameters, our value does not substantially narrow the range of uncertainty for the duration of galactic nucleosynthesis. However, the use of our half-life value to reinterpret the measurements of others on iron meteorites and chondrites suggests that iron meteorites might be 380 ± 160 million years younger than chondrites. Our half-life result may therefore have more profound implications for solar-system chronology than for galactic nucleosynthesis.
The Os concentration and isotopic composition of metalliferous carbonates deposited on the East Pacific Rise over the past 28 Ma are reported with complimentary Sr isotope data. Variations in the Os isotopic composition of these samples are interpreted as a record of past changes in the Os isotopic composition of seawater. These results are consistent with isotopic analyses of leachable Os in pelagic clays which have also been interpreted as a record of the 187Os/186Os ratio of seawater through time [1]. The metalliferous carbonate record clearly shows that seawater Os and Sr isotope systems are partially decoupled from one another over the past 28 Ma. Accelerated weathering of ancient organic-rich sediments is suggested as a possible mechanism to account for this decoupling and the rapid increase in the 187Os/186Os ratio of seawater over the past 15 Ma. This rapid increase suggests that the seawater Os record can potentially be used as a stratigraphic tool in some Neogene marine deposits.
Geochemical and mineralogical data have been obtained from black carbonaceous shales interbedded with dolomite in the Chuniespoort Group of the Late Archean-Early Proterozoic Transvaal Sequence. The black shales are fine grained and generally argillaceous and contain on average 1.2 percent total C organic, 0.7 percent S, 7.5 ppb Au, 60 ppm Cu, 330 ppm Cr, and 125 ppm Ni. These metal concentrations are similar to, or slightly higher than, the values given for the U.S. Geological Survey black shale standard SDO-1. All other heavy and transition metals, however, are depleted in the Chuniespoort shales relative to SDO-1. It is well known that high concentrations of Cr and Ni occur in Archean shales. The enhanced Cr and Ni values found in the Chuniespoort shales are a reflection of their antiquity and provenance and are not linked to preferential uptake of these metals during sedimentation and/or diagenesis. The Chuniespoort shales are characterized by excess Fe with respect to that required to form pyrite and have extremely variable C/S ratios as well. This, together with the wide-ranging S isotope ratios, is taken as an indication that biogenically produced sulfide was augmented by sulfide introduced from an extraneous source. Whole-rock S isotopes further indicate that S could have been derived from biogenic and inorganic reduction of sulfate as well as from hydrothermal fluids. Interelement correlations suggest that organic carbon does not play a role in hosting significant concentrations of base and transition metals. Rather, element associations indicate that V and Cr are preferentially contained in illite-hydromuscovite and that Ni, Co, Zn, Cu, Fe, Pb, and Mo probably reside in pyrite and/or chalcopyrite. Gold is only weakly correlated with other elements, which may indicate that its distribution is not solely determined by the presence of sulfide minerals. The fact that Au, Cu, and S/C ratios show an increase in samples with δ 34S values around 0 per mil suggests that these elements are likely to have an epigenetic hydrothermal origin. The limited syngenetic metal accumulation can be explained by the stable cratonic environment in which the sediments were deposited and the absence of submarine hydrothermal springs venting metalliferous fluids into the basin. Other factors, such as the low metal potential of the provenance, the rate of sedimentation, and the degree of oxygenation of the bottom waters of the basin, may have been additional factors limiting the concentration in, and extraction of, metals from seawater. The fact that the Chuniespoort shales host significant epigenetic gold occurrences, particularly in the Sabie-Pilgrim's Rest gold field, is largely related to their chemical characteristics, which promoted permeability and high absorption capacities for metals in hydrothermal fluids. Because of their low metal concentrations elsewhere in the basin, the shales cannot, however, be considered the source rocks for gold mineralization.
Proterozoic metal-rich black shales (metasiltstones) in the Bohemian Massif are always closely associated with submarine mafic volcanic rocks and occur in (semi)isolated basins that formed most likely in connection with the evolution of an Upper Proterozoic intracontinental rift. The black shales are characterized by an average of 3.5 wt percent C organic, and 5.2 wt percent S and carry up to 0.42 wt percent Zn, 0.21 wt percent V, 0.12 wt percent Ni, 0.12 wt percent Cr, 0.12 wt percent Cu, 0.06 wt percent As, 0.03 wt percent Mo, 132 ppb Au, 102 ppb Pd, 25 ppb Pt, 1.7 ppb Rh, and 1.3 ppb Ru. Their thickness varies from about 1 to 45 m. Sulfide minerals often include abundant framboidal pyrite, which occurs widely in C organic-rich layers. Grainy pyrite, Zn, Ni, Cu, and Mo sulfides appear to be restricted to late quartz and carbonate veinlets. Mo selenide, Mo telluride, galena, berthierite, and gold are very rare. Sulfur isotope results suggest the dominant source of sulfur to have been seawater sulfate, reduced by bacteria under different conditions. The δ 13C values of the organic matter (-24.2 to -37.5 ‰) reflect the combination of the carbon isotope composition of dissolved HCO 3-, its high concentration, and the type of organisms. The results of Rock-Eval pyrolysis as well as laser Raman spectroscopy show a high maturity, close to that of well-crystallized graphite. Fluid inclusion and stable isotope data from quartz and carbonate veinlets of very low grade regionally metamorphosed metal-rich black shales at Kamenec and highly metamorphosed facies at Hromnice indicate that they resulted from the interaction between 18O-rich fluids, volcanics, and metasediments. The interaction could have occurred during subsea-floor or later regional metamorphism.
Two depth profiles of the osmium concentration and the187Os/186Os isotopic ratio in the Indian Ocean showed that the osmium concentration seems to be unaltered by chemical or biological
processes occuring in seawater; accordingly, osmium is conservative. These data were obtained from an experimental method
that eliminated the problems related to osmium preconcentration. This method led to a new evaluation of the concentration
of osmium in seawater; the mean concentration of osmium and the187Os/186Os ratio are equal to 10.86 ± 0.07 picograms per kilogram and 8.80 ± 0.07, respectively. The results suggest the existence
of an organocomplex that dominates the speciation of osmium in seawater.
The Re-Os and complementary Sr, Nd and Pb systematics of 24 oceanic island basalts from the islands of Savaii, Tahaa, Rarotonga, Rurutu, Tubuai and Mangaia are investigated. Re concentrations range from 100 to 1621 ppt (parts per trillion), while Os concentrations vary from 26 to 750 ppt. The Re and Os concentration variations suggest that fractionation and accumulation of olivine, or a low Re/Os phase in conjunction with olivine, is important in determining the Os concentration and the Re/Os ratio of the erupted basalt. 187Os/186Os in EMII basalts from Samoa and Tahaa varies from 1.0261 to 1.1275. These ratios are mostly within estimates for depleted upper mantle, and do not constrain the involvement of recycled continental crust in the origin of the EMII signature. 187Os/186Os ratios in HIMU basalts from Rurutu, Tubuai and Mangaia range from 1.1159 to 1.2473, and provide strong evidence for the role of subducted oceanic crust in the HIMU source. The Pb-Pb systematics constrain the range of possible ages and 238U/204Pb and Th/U ratios of the subducted crust; this crust is estimated to pass through the subduction zone with Rb/Sr, Sm/Nd, Lu/Hf and Th/U ratios similar to fresh MORB. The homogeneity of the Os isotopic compositions in the Tubuai and Mangaia basalts indicates that interaction of these basalts with low 187Os/186Os mantle had an insignificant effect on the Os isotopic composition of the erupted magmas. This requires a network of channels, veins or cracks capable of delivering melt from the source region (plume) to the surface fast enough to avoid interaction with the depleted upper mantle and the oceanic lithosphere. The possible identification of the HIMU signature (high 206Pb/204Pb, low 87Sr/86Sr) with recycled oceanic crust suggests the possible presence of segments of recycled crust, with independent histories, in other oceanic mantle sources, including that of some mid-ocean ridge basalts.
Osmium is one of the rarer elements in seawater. Analytical difficulties have previously prevented the direct measurement of the osmium concentration and isotopic composition in seawater. We report a chemical separation procedure that yields quantitative extraction of osmium standard and of osmium tracer by iron hydroxide precipitation from seawater doped with osmium standard, osmium tracer, and FeCl3. The iron hydroxide precipitate is processed to extract osmium, using techniques developed for iron meteorites. Utilizing this procedure, water samples from the Pacific and Atlantic oceans were analyzed for osmium concentration and isotopic composition. Direct determination of the osmium concentration of seawater gives between 15 and 19 fM kg−1. Detailed experiments on different aliquots of one seawater sample from the North Atlantic Ocean, keeping the amounts of reagents constant, yield concentrations from 16 to 19 fM kg−1. The variability in concentration is outside the uncertainty introduced because of blanks and indicates a lack of full equilibration between the osmium tracer and seawater osmium. The most reliable osmium concentration of the North Atlantic deep ocean water is 19 fM kg−1 with the 187Os186Os ratio being 8.7 ± 0.2 (2σ). Detailed experiments on one seawater sample from the Central Pacific Ocean indicate that the most reliable osmium concentration of the deep ocean water from the Central Pacific is 19 fM kg−1 with the 187Os186Os ratio being 8.7 ± 0.3 (2σ). The directly measured osmium isotopic composition of the oceans is in good agreement with that obtained from the analysis of some rapidly accumulating organic rich sediments (Ravizza and Turekian, 1992). A sample of ambient seawater around the Juan de Fuca Ridge gave 187Os186Os= 6.9 ± 0.4. This is distinctly lower than the deep-sea water value and may reflect local hydrothermal activity or some analytical difficulty with this sample. The osmium isotopic composition of the deep oceans indicates that ∼80% of the osmium is derived from the continents and the rest from extraterrestrial and hydrothermal sources. Using the iridium data of Anbar et al. (1996, 1997) we find that the Os/Ir ratio is ∼22 for seawater. The mean residence time of osmium (τOs) cannot be derived directly from these data. However, using the Os/Ir ratio of seawater and the Os/Ir ratio of continental rocks, a lower limit can be established of τOs ≥ 4.4 × 104 years. The Os/Ir in modern deep sea sediments should be relatively constant and is distinctly less radiogenic than seawater osmium due to limited evaporation (26%) of osmium from infalling cosmic dust. Relative to the cosmic dust infall the hydrothermal sources may, however, play a smaller but significant role in the bulk economy of dissolved osmium and other Platinum Group Elements in the oceans.
We report new187Os/186Os data and Re and Os concentrations in metalliferous sediments from the Pacific to construct a composite Os isotope seawater evolution curve over the past 80 m.y. Analyses of four samples of upper Cretaceous age yield187Os/186Os values of between 3 and 6.5 and187Re/186Os values below 55. Mass balance calculations indicate that the pronounced minimum of about 2 in the Os isotope ratio of seawater at the K-T boundary probably reflects the enormous input of cosmogenic material into the oceans by the K-T impactor(s). Following a rapid recovery to187Os/186Os of 3.5 at 63 Ma, data for the early and middle part of the Cenozoic show an increase in187Os/186Os to about 6 at 15 Ma. Variations in the isotopic composition of leachable Os from slowly accumulating metalliferous sediments show large fluctuations over short time spans. In contrast, analyses of rapidly accumulating metalliferous carbonates do not exhibit the large oscillations observed in the pelagic clay leach data. These results together with sediment leaching experiments indicate that dissolution of non-hydrogenous Os can occur during the hydrogen peroxide leach and demonstrate that Os data from pelagic clay leachates do not always reflect the Os isotopic composition of seawater.
An efficient method is given for computing the best straight line by least squares when there are statistical errors in both coordinates. Exact expressions are obtained for the variances of the slope and intercept.
The Cambrian and Precambrian sedimentary strata in the centre and west part of the Yangtze Platform are in continuous successions, in which the Tiantzushan Member and the Meisuchun Formation are exceedingly rich in shelly fossils. In Co-operation with the stratigraphic study in depth and in order to determine the isotopic age for the Precambrian–Cambrian boundary in the Yangtze Platform, a number of isotopic age determinations by different techniques have been made in the laboratories of the Yichang Institute of Geology and Mineral Resources and the Research School of Earth Sciences, Australian National University, mainly on the sequences in the Yangtze Gorges area together with those from some other areas in the Yangtze Platform. In this paper, the authors summarize all of these results and infer 610±10 Ma as an age for the Sinian–Cambrian Boundary in the Yangtze Platform if the ‘golden spike’ is considered to be placed at the bottom of the Meishucun Stage.
An assemblage of problematical microfossils of Precambrian–Cambrian boundary age is redescribed from the Chert–Phosphorite Member, at the base of the Lower Tal Formation of Maldeota in the Lesser Himalaya of India. This assemblage has previously been ascribed to various ages, from Precambrian to Cretaceous, but is held by us to contain: Maldeotaia bandalica, Protohertzina anabarica group, trumpet-shaped elements, acicular elements A & B, ?Conotheca sp., Ovalitheca cf. multicostata, allathecid sp. A, Barbitositheca ansata, Hexangulaconularia cf. formosa, Coleoloides aff. typicalis, Hyolithellus aff. insolitus, H. cf. isiticus, H. vladimirovae, Spirellus shankari and Olivooides multisulcatus. These compare closely with assemblages found above the base of the first, Anabarites trisulcatus–Protohertzina anabarica Zone in China and in the second, Pseudorthotheca costata Zone of southern Kazakhstan. The stratigraphic setting of the Krol–Tal succession is reviewed and several similarities are noted between the Precambrian–Cambrian boundary successions of Lesser Himalaya in India and of Yunnan and Sichuan in Southwest China, indicating that correlation between them is possible at several levels.
Os isotopic compositions and Os and Re concentrations were measured in H2O2—H2SO4 leachates and bulk sediment samples from Holes 717C and 718C of ODP Leg 116 in the Bengal Fan. Os isotopic results indicate that, at the sediment surface, the leachable Os fraction is derived from seawater. In contrast, leachable Os from Ganges River sediments has 187Os/188Os ratios [1] much higher than the marine value. This difference suggests that the leachable radiogenic Os carried by the river sediments is completely released to the oceans prior to sediment deposition in the Fan. A simple calculation, assuming these sediments to be typical of those delivered by the Ganges-Brahmaputra river system, suggests that this process can account for a substantial part of the rise in the seawater Os isotopic ratio observed over the past 16 m.y.
The Re-Os isotope sytem, based on the long-lived β − transition of 187 Re to 187 Os, has matured to wide use in cosmochemistry and high-temperature geochemistry. The siderophilic/chalcophilic behavior of Re and Os is different from that of the elements that comprise most other long-lived radiogenic isotope systems. Mag-matic iron meteorites (IIIAB, IIAB, IVA, and IVB) have Re-Os isochrons that indicate asteroidal core crystallization within the first 10–40 million years of Solar System evolution. Rocks from Earth's convecting mantle show generally chondritic Re/Os evolution throughout Earth history that is explained by the ad-dition of highly siderophile elements to the mantle after core formation via late accretion. Oceanic basalts have Os-isotope systematics that improve the detailed geological interpretation of extant mantle components. Some portions of ancient subcontinental lithospheric mantle are severely depleted in Re and have corre-spondingly subchondritic 187 Os/ 188 Os, indicating long-term isolation from the convecting mantle during the Archean-Proterozoic. Magmatic ore deposits have differences in initial Os isotopic composition traceable to the crustal vs mantle sources of the platinum-group elements and base metals.
The decay of 187Re to 187Os provides a tool for determining depositional ages of black shales. Re and Os concentrations and Os isotopic compositions of whole rock samples of the Bakken Shale, a Mississippian/Devonian boundary black shale, yield a whole rock isochron with an age of 354 ± 49 Ma. This age is in agreement with the accepted age of the Bakken Shale (≈360 Ma). The initial ratio of the isochron is 6.2 ± 3.3. This value is indistinguishable from the ratios observed in modern Black Sea sediments. The concentration of common Os in Bakken Shale samples is strongly correlated with total nitrogen, indicating that a large fraction of the Os in these samples is associated with a hydrogenous component, which overwhelms any Os supplied to the sediment in association with detrital material or cosmic dust. The dominance of the hydrogenous component imparts a relatively homogeneous initial ratio to the sediment at the time of deposition. The degree of scatter about the isochron exceeds the degree of scatter expected from either analytical error or from the estimated degree of initial isotopic heterogeneity. The pattern of scatter is consistent with postdepositional mobilization of Re and/ or Os on a small spatial scale. We suggest that this mobilization may be a consequence of petroleum formation and migration in the Bakken Shale.
We use a high-resolution seawater Sr isotopic evolution curve for the last 100 m.y. in conjunction with modern riverine Sr flux measurements, and also geologic, tectonic and geochronological data, to make the case for a close relationship between seawater Sr isotopic composition and the India-Asia continental collision. Using a simple seawater Sr budget model we begin by showing that the Sr flux associated with alteration of seafloor basalts is too small and does not have the right time evolution to account for much of the seawater Sr isotopic curve of the last 100 m.y. The flux of dissolved Sr carried by rivers originating in the Himalaya-Tibet region on the other hand is presently a significant fraction of the global Sr budget. We calculate how this riverine flux would have had to change with time in order to match the observed seawater Sr isotopic curve and find that the riverine flux remains relatively constant prior to the collision of India with Asia but then increases very significantly after collision. We note that the period of most rapid change in seawater Sr isotopic ratio, from 20 Ma to 15 Ma, is also a period of exceptionally high erosion in parts of the Himalayas and the Tibetan Plateau. As further evidence that Sr derived from the collision of India with Asia plays a major role in the Sr isotopic evolution of seawater we show that the total amount erosion of the Himalaya-Tibetan Plateau since collision, which we calculate separately, represents a total amount of Sr that is very nearly the same as the cumulative amount required by the Sr isotopic change of seawater since collision. The relationship between erosion and riverine Sr flux allows us to use the Sr isotopic evolution of seawater to reconstruct a history of erosion since collision, and we find that the erosion rate accelerates with time since collision, with the present having the largest rate.
The transition from the Neoproterozoic Nemakit-Daldynian1 Krol-E dolomite to the Tommotian Lower Tal phosphorite-chert–black shale sequence denotes a period of drastic change in the depositional environment and the chemical milieu of the sedimentation basin. These changes are reflected by a marked negative excursion of carbon isotopes, phosphorus enrichment, trace-metal content, high organic carbon burial and notably the first appearance of prolific small shelly fauna and burrowing organisms. Recent studies have documented these changes along several stratigraphic profiles in the Mussoorie Hills of the Garhwal Himalaya for which a single profile from the Durmala phosphorite mine provides a key section discussed here.
A new technique for osmium isotope ratio determinations by negative thermal ionization mass spectrometry using the formation of OsO-3 ions is presented. Different filament materials and chemicals to reduce the electron work function are investigated. With Ba(OH)2 and platinum as filament material an ionization efficiency of more than 1% is obtained for nanogram sample amounts. The isotopic abundances of a laboratory standard are measured with relative standard deviations of less than 0.01% for the most abundant isotopes. This significant improvement in the precision of the osmium isotopic measurements, compared with previous investigations, provides a suitable tool to develop a ReOs age determination method on this basis. The new isotopic data could also be used recalculate the atomic weight of osmium to be 190.2251 ± 0.0001, which is much better in uncertainty than the current IUPAC data.
The ratios of acidic hydrogen peroxide leaches of river sediments are inferred to be representative of the isotopic composition of Os delivered to sea water from the continents. The values for river sediment leaches from the eastern and central U.S., the Ganges River, and the Rio Maipo range from 10.1 to 21.5. These results suggest that the of the soluble river load delivered from the major continental areas to the oceans is, on average, more radiogenic than average currently eroding continental crust (10.5). Sediment leaches from three rivers from the western U.S. that drain ophiolitic peridotites have distinctly lower values, 1.4–7.1; such rivers may be significant contributors to sea water of the relatively unradiogenic osmium required to yield the contemporary oceanic of 8.6.
The Os isotopic composition of recent sediments from the Black Sea are reported with the concentrations of Re, Os, and major and minor elements. Osmium concentrations range from 0.23 to 0.69 ppb and Re concentrations range from 21 to 85 ppb. Concentrations of both elements are large relative to average crustal material and are positively correlated with organic carbon. ratios range from 5.7 to 7.1; all of these values are less radiogenic than the estimated ratio of seawater and average continental crust material. Variations in the Os isotopic composition of these sediments are attributed to mixing of a hydrogenous component ( ratio between 7 and 9) and a less radiogenic detrital component ( ratio between 1 and 4). A mixing model is used to determine the fraction of Os in the sediment associated with each component, and the burial fluxes of Re and Os in the Black Sea are calculated. In addition the data are used to estimate the magnitude of systematic error which such initial Os isotopic heterogeneity may introduce into Re-Os age determinations of ancient organic-rich sediments.
The variation of sea water 187Os/186Os during the Cenozoic has been determined by analyzing the hydrogenous Os released by acidic hydrogen peroxide leaching of layers in an abyssal North Pacific pelagic clay core. Overall, sea water 187Os/186Os has increased from 3.2 at 58 Ma to the present-day value of ∼ 8.6. This pattern is similar to the well-known 87Sr/86Sr evolution of sea water. The increase in the proportion of continental Os (187Os/186Os⩾ 10) could be due to enhanced weathering rates of black shales associated with the uplift of the Himalayas. Other elements associated with black shale weathering, such as U and P, should show effects in the sedimentary record.
The Purana or Proterozoic-Lower Cambrian continental-margin sedimentary succession of the Lesser Himalaya comprises three lithological divisions: a lower argillo-arenaceous group, a middle carbonate succession and an upper argillo-calcareous assemblage. The poorly sorted basal sediments consisting of wacke-slate alternations of the lower group were emplaced by currents, including turbidity currents, which in the early stage flowed northwards from the Aravalli and Satpura mountains of Peninsular India. Lenticular deposits of diamictite within the flysch succession represent submarine slides. Penecontemporaneous basaltic lavas and associated tuffs, interbedded with shallow-water arenites making up the bulk of the upper part of the lower group, were related to rifting of the basins. The overlying dolomites and dolomitic limestones were laid down on a very shallow stable shelf and are characterised by columnar-branching stromatolites and lenticular units of crystalline magnesite and associated talc. Lenticular diamictites towards the top of the upper group of slates and limestone are harbingers of tectonic movements culminating towards the end of the Early Cambrian in an upheaval which abruptly terminated sedimentation throughout the Lesser Himalaya and Peninsular India, and caused a pronounced interruption of sedimentation in the Tethyan domain in the northern Himalaya.The Cambro-Ordovician (500±25 Ma) granites intruding the Lesser Himalayan thrust sheets of Lower Proterozoic metamorphic rocks and the upwarped northern front of the Himalaya signify an important tectonic event that terminated the Purana (Proterozoic-Lower Cambrian) sedimentation in the Indian subcontinent. This seems to be related to the tectonic movements that occurred towards the close of the Late Proterozoic or Early Cambrian.
Rhenium and osmium isotope and abundance data have been obtained on precisely-located samples from three suites of immature, organic-rich mudrocks from Jurassic coastal outcrops in England. The data provide accurate whole-rock ages of 207±12 Ma, 181±13 Ma and 155±4.3 Ma for suites of Hettangian, Toarcian (exaratum Subzone) and Kimmeridgian (sensu anglico, wheatleyensis Subzone) samples. These new Re–Os ages are indistinguishable, within the assigned analytical uncertainties, from interpolated depositional ages estimated from published geological timescales, and establish the importance of the Re–Os dating technique for chronostratigraphic studies. Early-diagenetic pyrite nodules possess levels of Re and Os which are ∼1–2 orders of magnitude lower than in the enclosing organic-rich mudrocks, indicating that these elements had already been removed from sediment pore waters at the time of nodule formation. Thus the Re–Os isotope system in these organic-rich mudrocks has been closed since, or from very soon after, the time of sediment deposition. Because most of the Re (98+%) and Os (95–99.8+%) in the mudrocks is shown to be hydrogenous, the 187Os/188Os(i) of the samples is interpreted to be that of contemporaneous seawater. The data thereby provide the first estimates of the Os isotope composition of Jurassic seawater. During the earliest Jurassic (Hettangian), the seawater 187Os/188Os ratio was extremely unradiogenic (∼0.15); it had increased to ∼0.8 at the end of the Early Jurassic (Toarcian) ∼20 Ma later, while in the Late Jurassic (Kimmeridgian) the seawater 187Os/188Os ratio was ∼0.59. The most likely explanation for the unradiogenic Os isotope composition of Hettangian seawater is that the contribution of unradiogenic Os to the oceans from the hydrothermal alteration of oceanic crust greatly exceeded the input of radiogenic Os from the continents at that time. This interpretation is in line with observations suggesting that global weathering rates were low in the Hettangian, and that increased hydrothermal and volcanic activity preceded the break-up of Pangea. The Re/Os ratios of Hettangian mudrocks (and by inference, of contemporaneous seawater) are similar to those of mudrocks deposited at later times during the Jurassic, and argues against the unradiogenic Os in Hettangian seawater being derived from extraterrestrial meteoritic sources.
We report on a technique for obtaining intense ion beams of negatively charged oxides of Os, Re and Ir by thermal ionization, in a conventional surface ionization mass spectrometer. It was found that the principal ion species of Os, Re and Ir produced are OsO3−, ReO4− and IrO2−. The sharp distinction in the masses of the dominant molecular species produced by this technique permits the measurement of isotopic compositions of each element from mixtures of platinum-group elements without significant isobaric interferences. For 187Re-187Os isotope studies, this technique offers the advantage of isotopic analyses without prior chemical separation of Re from Os, as no isobaric interference between the oxides of 187Os and 187Re exists under these conditions. For 4 ng Os, stable ion currents of 3 × 10−12 A can be maintained for over one hour, which allows determination of isotopic ratios with a Faraday collector to a precision of better than . For 70 pg Os, isotopic ratios can be measured with a precision of better than ±5‰ using a secondary electron multiplier. The detection limit for Os is estimated to be below 10−14 g. Osmium isotopic ratios have also been determined by direct loading of natural iridosmine with a precision of ±0.5‰ or better. We have obtained ionization efficiencies of 2–6% for Os and >20% for Re; these are superior to those reported for other techniques available to date and demonstrate that negative thermal ion mass spectrometry will have widespread application to 187Re-187Os chronometry and to studies of the geochemistry and environmental chemistry of the platinum-group elements.
Six years of geological investigations comprising studies of sedimentology, geomorphology, photo-interpretation and field checks carried out by a team of 6–8 geologists have led to a reappraisal of Lesser Himalayan geology.The Algonkian and Early Palaeozoic basin derived its sediments from the south. The Caledonian orogeny had a limited effect, the Himalayan basin was sliced into two parts by a central NW-SE trending arch which became a source of sediments during the later Palaeozoic and Mesozoic evolution. The Damta group of turbidites are of Algonkian-Cambrian age; they serve as a “supra-basement” over the whole region and may have partly been taken up in the later metamorphism of the central zone of the Himalaya. Facies changes between the Deoban-Shali carbonate sequence and the Chandpur-Basantpur black shales and shales are worked out. During the Mesozoic and Tertiary, facies changes are controlled by old arches and fundamental lines of weakness which can be recognized even in the present seismic and geomorphological evolution.The Blaini boulder beds, extensively quoted in literature as tillites and used as “fool-proof” marker beds are shown to be a slumped series at the base of the Krol foredeep sequence. The Krol belt rocks are autochthonous and find an extension across the Tehri-Meenus high in the Kophara black shale series that wedge out towards the north.The occurrence of the Eocene nummulitics is two-fold: 1.(a) in concordance with the Jurassic rocks, this position occurs in quiet synclinal areas;2.(b) in discordance over all the earlier deposited series, this position occurs over the arches witnessing late Mesozoic erosion.Reversal of metamorphism and production of gravity slides is a logical result in the evolution of the basin.
Two depth profiles of the osmium concentration and the 187Os/186Os isotopic ratio in the Indian Ocean showed that the osmium concentration seems to be unaltered by chemical or biological processes occuring in seawater; accordingly, osmium is conservative. These data were obtained from an experimental method that eliminated the problems related to osmium preconcentration. This method led to a new evaluation of the concentration of osmium in seawater; the mean concentration of osmium and the 187Os/186Os ratio are equal to 10.86 +/- 0.07 picograms per kilogram and 8.80 +/- 0.07, respectively. The results suggest the existence of an organocomplex that dominates the speciation of osmium in seawater.
An explosive episode of biological diversification occurred near the beginning of the Cambrian period. Evolutionary rates
in the Cambrian have been difficult to quantify accurately because of a lack of high-precision ages. Currently, uranium-lead
zircon geochronology is the most powerful method for dating rocks of Cambrian age. Uranium-lead zircon data from lower Cambrian
rocks located in northeast Siberia indicate that the Cambrian period began at approximately 544 million years ago and that
its oldest (Manykaian) stage lasted no less than 10 million years. Other data indicate that the Tommotian and Atdabanian stages
together lasted only 5 to 10 million years. The resulting compression of Early Cambrian time accentuates the rapidity of both
the faunal diversification and subsequent Cambrian turnover.
Precambrian–Cambrian boundary in the Tal Formation of Garhwal Lesser Himalaya: Rb-Sr age evidence from black shales underlying phosphorites
- K K Sharma
- D R Rao
- R J Azmi
- K Gopalan
- G V C Pantulu
Sharma K. K., Rao D. R., Azmi R. J., Gopalan K., and Pantulu G. V. C. (1992) Precambrian–Cambrian boundary in the Tal Formation of Garhwal Lesser Himalaya: Rb-Sr age evidence from black shales underlying phosphorites. Curr. Sci. 62, 528 –530.
1) are mainly from four regions selected from various formations of the outer (Nainital, Mussoorie, and Almora) and the inner (Theog, near Simla) belts and were collected Krishnaswami sitional mechanisms and environments of ancient deposits
- S K Singh
- J R Trivedi
Samples analysed in this study (Fig. 1) are mainly from four regions selected from various formations of the outer (Nainital, Mussoorie, and Almora) and the inner (Theog, near Simla) belts and were collected S. K. Singh, J. R. Trivedi, and S. Krishnaswami sitional mechanisms and environments of ancient deposits. Ann. Rev. Earth Planet. Sci. 22, 499 –551.
Miocene seawater 187 Os/ 186 Os ratios inferred from metalliferous carbonates
- G E Maasch
Reusch D. N., Ravizza G. E., Maasch K. A., and Wright J. D. (1998) Miocene seawater 187 Os/ 186 Os ratios inferred from metalliferous carbonates. Earth Planet. Sci. Lett. 160, 163–178.
Osmium isotope geochemistry of terrigenous and marine sediments
- B K Esser
Esser B. K. (1991) Osmium isotope geochemistry of terrigenous and marine sediments. Ph.D. dissertation, Yale Univ. Esser B. K. and Turekian K. K. (1993) The osmium isotopic composition of the continental crust. Geochim. Cosmochim. Acta 57, 3093–3104.