No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Stable-isotope evidence for low-temperature kaolinite weathering and post-formational hydrogen-isotope exchange in Permian kaolinites
Abstract
Depleted δ18O- and δD-values of +6.1 to +9.4‰ and −107 to −94‰, respectively, have been obtained from kaolinite samples of early Permian (Sakmarian-Artinskian) age from the Gunnedah Basin, eastern Australia.The samples come from kaolinite clayrocks representing both in situ kaolinitic weathering profiles and the eroded and re-deposited products of this weathering developed in the Quirindi-Wingen and Boggabri-Gunnedah areas of New South Wales, Australia. The clayrocks were produced by the intense surficial weathering of an underlying volcanic sequence, with subsequent erosion and re-deposition in basins flanking the volcanic pile.The calculated isotopic composition of meteoric water in equilibrium with kaolinite from the clayrocks had δ18O of ⩽ −17‰ and δD of ⩽ −125‰, indicating polar or sub-polar temperatures of condensation. As kaolinite formation cannot occur below the freezing point of water, the highly depleted isotopic results obtained from the samples suggest equilibrium with waters partly derived from deglaciation of the Gondwanan landmass, an event which consequently must have been well underway in eastern Australia by the Sakmarian-Artinskian.The climate prevailing at the time of kaolinite formation is unlikely to have been warmer than cool temperate, a conclusion which is difficult to reconcile with the classical interpretation of kaolinitic weathering as a tropical or subtropical phenomenon. This suggests that high rates of infiltration, efficient leaching of soluble cations, and good drainage are of more importance to kaolinite formation than the temperature at which the process operates.Oxygen-isotope exchange in the kaolinite samples since formation is thought to have been negligible, however, marked post-formational hydrogen-isotope exchange is demonstrable at temperatures not exceeding ∼ 80°C and probably considerably lower in some cases.
... The low temperatures of kaolinisation (20 8C T G 33 8C) and the positioning of the samples between the kaolinite and supergene/hypogene lines imply that kaolinisation took place in a supergene environment; therefore, Cretaceous-Tertiary kaolins of the Doula Sub-Basin are of weathering origin. As indicated by Bird and Chivas (1988), kaolins in a supergene environment generally have d 18 O values between +17 and +23%, and d 2 H values between À80 and À40%. This is sustained by the isotopic compositions of other kaolins formed by weathering, amongst which are the Cretaceous-Tertiary Georgia kaolins in the USA (Hassanipak and Elsinger, 1985), the Tertiary Lastarria kaolins (Gilg et al., 1999), the Cretaceous-Tertiary Rio Capim semi-flint kaolins in Brazil (Santos et al., 2007), the Permian Variscan kaolins in Spain (Ferná ndez-Caliani et al., 2010;Clauer et al., 2015), the Cambrian Burela kaolin deposit in Spain (Gá lan et al., 2016), and the Cretaceous-Tertiary Patagonia kaolins in Argentina (Domínguez et al., 2016). ...
... Even though warm temperatures are required for kaolin formation, precipitation and leaching are the main factors influencing the formation of kaolins (Bird and Chivas, 1988). According to the rainfall patterns during the Mesozoic to Cenozoic derived by Parrish et al. (1982), Douala and its environs fall in the high rainfall domain since the Cretaceous. ...
Increased interest in paleoenvironmental studies is a result of climatic changes occurring at present and predicted for the future. Such studies could be done using the stable isotope compositions (δ²H and δ¹⁸O) of kaolins, which provide knowledge on the paleoenvironmental conditions prevailing during the time of kaolinisation. In this study, the stable isotopic compositions of clay-size fraction of kaolins occurring in Cretaceous and Tertiary Formations of the Douala Sub-Basin in Cameroon are presented, with the aim of reconstructing the paleoenvironmental conditions of the Sub-Basin. To achieve this, the clay-size fraction (< 2 μm fraction) of 8 kaolinite-rich samples were analysed for their δ²H and δ¹⁸O compositions, and results were reported as part per mil (‰) relative to the SMOW standard. The δ¹⁸O values of kaolins found in the Cretaceous–Tertiary Formations of the Douala Sub-Basin varied between +18.2 and +21.0‰ whereas the δ²H values varied between –69 and –53‰. Nine of the eleven samples plotted on the right of the supergene–hypogene line. Five of these nine samples plotted very close to the kaolinite line, which represents the composition of kaolinite in equilibrium with meteoric water at 20 °C; suggesting a supergene weathering origin of these kaolins. The determination of the temperature of kaolinisation yielded mean formation temperatures of 22 ± 2 °C and 27 ± 6 °C for Cretaceous and Tertiary kaolins, respectively. Excluding the two samples falling in the hypogene field, averages of kaolinisation temperatures were 20 and 25 °C during the Cretaceous and Tertiary periods, respectively. These temperatures are slightly below the present mean annual temperature in Douala (27 °C), thereby suggesting that the climate was becoming warmer from the Cretaceous to the Present. Therefore, Douala had a cooler and rainy climate during the Cretaceous, and the climate is gradually becoming hotter and more humid, favouring the refinement of existing kaolins and the kaolinisation of kaolin-forming minerals in the Sub-Basin.
... The Rookery sample (ROOK) plots in the later part of the post-Eocene field, its position possibly represents a Miocene -Pliocene age and sample TJSC plots in the Pre-Mesozoic area of Bird and Chivas (1988a). Sample FGRC does not plot within any of the fields, but the δ 18 O (+8.1‰) is similar to Permian clays reported by Bird and Chivas(1988b ...
... Conditions conducive to the formation of kaolinitic weathering profiles can be found from the tropics to the polar regions. The formation of deep, clay-rich weathering profiles requires high rainfall and efficient leaching, taking place over extended periods of time (of the order of 0.01 -10 Ma)(Bird and Chivas, 1988b).Australia has drifted steadily northwards since splitting from Antarctica at ~95 Ma (figure 4.1).There is a strong correlation between δ 18 O compositions of meteoric waters and temperature, and also an increasing temperature gradient with decreasing latitude. These factors combine to result in a steady increase in δ 18 O of meteoric waters, as reflected in authigenic minerals formed in equilibrium with such waters. ...
... Oxygenisotope exchange is generally negligible below 100ºC (O'Neil and Kharaka, 1976;Yeh and Savin, 1977) but the likelihood of exchange increases at higher temperatures, with significant O-isotope exchange occurring by 300ºC (O'Neil and Kharaka, 1976). Hydrogen is more susceptible to isotopic exchange than oxygen, with measurable H-isotope exchange occurring at 100ºC (O'Neil and Kharaka, 1976) or even lower temperatures given sufficient time and/or high waterrock ratios (Bird and Chivas, 1988;Kyser and Kerrich, 1991;Mizota and Longstaffe, 1996). ...
... Hydrogen-isotope exchange between clay minerals and water typically occurs more readily and at lower temperatures than O-isotope exchange (O'Neil and Kharaka, 1976;Bird and Chivas, 1988;Sheppard and Gilg, 1996). Thus, H-isotope exchange was expected for berthierine from both nearinjector and distal-injector cores. ...
In situ thermal recovery methods such as cyclic steam stimulation (CSS) are required to extract highly viscous bitumen from the Clearwater Formation oil sands of Alberta, Canada. The injection of hot fluids during CSS has altered the mineralogy of the sands, resulting in the loss of some minerals (e.g. disseminated siderite, volcanic glass) and precipitation of others (e.g. zeolites and abundant hydroxy-interlayered smectite). The high temperatures and high water – rock ratios associated with CSS might also alter the oxygen and hydrogen isotopic compositions of pre-existing clay minerals even in the absence of mineralogical changes. The present study exploits this fact to track the movement of injected hot fluids during CSS. Berthierine, a common diagenetic clay mineral in the Clearwater sands, survived CSS but acquired substantially lower δ18O and δ2H values in cores located 4 10 m from the injection well. In contrast, the oxygen and hydrogen isotopic compositions of berthierine in cores located further from the injection well were generally unaffected, except at the depth of steam injection where horizontal fractures facilitate greater lateral penetration of hot fluids. Smectitic clays in near-injector cores also acquired lower δ18O values during CSS, but a systematic shift in δ2H values was not observed. While hydrogen-isotope exchange undoubtedly occurred, the particular combination of temperature and H isotopic composition of the injected fluid used during CSS appears to have yielded post-steam δ2H values that are indistinguishable from pre-steam values. Only samples from near-injector core G-OB3 that contain hydroxy-interlayered smectite have lower δ2H values as a result of CSS.
... Dickins, 1978), oxygen-isotope ratios (e.g. Rao & Green, 1982;Bird & Chivas, 1988) and palaeomagnetism (e.g. Irving, 1964;Embleton, 1973). ...
... Therefore, multiple generations of different ages of the same mineral (e.g., Fe-oxides and kaolinite) can coexist in the same unit of a lateritic profile (Balan et al., 2007;Monteiro et al., 2014;Mathian et al., 2019). Each generation will provide constraints on external chemical conditions, that is, intensity of rainfall and temperature, prevailing during duricrust formation (Bird and Chivas, 1988;Girard et al., 2000;Yapp and Shuster, 2011). Because laterites can be as old as hundreds of millions of years (Retallack, 2010), secondary minerals forming in such weathering systems are good long-term paleoclimatic indicators and can record discrete climatic events (Bird et al., 1992;Girard et al., 2000). ...
Thick regoliths developed under tropical climate, namely, laterites, resulting from long-term and pronounced geochemical and mineralogical rearrangement of the parent rock in response to environmental changes. Little information is available on the timing of laterite and bauxite formations, especially on the chronology of the main weathering episodes responsible for lateritic cover formation on the Guiana shield. For this purpose, we focused on both lateritic and bauxitic duricrusts developed over the Paleoproterozoic Greenstone Belt in the Brownsberg, Suriname. The duricrust samples have a relatively simple mineralogy (i.e., goethite, gibbsite, hematite, and kaolinite) but reveal, when observed at a microscopic scale, a complex history of formation with multiple episodes of dissolution/reprecipitation. The (U-Th)/He dating of 179 Fe-oxides subsamples shows that duricrusts sampled at the top of the Brownsberg plateau have ages ranging from <0.8 Ma to ∼19 Ma. In contrast, Fe-oxides extracted from detrital duricrust boulders collected downslope indicate formation ages up to 36 Ma. This age discrepancy may indicate that a main episode of physical erosion affected this region between ca. 30 and 20 Ma. Consistently, the bauxite sampled at the mountaintop indicates a younger phase of formation, with Fe-oxides recementing fragments of a preexisting bauxitic material older than ∼15 Ma. Geochronological data also reveal a long-lasting weathering history until the present day, with multiple generations of Fe-oxides in the bauxite and the duricrusts resulting from successive cycles of dissolution and reprecipitation of Fe-oxides associated with redox cycles. This long-lasting weathering history led to geochemical remobilization and apparent enrichment in some relatively immobile elements, such as REE, aluminum, and vanadium, especially in the duricrust sampled at the mountaintop. Our geochronological, mineralogical, and geochemical study of Fe- and Al-crusts from the Brownsberg mountain provide constraints on the evolution of environmental conditions prevailing since the early Oligocene in Suriname.
... Each generation will provide constraint on external chemical conditions, i.e. intensity of rainfall and temperature, prevailing during its formation (Bird and Chivas, 1988;Yapp and Shuster, 2011). Because laterites can be as old as hundreds of million years , secondary minerals forming in such weathering system are good long term paleoclimatic indicator and can record discrete climatic events (Bird et al., 1992;. ...
Les latérites sont des formations d’altération épaisses, se formant sous des climats chauds et humides pendant de longues périodes de temps, mais la chronologie de leur développement et les conditions climatiques prévalentes lors de leur formation demeurent complexes. Toutefois, les minéraux secondaires formés dans de ces contextes (e.g. la kaolinite, l’hématite ou la goethite) peuvent témoigner des conditions de formation des latérites et de leur fonctionnement, mais aussi enregistrer des informations sur les conditions paléoclimatiques au moment de leur formation. Alors que de nombreuses investigations géochimiques et minéralogiques ont été menées sur des couvertures latéritiques épaisses, seules quelques récentes études ont eu pour but d’apporter des contraintes temporelles sur ces formations et les relier aux variations paléoclimatiques ou paléoenvironnementales. En ce sens, le Bouclier amazonien est tout particulièrement intéressant car cette zone est restée en position équatoriale depuis le Crétacé et a ainsi enduré un climat favorable à la fois au développement et à la préservation des latérites. Néanmoins, des évènements géodynamiques à grande échelle durant le Cénozoïque ont affecté la région (e.g. la formation de la cordillère des Andes, la mise en place de l’actuelle Amazone), et ont pu impacté le climat local et, en conséquence, les régimes d’altération. Les travaux de cette thèse portent sur deux profils latéritiques du Bouclier amazonien, le premier profil développé sur un ensemble métamorphique paléoprotérozoïque au Brownsberg (Suriname), le second développé sur la formation sédimentaire crétacée Alter do Chão au nord de Manaus (Brésil). Des études minéralogiques, géochimiques, et géochronologiques ont été menées afin d’identifier et de dater les processus responsables de la formation de ces profils. Dans les deux sites d’études, la datation (U-Th)/He sur des oxydes de fer issus les matériaux latéritiques échantillonnés a révélé une longue histoire d’altération depuis la période Eocène – Oligocène. Des épisodes ultérieurs ont pu, par la suite, conduire à un rajeunissement de ces profils. Ces épisodes peuvent être corrélés à des évènements géodynamiques ou climatiques globaux ou locaux, confirmant le comportement épisodique de la formation des latérites et des conditions environnementales fluctuantes des systèmes d’altération. Au Brownsberg, de multiples cycles de dissolution – reprécipitation ont pu être enregistrés au sein de la cuirasse latéritique (~30 Ma, >13.5 Ma, 15 – 10 Ma, ~4 Ma), conduisant à un enrichissement progressif dans cette unité en éléments sensibles aux processus rédox, tel que le vanadium. Au contraire, dans la région de Manaus, la cuirasse s’est formée lors d’un court épisode d’altération empêchant un enrichissement aussi important en éléments. Toutefois, la kaolinite dominant l’ensemble du profil, montre une évolution progressive depuis une kaolinite large et bien ordonnée en bas de profil, vers une kaolinite peu ordonnée semblable à de la dickite dans le Ferralsol, attestant ici de la transformation d’une génération initiale de kaolinite en sommet de profil. Les compositions isotopiques en oxygène et hydrogène de ces kaolinites ont révélé deux périodes d’altération distinctes, probablement reliées à des évènements globaux et locaux. Bien qu’ils soient tous deux situés en Amazonie, l’étude de ces deux profils latéritiques a montré des mécanismes et des dynamiques de formation spécifiques, liés à des épisodes d’altération distincts d’âges et de durées différents.
... The temperature of formation (120 • C-~195 • C) coupled with δ 18 O (+17 to + 23‰) and δH (−46.90 to −38.80‰) values, in which samples plotted away from the kaolinite line but closer to the supergene-hypogene line (Table 5; Fig. 8), indicates that the investigated shales were formed in a supergene setting under a humid climate tropical condition, during the Cretaceous (Bird and Chivas, 1988, Mizota and Longstaffe, 1996)Mizota and Longstaffe, 1996. ...
Combined use of trace elements, rare earth elements (REEs) with hydrogen (δH) and oxygen (δ¹⁸O) stable isotope composition to elucidate origin and paleo-environment-conditions and reconstruction is a contemporary trend in the field of geochemistry. Geochemical investigations were carried out on the shale deposit from five wells within the Asu River Group Formation, exposed at Ikwo, Lower Benue Trough (LBT). Results of trace elements for the shale deposits examined showed averages of Co (19.10 ppm), Th (16.50 ppm), Zn (103.73 ppm), Sr (203.71 ppm) and Zr (292.80 ppm) compared to PAAS and UCC. Observed negative Eu anomalies, enriched LREEs and depleted HREEs patterns have shown that the shales are from rocks of continental origin. This is provided by Al/(Al + Fe + Mn) with a value > 0.2. Plots of Zr/Sc vs Th/Sc, La/Th vs. Hf and Cr/V vs. Y/Ni indicated felsic igneous rock precursors for the shale samples. C-parameter, ratios of Rb/Sr and Sr/Cu signify paleo-climatic conditions of semi-humid to arid, Ba/Al showed low paleo-productivity of the basin during shale deposition, largely within the freshwater setting according to the ratio of Sr/Ba. V/(V + Ni), U/Th, Ni/Co ratios and Ce/Ce∗ anomalies have revealed an oxic depositional environment with Fe/Ti > 20 suggesting hydrothermal activity. The temperature of formation (120 °C–∼195 °C) coupled with δ¹⁸O (+17 to + 23‰) and δH (−46.90 to −38.80‰) is consistent with materials of sedimentary origin from chemically weathered felsic precursors under humid climatic conditions with an influence of hydrothermal activity.
... The method, initially developed in Australia, is based on an assumption that D/H and 18 O/ 16 O ratios in secondary minerals formed during weathering, chiefly in kaolinite, reflect primarily the isotopic composition of groundwater in which they originated and the temperature of formation. These ratios, in turn, reflect the average isotope ratio of rainfall, which is dependent on the mean annual temperature (Bird and Chivas 1988). Application of this technique allowed the various generations of saprolites in Australia, consistent with Permian, pre-mid-Tertiary and post-mid-Tertiary ages of weathering, to be differentiated Chivas 1988, 1993). ...
This chapter reviews major advances in studies of long-term landform evolution in the last few decades of the 20 th century. These include in particular the development of etchplanation concept that evolved into a dynamic approach linking inheritance, environmental change, contemporary processes, realization of the importance of weathering mantles in deciphering landscape evolution at long timescales, recognition of vary ancient landforms, especially in the Southern Hemisphere, and insights into preglacial landscapes at high latitudes that survived successive glaciations.
... It is worth mentioning that there were large-scale unconformities between the Permian and Triassic strata in the Mahu Sag, and some areas even lacked the Triassic stratum. The distance from the reservoir of the Wuerhe Formation to unconformity was in the range of 10-200 m, which was conducive to meteoric freshwater leaching into the reservoir (Bird and Chivas, 1988;Purvis, 1995;Yuan et al., 2017). In contrast, the late calcite was more related to organic corrosion fluid. ...
The reservoir in the Wuerhe Formation in the Mahu Sag, northwestern Junggar Basin, China, exhibits complex dissolution and cementation related to zeolite. The source and mechanism of diagenetic fluids are crucial in studying the reservoir genesis. Thus we investigated the key reservoirs fluids related to the zeolite and discussed their significance in the zeolite-rich reservoir of the Permian Wuerhe Formation in the Mahu Sag. Based on thin sections and electron microscope observations of rock samples and analyses of physical properties, C-O isotopes, and major elements, it is found that the reservoir underwent mainly two stages of fluid-related dissolution and cementation processes, in which the hydrocarbon-bearing fluid played the primary role in forming the high-quality reservoir. Dissolution pores are the most important storage space, and zeolite cement is the most important dissolution mineral. The geochemical characteristics of zeolite and calcite cement indicate the presence of two diagenetic fluids. The iron-rich calcite and orange-red heulandite is related to early diagenetic fluids with high iron content and higher carbon isotope values, whereas the calcites, with high manganese content and lower carbon isotope values, are formed by late acidic organic diagenetic fluids related to oil and gas activities. The hydrocarbon-bearing fluids form different spatial diagenetic zones, including the dissolution zone, buffer zone, and cementation zone, and the dissolution zone near the oil source fault is the main site of zeolite dissolution. The late fluid has the characteristics of multi-stage activity, which makes the spatial zoning expand gradually, resulting in multiple superpositions of dissolution and cementation and increasing the complexity and heterogeneity of the reservoir diagenesis. This study expands the understandings of the dissolution activities of different fluids in zeolite-rich reservoirs and also has reference significance for dissolution activity of hydrocarbon fluid in other types of reservoirs.
... Geochemical and mineralogical studies (e.g., Psyrillos et al., 1998;Fernández-Caliani et al., 2010), stable isotope compositions (Sheppard, 1977;Bird and Chivas, 1988;Clauer et al., 2015), or fluid inclusion compositions in associated quartz (Alderton and Rankin, 1983;Webster et al., 1995) are used to determine the origin of kaolinitization. Combinations of these indicators that can provide stronger evidence and clear characteristics for each of these kaolinitization mechanisms are not common in the literature (e.g., Marumo, 2003;Kadir et al., 2011Kadir et al., , 2014Zirjanizadeh et al., 2018). ...
Clay fractions of the hypogene kaolin deposits from Ipoh granite and Jerai pegmatite are composed of kaolinite, while the greenish kaolin horizon of Ipoh area comprises of illite and kaolinite. Al2O3 and SiO2 are the main constituents in the studied kaolins with very low concentrations of other oxides. K2O occurs in relatively high contents in the illite-rich kaolin that formed by hydrothermal alteration of biotite-granite of the greenish kaolin horizon of Ipoh area. The higher contents of Ba, Cr, Cs, Ga, Rb, Sc, and Sn in the illite-rich horizon are inherited from its source rock. The relative enrichment of light rare earth elements (LREE) over heavy rare earth elements (HREE) as indicated from the high (La/Yb)n ratios and positive correlations between ΣREE and P2O5 in the studied kaolins suggest the occurrence of REE as authigenic florencite, churchite and/or rhabdophane and, therefore, reflect the kaolinitization process rather than the parent rocks. δH and δO values are similar in both Ipoh and Jerai kaolins with dH values range from −84 to −99‰ and positive δO values varying between 0.95 and 5.47%. H- and O-isotopes data fall close to the left side of the kaolinite line in equilibrium with meteoric water at temperatures of > 100 °C. The pronounced negative Eu anomalies, absence of positive Ce anomaly and high crystallization temperature (94–113 °C) indicate the hydrothermal (hypogene) origin of the studied kaolins as a result of the Triassic post-magmatic hydrothermal activities in Peninsular Malaysia. Mineralogical and geochemical variations among the studied deposits although they formed by the same kaolinitization process, suggest a significant role of parent rocks in their compositions.
... Caves et al., 2016). Note that we did not consider δ 18 O in Phanerozoic paleosols (e.g., Bird & Chivas, 1988;Tabor & Montañez, 2005) in Figure 8a (Figures 8b-8d). Relatively large error bars are introduced for the observation in Figures 8b-8d, reflecting all measured data from different depths of individual sections reported in the literature. ...
Two reactive transport models were developed to integrally simulate fluxes of oxygen isotopes from hydrothermal alteration of oceanic crust and weathering of continental rocks for a better understanding of the regulation of oxygen isotopes in the ocean. The hydrothermal alteration model consists of three boxes to represent pillow basalt, sheeted dike, and gabbro sections and simulates the steady‐state δ¹⁸O of solid rocks and porewaters in three individual sections along the spreading direction. The continental weathering model similarly calculates steady‐state δ¹⁸O profiles along the uplift direction. The two models were run in concert to simulate the evolution of seawater δ¹⁸O during the Phanerozoic, reflecting the relevant forcing factors to the corresponding parameter values of the models as functions of time. The seawater δ¹⁸O was calculated to have evolved from less than −5‰ in the early Paleozoic to between −2 and 0‰ for the Mesozoic and Cenozoic, as a result of declines in atmospheric CO2 and land area of igneous and high‐grade metamorphic rocks. The transition of seawater δ¹⁸O is similar to the δ¹⁸O trend that is commonly recorded in authigenic sedimentary rocks such as carbonates and cherts. The simultaneously simulated δ¹⁸O values for altered continental and oceanic rocks are decoupled from seawater δ¹⁸O because of kinetic inhibition and solid‐rock buffering at respective low and high temperatures and are similar to the shale and ophiolite δ¹⁸O records. Overall consistency between the simulation and observations supports the dynamic behavior of oceanic δ¹⁸O over the Phanerozoic eon.
... The relative stability of the δD values is taken as an indication that once formed the Ka+Kb kaolinites of the studied area remained isotopically stable. Under isotopic fractionation at low temperature, the hydrogen exchange between the mineral and the associated water occurs at much higher rates than with the oxygen, usually negligible (O'Neil and Kharaka, 1976;Bird andChivas, 1988, Longstaffe andAyalon, 1990). Thus, the cause for the wide oscillation of the δ 18 O values is not related to equilibrium with interacting waters, but it reflects mineralogical contaminations. ...
The kaolin deposits that occur in the Rio Capim area, east of Cametá Sub-Basin, are inserted in the Ipixuna Formation. This unit distinguishs for presenting one of the largest worldwide kaolin concentrations of excellent quality to the cellulose industry. Beyond the economic character, a great volume of academic works focusing these kaolin deposits had led to pedological and geochemical approaches, but without taking into account their sedimentologic aspects, which are important to understand their genesis. Detailed sedimentologic and stratigraphic studies of the Rio Capim kaolin have been increasingly carried out in the last years, which led to the paleoenvironmental interpretations for the Ipixuna Formation, as well as to discuss better the mode of formation of the soft and semi-flint kaolin units that are typical of this unit. These works served to motivate the integration of sedimentologic and stratigraphic data with optical studies combined with hydrogen and oxygen isotope geochemistry in order to discuss the geologic processes involved in the origin and evolution of the soft and semi-flint kaolin units. The sedimentological analysis consisted in a more detailed facies description and stratigraphic analysis of newly open quarries that were not available during previous investigations. The additional exposures led to a better characterization of the lower kaolin unit, known as the “soft kaolin”, which is well stratified, favoring facies analysis. Hence, the soft kaolin unit consists of kaolinitized sandstones and kaolinitized pelites that were formed in tidally influenced fluvial channels (Facies Association A), tidal channel (Facies Association B), tidal flat/mangrove (Facies Association C), and tidal sand bar/tidal sandy flat (Facies Association D). These depositional environments are attributed to a tide-dominated estuarine system. Petrographic studies and scanning electronic microscopy (SEM) of the kaolin deposits in the study area had their composition was strongly modified after sedimentation. The soft kaolin consists of kaolinitized quartz sandstone and either laminated or massive pelites, which are composed by fragments of meta-volcanic lithic and volcanic felsic rocks, as well as metamorphic and granitics rocks. These lithologies were strongly modified during kaolinitization, as revealed by the intense replacement of the framework grains by kaolinite of three types, named herein as Ka, Kb and Kc kaolinites. Ka kaolinite occurs dominantly associated with kaolinitized sandstones, being characterized by pseudohexagonal crystals 10-30 μm in diameter, which are organized as booklets or vermicular forms that reach up to 400 μm in length. Kb kaolinite dominantes in the pelites, and consists of pseudohexagonal crystals 1-3 μm in diameter, occurring as isolated, face-to-face and parallel to pseudo-parallel crystals. Kc kaolinite forms pseudohexagonal to hexagonal crystals of 200 nm in diameter. It occurs dispersed through the soft unit, increasing significantly in abundance in association with paleosols at the top of the unit. The semi-flint kaolin deposits are constituted mainly of reworked grains derived from the underlying soft kaolin unit that are mixed with grains derived from metamorphic and granitic sources. These deposits are dominantly composed of Kc kaolinite that was formed during weathering. The deuterium (δD) and oxygen (δO) isotope analysis of the kaolin deposits from the study area helped to discuss better the evolution of the different types of kaolinites described above. Hence, the soft kaolin deposits display δO values varying between 6.04 ‰ and 19.18 ‰ in the Ka+Kb kaolinites, and between 15.38 ‰ and 24.86 ‰ in the Kc kaolinite. The δD values from this unit vary from – 63.06 ‰ to 79.46 ‰, and from –68.85‰ to -244.35‰ in the Ka+Kb and Kc kaolinites, respectively. The semi-flint kaolin deposits are characterized by δO and δD values ranging from 15.08‰ to 21.77‰, and from -71.31‰ to -87.37‰, respectively. Based on these data and on the isotopic composition of both meteoric and ground waters, it was possible to conclude that the kaolinites had not been formed in balance with modern weathering. These values represent the isotopic composition during the time of formation of the kaolinites, as well as mineralogical contamination of framework grains that are now replaced by kaolinites.
... The relative stability of the δD values is taken as an indication that, once formed, the Ka + Kb kaolinites of the studied area remained isotopically stable. Under isotopic fractionation at low temperature, the hydrogen exchange between the mineral and the associated water occurs at much higher rates than with the oxygen, usually negligible (O'Neil and Kharaka, 1976;Bird and Chivas, 1988;Longstaffe and Ayalon, 1990). Thus, the cause for the wide oscillation of the δ 18 O values is not related to equilibrium with interacting waters, but it reflects mineralogical contaminations. ...
Deuterium (δD) and oxygen (δ18O) isotope data from the Rio Capim kaolin, northern Brazil, were combined with optical studies in order to better understand the genesis and evolution of the kaolinites. The results show that δ18O values from a lower soft kaolin unit range from 6.0‰to 19.2‰for Ka (size ranging from 1 to 3 μm) and Kb (size ranging from 10 to 30 μm) kaolinites, and from 15.4‰ to 24.9‰ for Kc (size b200 nm) kaolinites. The δD values range from −63.1‰ to 79.5‰ for the Ka+Kb kaolinites, and from −68.8‰ to −244.35‰ for the Kc kaolinites. An upper semi flint kaolin unit, dominated by Kc kaolinites, displays δ18O
and δD values ranging from 15.1‰to 21.8‰, and −71.3‰ to −87.4‰, respectively. Based on these data, and on the δ18O and δD
values obtained for the surface meteoric water and groundwater, it can be concluded that the kaolinites are not in equilibrium with
the modern weathering environment, but they reflect isotopic compositions of the formation time, probably due to the interaction
with fossil groundwater. However, mineralogical contaminations derived from replacements of framework grains also had great
influence in the isotopic composition of these kaolinites. In addition, the isotope values of the Kc kaolinites from the semi-flint
kaolin unit is variable, which is due to the presence of Kc kaolinites of different origins, including kaolinites derived from the
underlying soft kaolin unit, kaolinites formed during different phases of paleoweathering, as well as later phases of coarse-grained
kaolinites formed along fractures. Due to these complexities, binary diagrams contrasting δ18O and δD values, worldwide applied
for distinguishing supergenic from hypogenic kaolinites, as well as those formed under weathering conditions, can not be applied
to interpret the origin of the kaolinites in the Rio Capim Kaolin.
... Specifically, Vasconcelos et al. (2008) used geochronology to show that whilst eastern Australia experienced periods of volcanism, uplift and denudation, significant portions of the continent's surface west of the Tasman Line were not subject to major denudation. Using two lines of relative dating methods including stable isotopes (Bird & Chivas, 1993;1988) and palaeomagnetism (Pillans, 2005), Vasconcelos et al. (2008) argued for the longevity of weathering profiles across much of the interior of Australia dating at least back to the Mesozoic, if not the Paleozoic. The corollary of this is that erosion rates have been extremely low since at least this time and that the duricrusts formed in the region west of the Tasman Line are extremely resistant to weathering. ...
Today the eastern highlands of Australia are significantly more elevated than western Australia, but the continent's geodynamic evolution suggests that the opposite was the case during Cretaceous times, when the Eromanga Sea dominated the eastern Australian landscape. Previous geodynamic and surface processes models have been used to simulate the evolution of this seaway, but fell short in capturing its spatial and temporal characteristics, mainly because changes in tectonic topography were not sufficiently explored. Here we combine dynamic (mantle-driven) and tectonic topography across the Australian continent with global sea level change and surface processes since the late Triassic to understand the main driving factors of uplift, subsidence, erosion and coastline migration. A variety of initial and boundary conditions are tested and refined to place constraints on the evolution of Australian paleogeography. These include the paleotopography of the Australian continent during the Late Triassic at the starting time of our model, the magnitude and distribution of tectonic and dynamic topography following Gondwana break-up, the time dependence of precipitation and erodibility across the Australian continent and the combined effect of these parameters on erosion and coastline change through time. Iterative model refinement leads to a model with a variable erodibility grid considering major erodibility differences across the Tasman Line, and an initial topography that incorporates the influence of the African superswell on the elevation of western
... Although meteoric water can penetrate to considerable depths in sedimentary basins (Mahnheim, 1967;Habermehl, 1980;Giles, 1986), most meteoric diagenetic alterations have been suggested to occur closely beneath unconformities (Emery et al., 1990;Huang et al., 2003) or at buried depths of less than a few hundred meters (Bjørlykke, 1993;Mansurbeg et al., 2006;Bjørlykke and Jahren, 2012). Generally, stable isotopic compositions of authigenic kaolinite have been used as important evidence to support the meteoric diagenesis relevant to feldspar dissolution in sandstones (Bird and Chivas, 1988;Purvis, 1995). Aside from the hydrogen and oxygen isotope signatures of authigenic kaolinite, it is difficult to obtain other direct chemical evidence for meteoric water leaching (Yuan et al., 2017b). ...
The feldspar group is one of the most common types of minerals in the earth's crust. Feldspar alteration (including the whole processes of feldspar dissolution, transfer of released solutes, and secondary mineral precipitation) is ubiquitous and important in fields including resources and environmental sciences. This paper provides a critical review of feldspar alteration and its geological significance in shallow aquifers to deep hydrocarbon reservoirs, as assessed from peer-reviewed paper in the literature. A variety of mechanisms such as the surface reaction-controlled dissolution mechanism, the preferential leaching-diffusion controlled mechanism, the diffusion-precipitation controlled dissolution mechanism and the interfacial dissolution-reprecipitation mechanism have been proposed to be responsible for the dissolution of feldspars. Feldspar dissolution rates can be affected by the crystal structure, Al/Si ordering, temperature, pH, surface area, organic acids, chemical affinity, and precipitation of secondary minerals. Five main dissolution rate laws have been used to describe feldspar dissolution rates, including the linear transition state theory (L-TST) rate law, non-linear TST rate law, parallel rate law, stepwave model rate law, and partial equilibrium law. The rate inconsistency between laboratory experiments and field observations is interpreted with hypotheses that include the armoring effects of the coating secondary minerals on feldspar surfaces, the possible effects of leached layers, the approach to saturation with respect to feldspars, the inhibition by absorbed Al ³⁺ on the feldspar surface, and the inhibition by simultaneous slow clay precipitation rates. The inorganic-original (meteoric water and deep hot water) and organic-original (kerogen and hydrocarbon degradation) hydrogen ion (H ⁺ ) in a fluid can probably act as a significant catalyzer of fast dissolution of feldspars in shallow aquifers and deep hydrocarbon reservoirs. Various mineral assemblages including extensively leached feldspars with a wide range of associated amounts of clay minerals and quartz cements can be identified in subsurface reservoirs under different geological conditions. Feldspar dissolution can generate enhanced secondary porosities and rock permeability in open geochemical systems at shallow depth or at a moderate-deep depth where faults develop widely. While in closed geochemical systems at moderate-deep depth, feldspar dissolution is likely to generate redistributional secondary porosities and to decrease rock permeability. Authigenic clay minerals formed following feldspar dissolution alter rock wettability and affect the charging and entrapment of hydrocarbons in reservoir. Feldspar alteration may promote hydrocarbon degradation by promoting bioactivity or by consuming low molecular weight organic acids and CO 2 produced via oil degradation. Further work should be conducted to study hydrocarbon-water-feldspar interactions in deeply buried hydrocarbon reservoirs. Feldspar alteration may promote CO 2 sequestration by consumption of H ⁺ , generation of HCO 3⁻ , and pH buffering of formation water. K-feldspar alteration may also promote illitization in interbedded mudstones by supplying K ⁺ .
... 12 temperature-dependent isotopic fractionation for both 18 O/ 16 O and D/H (Sheppard and Gilg, 1996). (equivalent to 120 ‰ in D values), it has been noted that there has been a progressive age-related shift away from isotopic equilibrium in the D values of kaolinite with increasing age (Fig. 3 of Bird and Chivas, 1988b), owing to post-formational hydrogen isotope exchange. Accordingly, we have continued to work with 18 alone, and view the D values of ancient kaolinite with caution. ...
Oxygen isotope-inferred ages as old as the Jurassic have been derived from weathered monzogranite underlying the folded Mount Augustus Sandstone, which forms the massive anticlinal Burringurrah (or Mount Augustus) inselberg, in the Gascoyne district of Western Australia. This large inselberg stands ~700 m above the surrounding duricrusted Neogene Gascoyne planation surface at ~400 m. The 1150 m thick Mount Augustus Sandstone (c. 1620 Ma), buried by some 10 km of rock of the Edmund and Collier Groups, underwent folding during the Edmundian Orogeny (1030–950 Ma). Abundant Permian glacial deposits throughout western and southern Australia suggest that the landscape was extensively impacted by a continental ice mass during the Gondwanan Permian glaciation, and which provides a maximum age for most recorded chemical weathering profiles within Australia. Nine samples of kaolinite were collected from a 30 m deep exposure of weathered monzogranite cropping out in the core of the anticline beneath the overlying Mount Augustus Sandstone within the topographic amphitheatre of ‘The Pound’. Four samples from the profile were analysed, revealing two ages of weathering. The higher samples immediately below the unconformably overlying boulder ferricrete (δ¹⁸OVSMOW values of +12.0 and +14.0‰) imply a Jurassic to early Cretaceous weathering age, while the lower samples (δ¹⁸OVSMOW values of +17.6 and +18.3‰) are indicative of a Neogene age. These results suggest downward ‘younging’ of the profile consistent with a top-down advancing weathering front that developed after uncapping of the anticlinal inselberg and exposure of the underlying monzogranite to surficial chemical weathering. A long-term rate of landscape denudation of ~11 m/Ma is estimated from established geological events, rock ages and thicknesses. Dykes and quartz veins suggest that Burringurrah was still deeply buried 500 Ma ago, with the denudation rate indicating exposure of the upper surface of the Mount Augustus Sandstone by ~100 Ma. The oxygen isotope data suggest that weathering of the monzogranite beneath the Mount Augustus Sandstone occurred during Jurassic to early Cretaceous times (~200 to 100 Ma), ages broadly coincident with those derived from the application of denudation rates. This suggests that Burringurrah initially developed as an inselberg prior to at least the past 100 Ma. The younger ages from lower parts of the profile suggest that weathering continued into the Neogene/Quaternary (23 Ma to present), during which time the surrounding, now dissected, planation surface was also weathered.
... Temperature and isotopic composition of the ancient meteoric water At surficial temperatures, the stable isotope composition of kaolinite is related to its temperature of formation and the isotopic composition of the water from which it crystallized (e.g. Savin and Epstein, 1970;Lawrence and Taylor, 1972;Bird and Chivas, 1988;Sheppard and Gilg, 1996). Assuming that the global meteoric water line (GMWL) equation by Craig (1961) is a reasonable representation of the ancient meteoric water, which derived from the atmosphere as paleoprecipitation and percolated through the soil profile (e.g. ...
This study reports the impact of weathering on behavior of rare earth elements (REE) in a coastal lateritic profile developed on Pliocene sediments at mid-latitude location (~37° N), in the Guadalquivir Basin (Spain). It also explores the stable isotope signature of kaolinite providing constraints on paleotemperature and isotopic composition of the ancient meteoric water. The paleoweathering profile (~20m thick) comprises a white sandy clayey saprolite overlain by a red-mottled clay zone, which in turn is capped by a pisolitic ferricrete. The
kaolinitic regolith was practically reduced to a mixture of kaolinite and quartz by intense chemical weathering under warm and seasonally humid climate. The kaolinized material is markedly depleted in total REE (mean value of 77.30 mg kg−1 in the white saprolite and 72.70 mg kg−1 in the mottled zone) relative to the parent rock (168.96 mg kg−1). Development of acidic and oxidizing conditions promoted a suitable soil environment for REE release and leaching with percolating rainwater, leading to an apparent loss of REE from the profile (up to 87%). The parent rock-normalized REE patterns display concave-upward shapes tracing a remarkable depletion in middle REE (MREE), with no significant anomalies. The REE concentrations normalized against the North American Shale Composite (NASC) showed consistently similar fractionation patterns. The MREE-depleted signature (NASC-normalized ratios of La/Sm up to 3.32 and Gd/Lu as low as 0.45) probably arises from reductive dissolution of iron oxyhydroxides phases due to seasonal water saturation, as indicated from redoximorphic features. The<2 μm kaolinite separates showed δ18O values ranging from 17.3‰ to 20.0‰, and δD values between −71‰and −60‰. The δ18O and δD mean values indicated a crystallization temperature of about 24 °C, which is higher than the local present-day annual mean air temperature (~18 °C). For the calculated temperature, the oxygen isotope fractionation factor between kaolinite and water (αk-w=1.0248) implies that kaolinite formed in equilibrium with meteoric waters (δ18O=−5.4‰ and δD=−33‰) that closely reflect the regional meteoric precipitation. The lateritic weathering profiles evolved during the mid-Pliocene warm period in the southwestern Iberian margin could be useful for predicting potential future environmental effects of increased atmospheric CO2 on the Earth's critical zone.
... Similarly, in many other sandstones with large amounts of feldspars, homogeneous temperature of aqueous fluid inclusions in authigenic quartz suggested the precipitation of quartz cements when temperatures exceeded 80-90°C (Higgs et al., 2007;Guo et al., 2012;Yuan et al., 2013Yuan et al., , 2015a. However, isotopic data of authigenic kaolinite in these sandstones suggested that the precipitation of kaolinite started in a relatively shallow burial diagenetic stage with low temperatures (40-50°C), and generally in the presence of meteoric water (Bird and Chivas, 1988;Longstaffe and Ayalon, 1990;Bjørlykke and Jahren, 2012). ...
Dissolution of feldspars and precipitation of secondary minerals (kaolinite, illite and quartz) are significant diagenetic processes in arkosic sandstones. We examined moderately buried sandstones in the Eocene Shahejie Formation from two sags in the Bohai Bay Basin, East China. Three different types of mineral assemblages (MA) were identified: extensively leached feldspars with a large amount of authigenic kaolinite and quartz cement (MA-1), extensively leached feldspars with a large amount of authigenic kaolinite and minor quartz cement (MA-2), and extensively leached feldspars with a small amount of both authigenic kaolinite and quartz cement (MA-3).
... These low δD values in illites were attributed to radiolysis reactions on water from uranium by Halter et al. (1989). Such a process is unlikely, however, because (1) the expected changes in δ 18 O values from such a process are absent in the clays, (2) those minerals with aberrantly low δD values also occur far from mineralization and are in fractures where the integrated water/rock ratios have been high (Kotzer and Kyser, 1991), (3) this effect is absent in all other uranium deposits regardless of their location and (4) the preferential exchange of hydrogen isotopes has been documented in other hydrous minerals from a variety of geologic settings (Bird and Chivas, 1988;Longstaffe and Ayalon, 1990;Kyser et al., 1999;Longstaffe, this volume). The significance of the late kaolinites (K3) and minerals that have undergone preferential hydrogen isotope exchange with relatively modern waters is that late influx of meteoric waters occurred, but primarily along faults where permeabilities are enhanced. ...
... Savin and Epstein (1970) and Taylor (1971, 1972) also demonstrated that clay crystals are able to retain their isotopic signatures through further processes of erosion, transportation and deposition. Bird and Chivas (1988) showed that oxygen-isotope exchange in kaolinite being negligible, post-formational hydrogen-isotope exchange is possible at temperatures below~80°C. From δD and δ 18 O of diagenetic kaolinite and 10 Å clay minerals (illite, illite-smectite), Longstaffe and Ayalon (1990) reported that hydrogen-isotope re-equilibration between diagenetic kaolinite and formation water can be independent of oxygen-isotope exchange at temperatures as low as 40°C. ...
... Although it is theoretically possible to reconstruct both the δ 18 0 and ÖD of formation waters from single minerals, δ 18 0 can be reconstructed more reliably. Experiments on kaolinite, montmorillonite, and illite (O'Neil and Kharaka, 1976) and field studies on kaolinite (Bird and Chivas, 1988a;Longstaffe and Ayalon, 1990;Lawrence and Rashkes Meaux, 1993) show that oxygen isotope ratios of clay minerals are more resistant to post-formational exchange than hydrogen isotope ratios. Therefore studies employing δ 18 0 values to infer paleoclimate are preferred. ...
In addition to providing an understanding of processes within a catchment system, isotopic techniques have been instrumental in providing reconstructions of catchment climate and other environmental indicators at various time scales. Many recent changes are a direct consequence of anthropogenic activities. Isotopic analysis serves as a valuable tool for distinguishing between natural variations in long-term climatic patterns and anthropogenic effects, yielding improved understanding of natural feedback mechanisms and the development of realistic remediation strategies. This chapter discusses the examples of isotopic techniques that have been applied to understand several types of ongoing and recent environmental changes, and in paleo-environmental studies. It discusses isotope geochemistry, hydrology, and climatology to look at new ways of applying isotopic tracing techniques to provide information on environmental change. It also gives an overview on how isotopic indicators are being applied in investigations of environmental change in continental settings.
... With respect to this, it is interesting to note that, in aqueous systems with a large influx of meteoric water, pre-existing diagenetic kaolinites can be entirely re-equilibrated with respect to the 6D compositions of the formation waters at both high and low temperatures. They tend to show a narrow range of 6D values equivalent to the formation water value (Longstaffe & Ayalon, 1987;Bird & Chivas, 1988). In contrast, kaolinites and illites in the Magnus Sandstone, North Sea have a wide range of 6D values (-55 to -105%o) but a narrow range (--9 16%o) for the 61SO values (Fallick et al., 1993b). ...
Aeolian sandstones of the Lower Permian Leman Formation (Rotliegend Group) provide the best gas reservoir in the southern North Sea, but permeability is greatly reduced by the presence of authigenic fibrous illites. New radiogenic (K/Ar) and stable (oxygen and hydrogen) isotope data are presented for fibrous illite cements (<0.1 µm), so that the absolute timing and controlling diagenetic factors for their formation can be more fully evaluated. Thus, the expected quality of gas reservoirs in the southern North Sea might be better predicted. Samples have been analysed from five wells in areas with contrasting structural evolution: the Sole Pit Basin, and the Indefatigable Shelf. The K/Ar ages of between 160 and 190 Ma have been obtained from the Indefatigable Shelf illites, and between 120 and 160 Ma for those from the Sole Pit Basin, reflecting different times of basin inversion. These K/Ar ages are interpreted by reference to burial/thermal models for each well. The temperature of illite precipitation falls between 88 and 140°C. Calculated pore-fluid compositions derived from oxygen and hydrogen isotopic analyses give values of ∼ + 1 to +9‰ (SMOW) δ ¹⁸ O and +1 to −50‰ (SMOW) δD. The illite δD values have probably been affected by isotopic exchange and fractionation with the surrounding gaseous hydrocarbon. The δ ¹⁸ O values reflect the degree to which evaporative concentration had affected Zechstein marine waters which subsequently invaded the Leman Sandstone. Comparisons between δ ¹⁸ O and δD values in clays and in formation water for the Leman Field suggest that oxygen isotope exchange might have taken place, and that the initial K ⁺ and radiogenic ⁴⁰ Ar contents within illites may have been modified.
... Experimental evidence suggests that the amount of oxygen isotope exchange for kaolinite is insignificant below 350~ (O'Neil & Kharaka, 1976). Bird & Chivas (1988) have argued that 599 marked hydrogen exchange has occurred at <80~ in Permian kaofinites. However, they note that oxygen isotopes have not been exchanged, probably because change of structural H + and not (OH) was involved. ...
Fluid flow across the entire basin was driven by a hydrostatic head on the East Shetland Platform palaeo-landmass to the west. The development of the two kaolinite morphologies is possibly related to the degree of supersaturation at the time of precipitation. At low degrees of supersaturation, vermiform kaolinite precipitated slowly upon detrital mica surfaces. -from Authors
... C'est en particulier le cas des isotopes du carbone qui présentent des variations fortes au cours du Cénozoïque et dont les rapports ont souvent été utilisés pour les datations de calcrètes (Koch et al., 1985 ;Quade et al., 1989 ;Cerling, 1999 ;Cojan et al., 2000 ;Magioncalda et al., 2004 ). Les rapports isotopiques de l'oxygène et du deutérium ont également été utilisés en rapportant le rapport mesuré dans les kaolinites à la variation paléolatitudinale du rapport isotopique rapporté à la courbe de dérive des continents (Bird et Chivas, 1988;Bird et Chivas, 1989;Gilg, 2000). ...
Dating continental unconformities are essential to constrain geodynamical models. Paleomagnetism is a great tool to date ferrugineous paleoweathering. Datings of red "siderolithic" formations from the Massif central provide ages ranging from 160 Ma (Late Jurassic) in the Naussac Basin to 140 Ma (Early Cretaceous) in the Lembron region. On the southeast border of the Paris Basin, a hematitic layer located at the base of the Myennes Clays gave a recent age (0-3 Ma) which could be related to groundwater flow during the formation of the present-day landscapes. On the Massif Central and the Morvan crystalline basements albitized facies have been dated by paleomagnetism to be Triassic in age. Both datings and geographic distribution of the albitized formations argue for a superficial origin of these facies. The space and time distribution of the formations and the related unconformities gives elements to highlight the geodynamic evolution of the Massif Central. Triassic, Late Jurassic and Tertiary unconformities are superimposed on large areas. It implies very little erosion of the crystalline basement since Triassic times, as shown by the widespread albitized triassic unconformity outcroping at present. Datings points out that no Mesozoic cover was present on the basement during development of the red kaolinitic paleosoils during the Late Jurassic/Early Cretaceous period. Consequently, the Massif Central did probably not support an important (more than 500 m) sedimentary cover during the Mesozoic. Identification and dating of continental unconformities give evidences for long lasting continental evolution and landscape stability of the Massif Central all along the Mesozoic.
... However, such an impact could be due to either kaolinite infiltration into the band (Sternlof, 2006) or quartz grain indentation of pre-existing kaolinite during localized compaction. In order to understand the relative timing between the diagenetic evolution of kaolinite and the formation of compaction bands, further studies on the compositions of kaolinites inside and outside the compaction bands using oxygen isotope geochemistry (Bird and Chivas, 1988) would be necessary. ...
We measured the permeability of 30 samples extracted from 6 sets of compaction bands and the adjacent host rocks of the Jurassic aeolian Aztec Sandstone exposed in the Valley of Fire State Park in Nevada using core flooding experiments. The results show that the permeability within the high-angle compaction bands (three sets) is consistently three orders of magnitude lower than that of the host rocks. For the bed-parallel compaction bands, the measured permeability reduction is about half an order to three orders of magnitude for two sets of bands, and there is no detected permeability reduction for the samples from one set. For the samples that show permeability reduction within high-angle and bed-parallel compaction bands, the results are generally consistent with the data estimated from two-dimensional segmented image analyses in previous studies. Permeability of the samples used in the laboratory experiments was also obtained numerically based on three-dimensional tomographic images scanned from micro-samples and lattice-Boltzmann flow simulations. In addition, backscatter electron images (BET) and energy dispersive spectroscopy images (EDSI) of thin sections were used to estimate the clay content inside and outside the bands. Large differences exist between the lab-based and image-based permeability and porosity measurements of compaction bands and host rocks. Possible factors causing these differences are different sample sizes and heterogeneities within the host rocks, calibration on the image segmentation, and incomplete characterization of clay minerals and fines migration during lab-based experiments. Given the wide range of permeability reductions within compaction bands of different orientations by different investigators, their impact on fluid flow should be evaluated case by case, one should consider their dimensions and distributions.
... A growing number of authors now recognize that genesis of these end products do not require tropical conditions (cf. Paton and Williams, 1971;Bird and Chivas, 1988;Ollier, 1988;Taylor et al., 1992;Bourman, 1993Bourman, , 1995Bourman and Ollier, 2002). Abundant moisture and time appear to be key factors, as well as exposure to groundwater in the weathering profile. ...
Weathering processes are partially responsible for a characteristic geomorphology that occurs in the tropics and subtropics. Resistant landforms such as inselbergs, extreme solution processes such as silica karst, and deep weathering profiles with end stage weathering products such as laterite and kaolin are common features of tropical weathering. Many of these features also occur outside the tropics. In part, climate change and paleotectonics were responsible for tropical conditions in areas not now tropical. But, some processes assumed to require tropic conditions that are not so limited, sufficient moisture, and time sufficing for their development. This chapter reviews the weathering processes and distinctive landforms of the broadly defined tropics, and explores the debates over weathering factors as they pertain to tropic and extratropical environment.
... Ž . Bird and Chivas 1988 have argued that significant post-formational hydrogen isotope exchange has occurred at temperatures below 858C in some Permian Australian kaolinites of weathering origin. They found no evidence for oxygen isotope exchange of the same samples. ...
The 18O/16O and D/H ratios of the structural oxygen and hydrogen of naturally occurring clay minerals carry information about the conditions under which the minerals formed. Clays generally form in isotopic equilibrium with the environment, and hence have isotopic compositions that reflect the temperature of formation and the 18O/16O and D/H ratios of water in the micro-environment. The isotopic signature of water in the weathering environment is a reflection of the climate. Once formed, clays are resistant to isotopic alteration under surface and near-surface conditions, except when they undergo subsequent mineralogical alteration. In some instances, this alteration may be subtle, as in the case of conversion of kaolinite with a high degree of crystalline order (as measured by the Hinckley index) to kaolinite of low crystalline order in a pedogenic profile. Isotopic studies of pedogenic clays may be expected to yield more information about pedogenesis and paleoclimate when both 18O/16O and D/H ratios of the clays are measured. However, few studies have incorporated the use of the two isotope ratios.
... Savin and Epstein (1970) and Taylor (1971, 1972) also demonstrated that clay crystals are able to retain their isotopic signatures through further processes of erosion, transportation and deposition. Bird and Chivas (1988) showed that oxygen-isotope exchange in kaolinite being negligible, post-formational hydrogen-isotope exchange is possible at temperatures below~80°C. From δD and δ 18 O of diagenetic kaolinite and 10 Å clay minerals (illite, illite-smectite), Longstaffe and Ayalon (1990) reported that hydrogen-isotope re-equilibration between diagenetic kaolinite and formation water can be independent of oxygen-isotope exchange at temperatures as low as 40°C. ...
Clastic rock reservoirs in petroliferous basins are generally rich in feldspars. Feldspar dissolution has developed widely in clastic reservoirs, and the resulting secondary pores are crucial in deeply buried reservoirs. Based on a study of the diagenesis of clastic reservoirs in the Bohai Bay Basin, Tarim Basin, and Pearl River Mouth Basin and physical and numerical simulation experiments of fluid-rock interactions, this paper proposed a successive formation model of secondary pores via feldspar dissolution in deeply buried clastic reservoirs, considering the global research progresses in feldspar dissolution in clastic rocks. Feldspar dissolution can occur from shallow open systems to deep-ultra deep closed systems in petroliferous basins, resulting in the successive formation of secondary pores at different diagenetic stages. The successive mechanism includes three aspects. The first aspect is the succession of corrosive fluids that dissolve minerals. Meteoric freshwater dominates at the Earth’s surface and the early diagenetic A stage. Subsequently, organic acids and CO2 formed via kerogen maturation dominate at the early diagenetic B stage to the middle diagenetic stage. CO2 and organic acids formed via hydrocarbon oxidation in hydrocarbon reservoirs dominate at the middle diagenetic B stage to the late diagenetic stage. The second aspect is the successive formation processes of secondary pores via feldspar dissolution. Large-scale feldspar secondary pores identified in deep reservoirs include secondary pores formed at shallow-medium depths that are subsequently preserved into deep layers, as well as secondary pores formed at deep depths. Existing secondary pores in deeply buried reservoirs are the superposition of successively feldspar dissolution caused by different acids at different stages. The third aspect is a successive change in the feldspar alteration pathways and porosity enhancement/preservation effect. Open to semi-open diagenetic systems are developed from the Earth’s surface to the early diagenetic stage, and feldspar dissolution forms enhanced secondary pores. Nearly closed to closed diagenetic systems develop in the middle to late diagenetic stages, and feldspar dissolution forms redistributional secondary pores. The associated cementation causes compression resistance of the rock, which is favorable for the preservation of secondary pores in deep layers. These new insights extend the formation window of secondary pores in petroliferous basins from the traditional acid-oil generation window to a high-temperature gas generation window after hydrocarbon charging. The proposed model explains the genesis of deep-ultra deep high-quality reservoirs with low-permeability, medium-porosity and dominating feldspar secondary pores, which is significant for hydrocarbon exploration in deep to ultra-deep layers.
https://doi.org/10.1360/SSTe-2020-0157
含油气盆地碎屑岩储层中富含长石等大量铝硅酸盐矿物，长石溶蚀作用普遍发育，且长石溶蚀形成的次生孔隙是深层碎屑岩储层重要的储集空间。本文基于对渤海湾盆地、塔里木盆地和珠江口盆地等碎屑岩储层地质实例成岩作用和流体-岩石相互作用物理及数值模拟实验的研究，结合国内外碎屑岩储层长石溶蚀作用研究进展，提出了含油气盆地深层碎屑岩储层“长石溶蚀接力成孔”认识。即长石溶蚀作用从含油气盆地中浅层开放体系到深层-超深层封闭体系均可发生，表现出接力成孔效应，包含三层含义。一是溶蚀矿物的侵蚀性流体的接力：地表-早成岩A期主要是大气淡水，早成岩B期-中成岩A期主要是配套烃源岩中干酪根热演化生成有机酸和CO2等酸性流体，中成岩B期-晚成岩期主要是油气储层中烃类高温水氧化生成的CO2和有机酸等酸性流体。二是长石溶蚀成孔过程的接力：深层储层中的规模性长石次生孔隙既有中/浅层溶蚀形成后保存到深层的，又有直接在深层溶蚀形成的，是不同期次、不同来源酸性流体对长石矿物多期持续溶蚀改造的叠合。三是长石溶蚀路径和增孔-保孔效应的转换接力：地表-早成岩阶段发育开放-半开放成岩体系，长石溶蚀形成增孔型次生孔隙；中成岩-晚成岩阶段发育近封闭-封闭成岩体系，长石溶蚀形成调配型次生孔隙，伴生的胶结作用使岩石具有更强抗压性，利于次生孔隙有效保存。这一认识的提出，将含油气盆地中次生孔隙形成窗口从传统认识的生酸-生油窗拓展到成藏后高温生气窗，合理解释了深层-超深层长石溶蚀次生孔隙主导的低渗-中孔型优质储层的发育机理，对拓展深层-超深层油气勘探领域有重要意义。
Tracing interactions during burial-induced organic maturation and associated clay-material alteration is of prime importance for understanding both the individual and combined mineral and organic processes. In the present study the light elements B, Li, O, and H of a sample from oil-prone Eocene Kreyenhagen Shale from San Joaquin Basin (California) were examined. The natural burial-induced temperature increase was simulated by pyrolysis experiments at progressively increasing temperatures (270–365°C) and for varied durations (72–216 h) applied to the whole rock and its <2 μm fraction. The illite structure as well as the K-rich interlayers of the illite-smectite mixed layers were not affected by the pyrolysis experiments and the smectite-rich interlayers did not collapse, while the soluble minerals and the organic matter were altered. The distribution pattern of the rare-earth elements (REEs) from untreated whole rock and of its pyrolyzed equivalents are within analytical uncertainty, which confirms that the changes induced by pyrolysis experiments were minimal in the bulk sample. Conversely, the REEs from the <2 μm fractions were modified significantly, suggesting that the whole rocks and the <2 μm fractions may contain different types of organic materials. Also, only the carbonates, oxides, chlorides, and organic matter were affected together with the smectite-rich interlayers of the illite-smectite structure. Bitumen coating of the smectite interlayers probably increased the amount of B of organic origin in their sites. The δ11B and δ7Li of the successively expelled hydrocarbon phases changed with increasing pyrolysis temperatures, together with the B and Li contents of the hydrocarbon-related fluids. On the basis of the δ11B and δ7Li from pyrolyzed clay fractions, the B released successively was not isotopically homogeneous, probably depending on how the type of organic matter decomposed during the successive pyrolysis steps, and on which components were released. The δ11B of organic-B increased progressively from –2‰ at low experimental temperature up to +9‰ at the highest temperature. The calculated δ7Li that was released also increased relative to the value of the outcropping sample used as a reference, but it remained almost constant from –7‰ at 310°C for 72 h to –8‰ at 365°C for 216 h. The δ18O values of the <2 μm size fractions decreased significantly during pyrolysis above 300°C, but the δD changes were rather modest. The total organic carbon (TOC) remained statistically constant after pyrolysis to 300°C, as did the δ7Li values. The pyrolysis experiments in the present study suggest the presence of bitumen-coated smectite interlayers that could have been misidentified as dehydrated smectite in the literature. Together with abnormal illite K-Ar ages, the occurrence of such bitumen-coated illite-smectite interlayers occurring in source and reservoir rocks could indicate the timing of hydrocarbon maturation relative to illitization.
Weathering processes are partially responsible for a characteristic geomorphology that occurs in the tropics and subtropics. Resistant landforms such as inselbergs, extreme solution processes resulting in silica karst, and deep weathering profiles with end stage weathering products such as laterite and kaolin are common features of tropical weathering. Many of these features also occur outside the tropics. In part, climate change and paleotectonics were responsible for tropical weathering conditions in areas not now tropical. But, some processes assumed to require tropical conditions that are not so limited, sufficient moisture and time sufficing for their development. This chapter reviews the weathering processes and distinctive landforms of the broadly defined tropics, and explores the debates over weathering factors as they pertain to tropical and extratropical environments.
Although abundant evidence exists for past liquid water on Mars, the composition and amount of that liquid water remain unconstrained. The characteristics of liquid water are preserved within weathering profiles, which can be used to interpret past aqueous conditions. Putative weathering profiles consisting of Al-rich units above Fe/Mg-rich units have been documented in numerous locations on Mars. Of these locations, Mawrth Vallis is the largest in area; therefore, understanding the aqueous conditions that could cause such a profile may also help interpret similar transitions in clay mineralogy elsewhere on Mars. The stratigraphy of the Mawrth Vallis region is characterized by an Al-rich unit dominated by kaolinite and/or montmorillonite with Fe-(oxy)hydroxides and/or oxides, overlying an Fe/Mg-rich unit composed of nontronite and/or saponite. To interpret the implications of the possible formation of Mawrth Vallis by weathering, we used the reactive transport code CrunchFlow to model alteration of parent material consisting of: (1) nontronite; (2) nontronite and montmorillonite; (3) nontronite and saponite, and (4) saponite. Results indicate either the enrichment of Al-rich clay minerals in an upper weathered unit from the weathering of Fe/Mg-rich clay minerals, or weathering of saponite producing a nontronite-rich alteration product which would be anticipated to subsequently alter to an Al-clay mineral rich capping unit. Results indicate the locations that have experienced the most aqueous/acidic alteration would be characterized by a thick Al-rich capping unit composed of kaolinite with Fe-(oxy)hydroxides, overlying nontronite. Nontronite overlying saponite would be consistent with less weathering than an Al-capping unit, as an Al-rich capping would likely develop with the continued weathering of nontronite. Al-rich units are observed in the areas around Jezero crater, Mars the landing area of the Mars2020 rover Perseverance. Characteristics of weathering profiles such as those examined here can be used in the examination of locations on Mars to help choose environments that contained liquid water with characteristics most conducive to habitability and biosignature preservation.
The McArthur River unconformity-related uranium deposit, located in the Athabasca Basin of Saskatchewan, Canada, is structurally hosted near the unconformity between Archean to Paleoproterozoic metasedimentary basement and the Proterozoic Athabasca Group sandstones. In this study, the mineralogy and geochemistry of fracture materials within the entire ca. 550 m thickness of the Athabasca Group sandstones and the metasedimentary (host) rocks from the McArthur River area were used to determine the paragenetic sequence and origin of minerals in and near the fractures. Our work sought to determine if the host minerals record elements associated with the uranium deposit at depth and if they could be used to guide exploration (vectoring). Fracture orientations indicate that most are moderately dipping (<50°) and provided permeable pathways for fluid movement within the basin, from below, and through the overlying sedimentary rocks. Many of the fractures and adjacent wall rocks record evidence of multiple distinct fluid events.
Seven types of fracture fillings were identified from drill core intersecting the Athabasca Basin and present distinct colors, mineralogy, and chemical features. Brown (Type 1) and pink (Type 7) fractures host paragenetically late botryoidal goethite, Mn oxide minerals, and poorly crystallized kaolinite that formed from relatively recent low-temperature meteoric fluids, as indicated by poor crystallinity and low δ2H values of –198 to –115‰. These minerals variably replaced higher temperature minerals that are rarely preserved on the fractures or in wall rock near the fractures. Hydrothermal alteration associated with the mineralizing system at ca. 200 °C is recorded in assemblages of dickite, well-crystallized kaolinite, and spherulitic dravite in some white and yellow (Type 2) and white (Type 3) fractures, as reflected by the crystal habits and variable δ2H values of –85 to –44‰. Fibrous goethite in white and yellow (Type 2) and black and orange (Type 5) fractures and microfibrous Mn oxy-hydroxide minerals in black (Type 4) fractures also crystallized from hydrothermal fluids, but at temperatures less than 200 °C.
White and yellow fractures (Type 2) containing fibrous goethite reflect fracture networks indicative of hydrothermal fluids associated with the mineralizing system during primary dispersion of pathfinder elements and therefore extend the deposit footprint. Brown (Type 1) and pink (Type 7) fractures have low δ2H values in botryoidal goethite and poorly crystallized kaolinite and are indicative of the movement of meteoric waters. Secondary dispersion of elements from the deposit to the surface on some fractures is evidence that fractures are pathways for element migration from the deposit to the surface, over distances exceeding ∼500 m.
Detrital zircon grains within four (4) deposits including two (2) Cretaceous and two (2) Paleogene/Neogene kaolins in Nigeria were analysed using U–Pb LA-SFICP-MS to determine their provenance. The zircon textures were dominated by xenocrystic cores and oscillatory zoning in the Cretaceous and Paleogene/Neogene kaolins, respectively. The Th/U ratios obtained for the detrital zircon grains within the kaolins were predominantly within known values for rocks with magmatic origin. The age populations obtained for the detrital zircon grains were dominated by values from 529 to 978 Ma within the Neoproterozoic, followed by values from 1754 to 2497 Ma of the Paleoproterozoic. Detrital zircon ages obtained between 553.2 ± 6.2 and 583.5 ± 2.0 Ma represent part of the minimum provenance ages for the primary minerals that were kaolinised. The Cretaceous–Paleogene/Neogene kaolins were derived from parent rocks of Eburnean and Pan African ages within the Western and Northern Nigeria Basements.
Braided river sandstones in the Permian Upper Shihezi (US) Formation in the Huanghua Depression, Bohai Bay Basin, East China, are reservoirs with large accumulations of hydrocarbons. The Upper Paleozoic formations, including the Carboniferous and the Permian strata experienced multi-stage of subsidence and uplift in the past 320 Ma, leading to occurrence of subaerial exposure and deep burial of the Permian US sandstones. Due to complex tectonic evolution, the Upper Paleozoic formations in different tectonic zones of the Huanghua Depression experienced various burial-thermal histories, resulting in the development of long-term open (LTO), early-open to late-closed (EOLC), and long-term closed (LTC) geochemical systems in different Permian US sandstones, as well as different evolutionary patterns of diagenesis and reservoir quality. After extensive diagenetic alteration, the present sandstones are primarily quartzose, subfeldsarenite, and sublitharenite sandstones. Pores in shaly sandstones and fine-grained sandstones were destroyed extensively, only pebbly to medium-grained sandstones can be potential effective reservoirs. The Permian US sandstones with the LTO systems, which are currently overlain by the Paleogene strata and buried less than 2500 m, experienced two stages of meteoric freshwater leaching and are characterized by extensive feldspar dissolution and weak precipitation of authigenic kaolin and quartz. Intergranular and intragranular pores dominate the 10–20% porosity, and permeability ranges from 1mD to 800 mD. The Permian US sandstones with the EOLC systems, which are currently overlain by thick Jurassic and Cretaceous strata on a regional scale and buried deeper than 3000 m, experienced both meteoric freshwater leaching and deep burial leaching and are currently characterized by extensive feldspar dissolution, extensive kaolin precipitation, and weak-moderate quartz cementation. Intragranular secondary pores and kaolin intercrystal pores dominate the 5–15% porosity, and permeability ranges from 0.1 mD to 10 mD. The Permian US sandstones with the LTC systems, which are currently overlain by a thick Permian Shiqianfeng Formation on a regional scale and buried deeper than 3000 m, experienced no meteoric freshwater leaching but only burial dissolution; they are currently characterized by extensive feldspar dissolution and extensive precipitation of clays and quartz cements. Intercrystal pores in the authigenic clay and intragranular secondary pores dominate the 5–10% porosity, and permeability ranges from 0.01mD to 1mD. Since the unconformities, overlying strata, and tectonic evolution relevant to the Permian US sandstone unit can be analyzed using seismic data, the proposed genetic models can support the predrill evaluation of reservoir quality of the Permian sandstones.
The Paleozoic Wajid Sandstone in southwestern Saudi Arabia comprises four members known as Dibsiyah, Sanamah, Khusayyayn and Juwayl. The study characterizes the sedimentary facies of the Juwayl Member from five outcrop localities in Wadi Ad-Dawasir area. The objective is to reveal the influence of facies heterogeneity on the reservoir distribution and quality. The field investigations involve facies description, sampling and architectural elements identification. The second step involves the interpretation of facies associations to predict the depositional environments and controls. The petrographic analysis, of thin section, SEM and XRD, helped to define the diagenetic controls that affected the quality and heterogeneity of rock properties such as porosity and permeability. The facies analysis revealed ten lithofacies corresponding to glacio-fluvial and braided-fluvial depositional environments. The glacio-fluvial deposits consist of pebbly to conglomeratic sandstone lithofacies that suggest high to medium energy conditions within a proximal fluvial setting. The braided-fluvial deposits are of medium to coarse-grained, trough to planner cross-bedded sandstone lithofacies. These lithofacies indicate a lower energy distal condition of laterally switching channels. The remaining lithofacies include overbank deposits of fine-grained argillaceous-ferruginous sandstone lithofacies, observed at the top of fining upward sequences. The study classified Juwayl sandstone as mineralogically mature quartz arenite with good to very good porosity and permeability. It dominates by (90%) quartz, and other components of calcite, clay and iron oxides with subordinate K-feldspars. The diagenetic controls including cementation, grain-size reduction, quartz overgrowth and clay minerals are affecting the Juwayl sandstone porosity and permeability. The depositional environments of Juwayl sandstone indicate a system switching from high energy glacio-fluvial to lower energy braided-fluvial setting. The thickness and width of the sandstone bodies tend to vary in this model implying vertical and lateral sandstone heterogeneity. Clay, hematite and calcite cements contributed to porosity reduction, while grain dissolution, clay coats and rims contributed to porosity enhancement. Macro-scale outcrop heterogeneity in terms of facies, paleonvironments and stratigraphic architecture and microscopic scale in terms of texture, composition and diagenesis, all might provide guides for subsurface reservoir assessment, comparison and correlations.
This study evaluated the Cretaceous (Campanian–Maastrichtian) kaolinitic sediments of the Ajali/Mamu and Enugu/Nkporo Formations from the Lower Benue Trough of Nigeria. A combined method of inductively coupled plasma–mass spectrometry and isotope ratio mass spectrometry was used to investigate trace and rare-earth element geochemistry and hydrogen and oxygen isotopic compositions. These data were then used to infer the sediments’ provenance and paleoclimatic conditions during their deposition. The sediments contained low concentrations of most trace elements, with the exceptions of Zr (651–1352 ppm), Ba (56–157 ppm), V (38–90 ppm), and Sr (15.1–59.6 ppm). Average values of Co and Ni were 1.5 and 0.7 ppm, respectively. Trace and rare earth element values were lower than corresponding values for upper continental crust and Post-Archean Australian Shale, with the exception of Zr. The samples showed only slight light rare-earth enrichment and nearly flat heavy rare-earth depletion patterns, with negative Eu and Tm anomalies, typical of felsic sources. Geochemical parameters such as La/Sc, Th/Sc, and Th/Co ratios support that the kaolinitic sediments were derived from a felsic rock source, likely deposited in an oxic environment. ¹⁸O values ranged from + 15.4 to + 21.2‰ for the investigated samples, consistent with a residual material derived from chemical weathering of felsic rock and redeposited in a sedimentary basin (typical values of + 19 to + 21.2‰). While in the basin, the sediments experienced extended interactions with meteoric water enriched in δD and δ¹⁶O. However, the variation in δD and δ¹⁶O values for the investigated samples is attributed to the high temperature of formation (54–91 °C). The δD and δ¹⁸O values suggest that the sediments, although obtained from different localities within the Lower Benue Trough, formed under similar hot, tropical climatic conditions.
This conference paper (p53-56) is about swales (inverted yardangs) discovered between sand dunes of the Hunder dunefield, in Ladakh, in the Northern Himalayas.
Proceedings of the 5th Australian Regolith Geoscientists Association
Conference, from Wallaroo, South Australia, are also included.
The proceedings cover topics including:
regolith geochemistry
hydrogeology
soils
critical zone science
geomorphology
landscape evolution and
UNCOVER
Metasedimentary rocks of the Paleoproterozoic Hutchison Group on southern Eyre Peninsula, South Australia contain zones of high-grade flake graphite, distributed along a north-south belt of tightly folded upper amphibolite to granulite facies biotite-quartz schist and quartz-feldspar-biotite-garnet gneiss, immediately west of the Kalinjala shear zone. During early 1990s, open-cut mining for graphite at the Uley mine, 18 km southwest of Port Lincoln, exposed extensive patches of nontronite (Fe-rich smectite) alteration, with minor celadonite (Fe-rich white mica), in weathered biotite-graphite schist and amphibolite. Quartz-biotite-graphite schist in coastal cliffs at Clem Cove on Sleaford Bay, 7 km south of Uley mine, is also intensely altered with veins and patches of celadonite, Fe-oxy/hydroxides, silica and Mn-oxides.
Alteration of biotite to nontronite or celadonite is unusual and was concluded to result from low temperature, saline hydrothermal fluids circulating within shear zones. This was based on the persistence of alteration with depth, increased intensity near prominent shear zones, and environmental conditions most commonly associated with nontronite/celadonite formation (i.e. conditions of hydrothermal alteration of basalt in mid-ocean ridge spreading centres).
Celadonite samples from Uley and Sleaford Bay were dated subsequently using K-Ar method, which indicated early Eocene to early Miocene ages of crystallisation. The timing is not linked to any recorded thermal event in the region but corresponds broadly with episodes of Cenozoic deep weathering. The timing and apparent restriction of this style of alteration to zones with high graphite content suggests an alternative explanation is required that involves some local modification of weathering processes.
Chemical weathering and oxidation creates a potential difference between the weathered zone and fresh rock. Graphitic shear zones can act as conductive bodies that focus the flow of negative charge towards the zone of oxidation. This results in a more reduced environment in the area around the top of the conductor. Under conditions of saline (neutral to slightly alkaline) groundwater, a weathering environment is envisaged where conditions are sufficiently reducing, such that Fe released during biotite weathering is available to substitute in silicate structures to form celadonite and nontronite, in addition to forming iron oxy/hydroxides.
Twelve representative human geophagic soil samples collected from selected rural communities in Gauteng and Limpopo Provinces, South Africa, were analyzed using X-ray diffractometry to identify, quantify and characterize their minerals constituents. Kaolinite, smectite, talc, muscovite, quartz, calcite, dolomite, microcline, goethite and hematite were the clay and non-clay minerals identified. Two most dominant minerals in the geophagic soil samples were quartz (35-52 wt%) and kaolinite (31-53 wt%). Abundances for the other minerals were 8-27wt%. Based on the results, the clay minerals and mineral oxides could serve as elemental sources for supplementation of Al, Na, K, Ca, Mg, and Fe in the bodies of the geophagic individuals. There could however be influential factors such as the soil matrix itself, soil type, chemical form of the element, stomach and intestinal pH, and the soil to solution ratio, which could affect the elemental bio-accessibilities.
Twelve representative human geophagic soil samples collected from selected rural communities in Gauteng and Limpopo Provinces, South Africa, were analyzed using X-ray diffractometry to identify, quantify and characterize their minerals constituents. Kaolinite, smectite, talc, muscovite, quartz, calcite, dolomite, microcline, goethite and hematite were the clay and non-clay minerals identified. Two most dominant minerals in the geophagic soil samples were quartz (35-52 wt%) and kaolinite (31-53 wt%). Abundances for the other minerals were 8-27wt%. Based on the results, the clay minerals and mineral oxides could serve as elemental sources for supplementation of Al, Na, K, Ca, Mg, and Fe in the bodies of the geophagic individuals. There could however be influential factors such as the soil matrix itself, soil type, chemical form of the element, stomach and intestinal pH, and the soil to solution ratio, which could affect the elemental bio-accessibilities.
Weathered rock holds groundwater. The zone of groundwater saturation is topped by a watertable which separates an upper oxidised zone from a lower, reducing zone. Some elementary weathering scenarios envisage weathering in only the acid, oxidizing upper zone, but weathering can proceed at the base of the saturated zone by hydrolysis. Weathering products must be removed if the reaction is to continue: in upper groundwater zones they are washed away; in lower zones chemical diffusion removes weathering products from the weathering front to the zone of mobile groundwater. Since deep weathering profiles may be tens of metres thick, the temperature at the ground surface does not affect the temperature at depth.
“Tropical” and “lateritic” are usually undemonstrated and unhelpful adj actives used in describing deep weathering profiles. Independent evidence shows that the climate at the time of formation of some deep weathered profiles was warm and wet, but this relationship is not necessary and some deep weathering occurred under cold conditions. In Australia some deep weathering is as old as Mesozoic, and formed before the breakup of Gondwanaland. The composition of alluvial gravels changed through the Tertiary as deep regolith was progressively stripped. Hydrothermal alteration has seeldom been demonstrated to be the cause of deep kaolin deposits: stable isotope studies indicate they are more likely to be caused by weathering.
The oxygen and hydrogen isotope compositions of diagenetic minerals can be used to provide constraints on the origin and evolution of porewaters during the life cycle of a sedimentary basin. In the first part of this chapter, general controls on the oxygen and hydrogen isotope fractionations in diagenetic systems are discussed. In the second part, stable isotope results for diagenetic minerals from sandstones and conglomerates of the western Canada sedimentary basin are reviewed as an example of this approach. This review illustrates the importance of meteoric water throughout the diagenetic evolution of these rocks. For many units, the isotopic data show that meteoric water was abundant during early diagenesis, either because of depositional environment or because of early, post-depositional processes related to sea-level fluctuation. During burial, the oxygen isotope values of porewaters increased not only because of rock-water interaction, but also because of mixing with brines derived from underlying Paleozoic carbonates. The oxygen isotopic compositions of late diagenetic minerals indicate crystallization during regional, gravity-driven flow of meteoric water that was initiated in response to uplift of the western Canada sedimentary basin in early Eocene time.
This study aimed at employing differential thermal analyses/ thermogravimetric analyses (DTA/TGA) and stable isotopes δ18O and δD analytical techniques in understanding the mineral genesis of kaolinite from two genetically different sources in Botswana. The mineral contents of the samples were determined by X-ray powder diffraction (XRPD) technique and the loss on ignition by heating. Thermal characterization studies were conducted using differential thermal analyses/thermogravimetric analyses (DTA/TGA) techniques. Kaolinite was the dominant mineral in both deposits. Mean temperatures for endothermic peaks for Makoro kaolinite was 589°C and for Kgwakgwe kaolinite 604°C; and the mean temperatures for their exothermic peaks were 1025°C for Makoro kaolinite and 1010°C for Kgwakgwe kaolinite. Stable isotopes mean values for kaolinite from both Makoro and Kgwakgwe were as follows: δ18O for Makoro = +14.0 ± 0.5%o and for Kgwakgwe = +14.8 ± 0.5 ‰; and the δD for Makoro = -71 ‰ and for Kgwakgwe = -77 ‰. Low temperatures are inferred from the stable isotope values to have been involved in the kaolintisation, thereby eliminating hydrothermal fluids playing any major role. Whereas Makoro kaolin is secondary, Kgwakgwe kaolin is primary but residual.
Distinct lateral and stratigraphic trends in paleosol morphology and clay mineralogy, in conjunction with the stable-isotope composition of sub-millimeter-scale spherulitic siderite (sphaerosiderite) and flint-clay kaolinite in middle-upper Pennsylvanian cyclic coal-bearing strata of the Illinois basin (IB) presented herein provide proxy records of middle-late Pennsylvanian equatorial terrestrial environments. Collectively, these data provide a better understanding of the polygenetic history of ancient soils preserved in cyclic strata and indicate that low-latitude Pennsylvanian IB soil profiles were influenced by a combination of autogenic and allogenic controls. Lateral variations in paleosol morphology from the interior of the basin to the northern margin indicate that soil development was dominantly influenced by autogenic controls, such as differences in local paleohydrology (i.e., groundwater fluctuations) and subsidence rates across the basin. Conversely, allogenic controls (i.e., glacioeustasy and climate) are interpreted to be principally responsible for the preservation of features in gleyed Protosols, gleyed Vertisols, and gleyed calcic Vertisols that occur in the interior of the IB. These paleosols are characterized by a polygenetic development history that includes (1) an initial period of soil formation in well-drained environments with seasonal wetting and drying of the profile, followed by (2) a subsequent period of waterlogging and development of reducing conditions. Paleosols that lack evidence for extensive periods of waterlogging are restricted, with the exception of two examples, to the northern margin of the basin, and they become abundant in the stratigraphic record near the middle-upper Pennsylvanian boundary (uppermost Desmoinesian). The stratigraphic distribution of these paleosols, in conjunction with a decrease in the relative abundance of kaolinite in the < 2 mu m size fraction of IB paleosols near the Desmoinesian-Missourian (D-M) boundary, is interpreted to reflect a shift in the overall low-latitude regional climate to drier conditions. The stable-isotope compositions of sphaerosiderite and flint-clay kaolinite in middle-upper Pennsylvanian strata of the IB provide an additional proxy record of past equatorial terrestrial environments and are used to constrain the magnitude of paleoclimate change across the D-M (similar to Moscovian-Kasimovian) boundary. Sphaerosiderite delta O-18(V-PDB) and delta C-13(V-PDB) values range from -3.6 parts per thousand to -0.5 parts per thousand and -12.8 parts per thousand to -3.2 parts per thousand, respectively. In particular, sphaerosiderite delta O-18 values from the B horizons of paleosols display an approximate -1.5 parts per thousand shift across the D-M boundary. This stratigraphically short isotopic shift occurs across the D-M boundary, on the order of one cyclothem ( similar to 400 kyr), and is followed by a subsequent shift on the order of one cyclothem, back to more positive siderite delta O-18 values. The average delta O-18(V-SMOW) and delta DV-SMOW values of kaolinite isolated from the < 0.2 mu m size fraction of a latest Desmoinesian flint-clay breccia are +21.0 parts per thousand and -46.5 parts per thousand. These values correspond to a latest Desmoinesian crystallization temperature of 27 +/- 2 degrees C and a meteoric-water delta O-18(V-SMOW) value of -3.1 parts per thousand. Combination of sphaerosiderite delta O-18 values and co-occurring flint-clay kaolinite delta D and delta O-18 values suggest a possible warming of up to 6 degrees C across the D-M boundary, corresponding to an approximate temperature change of siderite crystallization from 27 to 33 degrees C. This temperature increase, in conjunction with distinct stratigraphic trends in the clay mineralogy and morphology of IB paleosols, is consistent with previous paleoclimate interpretations, and it suggests that low-latitude Pennsylvanian paleoclimate became drier and warmer across the D-M boundary.
Stable isotopic and petrographic data have been used to interpret conditions for the formation of authigenic kaolinite within Lower Palaeocene sands, Central North Sea. Two wells within the Witch Ground Graben were sampled (1975 m to 2795 m). Texturally early calcite concretions have isotopic compositions (61So = 18.3-21.6%o SMOW) which indicate that they were precipitated in predominantly meteoric waters. The isotopic composition of later vermiform kaolinite (~tSO = 14.8-17.7%o SMOW and ~)D = -53 to -71%o SMOW) indicates that kaolinite precipitated at around 45-70~ from a mixed meteoric- marine pore-water (alSO = -5 to -3%0 SMOW). These modelled precipitation temperatures are consistent with the paragenetic sequence and consequently post-precipitation hydrogen isotope exchange between kaolinite and the pore-waters is presumed not to have occurred. It is inferred that the original depositional marine pore-waters were flushed out during the late Palaeocene (54.8 Ma) by a head of meteoric water from the East Shetland Platform. The Lower Palaeocene aquifer became closed to meteoric influx after marine transgression during the late Palaeocene (54.0 Ma). The remaining meteoric pore-waters in the sandstones became mixed with water from compacting marine muds surrounding the hydrostatically pressured sandstones.
Mineralogic, crystallographic, and oxygen isotopic analyses were made of kaolinites from a 20 m thick lateritic profile developed on the detrital sands and clays of the mid-Cretaceous to Tertiary Alter-do-Chão formation, central Amazon Basin, Brazil. A detailed δ18O profile of soil water from the upper 4 m of the same section was also measured.Kaolinite separates were prepared from different pedogenic facies of small hand specimens. Most of these kaolinite separates exhibit polymodal size distributions. The separates were further divided into individual particle size modes for subsequent isotopic and crystallographic analysis. In general, Hinckley Index and δ18O values of different size fractions of the same sample differ. Hinckley Index values decrease strongly, and Fe content increases in the upward direction in the profile.We conclude that (1) the kaolinite in the profile has progressively evolved in response to changing microenvironments as weathering fronts have moved progressively downward, (2) different kaolinite fractions, even within a single facies of a single sample a few cm3 in size, typically formed under somewhat different conditions, (3) the bulk of the fine-grained kaolinite in the lateritic profile does not constitute a continuous, inverted time record; that is, lower levels do not represent more recent kaolinite formation, and higher levels do not represent older kaolinite formation, and (4) isotopic compositions of kaolinites from lateritic paleosols should provide an indication of mean climatic conditions and some measure of seasonality during the interval preceding the removal of the laterite from the active weathering environment.We speculate that the different conditions under which the different kaolinite fractions have formed may be related, at least in part, to variations in soil microenvironments that are ultimately derived from seasonal and year-to-year variations in the chemistry and isotopic composition of infiltrating rainfall.
This study examines the oxygen and carbon isotopic distribution in the brachiopods, crinoids, rugose corals, and matrix/cement from the Mississippian Burlington Limestone (Iowa and Missouri) and the Silurian Read Bay Formation (Arctic Canada). The isotopic data can be reconciled with textural and trace element observations only if the 18O content of ancient oceans differed from that of the present oceans. This postulated secular variation in the 18O content of seawater requires a separate evaluation of the Mississippian and Silurian carbonate components. Examination of the 18O content of the least-altered low-Mg calcite fossil components of the two formations suggests that the delta18O of Mississippian and Silurian seawater was lighter than the delta18O of present day ocean water. The delta13C values are bimodally distributed. This distribution is independent of geologic age, inferred original mineralogy and degree of diagenetic alteration.-from Authors
The depositional age of the samples spans the last 300 000 yr. The minerals present are quartz, feldspar, illite, chlorite and/or non-expandable vermiculite, and expandable vermiculite and/or mixed-layer illite/expandable vermiculite. The relative amounts of quartz, feldspar, and total clay vary with particle size. del O18 values of the 4 coarser size fractions range from +9.7 to +12.0o/oo with variations attributable to changes in quartz/feldspar and clay/(quartz + feldspar) abundances. Values of del O18 for the expandable vermiculite-rich less than 0.1 micron size fraction range from *12.1 to +16.3o/oo which indicates some oxygen isotope exchange at surface temperatures between meteoric waters and the parent rock during vermiculite formation. Values of del D range from -46 to -47o/oo with variations attributable to changes in amounts of different clay minerals present.-from Authors
Petrographic observations show that disseminated, fine-grained kaolinite and quartz are common in the coal. On the basis of oxygen isotope ratios, it appears that the disseminated kaolinite and siderite nodules were precipitated from Permian pore water early in the deposition of peat at calculated temperatures of 16-32oC. Secondary cleat-filling kaolinite at Leichhardt colliery in the Bowen Basin has a constant delta ¹⁸O value of around +6mu , which is consistent with formation from mine water of the present isotopic composition at a temperature of 56oC. Lower temperatures are indicated for secondary kaolinitization in coals of the S Sydney Basin. Detrital quartz grains and chemical quartz have similar isotopic compositions and therefore cannot be distinguished isotopically. However, the oxygen isotope ratio of disseminated, fine-grained silica is consistent with formation in the brown coal stage at a temperature of 52oC and depth of approx 700m. The ability to distinguish between primary and secondary kaolinite appears to hold promise as a guide to prediction of outbursting conditions in underground coal mines.-from Authors
A technique has been developed in which bromine pentafluoride is used as a reagent for quantitative liberation of oxygen from oxides and silicates. For all of the rocks and minerals analysed, the oxygen yields are 100 ± 2 per cent of the theoretical amount. The advantage over techniques involving reduction with carbon lies in the consistently better oxygen yields, with consequent decrease in systematic errors in isotopic composition. Bromine pentafluoride has advantages over fluorine in being easier and safer to handle in the laboratory, in being readily purified, and in reacting with some minerals which do not react completely with fluorine. The results of isotopic analyses are compared with measurements made in other laboratories by other procedures.
The Koogah Formation, which is the equivalent of the Greta Coal Measures of the lower Hunter Valley, is restricted in outcrop to the area lying immediately to the northeast of Wingen. The unit increases in thickness from 30 to 230 m in an easterly direction and consists for the most part of kaolinite clayrocks that range in texture from fine-grained to coarsely conglomeratic. Residual volcanic textures are characteristic of most of the kaolinite clasts and there seems little doubt that the detritus was derived from weathered zones developed on the Werrie Basalts, and accumulated in an alluvial fan environment. Nevertheless, in parts of the Koogah Formation lithic-quartz sediments that presumably represent fluvial material transported from a geosynclinal source which lay some distance to the north or west are abundant, and in the cores of two boreholes these sediments predominate to the virtual exclusion of the kaolinite clayrocks.
The Permian carbonates of Tasmania formed at a paleolatitude near 80 oS during the Gondwanan Ice Age, contain abundant dropstones, and are associated with glaciomarine sediments. A shallow-marine carbonate depositional model with floating icebergs, calved from an adjacent ice shelf grading into an ice sheet, has been proposed. As the sea level rose during relatively warmer periods, large volumes of melt waters mixed with marine waters retarding limestone deposition and causing deposition of shales/siltstones. These phases are preserved in the stratigraphic record as interbedded units (less than 1m thick) of limestones and shales/siltstones. The delta O 18 values of Permian fauna, if one assumes no diagenetic equilibration, give unrealistic paleotemperatures because of this variable melt-water dilution of the Permian sea. However, calculated delta O 18 values, corresponding to marine delta C 13 values of brachiopods and Eurydesma and extrapolated from a model based on the linear trend of delta C 13-delta O 18 in modern and last-glacial cold-water carbonates, give reasonable estimates of Australian Permian temperatures of up to 15 oC with the coldest waters of less than 4 oC around Tasmania.-from Authors
This chapter discusses mineralogy and petrology of burial diagenesis and incipent metamorphism in clastic rocks. The phase-modification processes referred to as “late-diagenetic” or “epigenetic” (or catagenetic) are metamorphic in nature. Hence, the distinction between burial diagenesis and lowest-grade regional metamorphism is largely artificial and arbitrary. The modification of various kinds of sedimentary rocks upon burial has largely been studied by investigators from different sub-disciplines within the earth sciences: (1) changes in mineralogy of clay-rich sedimentary rocks by sedimentary petrographers and clay mineralogists; (2) authigenic mineral assemblages in tuffs and volcanic sediments by metamorphic petrologists; and (3) changes in the constitution of coaly and other organic matter by coal petrologists, and organic geochemists. The growing awareness of the overlap in space of the physical conditions and of the chemical influences controlling these phenomena warrants renewed efforts to correlate the changes in various types of sedimentary rocks in order to arrive, ultimately, at an integrated picture of the phase changes in different types of sedimentary rocks upon burial and incipient metamorphism. This chapter intends to contribute to such an integrated picture.
Mineralogy, kaolin crystallinity, Fe content, deltaO18, and delta D were determined for late Cretaceous 'soft' and early Tertiary 'hard' Georgia kaolins. Data indicate that the Tertiary kaolins could not have been simply derived from the Cretaceous kaolins by winnowing unless post-sedimentation recrystallization of one or both occurred. Delta D vs. deltaO18 systematics indicate that the late Cretaceous to early Tertiary Georgia kaolins crystallized over a temperature range of about 15oC in the presence of waters that varied little in isotopic composition.-Authors
Illustrates how oxygen- and hydrogen-isotope methods can be used to deduce the physicochemical conditions that existed during various stages of diagenesis in sandstones and shales. Discusses: i) the principles of stable isotope geochemistry, ii) the oxygen- and hydrogen-isotope variations of waters that may be involved in clastic diagenesis (ocean water, meteoric water, formation fluids, etc.) and iii) the whole-rock oxygen-isotope geochemistry of clastic rocks. Case studies illustrate particular concepts. -from Author
δ^(18)O values have been determined for 53 coexisting chert-phosphate pairs from the Phanerozoic and Precambrian. In addition, δ^(18)O values were also determined for 18 biogenic phosphates from the Phanerozoic.
The oxygen isotope data for the chert-phosphate pairs permit the calculation of the temperatures of formation of the chert-phosphate pairs as well as the δ^(18)O of the waters in which they equilibrated. In turn the calculated δ^(18)O of the waters permit the calculation of temperatures from the isotopic data of the biogenic phosphates. The temperatures for the Tertiary and Mesozoic samples are compatible with the literature values determined from oxygen isotope data for well-preserved calcareous marine skeletal remains. The temperatures calculated from the chert-phosphate pairs of the Paleozoic are usually higher and vary in cycles of about a 150 million year period. The early Precambrian temperatures are as high as 80°C and decrease with time. The use of chert-phosphate pairs for temperature determinations are sufficiently promising to warrant further investigations of this type on well preserved chert-phosphate or carbonate-phosphate pairs.
Authigenic minerals in the sandstones have low delta 18O and delta 13C values: clay minerals (<2 mu m), dominated by authigenic kaolinite, delta 18O = +11.3 - +14.2per mille (SMOW); authigenic calcite, delta 18O = +15.3 - +18.5 (SMOW) , delta 13C = -9.9 to -2.6 (PDB). Apparently the authigenic minerals formed at +15 - +20oC.-K.A.R.
Many problems remain in the understanding of the Late Palaeozoic glaciation. Existing and new evidence suggests that the period and extent of glaciation were more limited than sometimes supposed. The main glaciation, identified, especially by moraine, in southern Africa, the Arabian Peninsula, India, Tibet, Australia, South America, and Antarctica may have been confined to the Early Permian (Asselian). Data for continental ice sheet conditions are lacking. The main glaciation was followed in the Sakmarian (Tastubian) by a eustatic rise in sea level. Subsequent to this, neither moraine nor glaciers can be identified, although in Australia and, possibly, Siberia the climate remained cold. In the Upper Carboniferous, deposited before the main glaciation, evidence for moraine is found in South America and Australia and, possibly, northeastern Siberia, but this appears to be mountain glaciation and there is no evidence for any extensive ice sheet. -Author Palaeozoic glaciation moraines southern Africa Arabian Peninsula India Tibet Australia South America Antarctica
The magnetizations of many rock units in southeastern Australia have been severely overprinted. Interpretation of early work was hindered by this, and one of the aims of the current research is to ascertain the nature of the overprint mechanism, its geological significance, and its regional extent. Results from three igneous bodies from the Sydney Basin, which have been studied previously in a reconnaissance fashion, have provided a new insight into the reason for such widespread magnetic overprinting. The Hornsby Breccia (HB) and Milton Monzonite (MM) yield similar magnetic signatures, both containing discrete high- and low-temperature magnetizations with directions of dec = 72°, inc = +83° (α95 = 4°) and dec =13°, inc = −76° (α95 = 2°), respectively, for HB, and dec = 60°, inc = +78° (α95 = 4°) and dec = 348°, inc = −79° (α95 = 6°) respectively for MM. The corresponding pole positions are 29°S, 166°E (A95 = 7°) and 59°S, 139°E (A95 = 4°) for HB, and 22°S, 171°E (A95 = 7°) and 55°S, 158°E (A95 = 11°) for MM. The paleomagnetic direction and pole from the third body studied, the Mogo Hill Basalt (MH), are dec = 241°, inc = 81° (α95 = 5°) and 41°S, 130°E (A95 = 9°). This pole position and the two low-temperature overprint pole positions from HB and MM are consistent with ages ranging from 100 m.y. to approximately 70 m.y. when compared to the Late Mesozoic apparent polar wander path. We relate the overprinting to uplift, erosion, and rapid supracrustal cooling of the southeastern margin of Australia associated with initial phases of marginal rift development prior to seafloor spreading in the Tasman Sea. The temperatures of rocks now exposed may have reached 200°C or higher, before the rapid cooling blocked and stabilized the overprint magnetizations.
All of the analyzed igneous rocks from DSDP (Deep-Sea Drilling Project)
legs 34 and 37 are altered. Their primary oxygen isotope composition,
which can be estimated from the analysis of separated minerals, was
indistinguishable from that of fresh unaltered basalts dredged from
midocean ridges (5.7 ± 0.2‰ SMOW), but the alteration of
the rocks has either enriched or depleted them of 18O.
Low-temperature weathering of basalts by seawater has increased their
18O content by 1 to 3‰. Basalts recovered from 600 m
within the oceanic crust in site 332B have a δ18O value
of 8.3‰. On the other hand, the 18O contents of
intrusive rocks from site 334 have been lowered 1 to 3‰ by
high-temperature, postsolidus exchange with seawater. Both kinds of
altered rocks are already known from the study of dredged materials, but
only the ocean drilling program can determine the depth to which
seawater penetrates in the oceanic crust and the prevalence of altered
rock. Implicit in current tectonic models is the hypothesis that
sufficiently large volumes of oceanic crust are produced each year, so
that any appreciable exchange between the crust and the oceans will
influence the chemical composition of seawater. If site 332B has sampled
typical oceanic crust, then weathering of the crust is indeed a major
sink for 18O. However, if intrusives are prevalent in the
crust, then their alteration is a major source of 18O to the
ocean. Estimates of the volumes of materials involved in the formation
and alteration of the oceanic crust (1 km of hydrothermally altered
rock, 600 m of weathered rock), cycling of water through the mantle (2%
of the subducted crust), and weathering of the continents indicate that
the isotopic composition of the ocean may be held at its present value
as a consequence of the 18O enrichments balancing the
18O depletions. Approach to the present isotope ratio is
governed by a time constant of 200 or 300 m.y.
The oxidation potential of dithionite (Na 2 S 2 O 4 ) increases from 0.37 V to 0.73 V with increase in pH from 6 to 9, because hydroxyl is consumed during oxidation of dithionite. At the same time the amount of iron oxide dissolved in 15 minutes falls off (from 100 percent to less than 1 percent extracted) with increase in pH from 6 to 12 owing to solubility product relationships of iron oxides. An optimum pH for maximum reaction kinetics occurs at approximately pH 7.3. A buffer is needed to hold the pH at the optimum level because 4 moles of OH are used up in reaction with each mole of Na 2 S 2 O 4 oxidized. Tests show that NaHCO 3 effectively serves as a buffer in this application. Crystalline hematite dissolved in amounts of several hundred milligrams in 2 min. Crystalline goethite dissolved more slowly, but dissolved during the two or three 15 min treatments normally given for iron oxide removal from soils and clays.
A series of methods for the extraction of iron oxides from soils and clays was tested with soils high in free iron oxides and with nontronite and other iron-bearing clays. It was found that the bicarbonate-buffered Na 2 S 2 O 4 -citrate system was the most effective in removal of free iron oxides from latosolic soils, and the least destructive of iron silicate clays as indicated by least loss in cation exchange capacity after the iron oxide removal treatment. With soils the decrease was very little but with the very susceptible Woody district nontronite, the decrease was about 17 percent as contrasted to 35–80 percent with other methods.
Evidence from overprinting of magnetizations of Late Permian and Mesozoic rocks and from the rank of Permian coals and Mesozoic phytoclasts (coal particles) suggests that surface rocks in the Sydney Basin, eastern Australia, have been raised to temperatures of the order of 200°C or higher. As vitrinite reflectance, an index of coal rank or coalification, is postulated to vary predictably with temperature and time, estimates of the paleotemperatures in the Sydney Basin based on observed vitrinite reflectance measurements can be made in conjunction with reasonable assumptions about the tectonic and thermal histories of the basin. These estimates give maximum paleotemperatures of present day surface rocks in the range 60-249°C, depending on factors such as location in the basin, the thickness of the sediment eroded, and the maximum paleogeothermal gradient. Higher coal rank and, consequently, larger eroded thicknesses and paleogeothermal gradients occur along the eastern edge of the basin and may be related to seafloor spreading in the Tasman Sea on the basin's eastern margin. A theory of thermal activation of magnetization entailing the dependence of magnetic viscosity on the size distribution of the magnetic grains is used to obtain an independent estimate of the maximum paleotemperatures in the Sydney Basin. This estimate places the maximum paleotemperature in the range 250-300°C along the coastal region. Both coalification and thermal activation of magnetization models provide strong evidence of elevated paleotemperatures, which in places exceed 200°C, and the loss of sediment thicknesses in excess of 1 km due to erosion.
Isotopic analyses of 75 samples for the Samail ophiolite indicate that pervasive subsolidus hydrothermal exchange with seawater occurred throughout the upper 75% of this 8-km-thick oceanic crustal section; locally, the H2O even penetrated down into the tectonized peridotite. Pillow lavas (delta18O = 10.7 to 12.7) and sheeted dikes (4.9 to 11.3) are typically enriched in 18O, and the gabbros (3.7 to 5.9) are depleted in 18O. In the latter rocks, water/rock?0.3, and delta18Ocpx~2.9+0.44delta18Ofeld, indicating pronounced isotopic diequilibrium. The mineral delta18O values approximately follow an exchange (mixing) trajectory which requires that plagioclase must exchange with H2O about 3 to 5 times faster than clinopyroxene. The minimum delta18Ofeld value (3.6) occurs about 2.5 km below the diabase-gabbro contact. Although the gabbro plagioclase appears to be generally petrographically unaltered, its oxygen has been thoroughly exchanged; the absence of hydrous alteration minerals, except for minor talc and/or amphibole, suggests that this exchange occurred at T>400°-500°C. Plagioclase delta18O values increase up section from their minimum values, becoming coincident with primary magmatic values near the gabbro-sheeted diabase contact and reaching 11.8 in the diabase dikes. These 18O enrichments in greenschist facies diabases are in part due to exchange with strongly 18O-shifted fluids, in addition to retrograde exchange at much lower temperatures. The delta18O data and the geometry of the mid-ocean ridge (MOR) magma chamber require that two decoupled hydrothermal systems must be present during much of the early spreading history of the oceanic crust (approximately the first 106 years); one system is centered over the ridge axis and probably involves several convective cells that circulate downward to the roof of the magma chamber, while the other system operates underneath the wings of the chamber, in the layered gabbros. Upward discharge of 18O-shifted water into the altered dikes from the lower system, just beyond the distal edge of the magma chamber, combined with the effects of continued low-T hydro-thermal activity, produces the 18O enrichments in the dike complex. Integrating delta18O as a function of depth for the entire ophiolite establishes (within geologic and analytical error) that the average delta18O(5.7+/-0.2) of the oceanic crust did not change as a result of all these hydrothermal interactions with seawater. Therefore the net change in delta18O of seawater was also zero, indicating that seawater is buffered by MOR hydrothermal circulation. Under steady conditions the overall bulk 18O fractionation (Delta) between the oceans and primary mid-ocean ridge basalt magmas is calculated to be +6.1+/-0.3, implying that seawater has had a constant delta18O~-0.4 (in the absence of transient effects such as continental glaciation). Utilizing these new data on the depth of interaction of seawater with the oceanic crust, numerical modeling of the hydrothermal exchange shows that as long as worldwide spreading rates are greater than 1 km2/yr, 18O buffering of seawater will occur. These conclusions can be extended as far back in time as the Archena (>2.6 eons) with the proviso that Delta may have been slightly smaller (about 5?) because of the overall higher temperatures that could have prevailed then. Thus ocean water has probably had a constant delta18O value of about -1.0 to +1.0 during almost all of earths's history.
The extent of hydrogen and oxygen isotope exchange between clay minerals and water has been measured in the temperature range 100-350° for bomb runs of up to almost 2 years. Hydrogen isotope exchange between water and the clays was demonstrable at 100°. Exchange rates were 3-5 times greater for montmorillonite than for kaolinite or illite and this is attributed to the presence of interlayer water in the montmorillonite structure. Negligible oxygen isotope exchange occurred at these low temperatures. The great disparity in D and O 18 exchange rates observed in every experiment demonstrates that hydrogen isotope exchange occurred by a mechanism of proton exchange independent of the slower process of O 18 exchange. At 350° kaolinite reacted to form pyrophyllite and diaspore. This was accompanied by essentially complete D exchange but minor O 18 exchange and implies that intact structural units in the pyrophyllite were inherited from the kaolinite precursor.
Studies of isotopes in groundwater using 14C, 2H (deuterium) and 18O have provided significant insight into the paleoclimate of middle and late Wisconsin age in a typical arid environment of southern Nevada, the west-central Amargosa Desert. Evidence indicates that recharge probably was through infiltration of runoff in paleostream channels and that this runoff was important only from ∼ 17,000 to ∼ 10,000 yr. B.P. Mean annual temperature at 17,000 yr. B.P. was ∼ 8°C less than present mean annual temperature; some summer moisture was effective in recharge. Winter (October–May) temperature at ∼ 10,000 yr. B.P. was ∼ 1°C less than present; summer moisture did not contribute to recharge.
In siliciclastic sediments only a minor amount of cement is internally derived. This implies that an external source has to be sought in sandstone diagenesis. Controversy exists about the external source of cement and the nature of the aqueous solution, viz. meteoric, hydrothermal and marine, and/or dewatering of shale. Isotope geochemistry may prove useful in solving this controversy.The oxygen isotopic compositions of neoformed clay minerals are related to the isotopic composition of the coexisting aqueous solution. Aqueous solutions of diverse origin such as meteoric, hydrothermal and marine, and dewatering of shale, in general, bear characteristic isotopic signatures. Thus, the δ18O-values of authigenic clay minerals may be used to identify the nature of the original aqueous solution, provided the temperature of the environment is known and the extent and the nature of isotopic exchange between the minerals and fluids following crystallization can be evaluated.Oxygen isotopic compositions of early authigenic clay minerals in Permo-Triassic Gondwana sandstones of Peninsular India have been used to infer the cement source. Sandstones of Sakmarian age have an average δ8O-value of +5.0%o, whereas a relatively high average δ8O-value of +13.2%o is observed in sandstones of Rhaetic age. δ18O-values of sandstones ranging in age between Sakmarian and Rhaetic have values between +5.0 and +13.2%0. This gradual increase of δ180-values with decreasing age shows a strong correlation with the changing latitudinal location of the sample site from a position around 60°S during Sakmarian time to a position around 38°S during Rhaetic time. The changing pattern of oxygen isotopic composition of suthigenic clay has been interpreted as a result of corresponding change in isotopic composition of meteoric water due to the northerly drift of the Indian plate during Gondwana sedimentation. This suggests that the aqueous solution, involved in the early diagenesis of Gondwana sandstones, is of meteoric origin.
Early diagenetic chlorite has delta 18O values (+4.2 to +6.3, SMOW) compatible with low-temperature crystallization from brackish water. The delta 18O values of diagenetic calcite (+11.8 to +14.7, SMOW) and kaolinite (+10.4 to +12.8, SMOW) are best explained by crystallization at temperatures of at least 45 o to 70 oC from pore fluids similar in delta 18O to the existing formation water (-9, SMOW). The implication of such temperatures is that the sandstone was much more deeply buried in the past than it is at present.-from Author
From a study of the association of flint clays and thick, lateritic palaeosols developed on Early Permian basalts at a depth of more than 1,200 m in the Quirindi Bore, it is concluded that during at least part of the Artinskian Stage in the Sydney Basin the climate was humid and tropical to subtropical. The concept however, is at variance with the popular belief that in accordance with other Gondwana countries, alternations of frigid and cold temperate conditions prevailed in Australia during the Early Permian and may have persisted through to the close of the period.
The Broughton Sandstone, a 500-foot thick horizontal sequence of Permian conglomerates, sandstones, and siltstones, interstratified with contemporaneous submarine volcanics (latites) occurs at Kiama, N.S.W. The sequence contains authigenic mineral assemblages characteristic of the laumontite zone of the zeolite facies. Albite, calcite, chlorite, quartz, clay minerals, and laumontite are the common authigenic minerals in the sediments, whereas in addition, prehnite, pumpellyite, epidote, and sphene occur in the latites. The clean-washed sediments are largely composed of volcanic rock fragments and detrital plagioclase with minor amounts of ferromagnesian minerals and quartz. The composition, sedimentary structures, and faunal assemblage indicate that the sediments were derived from a volcanic provenance, and deposited in a shallow-water marine, possibly near-shore environment. Physicochemical parameters of the early diagenetic environment appear to have significantly influenced the nature of the final authigenic minerals. Connate water and the breakdown of detrital constituents, especially calcic plagioclase, provided the chemical elements for the formation of the authigenic minerals. The low-grade 'burial' metamorphism, induced by increasing temperatures in response to increasing load pressures, alone, could not have produced the observed mineral assemblages, since the rocks have been buried to a maximum depth of only about 3000 feet.
Mineralogy, kaolin crystallinity, Fe content, δ O ¹⁸ , and δ D were determined for late Cretaceous “soft” and early Tertiary “hard” Georgia kaolins. The crystallinity of the <0.5-, 0.5–1.0-, and 1.0–2.0- μ m size fractions of soft kaolins was higher than that of equivalent size fractions of hard kaolins. δ O ¹⁸ and δ D of the soft and hard kaolins ranged between 18.5 to 23.1‰, and −64 to −41‰, respectively, and could not be used to discriminate soft from hard kaolins. The trends of crystallinity vs. δ O ¹⁸ were different for kaolins collected at different localities, and, for a given sample, δ O ¹⁸ generally decreased with increasing crystallinity and with increasing crystallite size. These data indicate that the Tertiary kaolins could not have been simply derived from the Cretaceous kaolins by winnowing unless post-sedimentation recrystallization of one or both occurred. δ D vs. δ O ¹⁸ systematics indicate that the late Cretaceous to early Tertiary Georgia kaolins crystallized over a temperature range of about 15°C in the presence of waters that varied little in isotopic composition.
Resilication of bauxite produced kaolin at and beneath an old erosion surface on bauxite at the Alabama Street Mine of ALCOA in Saline County, Arkansas. The transitional alteration can be traced in morphology by scanning electron microscopy, (SEM) and in Al:Si ratio by energy dispersive analysis. In one illustrated example, the sequence of resilication took place within 1 mm thickness; in another, across 80 mm. The first morphologic alteration of gibbsite (bauxite) appears to be to allophane that occurs in micrometer-size plates which show elongate cracking and/or straight to highly curved elongate edges. The next phase is kaolinite, first in micrometer-size flakes followed by coarser flakes that grade into a zone of typical stacked kaolinite, likewise identified by X-ray powder diffraction. Notably large stacks and small flakes of kaolinite are intimately mixed in the SEMs, thus suggesting that unequal sizes of kaolinite crystals can grow during one episode of an in-situ genesis.
The characteristic structural, textural and compositional features of the various kaolinite-rich clayrocks known as flint clay, toasted clay, kaolinite tonstein and bauxitic clay are briefly reviewed together with their mode of occurrence. It is considered that these rocks have much in common; sufficient to warrant their grouping in a single sedimentary facies. The origin of these clayrocks is discussed and it is concluded that whereas some modifications apparently took place within the depositional area, the concept of development in Situ for the bulk of the material is difficult to substantiate. Rather, the evidence appears consistent with an essentially allochthonous origin.
O-isotope analysis of shales sampled from wells drilled through sedimentary deposits in the Gulf of Mexico region indicates that the sediments and rocks are not isotopically equilibrated systems - even those that have been buried to depths where temperatures are as high as 170 °C. In comparison with the coarser fractions, the finer fractions of both clay minerals and quartz are almost always richer in O18. O-isotope disequilibrium among the clay fractions becomes less marked as burial temperature increases. O-isotope exchange between clay and pore water become more extensive at higher temperatures; this corresponds to more extensive diagenetic alteration of mixed-layer illite-smectite. There is no evidence for O-isotope exchange between detrital quartz and pore water. However, quartz that forms diagenetically as an accompaniment to the conversion of smectite to illite layers in the mixed-layer clay forms in equilibrium with the pore water. The usefulness of O-isotope geothermometry for determination of the maximum temperatures to which shales have been heated during burial was investigated. Temperatures were calculated from the O-isotope fractionations between coexisting fine-grained quartz and clay from three wells; these calculated temperatures progressively approached the measured well (logged) temperatures as depth of burial and temperature increased. In one well, good agreement between calculated and measured temperatures was obtained for measured temperatures between 100 and 180 °C. In two other wells, satisfactory agreement was approached but not obtained at measured temperatures as high as 120 °C. Temperatures calculated from the O-isotope fractionations of quartz and calcite or calcite and clay were not reasonable. This probably reflects isotope exchange between calcite and pore water after the silicates attained their measured isotope ratios. Consequently, calcite is not a suitable mineral for use in isotope geothermometry of diagenetically altered shales.
A deep‐weathering profile is developed in the Mesozoic sedimentary rocks of the Roma‐Amby region, where it forms a capping to dissected tablelands. The profile consists of up to 25m of mottled and leached sediments, rarely in well‐defined zones; crust in place with respect to the profile is seldom preserved. Tholeiitic basalts and olivine basalts with possible alkaline affinities were erupted along the axis of the Merivale Syncline in the Amby‐Mt Hutton area; K‐Ar dates indicate that they are about 23 m.y. old. The basalts were extruded after the period of deep‐weathering, which occurred therefore in or before Early Miocene. The abundant silcrete in the area has formed at various times during the Cainozoic and is not necessarily related to the deep‐weathering.
The requirements of apparatus to measure stress relaxation of rubbers in compression are outlined. Apparatus available commercially are described, in particular a universal semi-automatic force measuring head and jigs which accept button test pieces and have no sliding bearings to introduce friction.
The extent of oxygen isotopic exchange between detrital clay minerals and sea water was investigated by analyzing O 18 / O 16 ratios of separated fine-grained size fractions of deep-sea sediments from three North Pacific ocean cores. Isotopic results were interpreted according to models based on the assumption that the extent of isotopic exchange should increase with decreasing particle size and increasing time of exchange between the sediment and sea water. The data indicate that information concerning the provenance and mode of formation of detrital clay minerals can be obtained from the O 18 / O 16 ratios of the coarser-than-0.1 m fraction of deep-sea sediments younger than several million years and the finer-than-0.1 m fraction of deep-sea sediments younger than several tens of thousands of years. Furthermore, if the extent of chemical reaction between detrital clays and sea water is similar to the extent of oxygen isotopic exchange, such reaction may be important in regulating the chemistry of sea water.
The D/H ratios of separated size fractions of clay minerals in two deep sea sediments taken from depths of 30 and 1100cm in a North Pacific Ocean core were measured to investigate the extent of hydrogen isotope exchange between detrital clay minerals and sea water. The D/H ratio of each size fraction of the shallower sample was compared with that of the corresponding size fraction of the deeper sample. No differences were detected between D/H ratios of corresponding size fractions from the two levels in the core except for the
Oxygen isotope exchange experiments at 22°C have been made on a series of illites displaying a range of perfection of crystallinity and degree of interstratification. Structurally bound oxygen underwent significant non-equilibrium exchange in time periods ranging from 2 to 70 months. Exchange was found to be controlled by accessibility of water to the interlayer cation region.1000 In α appears to be in the general neighborhood of +23.4 at 22°C for the illite-water system; however, it was found that equilibrium fractionation cannot be represented by a single value for structurally and chemically distinct illites.
Samples from a profile of the Paragominas bauxite (Brazil) were analyzed extensively. Principal minerals were determined as well as the concentrations of 56 elements. The major-element chemistry can be described in terms of the oxides and hydroxides of Al, Si, Ti and Fe, bound mineralogically as gibbsite, kaolinite and hematite. 30 of the 45 minor and trace elements analyzed often maintain their average crustal concentrations. Depletions are most obvious for the alkali metals, the alkaline earths and the metals of the first-row transition series. Enrichment is observed for B, Sc, Zr, Nb, Sn, Sb, I, Hf and Th. Differences in concentration patterns for some refractory metals (Al, Ti, Zr, Hf, etc.) lead to the suggestion that some (e.g., Ti, Zr, Hf) of these metals may be participating in biological reactions.The economic bauxite zone is bracketed above and below by kaolinitic horizons, and this succession of alteration could be explained by extraordinarily high flow rates of meteoric waters for millions of years through highly permeable terrains. The formation and preservation of these highly leached profiles also require a quiescent tectonic setting for 10–100 Ma. Such a model is consistent with conditions which may have prevailed in eastern Amazonia for at least 100 Ma.
The geochemistry analyses of the clay fraction (< 1 μm) of Tertiary alterites overlying tholeiitic basalt (Site 336) on the Iceland—Faeroe Ridge, revealed — close to the parent rock — the predominance of ferriferous montmorillonite (80%) over kaolinite and halloysite. Conversely, the top of alterite redbeds proved to be exclusively composed of kaolinite. A comparative study of these redbeds with the present-day ones derived from the same type of parent rock, suggested genesis under tropical climatic conditions. Compared to the basaltic parent rock, alterites are characterized by moderate desilication and partial elimination of Ca, Na, Mg, Mn, Sr, Cu and Sr and on the other hand by a concentration of some elements such as Al, Fe, Ti, K, Rb and Zr.
From the evidence of the marine invertebrate faunas, general changes in water temperature can be reconstructed with some degree of assurance. The reconstruction is based mainly on changes at the generic level. With advances in stratigraphic knowledge it may become possible to use changes at the specific level to detect shorter term fluctuations in water temperature.In Stage A (Sakmarian) time, cold water is indicated all over Australia from about present latitude 20° southwards — by close association of the faunas with glacial deposits, low diversity and genera such as Deltopecten, Eurydesma, Keeneia and Trigonotreta. At the end of this time a eustatic rise in sea level apparently marked the end of the main glacial period. Evidence for this eustatic rise is found also in South Africa, India and South America.Stage B (Sakmarian to early Artinskian) is marked by an amelioration in climate, but cool temperatures prevailed. The amelioration is particularly distinctive in western Australia with the entry of many forms with Tethyan relationships. Although some forms such as Eurydesma and Keenia have not been found at this level in western Australia, they persist in eastern Australia. Isolation of eastern Australia might be a complicating factor, but the presense of Eurydesma and Keeneia seems to indicate cooler conditions in eastern Australia. The faunas are more diverse and glaciation at sea level is no longer in evidence.In Stage C and D (Artinskian and Kungurian) time a slow general warming is apparent in western Australia: Tethyan faunal links are strengthened and the fauna diversified. In eastern Australia the faunas become more diverse. In New South Wales and Tasmania persistently cooler conditions than in Queensland are indicated by the continued occurrence of such forms as Deltopecten, Eurydesma and Keeneia.In western Australia no marine faunas are referred to Stage E (early late Permian) but in eastern Australia, the trends established earlier persist. In Queensland the close link with western Australia and Tethyan forms seems to indicate both a northern sea connection and somewhat warmer conditions. Nowhere in Australia up to this time are any truly warm water faunas recognized.Stage F (late late Permian) fauna are known only from western Australia and can hardly be differentiated from other Tethyan faunas. The presence of Leptodus at the northernmost occurrences of Stage F can be taken to indicate tropical rather than warm temperate conditions.
Bauxites and latentes are formed under weathering conditions where alkalies, alkali earths, and Si are more mobile than Al and Fe. Using techniques of Garrels and Christ (1965) for the construction of Eh-pH diagrams, it has been shown that 1) under a wide range of Eh and pH conditions, both Al and Fe have very low mobilities and the soil type will reflect the Al/Fe ratio of the protore, 2) under conditions of low pH and high Eh, lateritization occurs if Fe is available in the protore, 3) under conditions of low to intermediate pH and low Eh, bauxitization occurs if Al is available in the protore. Points 2) and 3) were generally recognized by Petersen (1971). Subsequent changes in Eh and particularly increases in pH result in precipitation of Fe or Al, the amounts depending upon the initial groundwater quality. Mn, Co, and Ni behave in a manner similar to Fe but are more mobile than Fe and Al for any given Eh and pH. Residual enrichment of Mn, Co, or Ni will be accompanied by Fe enrichment for any waters and Al enrichment if the initial pH of the groundwater is greater than 4. Bauxites formed by residual enrichment of Al relative to Fe will be depleted in Mq, Co, and Ni relative to Fe.
The Climax mine contains three molybdenite ore bodies and widespread late quartz-pyrite-sericite barren mineralization, each related to separate intrusive phases of the Climax stock. Alteration zones spatially related to each molybdenite ore body include a silica zone below, a potassium-silicate zone that approximately coincides with ore, and overlying quartz-sericite-pyrite-topaz, argillic, and propylite zones. δ 18O values of quartz range from +8.4 to 10.9 per mil; δ 18O values of muscovite and sericite range from -0.8 to +7.5 per mil. Eight of ten sericite values are between +5.8 and +7.5. All of the quartz and most of the sericite is in isotopic equilibrium with a calculated water at 350°C with δ 18O values of +3.0 to 5.5 per mil. δ 18O values of K-feldspars range from -4.5 to +7.5 per mil. This wide range indicates partial reequilibration of the feldspar with later, isotopically light, waters. Two muscovites have δD values of -146 and -91 per mil; sericite ranges from -168 to -116 per mil. The calculated δD values of water at 275°C in equilibrium with sericite are -144 to -92. This range is the same as that of fluid inclusions in ore samples. Kaolinite in the argillic zone has δ 18O values of -0.9 to -2.2 and δD values of -162 per mil. Calculated water at 250°C in equilibrium with kaolinite δ 18O values of -4.7 to -6 and δD of about -130 per mil. The isotopic and fluid inclusion data best fit a model whereby the Climax ore bodies were formed from a hydrothermal fluid that originated by mixing light and heavy waters. The light water is isotopically similar to present-day meteoric water; the heavy water has the isotopic composition postulated for magmatic water, although it could be meteoric water that was heavier than present-day meteoric water.