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Erratum to ``Trace element evidence from seamounts for recycled oceanic crust in the Eastern Pacific mantle''[Earth Planet. Sci. Lett. 148 (1997) 471-483]
... Mid-ocean ridge basalts (MORB) are principally the products of magmas that erupted along mid-ocean ridges (e.g., Klein and Langmuir, 1987;Johnson and Dick, 1992;Kinzler, 2006;Asimow, 2001;Ito and Mahoney, 2005;Niu and Batiza, 1997;Hofmann, 2014). Studying midocean ridge crustal construction provides opportunities to better model the movements of magmas beneath mid-ocean ridges, which helps to develop our understanding of key relationships between mantle dynamics and the generation of new ocean crust. ...
... Many of these works emphasize connecting MORB genesis with a single factor (such as mantle temperature, mantle source heterogeneity, or melting regime). For example, Klein and Langmuir (1987) stated that mantle source temperature principally controls the extent of melting and the thickness of the crust; while Niu and Batiza (1997) demonstrated that the mantle source of the East Pacific Rise (EPR) is heterogeneous on very small scales from trace element analyses. Despite the fact that some researchers consider the influences of multiple factors on the final composition of MORB (e.g., Standish et al., 2008;Komiya et al., 2004Komiya et al., , 2002Dick and Zhou, 2014;Woodhead, 1989;Haase, 2002;Niu et al., 2002), few of them consider the relationships between these indicators and faulting styles at these ridge segments. ...
... Mid-ocean ridge basalts derive from mantle reservoirs that have been significantly depleted by the extraction of the continental crust in Earth's early history, and then reenriched by recycling of crust into the convecting mantle (Arevalo and McDonough, 2010;Hofmann, 1988). Studying trace elements is a critical way to examine mantle source heterogeneity, since (1) much like TiO2, the melt partitioning behavior of trace elements in peridotite and pyroxenite are different (e.g., Stracke and Bourdon, 2009;Langmuir et al., 1992), and (2) abundances of trace elements are different in recycled and residual mantle reservoirs (e.g., Niu and Batiza, 1997;Saunders et al., 1988). Elemental ratios involving Ba (such as Ba/Th) are thus potentially useful for evaluating the chemical differences in MORB compositions due to source heterogeneity (Stracke and Bourdon, 2009). ...
Faulting style along slow-spreading ridge segments exerts a major control over oceanic crustal construction and ridge morphology. Seafloor spreading along slow-spreading ridges is classified into asymmetrical detachment faulting and symmetrical spreading styles. Magma supply variations may influence the formation of symmetric versus asymmetric segments, but the factors that derive such variations remain unclear. We present U and Th concentration data by isotope dilution for basalts from 24 to 30 oN MAR, of which 18 samples were retrieved from detachment faulted and 10 from symmetrical segments. The U-Th concentration results from this study are further compiled with prior published data from Gale et al. (2013) and data from Langmuir (personal communication). The compiled dataset provides robust combined major and trace element analyses of basalts from the study area. Statistical analyses show that basalt samples from asymmetrical segments have higher mean MgO, but lower mean Na2O, Sr and Sr/Nd contents compared to samples from symmetrical segments (at the 95% confidence level). The differences in these major and trace elements may indicate a higher proportion of pyroxenites and/or a more trace element enriched peridotite in the source mantle beneath symmetrical segments, which increases the magma supply and prevents the formation of detachment faults beneath these segments. Furthermore, the larger amount of magma supply facilitates the crust to fully accommodate seafloor spreading and generate symmetrical accretion along the Kane-Atlantis segments. On the other hand, the detachment faults beneath asymmetrical segments may decrease the degrees of assimilation and fractional crystallization at shallow levels. Advisor: Lynne J. Elkins
... To help address issues on MORB genesis and the variable MORB Hf– Nd isotopic correlations at different ridges (Salters et al., 2011), we study Hf isotope compositions of near-ridge seamounts flanking the East Pacific Rise (EPR) (see Figs. 1 and A1). We choose these seamount lavas because (1) they have already been well-characterized for major elements, trace elements, and Sr–Nd–Pb isotope compositions (Niu and Batiza, 1997; Niu et al., 2002); (2) they represent an integral part of EPR ridge magmatism as they were derived from the same upper mantle source of EPR axial MORB; (3) they have avoided melt homogenization during melt aggregation in the mantle and in the long-lived axial magma chambers, and thus more faithfully record the nature of the MORB mantle source beneath the EPR than axial lavas; and importantly , (4) they display a large compositional spectrum that encompasses much of the global MORB compositional variability from ridges unaffected by mantle plumes/hotspots, and can thus help answer first-order questions on MORB mantle sources and processes, especially beneath the fast-spreading EPR. ...
... (a) Tectonic framework of the northern (5°–15°N) EPR and vicinity; (b) simplified map of the study area showing the locations of near-ridge seamounts. The sizes of the circles (sample locations) are not to scale (Niu and Batiza, 1997; Niu et al., 2002 Niu et al., ). 1990). ...
... They were collected from near-ridge seamounts within the 1 Ma isochron (b ~ 60 km) of the EPR axis between 5° and 15°N on both the Pacific and Cocos Plates (Fig. 1). These 36 samples have been studied for major and trace elements (Batiza and Niu, 1992; Batiza et al., 1990; Niu and Batiza, 1997) and Sr–Nd–Pb isotopes (all samples were analyzed for Sr isotope, 28 of them were analyzed for Nd isotope and 34 of them were analyzed for Pb isotope, see Niu et al., 2002). All the samples were carefully hand-picked under a binocular microscope before they were leached at room temperature in 10% H 2 O 2 for a few minutes to remove Mn oxides in possible micro-fractures and other potential labile contaminants. ...
We report new Hf isotopic data for basaltic glasses from seamounts flanking the East Pacific Rise (EPR) between 5° and 15°N that have been previously analyzed for Sr-Nd-Pb isotopes as well as major and trace elements. The Hf isotopic data offer new perspectives on the petrogenesis of these samples in a broader context on mantle dynamics. The Hf isotope compositions show significant correlations with Sr-Nd-Pb isotopes and with both abundances and ratios of incompatible elements. The seamount lavas are thus best interpreted as products of melting-induced mixing in a two-component mantle.
... The Gcha and HY granitoids show a large compositional and lithological spectrum with significant correlations between major elements (Fig. 2) and between isotopes (Fig. 7), e.g., significant positive Nd(450) -Hf(450) correlation (Fig. 7A). These correlations are best explained as reflecting mixing between the relatively depleted and enriched endmembers, consistent with melting-induced source mixing of varying lithologies (see Niu and Batiza, 1997). ...
... The correlations between isotopes, however, point to the importance of mixing between different lithologies. As the two plutons are N150 km apart, the correlated trends are best explained as resulting from varying degrees of melting-induced mixing of varying source lithologies (see Niu and Batiza, 1997 ), e.g. the subducted oceanic crust, heterogeneous old crustal material and sediments, with superimposed effect of varying extent of fractional crystallization and crustal assimilation. ...
... Additional materials from the lower crust and subducted sediments can explain the inherited zircons with large negative zircon Hf(450) values and wide compositional range. The significant correlations between major elements and between isotopes can be readily explained by meltinginduced mixing in a compositionally heterogeneous source (Niu and Batiza, 1997). The dominant age peaks at 450 Ma for both HY and Gcha samples may indicate the extensive heat supply from the convecting asthenosphere due to slab break off (Fig. 8C). ...
In this paper we present a new model that can explain the large zircon age spectrum of ~ 510 – 420 Ma within a single sample from the Gangcha (Gcha) biotite granodiorite and the Huangyuan (HY) two-mica monzogranite on the northern Tibetan Plateau. The large age spread recorded in zircons is characteristic of granitoid samples from the studied region, which is best explained by the long-lasting magmatism since the onset of continental collision at ~ 500 Ma, followed by slab steepening and the ultimate slab break-off at ~ 450 Ma. These granitoids have a large major and trace element compositional variation, but limited initial Sr (ISr[450] = 0.709 to 0.715), Nd (ԐNd[450] = -6.5 to -3.7), Hf (ԐHf[450] = -4.3 to 1.5,) and Pb (206Pb/204Pb[450] = 17.70 to 17.17; 207Pb/204Pb[450] = 15.60 to 15.69; 208Pb/204Pb[450] = 38.04 to 38.73) isotopic variation. The small negative whole rock ԐNd[450] and ԐHf[450] values are most consistent with the granitoid source being dominated by subducted seafloor materials. The inherited zircons with large negative ԐHf[450] values (e.g. up to -50) are indicative of input from the lower continental crust and subducted sediments. The correlated variations among major elements, trace elements and radiogenic isotopes are best interpreted as reflecting melting-induced mixing of a compositionally heterogeneous source with superimposed effect of varying extent of fractional crystallization and crustal assimilation. The inherited zircons of Palaeo-Proterozoic age and the Archean crustal model ages signify the involvement of ancient basement rocks.
... The samples in this study are glassy basalts from the NEPR off-axis seamounts, the QDG transform fault system , and the Macquarie Island. Major elements, trace elements , and isotopic compositions of NEPR seamounts (Niu and Batiza, 1997; Niu et al., 2002) and Macquarie Island samples (Kamenetsky et al., 2000; Kamenetsky and Maas, 2002) have been previously analyzed and discussed, while those for the QDG transform fault samples are new data. Volatile elements for NEPR seamounts and QDG transform fault samples are new data, while those of Macquarie Island lavas (except for CO 2 ) have been previously analyzed and discussed (Kamenetsky et al., 2000). ...
... Standard used were Smithsonian basalts VG-A99B (USNM 113498) and VG-2B (USNM 111240) (Jarosewich et al., 1980 ). Average analytical error (2r SE) of the major element analyses were <1% for SiO 2 , Al 2 O 3 , and MgO, <2% for FeO and CaO, <3% for Na 2 O and TiO 2 ; for K 2 O and P 2 O 5 the errors were significantly larger (1–20% and 2–30% respectively). Major element analyses of the NEPR seamount samples are on average within 6% of those from Niu and Batiza (1997) for all elements except MnO and K 2 O (within 13% of published analyses). Analyses of the Macquarie Island seamount samples are on average within 8% of those from Kamenetsky et al. (2000) for all elements except MnO (within 18% of published analyses). ...
... Symbols are as in shallow level melt infiltration origin for the isotopic composition observed in clinopyroxenes from the lithospheric mantle rather than representing the compositional variation of the actual convecting mantle (Warren et al., 2009; Warren and Shimizu, 2010). All these observations strongly suggest that the NMORB might represent mixing of melts from a two-component D-DMM and E-DMM mantle (Niu et al., 1996; Niu and Batiza, 1997) rather than an actual DMM mantle source. To test this hypothesis we present a two-component mantle melting-mixing model in Section 5.4. ...
We report major, trace, and volatile element (CO2, H2O, F, Cl, S) contents and Sr, Nd, and Pb isotopes of mid-ocean ridge basalt (MORB) glasses from the Northern East Pacific Rise (NEPR) off-axis seamounts, the Quebrada-Discovery-GoFar (QDG) transform fault system, and the Macquarie Island. The incompatible trace element (ITE) contents of the samples range from highly depleted (DMORB, Th/La ⩽ 0.035) to enriched (EMORB, Th/La ⩾ 0.07), and the isotopic composition spans the entire range observed in EPR MORB. Our data suggest that at the time of melt generation, the source that generated the EMORB was essentially peridotitic, and that the composition of NMORB might not represent melting of a single upper mantle source (DMM), but rather mixing of melts from a two-component mantle (depleted and enriched DMM or D-DMM and E-DMM, respectively). After filtering the volatile element data for secondary processes (degassing, sulfide saturation, assimilation of seawater-derived component, and fractional crystallization), we use the volatiles to ITE ratios of our samples and a two-component mantle melting-mixing model to estimate the volatile content of the D-DMM (CO2 = 22 ppm, H2O = 59 ppm, F = 8 ppm, Cl = 0.4 ppm, and S = 100 ppm) and the E-DMM (CO2 = 990 ppm, H2O = 660 ppm, F = 31 ppm, Cl = 22 ppm, and S = 165 ppm). Our two-component mantle melting-mixing model reproduces the kernel density estimates (KDE) of Th/La and 143Nd/144Nd ratios for our samples and for EPR axial MORB compiled from the literature. This model suggests that: (1) 78% of the Pacific upper mantle is highly depleted (D-DMM) while 22% is enriched (E-DMM) in volatile and refractory ITE, (2) the melts produced during variable degrees of melting of the E-DMM controls most of the MORB geochemical variation, and (3) a fraction (∼65 to 80%) of the low degree EMORB melts (produced by ∼1.3% melting) may escape melt aggregation by freezing at the base of the oceanic lithosphere, significantly enriching it in volatile and trace element contents. Our results are consistent with previously proposed geodynamical processes acting at mid-ocean ridges and with the generation of the E-DMM. Our observations indicate that the D-DMM and E-DMM have (1) a relatively constant CO2/Cl ratio of ∼57±8, and (2) volatile and ITE element abundance patterns that can be related by a simple melting event, supporting the hypothesis that the E-DMM is a recycled oceanic lithosphere mantle metasomatized by low degree melts. Our calculation and model give rise to a Pacific upper mantle with volatile content of CO2 = 235 ppm, H2O = 191 ppm, F = 13 ppm, Cl = 5 ppm, and S = 114 ppm.
... Compared with the peraluminous rhyolites (e.g., Samples LGN-7 and LGN-9A), peralkaline rhyolites from the GHM area and elsewhere characteristically have high Nb and Ta (Figs. 4, S3b). This is best shown in Nb*–Ta* space (Fig. 5a; Niu et al., 1999). Niu and Batiza (1997) show that during mantle melting and much of the basaltic magma evolution (e.g., olivine–clinopyroxene–plagioclase crystallization ) D Nb ≈ D Th b D Ta ≈ D U , suggesting that Nb/Th and Ta/U ratios in magmatic processes do not change. It follows that the varying Nb/Th (or Nb*) and Ta/U (or Ta*) in magmatic rocks must have been inherited ...
... 5b further demonstrates that in contrast to the low Nb/Ta (b~12) of continental crust and peraluminous rhyolites, the GHM trachybasalts, trachytes and peralkaline rhyolites all have elevated Nb/Ta ratios as high as or higher than that of MORB and OIB, manifesting the inherited high Nb/Ta from OIB-like alkali basalts. The apparently higher Nb/Ta of trachytes and peralkaline rhyolites than trachybasalts is consistent with the observation that Nb is slightly more incompatible than Ta during magma evolution ( Batiza, 1997 and Hékinian, 1997). ...
... Because the partition coefficient Kd Nb,Ta /Kd Th,U N 1 in the ilmenite , the evolved trachytes and peralkaline rhyolites are expected to have slightly lower Nb/Th and Ta/U than in the trachybasalts (Fig. 5a). Furthermore, in the protracted fractionation processes with D Nb / D Ta b 1 (Niu and Batiza, 1997), it is anticipated that the highly evolved trachytes and peralkaline rhyolites have higher Nb/Ta than the parental trachybasalts (Fig. 5b). After a large amount of plagioclase separation (Fig. 7f–i), the positive Sr and Eu anomalies in the primitive mantle melts (Niu and O'Hara, 2009) gradually disappear and the more evolved syenite, trachytes, and peralkaline rhyolites become progressively and highly depleted in Ba, Sr and Eu with strongly developed negative anomalies (Figs. 4, S5). ...
We report petrological and geochemical data on coeval trachybasalts, syenites with enclaves, trachytes, peralkaline rhyolites and peraluminous rhyolites from the Glass House Mountains–Maleny–Noosa area, southeast Queensland, Australia. This rock association and the unique characteristics of the peralkaline rhyolites offer convincing lines of evidence that the petrogenesis of the peralkaline rhyolites is a straightforward consequence of protracted fractional crystallization from basaltic melts of alkali-rich composition. Compared to the common peraluminous rhyolites elsewhere, the peralkaline rhyolites here are characterized by elevated abundances of most incompatible elements, especially the very high Nb (vs. Th) and Ta (vs. U), the very low Ba, Sr and Eu and the extremely high 87Sr/86Sr ratio. The high Nb and Ta are inherited from the parental alkali basaltic melts. The low Ba, Sr and Eu result from removal of plagioclase during the protracted fractional crystallization. These rocks altogether define a Rb–Sr isochron of ~28 Ma, which is similar to Ar–Ar age data on these rocks in the literature. The extremely high 87Sr/86Sr ratio of the peralkaline rhyolites (up to 1.88) is actually characteristic of peralkaline rhyolites because of extreme Sr (also Eu and Ba) depletion and thus the very high Rb/Sr ratio. That is, the Sr in these rocks is essentially radiogenic 87Sr accumulated from the 87Rb decay since the volcanism. We suggest that the petrogenesis of the peralkaline rhyolites from the Glass HouseMountain area may be of general significance globally. The coeval peraluminous rhyolites apparently result from crustal anatexis in response to the basaltic magma underplating. The small “Daly Gap” exhibited in this rock association is anticipated during the protracted fractional crystallization from basaltic parent to the more evolved felsic varieties.
... The data are presented in Electronic Appendix 1 on an anhydrous basis. Bulk-rock minor and trace element (Li, Be, Sc, Ti, V, Cr, Co, Ni, Cu, Zn, Ga, Rb, Sr, Y, Zr, Nb, Cs, Ba, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, Pb, Th and U) abundances in these same samples were analyzed by ICP-MS on a Fisons PQ2 þ system at UQ with analytical conditions and procedures following Eggins et al. (1997) and Niu & Batiza (1997) except for sample digestion, which was done using high-pressure bombs to ensure complete digestion/dissolution (Niu et al., 2002b). Some samples were digested and analyzed more than once or twice, and the reported values are reproducible within analytical uncertainties. ...
... Bougault et al., 1979; Hofmann et al., 1986; Jochum et al., 1986; Sun & McDonough, 1989). Although it has been noted that the Zr/Hf ratio varies in carbonatites, and is super-chondritic in some alkali basalts, Niu & Batiza (1997) showed for the first time that these two ratios do vary significantly in lavas from seamounts near the axis of the East Pacific Rise (seafloor basalts) with Zr/ Hf $ 25–50 and Nb/Ta $ 9–18, respectively. These two ratios are correlated with each other (R Zr/Hf–Nb/Ta ¼ 0Á75, statistically significant at >99Á9% confidence levels for N > 80 samples), and with commonly used index ratios such as Th/U, Nb/U, Rb/Cs, La/Sm, Ce/Yb and Ce/Pb (Hofmann et al., 1986; Hofmann, 1988 Hofmann, , 1997 Sun & McDonough, 1989 ) as well as the abundances of progressively more incompatible elements. ...
... etc., which disagrees with what is generally accepted. For example, D Cs < D Rb is expected both theoretically and experimentally (Blundy & Wood, 1994), but D Rb < D Cs is observed instead (Niu & Batiza, 1997; Niu et al., 2002a). Recent studies of various terrestrial rocks (e.g. ...
This paper presents the first comprehensive major and trace element data for $130 abyssal peridotite samples from the Pacific and Indian ocean ridge–transform systems. The data reveal important features about the petrogenesis of these rocks, mantle melting and melt extraction processes beneath ocean ridges, and elemental behaviours. Although abyssal peridotites are serpentinized, and have also experienced seafloor weathering, magmatic signatures remain well preserved in the bulk-rock compositions. The better inverse correlation of MgO with progressively heavier rare earth elements (REE) reflects varying amounts of melt depletion. This melt depletion may result from recent sub-ridge mantle melting, but could also be inherited from previous melt extraction events from the fertile mantle source. Light REE (LREE) in bulk-rock samples are more enriched, not more depleted, than in the constituent clinopyr-oxenes (cpx) of the same sample suites. If the cpx LREE record sub-ridge mantle melting processes, then the bulk-rock LREE must reflect post-melting refertilization. The significant correlations of LREE (e.g. La, Ce, Pr, Nd) with immobile high field strength elements (HFSE, e.g. Nb and Zr) suggest that enrichments of both LREE and HFSE resulted from a common magmatic process. The refertilization takes place in the 'cold' thermal boundary layer (TBL) beneath ridges through which the ascending melts migrate and interact with the advanced residues. The refertilization appar-ently did not affect the cpx relics analyzed for trace elements. This observation suggests grain-boundary porous melt migration in the TBL. The ascending melts may not be thermally 'reactive', and thus may have affected only cpx rims, which, together with pre-cipitated olivine, entrapped melt, and the rest of the rock, were subsequently serpentinized. Very large variations in bulk-rock Zr/Hf and Nb/Ta ratios are observed, which are unexpected. The correlation between the two ratios is consistent with observa-tions on basalts that D Zr /D Hf < 1 and D Nb /D Ta < 1. Given the identical charges (5 þ for Nb and Ta; 4 þ for Zr and Hf) and essentially the same ionic radii (R Nb /R Ta ¼ 1Á000 and R Zr / R Hf ¼ 1Á006–1Á026), yet a factor of $2 mass differences (M Zr /M Hf ¼ 0Á511 and M Nb /M Ta ¼ 0Á513), it is hypothe-sized that mass-dependent D values, or diffusion or mass-transfer rates may be important in causing elemental fractionations during porous melt migration in the TBL. It is also possible that some 'exotic' phases with highly fractionated Zr/Hf and Nb/Ta ratios may exist in these rocks, thus having 'nugget' effects on the bulk-rock analyses. All these hypotheses need testing by constraining the storage and distribution of all the incompatible trace elements in mantle peridotite. As serpentine contains up to 13 wt % H 2 O, and is stable up to 7 GPa before it is transformed to dense hydrous magnesium silicate phases that are stable at pressures of $5– 50 GPa, it is possible that the serpentinized peridotites may survive, at least partly, subduction-zone dehydration, and transport large amounts of H 2 O (also Ba, Rb, Cs, K, U, Sr, Pb, etc. with elevated U/Pb ratios) into the deep mantle. The latter may contribute to the HIMU component in the source regions of some oceanic basalts. KEY WORDS: abyssal peridotites; serpentinization; seafloor weathering; bulk-rock major and trace element compositions; mantle melting; melt extraction; melt–residue interaction; porous flows; Nb/Ta and Zr/Hf fractionations; HIMU mantle sources
... They also stressed that this recycled ancient oceanic crust, the plume material, which is the source of OIB is geochemically more ''enriched'' in K, Rb, U, Th, and light rare earth elements relative to the more ''depleted'' source of mid-ocean ridge basalts (MORB). While some details are considered conjectural , the principal idea of the model has been widely accepted by the solid Earth community as being, to a first order, correct [e.g., Hofmann, 1988 Hofmann, , 1997 Chauvel et al., 1992; Hauri, 1996; Hauri et al., 1996; Rehkämper and Hofmann, 1997; Niu and Batiza, 1997; Lassiter and Hauri, 1998; Sobolev et al., 2000; Cordery et al., 1997; Leitch and Davies, 2001]. In this paper, instead of reviewing numerous works following this line in the literature, we argue that there is no obvious association between ancient subducted oceanic crusts and source materials of OIB. ...
... Given the known composition of the present-day DM and the decay constants of the radioactive parents (i.e., 87 Rb, 147 Sm, and 176 Lu), the P/D ratios (i.e., 87 Rb/ 87 Sr, 147 Sm/ 143 Nd, and 176 Lu/ 176 Hf) of the DM along these curves are then constrained at any given time in the last 2.5 Gyr. [8] The significant coupling between radiogenic isotopes and incompatible element abundances and ratios in many OIB suites, seamount lavas, and MORB [e.g., Saunders et al., 1988; Sun and McDonough, 1989; Niu and Batiza, 1997; Niu et al., 1999 Niu et al., , 2002a suggests that the source materials of these basalts are ancient and have developed their isotopic characteristics over a period in excess of 1.0 Gyr. If recycled ancient oceanic crusts played a role in the source regions of these oceanic basalts, then these ancient oceanic crusts would have to be formed >1.0 Ga. ...
... A Two-Component Mantle Source (Composite Lithologies) for Oceanic Basalts [30] Many studies suggest that fertile sources of oceanic island basalts are very heterogeneous on variably small scales and also vary with time beneath a given ocean island, a given seamount and a given ridge segment. A common view is that such a heterogeneous source essentially has two components, a volumetrically small and easily melted component enriched in volatiles, alkalis and other incompatible elements dispersed in ambient depleted and refractory peridotitic matrix [Hanson, 1977; Wood, 1979; Le Roex et al., 1983; Prinzhofer et al., 1989; Hirschmann and Stolper, 1996; Niu et al., 1996 Niu et al., , 1999 Niu et al., , 2001b Niu et al., , 2002a Niu and Batiza, 1997; Phipps Morgan and Morgan, 1999]. The two components themselves can each be compositionally heterogeneous [Niu et al., , 2002a. ...
Consideration of petrology, geochemistry, and mineral physics suggests that ancient subducted oceanic crusts cannot be the source materials supplying ocean island basalts (OIB). Melting of oceanic crusts cannot produce high‐magnesian OIB lavas. Ancient oceanic crusts (>1 Ga) are isotopically too depleted to meet the required values of most OIB. Subducted oceanic crusts that have passed through subduction zone dehydration are likely to be depleted in water‐soluble incompatible elements (e.g., Ba, Rb, Cs, U, K, Sr, Pb) relative to water‐insoluble incompatible elements (e.g., Nb, Ta, Zr, Hf, Ti). Melting of residual crusts with such trace element composition cannot produce OIB. Oceanic crusts, if subducted into the lower mantle, will be >2% denser than the ambient mantle at shallow lower mantle depths. This negative buoyancy will impede return of the subducted oceanic crusts into the upper mantle. If subducted oceanic crusts melt at the base of the mantle, the resultant melts are even denser than the ambient peridotitic mantle, perhaps by as much as ∼15%. Neither in the solid state nor in melt form can bulk oceanic crusts subducted into the lower mantle return to upper mantle source regions of oceanic basalts. Deep portions of recycled oceanic lithosphere are important geochemical reservoirs hosting volatiles and incompatible elements as a result of metasomatism taking place at the interface between the low‐velocity zone and the cooling and thickening oceanic lithosphere. These metasomatized and recycled deep portions of oceanic lithosphere are the most likely candidates for OIB sources in terms of petrology, geochemistry and mineral physics.
... Consequently, lavas from these seamounts are not e⁄ciently mixed, and thus a¡ord promising material to test the relative e¡ect of fertile mantle compositional variation and the effect of physical conditions on MORB composi- tions. In this study we present Sr, Nd and Pb isotope data for near-EPR seamounts (Fig. 1) previously studied for major and trace elements [1,29]. Our data indeed show signi¢cant correlations among radiogenic isotopes, incompatible trace elements and major elements, which points to lithological controls on the fertile mantle source Seamounts and axial locations with HIMU-like, Hawaiianlike and alkali lavas are indicated (seeFig. ...
... Depth Selected major elements (wt%) and trace element ratios are taken from [1] Rock type: N, N-MORB (K/Ti 6 0.11); E, E-MORB (K/Ti s 0.11); and alkali basalts with K 2 O s 1wt%. Sr, Nd and Pb isotope analyses were done on fresh, hand-picked glasses, lightly leached at room temperature in HCl^H 2 O 2 for a few minutes to remove Mn oxides in possible micro-fractures and other potentially labile contaminants. ...
... They were collected from near-ridge seamounts within the 1 Ma isochron ( 6 V60 km) of the EPR axis between 5 ‡ and 15 ‡N on both the Paci¢c and Cocos Plates (Fig. 1). These samples have been previously studied for major and trace elements [1,24,29,30]. Subsets of these samples that cover the entire compositional spectrum de¢ned by trace elements are studied here for Sr^ Nd^Pb isotopes. ...
Niu and Batiza [Earth Planet. Sci. Lett. 148 (1997) 471^483] show that lavas from the seamounts on the flanks of the East Pacific Rise (EPR) between 5 and 15N vary from extremely depleted tholeiites to highly enriched alkali basalts. The extent of depletion and enrichment exceeds the known range of seafloor lavas in terms of the abundances and ratios of incompatible elements. New Sr^Nd^Pb isotope data for these lavas show variations (87 Sr/ 86 Sr = 0.702362^0.702951; 206 Pb/ 204 Pb = 18.080^19.325 and 143 Nd/ 144 Nd 0.512956^0.513183) larger than observed in lavas erupted on the nearby EPR axis. These isotopic ratios correlate with each other, with the abundances and ratios of incompatible elements, with the abundances of measured major elements such as MgO, CaO, Na 2 O and TiO 2 contents, and with the abundances and ratios of major elements corrected for crystal fractionation to Mg# = 0.72 (Ti 72 , Al 72 , Fe 72 , Ca 72 , Na 72 , and Ca 72 /Al 72). These coupled correlations and the spatial distribution of seamounts require an EPR mantle source that has long-term (s 1 Ga) lithological heterogeneities on very small scales [Niu and Batiza, Earth Planet. Sci. Lett. 148 (1997) 471^483]. Mid-ocean ridge basalt (MORB) major element systematics are, to a great extent, inherited from their fertile sources, which requires caution when using major element data to infer melting conditions. The significant correlations in elemental and isotopic variability (defined as RSD% = 1c/ meanU100) between seamount and axial lavas suggest that both seamount and axial volcanisms share a common heterogeneous mantle source. We confirm previous interpretations [Niu 7067^7087] that the geochemical variability of lavas from the broad northern EPR region results from melting-induced mixing of a two-component mantle with the enriched (easily melted) component dispersed as physically distinct domains in a more depleted (refractory) matrix prior to the major melting events. The data also allow the conclusion that recycled oceanic crust cannot explain elevated abundances of elements such as Ba, Rb, Cs, Th, U, K, Pb, Sr etc. in enriched MORB and many ocean island basalts. These elements will be depleted in recycled oceanic crust that has passed through subduction zone dehydration reactions. We illustrate that deep portions of recycled oceanic lithosphere are important geochemical reservoirs hosting these and other incompatible elements as a result of metasomatism taking place at the interface between the low velocity zone and the cooling and thickening oceanic lithosphere. ß 2002 Elsevier Science B.V. All rights reserved.
... Melting cessation beneath ocean ridges may be caused by the increased heat of fusion of residual solid as a result of progressive depletion (Niu and Batiza, 1991), and the effect of pressure-induced solid–solid phase transitions on suppressing melting (Asimow et al., 1995). In addition, conductive cooling and spreading rate are also considered important factors to determine the final extent of melting (Shen and Forsyth, 1995; Niu and Batiza, 1997). However, the partial melts enter the period of fractional crystallization in the Cold Thermal Boundary Layer (CTBL) after melting cessation. ...
... MORBs are anhydrous melts derived from a mantle source region depleted in incompatible elements and volatile components (Niu et al., 1999Niu et al., , 2002). The path of cumulates in anhydrous MORB melt fractionation follows the order of Ol (dunite) → Ol+Pl (troctolite) → Ol+Pl+Cpx (gabbro) (Yoder, 1965; Gaetani, 1993; Niu and Batiza, 1997; Niu et al., 1999 Niu et al., , 2002 Niu et al., , 2008) (Fig. 5). The effect of dunite crystallization on Ca/Al ratio is insignificant. ...
The composition of melt inclusions in basalts erupted at mid-ocean ridges may be modified by post-entrapment processes, so the present composition of melt inclusions may not represent their original composition at the time of entrapment. By combining the melt inclusion composition in samples from the South Mid-Atlantic Ridge at 19ºS analyzed in this study, and from the Petrological Database, we found that post-entrapment crystallization processes resulted in higher Ca/Al, Mg#[100×atomic Mg2+/(Mg2++Fe2+)], MgO and FeO contents, and lower CaO and Al2O3 contents of plagioclase-hosted melt inclusions relative to those hosted in olivine. In addition, melt inclusions hosted in plagioclase with anorthite content larger than 80mol.% had been modified more readily than others. By discussing the relationships between Ca/Al and fractional crystallization, post-entrapment crystallization, and the original melt composition, we propose that Ca/Al can be regarded as an indicator of the effect of post-entrapment processes on melt inclusion composition. Specifically, i) when Ca/Al<0.78, melt inclusion compositions corrected for fractional crystallization to Mg#=72 can represent the primary magma at mid-ocean ridges; ii) when 0.78<Ca/ Al<1.0, melt inclusions are mainly modified by post-entrapment crystallization effects, and can reveal the original melt composition after correcting for these effects; iii) when Ca/Al>1.0, the compositions of melt inclusions do not reflect the original melt composition nor preserve information about the mantle source. According to these criteria, plagioclase-hosted melt inclusions with Ca/Al>1.0 in basalts from the South Mid-Atlantic Ridge at19ºS cannot represent the composition of the melt at the moment of their entrapment.
... Experiments, however, show that rutile preferentially incorporates Ta over Nb Klemme et al. 2005;Schmidt et al. 2004;Xiong et al. 2005), such that melts from a rutile-bearing oceanic slab should have even higher Nb/Ta than that of MORB and the residue, which cannot reconcile subchondritic Nb/Ta of the CC . Although melting of hydrous rutile-bearing eclogites with initially low Nb/Ta 474 X. Ding et al. ratios can produce subchondritic Nb/Ta characteristics (Rapp et al. 2003;Xiong 2006;Xiong et al. 2005), sources such as arc crust and very depleted mid-oceanic ridge basalts (Niu and Batiza 1997;Rapp et al. 2003;Sun et al. 2003) are not volumetrically important enough to quantitatively account for the low Nb/Ta of the CC. Therefore, a simple model of melting of the subducted slab cannot convincingly explain the Nb and Ta characteristics of the CC. ...
... Even though Nb and Ta have long been considered to have identical geochemical behaviours during most mantle processes (Green 1995;Sun and McDonough 1989), considerable Nb/Ta variations have been reported in different rocks formed in various geological settings, including arc rocks (Eggins et al. 1997;Munker 1998), oceanic basalt (Niu and Batiza 1997;Sun et al. 2003), granitoid suites (Collins et al. 1982;Dostal and Chatterjee 2000;Zhao et al. 2008), and metamorphic/metasomatic rocks (Aulbach et al. 2008;Gao et al. 2007;Ionov and Hofmann 1995;Rudnick et al. 2000;Xiao et al. 2006). Corresponding processes that fractionate Nb and Ta have long been argued. ...
... Trace element and isotopically enriched mid-ocean ridge basalts are found at locations far from the influence of hotspots and are commonly coeval with more voluminous normal (N-)MORB material (Zindler et al., 1984; Fornari et al., 1988a; Allan et al., 1989; Niu & Batiza, 1997; Niu et al., 1999; Donnelly et al., 2004). The similarity of enriched (E-)MORB and N-MORB in terms of their major element geochemistry suggests that they share a similar melting history. ...
... The positive correlations of Rb–Sr, Sm–Nd, and 238 U– 206 Pb isochron plots for basalts from all ocean basins indicate an apparent source age of $300 Ma (Donnelly et al., 2004). This age defines the continuous creation and destruction of the enriched source, which has been interpreted as recycled oceanic crust resulting in the formation of mafic veins (pyroxenites) within a depleted peridotite matrix (Allè gre & Turcotte, 1986; Niu & Batiza, 1997; Niu et al., 1999; Sobolev et al., 2007; Mallik & Dasgupta, 2012). There have been numerous recent studies inferring the role of pyroxenite heterogeneities in the generation of mid-ocean ridge basalts (Hirschmann & Stolper, 1996; Yaxley & Green, 1998; Pertermann & Hirschmann, 2003b; Sobolev et al., 2007; Lambart et al., 2009 Lambart et al., , 2012 Lambart et al., , 2013 Stracke & Bourdon, 2009; Waters et al., 2011; Zhang et al., 2012 ), although the origin of the lithological variation remains controversial. ...
Seamounts formed adjacent to mid-ocean ridges are the most abundant on Earth, numbering several orders of magnitude higher than hotspot-related seamounts. The Taney Seamounts are a linear NW–SE-trending, near mid-ocean ridge chain consisting of five volcanoes located on the Pacific plate 300 km west of San Francisco, California. Taney Seamount-A, the largest and oldest in the chain, has four well-defined calderas. These calderas have clear cross-cutting relationships, creating a relative chronology. The caldera walls and intracaldera pillow mounds were sampled systematically by a remotely operated vehicle to obtain stratigraphically controlled samples, a unique aspect of this study. Changes in lava geochemistry are consistent with an open-system sub-caldera reservoir that undergoes periodic collapse, replenishment, shallow crystallization, and eruption. Replenishing magmas contain large, anorthite-rich plagioclase crystals that exhibit sieve textures and zoning indicating interaction with percolating melt. The enrichment of elements in the lavas that are incorporated in plagioclase (e.g. aluminum, strontium) provides chemical evidence for the interaction between mantle-derived melts and plagioclase cumulates in the lower oceanic crust or upper mantle (8–12 km), prior to magmas entering the sub-caldera plumbing system. Based on trace element variations, the erupted lavas vary from typical peridotite-derived normal mid-ocean ridge basalt (N-MORB) compositions to those with an apparent residual garnet signature. Geochemical and thermodynamic modeling shows that decompression melting of a MORB mantle peridotite re-fertilized by garnet pyroxenite partial melts can reproduce the garnet signature observed in the Taney-A edifice lavas. Hence the magmatic architecture of Taney Seamount-A is characterized by the melting of a mixed lithology mantle, melt–rock interaction in the upper mantle to lower oceanic crust, and open-system evolution in a sub-caldera magma reservoir.
... It is possible that such seamount crust was part of the largely inaccessible continental fore-arc basement. Alternatively, it could be part of subducting Cocos plate where clusters of intraplate seamounts are common (e.g., Bohrson and Reid, 1995; Niu and Batiza, 1997; Castillo et al., 2010). Local recycling of seamount material, mingling to some extent with granodiorite, could account for the limited distribution of the Nb-rich magmas in space and time in the Sierra Chichinautzin (Straub et al., 2013b) as well as along the volcanic front of the entire MVB. ...
... While mantle, AOC and granodiorite compositions are reasonably well known (Table 2 ), the composition of the inferred recycled seamount component is unknown, and therefore its quantification is tentative. For an estimate, we used the Sr, Nd, Pb and Hf abundances of off-axis seamounts with Nb >13–46 ppm from Niu and Batiza (1997)Sr (1997), Nb >10 ppm a Model in Fig. 14, uses increased Sr abundances, by factor of 3 for calc-alkaline series (Sr = 368 ppm; Sr/Nd = 36), and by a factor of 2.5 for the NEAB (Sr = 306 ppm; Sr/Nd = 30). b Model in Fig. 14, uses increased Sr abundances, by factor of 4 (Sr = 1177 ppm; Sr/Nd = 36). ...
Recycling of upper plate crust in subduction zones, or ‘subduction erosion’, is a major mechanism of crustal destruction at convergent margins. However, assessing the impact of eroded crust on arc magmas is difficult owing to the compositional similarity between the eroded crust, trench sediment and arc crustal basement that may all contribute to arc magma formation. Here we compare Sr-Nd-Pb-Hf and trace element data of crustal input material to Sr-Nd-Pb-Hf-He-O isotope chemistry of a well-characterized series of olivine-phyric, high-Mg# basalts to dacites in the central Mexican Volcanic Belt (MVB). Basaltic to andesitic magmas crystallize high-Ni olivines that have high mantle-like 3He/4He = 7-8 Ra and high crustal δ18Omelt = +6.3-8.5‰ implying their host magmas to be near-primary melts from a mantle infiltrated by slab-derived crustal components. Remarkably, their Hf-Nd isotope and Nd/Hf trace element systematics rule out the trench sediment as the recycled crust end member, and imply that the coastal and offshore granodiorites are the dominant recycled crust component. Sr-Nd-Pb-Hf isotope modeling shows that the granodiorites control the highly to moderately incompatible elements in the calc-alkaline arc magmas, together with lesser additions of Pb- and Sr-rich fluids from subducted mid-oceanic ridge basalt (MORB)-type altered oceanic crust (AOC). Nd-Hf mass balance suggests that the granodiorite exceeds the flux of the trench sediment by at least 9-10 times, corresponding to a flux of ⩾79-88 km3/km/Myr into the subduction zone. At an estimated thickness of 1500-1700 m, the granodiorite may buoyantly rise as bulk ‘slab diapirs’ into the mantle melt region and impose its trace element signature (e.g. Th/La, Nb/Ta) on the prevalent calc-alkaline arc magmas. Deep slab melting and local recycling of other slab components such as oceanic seamounts further diversify the MVB magmas by producing rare, strongly fractionated high-La magmas and a minor population of high-Nb magmas, respectively. Overall, the central MVB magmas inherit their striking geochemical diversity principally from the slab, thus emphasizing the importance of continental crust recycling in modern solid Earth relative to its new formation in modern subduction zones.
... Therefore, continental crust derived melts are expected to have lower Nb/Th, Ta/U, and ε Nd(t) and higher 87 Sr/ 86 Sr than mantle-derived melts (Fig. 8; Goldstein and Jacobsen, 1988; Rudnick and Gao, 2003). Given the relative incompatibility of D Nb ≈ D Th b D Ta ≈ D U during basaltic magmatism (Niu and Batiza, 1997; Niu and O'Hara, 2009 ), Nb/Th and Ta/U ratios will remain constant during magmatism and their variation, if any, in samples would be inherited from the source rocks or source histories. If such mafic magmas were contaminated by the crust, the Nb/Th, Ta/U and ε Nd(t) would decrease whereas 87 Sr/ 86 Sr would increase with increasing SiO 2 . ...
... Partial melting of such juvenile crustal material produced felsic melts parental to the felsic volcanic rocks in the EKOB. That is, many of the original materials were derived directly (alkaline melts parental to the Niu and Batiza, 1997). Compared with common basalts, the diabasic dikes have Ta and Nb deficiencies, but significantly less so than continental crustal materials as well as the felsic volcanic rocks. ...
We present zircon U-Pb ages and geochemical data on the late Triassic mafic dikes (diabase) and felsic volcanic rocks (rhyolite and rhyolitic tuff) in the East Kunlun Orogenic Belt (EKOB). These rocks give a small age window of 228-218 Ma. The mafic dikes represent evolved alkaline basaltic melts intruding ~ 8-9 Myrs older and volumetrically more abundant A-type granite batholith. Their rare earth element (REE) and multi-element patterns similar to those of the present-day ocean island basalts (OIB) except for a weak continental crustal signature (i.e., enrichment of Rb and Pb and weak depletion of Nb, Ta and Ti). Their trace element characteristics together with the high 87Sr/86Sr (0.7076- 0.7104), low εNd(t) (-2.18 to -3.46), low εHf(t) (-2.85 to -4.59) and variable Pb isotopic ratios are consistent with melts derived from metasomatized subcontinental lithospheric mantle with crustal contamination. The felsic volcanic rocks are characterized by high LREE/HREE (e.g., [La/Yb]N of 5.71-17.00) with a negative Eu anomaly and strong depletion in Sr and P, resembling the model upper continental crust (UCC). Given the high 87Sr/86Sr (0.7213-0.7550) and less negative εNd(t) (-3.83 to -5.09) and εHf(t) (-3.06 to -3.83) than the UCC plus the overlapping isotopes with the mafic dikes and high Nb-Ta rhyolites, the felsic volcanic rocks are best interpreted as resulting from melting-induced mixing with 45-50% crustal materials and 50-55% mantle-derived mafic melts probably parental to the mafic dikes. Such mantle-derived melts underplated and intruded the deep crust as juvenile crustal materials. Partial melting of such juvenile crust produced felsic melts parental to the felsic volcanic rocks in the EKOB. We hypothesize that the late Triassic mafic dikes and felsic volcanic rocks are associated with post-collisional extension and related orogenic collapse. Such processes are probably significant in causing asthenospheric upwelling, decompression melting, induced melting of the prior metasomatized mantle lithosphere and the existing crust. This work represents our ongoing effort in understanding the origin of the juvenile crust and continental crustal accretion through magmatism in the broad context of orogenesis from seafloor subduction to continental collision and to post-collisional processes.
... Because mantle source materials are probably heterogeneous on all scales and geochemically enriched OIB-like basalts are widespread (although volumetrically small) along ocean ridges and at near-ridge seamounts (e.g. Batiza & Vanko, 1984; Zindler et al., 1984; Langmuir et al., 1986; Castillo & Batiza, 1989; Sinton et al., 1991; Mahoney et al., 1994; Niu et al., 1996 Niu et al., , 1999 Niu et al., , 2001 Niu et al., , 2002 Niu & Batiza, 1997; Castillo et al., 1998 Castillo et al., , 2000 Castillo et al., , 2010), OIB geochemistry alone cannot convincingly resolve whether their source materials originate from deep 'mantle plumes' or from concentrated (versus diluted beneath ocean ridges) heterogeneities in the upper mantle. In this contribution, we do not attempt to resolve the 'mantle plume' debate, nor to model the petrogenesis of any particular OIB suite, but instead we discuss some geodynamic implications of a recent finding by Humphreys & Niu (2009) that oceanic lithosphere thickness variation exerts the first-order control on the geochemistry of OIB on a global scale, despite the importance of other effects such as mantle compositional heterogeneity and mantle T P variations. ...
... The enriched component in the melt is diluted progressively with continued decompression melting of the more depleted, or, rather, less enriched, source component(s). The 'dilution' effect, reflected in geochemical variation diagrams, is equivalent to mixing of melts from a compositionally heterogeneous mantle source containing an enriched or easily melted component dispersed in a more depleted peridotitic matrix (Niu et al., 1996Niu et al., , 2002 Niu & Batiza, 1997sphere. This is indeed broadly the case as shown inFig. ...
Based on an evaluation of major and trace element data for ocean island basalts (OIB), we demonstrate that oceanic lithosphere thick-ness variation, which we refer to as the lid effect, exerts the primary control on OIB geochemistry on a global scale. The lid effect caps the final depth (pressure) of melting or melt equilibration. OIB erupted on thick lithosphere have geochemical characteristics consist-ent with a low extent and high pressure of partial melting, whereas those erupted on thin lithosphere exhibit the reverse; that is, a high extent and low pressure of melting cessation. This observation re-quires that mantle melting beneath intra-plate volcanic islands takes place in the asthenosphere and results from dynamic upwelling and decompression. Melting beneath all ocean islands begins in the garnet peridotite facies, imparting the familiar ' garnet signature' to all OIB melts (e.g. [Sm/Yb] N 41); however, the intensity of this signature decreases with increasing extent of melting beneath thinner lithospheric lids as a result of dilution.The dilution effect is also re-corded in the radiogenic isotope composition of OIB, consistent with the notion that their mantle source regions are heterogeneous with an enriched component of lower solidus temperature dispersed in a more refractory matrix. High-quality data on the compositions of olivine phenocrysts from mid-ocean ridge basalt and global OIB sample suites are wholly consistent with the lid effect without the need to invoke olivine-free pyroxenite as a major source component for OIB. Caution is necessary when using basalt-based thermobaro-metry approaches to estimate mantle potential temperatures and solidus depth because OIB do not unequivocally record such informa-tion. For plate ages up to $80 Ma, we demonstrate that the geophy-sically defined base of the growing oceanic lithosphere corresponds to both an isotherm ($11008C) and the pargasite (amphibole) dehydration solidus of fertile mantle peridotite. As pargasite in H 2 O^CO 2 -bearing mantle peridotite is stable under conditions of T 11008C and P 3 GPa ($90 km), this solidus is essentially isothermal (i.e. dT/dP $ 0 in P^T space) with T $11008C) at depths 90 km, but becomes isobaric (i.e. dP/dT $ 0 in P^T space) at the $90 km depth. The latter explains why older (470 Ma) oceanic lithosphere cannot be thicker than $90 km with-out the need to invoke physically complex processes such as convective removal.
... Th) and Ta (vs. U) (Figure 4b; also see Niu and Batiza, 1997). ...
... However, because melting regions are finite, low-F melt may exist and metasomatism may thus take place in the peripheral areas of these major melting regions (Niu et al., 1996;Pilet et al., 2011). Mantle wedge overlying subduction zones is surely metasomatized (see Wyllie and Sekine, 1982;Donnelly et al., 2004), but the connection to OIB petrogenesis is obscured because the metasomatic agent there may have an arc-melt signature (i.e., [Nb/Th] PM (IAB) < 1 and [Ta/U] PM (IAB) < 1), whereas both MORB and OIB all have [Nb/Th] PM (MORB, OIB) ≥ 1 and [Ta/U] PM (MORB, OIB) ≥ 1 (Niu and Batiza, 1997;Niu et al., 1999;Niu and O'Hara, 2009;Sun and McDonough, 1989;Halliday et al., 1995;Hofmann, 1997). ...
A trace element perspective on the source of ocean island basalts (OIB) and fate of subducted ocean crust (SOC) and mantle lithosphere (SML) The history of petrogenesis has been the history of re-interpretations of origins – Peter J. Wyllie important enriched geochemical reservoir. We argue that ancient subducted metasomatized mantle lithosphere (SML) provides the major source component for OIB. The metasomatic agent is an H 2 O-CO 2 -rich silicate melt derived from within the LVZ. Upward migration and concentration of the melt at the lithosphere-LVZ interface (i.e., the lithosphere-asthenosphere boundary or LAB) results in chemical stratification in the LVZ with the deeper portion being more depleted (i.e., DMM), providing the source for MORB. The widespread metasomatized peridotites, pyroxenites and hornblendites from xenolith suites exhumed from the deep lithosphere (both oceanic and continental) and in orogenic peridotite massifs confirm the role of a low-F silicate melt phase as the metasomatic agent. The SOC, if subducted into the lower mantle, will be too dense to return in bulk to the upper mantle source regions of oceanic basalts, and may have contributed to the two large low shear wave velocity provinces (LLSVPs) at the base of the mantle beneath the Pacific and Africa over Earth's history.
... Th) and Ta (vs. U) (Figure 4b; also see Niu and Batiza, 1997). ...
... However, because melting regions are finite, low-F melt may exist and metasomatism may thus take place in the peripheral areas of these major melting regions (Niu et al., 1996;Pilet et al., 2011). Mantle wedge overlying subduction zones is surely metasomatized (see Wyllie and Sekine, 1982;Donnelly et al., 2004), but the connection to OIB petrogenesis is obscured because the metasomatic agent there may have an arc-melt signature (i.e., [Nb/Th] PM (IAB) < 1 and [Ta/U] PM (IAB) < 1), whereas both MORB and OIB all have [Nb/Th] PM (MORB, OIB) ≥ 1 and [Ta/U] PM (MORB, OIB) ≥ 1 (Niu and Batiza, 1997;Niu et al., 1999;Niu and O'Hara, 2009;Sun and McDonough, 1989;Halliday et al., 1995;Hofmann, 1997). ...
A trace element perspective on the source of ocean island basalts (OIB) and fate of subducted ocean crust (SOC) and mantle lithosphere (SML) The history of petrogenesis has been the history of re-interpretations of origins – Peter J. Wyllie important enriched geochemical reservoir. We argue that ancient subducted metasomatized mantle lithosphere (SML) provides the major source component for OIB. The metasomatic agent is an H 2 O-CO 2 -rich silicate melt derived from within the LVZ. Upward migration and concentration of the melt at the lithosphere-LVZ interface (i.e., the lithosphere-asthenosphere boundary or LAB) results in chemical stratification in the LVZ with the deeper portion being more depleted (i.e., DMM), providing the source for MORB. The widespread metasomatized peridotites, pyroxenites and hornblendites from xenolith suites exhumed from the deep lithosphere (both oceanic and continental) and in orogenic peridotite massifs confirm the role of a low-F silicate melt phase as the metasomatic agent. The SOC, if subducted into the lower mantle, will be too dense to return in bulk to the upper mantle source regions of oceanic basalts, and may have contributed to the two large low shear wave velocity provinces (LLSVPs) at the base of the mantle beneath the Pacific and Africa over Earth's history.
... There are several opinions regarding to the genesis on the E-MORB feature of a mantle derived magma far from the hotspot. One school of thought argues that the enrichment of highly incompatible elements in E-MORB far from plumes can be explained by recycling of either oceanic crust or seamount (OIB) (Ulrich et al., 2012;Hofmann, 1997;Niu and Batiza, 1997). Another group of researchers propose that it is enriched by refertilization by small degrees of melt from the subducted slab (Donnelly et al., 2004;Niu et al., 1999). ...
The nature of lithosphere mantle beneath North China Craton (NCC) has been changed during the Mesozoic. We investigated the mafic dykes in Jiaodong to put constraint on the origin of them and reveal the mantle evolution of the NCC. Zircons from Kunyushan and Nansu mafic dykes give concordia U−Pb ages of 114.9 and 52.8 Ma, respectively. The Kunyushan mafic dyke has high MgO (∼8.0 wt.%) and Cr (>380 ppm) but lower FeO (<10 wt.%) contents. Large ion lithophile elements (LILE) are enriched compared to primitive mantle. The εHf(t) values of zircons are between −15.0 and −20.1. All these geochemical data imply an old and enriched lithosphere mantle source. In contrast, the Nansu mafic dyke has lower concentrations of MgO (<7.0 wt.%) and Cr (200 ppm–230 ppm) but higher FeO (>11 wt.%) contents. The LILE and light rare earth elements (LREE) are not so strongly enriched relative to the mafic dyke from Kunyushan but display slight enrichment compared to the primitive mantle. Positive zircon εHf(t) values are obtained (from +11.7 to +19.5, with one outlier of +3.4). It is suggested that the Nansu dyke sourced from an enriched MORB mantle and was generated by fluid flux melting.
... This is because slab dehydration occurs during the subduction of oceanic crust in the rutile stability field, and fluid-mobile elements (U, Ba, and Sr) are transported to the mantle wedge peridotite, whereas HFSEs (e.g., Nb and Ta) remain in the residual oceanic crust (e.g., Porter and White 2009). Recycling of such residual oceanic crust with high (Ta/U) N > 1 and (Nb/Th) N > 1 into the deep mantle can form the source of enriched (E-)MORBs and OIBs with positive Nb and Ta anomalies (e.g., Niu and Batiza 1997). Therefore, the high (Ta/U) N and (Nb/Th) N values of the Zhongba basalts may indicate that recycled oceanic crust is present in their mantle source (Fig. 8a). ...
The timing and origins of the Early Cenozoic magmatic flare-up in southern Tibet remain controversial, mainly because of the lack of available data for the rare, small-volume, ocean island basalt (OIB)-like mafic rocks that can shed insights into this geological event. We have confirmed that Early Cenozoic OIB-like basalts in the Zhongba area, southern Tibet, were closely related to this magmatic flare-up, which occurred in response to deep mantle geodynamics. In this study, we present detailed geochronological and geochemical data for the Zhongba basalts. These Eocene (ca. 52 Ma) OIB-like basalts have negative K, Zr, Hf, and Ti anomalies, low Fe/Mn and high Fe/Zn ratios, and uniform and enriched Sr–Nd–Hf and HIMU-like Pb isotopic compositions (87Sr/86Sr(i) = 0.7052–0.7053; εNd(t) = − 1.7 to − 1.3; εHf(t) = − 1.1 to − 0.3; 206Pb/204Pb = 19.83–19.87), indicative of derivation from carbonated peridotite. Based on the regional tectonic evolution, we propose that the carbonated peridotite source of the Zhongba basalts was generated by the reaction between depleted mid-ocean ridge basalt-like mantle peridotite and melts from subducted Neo-Tethys carbonate-bearing oceanic crust. This carbonated peridotite was possibly preserved at the thermal boundary between the lithosphere and asthenosphere and underwent partial melting to produce the Zhongba basalts in response to slab break-off of the Neo-Tethys oceanic lithosphere at ca. 52 Ma. The slab break-off triggered the magmatic flare-up and other geological processes (e.g., a decrease in the velocity of the Indian Plate) in southern Tibet during the Eocene.
... At a minimum, oceanic crust equivalent to ~2.2% of Earth's total mass has been subducted over the last 2.5 b.y. ( Rudnick et al., 2000). Although the paradigm is that this crust ultimately gets recycled back into the convecting mantle ( Staudacher and Allègre, 1988;Liu et al., 2010;Becker, 2000;Hofmann and White, 1982;Niu and Batiza, 1997;Sobolev et al., 2000;Hoernle et al., 2002;Kellogg et al., 1999;Kelley et al., 2005), there is also ample evidence that a portion of it accumulates at the core-mantle boundary over billion-year time scales (Burke et al., 2008;Hirose et al., 1999;Rao and Kumar, 2014;Spasojevic et al., 2010). Therefore, the geochemical and isotopic signature of this subducted oceanic crustal reservoir must be accounted for during terrestrial-scale geochemical mass-balance calculations (Kamber and Collerson, 2000;Sun and McDonough, 1989;Christensen and Hofmann, 1994;Aulbach et al., 2008). ...
... patible element enrichment and suggests no role for involvement of subduction-related metasomatism. The magmas generated at midocean ridges are commonly of depleted nature (N-MORBs), though enriched magma compositions also exist (E-MORB) (e.g.Hoernle et al., 2011;Niu & Batiza, 1997;Zindler, Staudigel, & Batiza, 1984). Even when the extreme end of E-MORBs is considered, the enrichment levels observed in the lavas from Karincali-Southwest section remains considerably higher(Figure 15), thus eliminating the possibility of generation of these lavas in a mid-ocean ridge setting. ...
Abstract
Blocks and tectonic slices within the Mersin Mélange (southern Turkey), which are of Northern Neotethyan
origin (Izmir–Ankara–Erzincan Ocean (IAE)), were studied in detail by using radiolarian, conodont,
and foraminiferal assemblages on six different stratigraphic sections with well-preserved
Permian succesions. The basal part of the Permian sequence, composed of alternating chert and mudstone
with basic volcanics, is assigned to the late Asselian (Early Permian) based on radiolarians. The
next basaltic interval in the sequence is dated as Kungurian. The highly alkaline basic volcanics in the
sequence are extremely enriched, similar to kimberlitic/lamprophyric magmas generated at continental
intraplate settings. Trace element systematics suggest that these lavas were generated in a continental
margin involving a metasomatized subcontinental lithospheric mantle source (SCLM). The middle part
of the Permian sequences, dated by benthic foraminifera and conodont assemblages, includes detrital
limestones with chert interlayers and neptunian dykes of middle Wordian to earliest Wuchiapingian
age. Higher in the sequence, detrital limestones are overlain by alternating chert and mudstone with
intermittent microbrecciated beds of earlyWuchiapingian to middle Changhsingian (Late Permian) age
based on the radiolarians. A large negative shift at the base of the Lopingian at the upper part of
section is correlated to negative shifts at the Guadalupian/Lopingian boundary associated with the
end-Guadalupian mass extinction event. All these findings indicate that a continental rift system
associated with a possible mantle plume existed during the late Early to Late Permian period. This
event was responsible for the rupturing of the northern Gondwanan margin related to the opening
of the IAE Ocean. When the deep basinal features of the Early Permian volcano-sedimentary
sequence are considered, the proto IAE oceanic crust formed possibly before the end of the Permian.
This, in turn, suggests that the opening of the IAE Ocean dates back to as early as the Permian.
KEYWORDS
biochronology, fossil assemblages, magmatic geochemistry, Permian, Tethyan evolution, Turkey
... (c) interrelations (e.g. Niu and Batiza 1997;Phipps Morgan 1999;Niu et al. 2002;Anderson 2006;Fitton 2007;Yamamoto et al. 2007). Alkali basalts on the ocean floor are mainly considered to be the result of: 1) the partial melting of relatively primordial mantle or recycled oceanic lithosphere with mantle associated with a mantle plume; 2) off-axis magmatism in a divergent tectonic setting with low melt fraction; 3) the partial melting of fertile blobs from oceanic or continental lithosphere with additional depleted upper mantle and 4) a combination of these processes (e.g. ...
A multi-disciplinary study of the KR1 segment of the Australian–Antarctic Ridge has been conducted since 2011. We present geochemical and age dating results for samples dredged from three sites on the KR1 seamount trail. The majority of the samples are alkaline ocean island basalts with subdominant enriched tholeiites. The samples from the DG05 bathymetric depression include ice-rafted erratics from Antarctica, which consist of gabbro, diabase, various granitoids, volcanic rocks such as trachyte and rhyolite and deformed or undeformed sedimentary rocks. The main provenance of glacial erratics is considered to be the Ross Sea region. However, Carboniferous to Cretaceous ages of erratics indicate that some of these may originate from the western regions of West Antarctica. Based on the size and topography of the volcanic features and geochemical characteristics of the alkaline ocean island basalts (La/SmN = 2.62–3.88; Tb/YbN = 1.54–2.67) and the enriched tholeiites, the KR1 seamount trail is interpreted to be a submarine hotspot chain that is the product of alkaline volcanic eruption and seafloor spreading.
... These trace element systematics are consistent with trends caused by dehydration of oceanic crust during subduction, which results in net loss of Cs, Rb, Ba, K, LREE, Pb, and Sr due to removal of fluid-mobile elements, but in relative enrichment of Nb and Ta due to retention of Nb and Ta in residual rutile (Ayers, 1998;Brenan et al., 1994Brenan et al., , 1995Bromiley & Redfern, 2008;Foley et al., 2000;Keppler, 1996;Klemme et al., 2002Klemme et al., , 2005Kogiso et al., 1997;McCulloch & Gamble, 1991;Schmidt et al., 2004a;Stalder et al., 1998). Thus, HIMU basalts are most likely formed by direct melting of recycled ancient subduction-modified oceanic lithosphere (e.g., Chase, 1981;Chauvel et al., 1992Chauvel et al., , 1997Fitton, 2007;Halliday et al., 1988;Hart, 1988;Hauri et al., 1994;Hauri & Hart, 1993;Hofmann, 1997Hofmann, , 2014Hofmann & White, 1982;Lassiter & Hauri, 1998;Niu & Batiza, 1997;Niu et al., 1999;Palacz & Saunders, 1986;Roy-Barman & All egre, 1995;Salters & White, 1998;Stracke et al., 2003Stracke et al., , 2005Vidal et al., 1984;White, 1985;Zindler & Hart, 1986). ...
Volatile and stable isotope data provide tests of mantle processes that give rise to mantle heterogeneity. New data on enriched mid-oceanic ridge basalts (MORB) show a diversity of enriched components. Pacific PREMA-type basalts (H2O/Ce = 215 ± 30, δDSMOW = −45 ± 5 ‰) are similar to those in the northern Atlantic (H2O/Ce = 220 ± 30; δDSMOW = −30 to −40 ‰). Basalts with EM-type signatures have regionally variable volatile compositions. Northern Atlantic EM-type basalts are wetter (H2O/Ce = 330 ± 30) and have isotopically heavier hydrogen (δDSMOW = −57 ± 5 ‰) than northern Atlantic MORB. Southern Atlantic EM-type basalts are damp (H2O/Ce = 120 ± 10) with intermediate δDSMOW (−68 ± 2 ‰), similar to δDSMOW for Pacific MORB. Northern Pacific EM-type basalts are dry (H2O/Ce = 110 ± 20) and isotopically light (δDSMOW = −94 ± 3 ‰). A multistage metasomatic and melting model accounts for the origin of the enriched components by extending the subduction factory concept down through the mantle transition zone, with slab temperature a key variable. Volatiles and their stable isotopes are decoupled from lithophile elements, reflecting primary dehydration of the slab followed by secondary rehydration, infiltration, and re-equilibration by fluids derived from dehydrating subcrustal hydrous phases (e.g., antigorite) in cooler, deeper parts of the slab. Enriched mantle sources form by addition of <1% carbonated eclogite ± sediment-derived C-O-H-Cl fluids to depleted mantle at 180–280 km (EM) or within the transition zone (PREMA).
... As shown in Figures 11D–11F, trace-element variations in the upper group basalts are also refl ective of binary mixing. The mixing curves can be either linear or hyperbolic (Niu and Batiza, 1997, and references therein) on element ratio-ratio diagrams, depending on the differences in relative incompatibility between the two elements in the numerator and denominator . The hyperbolic plots, however, constrain the nature of the mixing end members. ...
... Previous studies have shown that the upper mantle beneath spreading ridges is chemically, mineralogically and isotopically heterogeneous, consisting of more enriched , relatively fertile mantle lithologies within a more depleted, refractory matrix (e.g. Haase et al., 2011; Wendt et al., 1997; Niu and Batiza, 1997; Salters and Dick, 2002). It is possible that at the smaller degrees of mantle melting beneath thicker lithosphere at very slow-spreading ridges, the effects of more fertile mantle lithologies may dominate melt composition. ...
... The Zr/Hf fractionation in shergottites seems to be linked to depletion processes taking place in the martian mantle (a garnet effect?), although we cannot quantify the trend we obtained. The same type of relationship was recently observed for terrestrial basalts (Niu and Batiza, 1997; David et al., 2000), but its cause is unclear. Progressive melting of mantle sources with similar Zr/Hf ratios is a possible explanation, even though, as pointed out by David et al. (2000), it cannot be reconciled with any realistic batch melting model. ...
North West Africa (NWA) 480 is a new martian meteorite of 28 g found in the Moroccan Sahara in November 2000. It consists mainly of large gray pyroxene crystals (the largest grains are up to 5 mm in length) and plagioclase converted to maskelynite. Excluding the melt pocket areas, modal analyses indicate the following mineral proportions: 72 vol% pyroxenes extensively zoned, 25% maskelynite, 1% phosphates (merrillite and chlorapatite), 1% opaque oxides (ilmenite, ulvospinel and chromite) and sulfides, and 1% others such as silica and fayalite. The compositional trend of NWA 480 pyroxenes is similar to that of Queen Alexandra Range (QUE) 94201 but in NWA 480 the pyroxene cores are more Mg-rich (En(77)-En(65)). Maskelynites display a limited zoning (An(42-50)Ab(54-48)Or(2-4)). Our observations suggest that NWA 480 formed from a melt with a low nuclei density at a slow cooling rate. The texture was achieved via a single-stage cooling where pyroxenes grew continuously. A similar model was previously proposed for QUE 94201 by McSween et al. (1996). NWA 480 is an Al-poor ferroan basaltic rock and resembles Zagami or Shergotty for major elements and compatible trace element abundances. The bulk rock analysis for oxygen isotopes yields Delta(17)O = +0.42parts per thousand, a value in agreement at the high margin, with those measured on other shergottites (Clayton and Mayeda, 1996; Romanek et al., 1998; Franchi et al., 1999). Its CI-normalized rare earth element pattern is similar to those of peridotitic shergottites such as Allan Hills (ALH)A77005, suggesting that these shergottites shared a similar parent liquid, or at least the same mantle source.
... Hence, aqueous fluids derived from the subducting seafloor are charac- terized by high U/Th ratios with relatively low Th. However, during partial melting of crustal rocks, Th is slightly more incompatible than U (Beattie 1993; LaTourrette et al. 1993;Hawkesworth et al. 1997;Niu and Batiza 1997;Niu et al. 1999;Klemme et al. 2005;Elkins et al. 2008). As a consequence, the melts derived from subducting seafloor should have low U/Th ratios with relatively high Th. ...
Seafloor subduction and subduction-zone metamorphism (SZM) are understood to be the very cause of both subduction-zone magmatism and mantle compositional heterogeneity. In this article, we compile geochemical data for blueschist and eclogite facies rocks from global palaeo-subduction-zones in the literature, including those from the Chinese Western Tianshan ultrahigh pressure (UHP) metamorphic belt. We synthesize our up-to-date understanding on how chemical elements behave and their controls during subduction-zone metamorphism. Although the compositional heterogeneity of metamorphic minerals from subducted rocks has been recently reported, we emphasize that the mineral compositional heterogeneity is controlled by elemental availability during mineral growth, which is affected by the protolith composition, the inherited composition of precursor minerals, and the competition with neighbouring growing minerals. In addition, given the likely effects of varying protolith compositions and metamorphic conditions on elemental behaviours, we classify meta-mafic rocks from global palaeo-subduction-zones with varying metamorphic conditions into groups in terms of their protolith compositions (i.e. ocean island basalt (OIB)-like, enriched mid-ocean ridge basalt (MORB)-like, normal [N]-MORB-like), and discuss geochemical behaviours of chemical elements within these co-genetic groups rather than simply accepting the conclusions in the literature. We also discuss the geochemical consequences of SZM with implications for chemical geodynamics, and propose with emphasis that: (1) the traditionally accepted ‘fluid flux induced-melting’ model for arc magmatism requires revision; and (2) the residual subducted ocean crust cannot be the major source material for OIB, although it can contribute to the deep mantle compositional heterogeneity. We also highlight some important questions and problems that need further investigations, e.g. complex subduction-zone geochemical processes, different contributions of seafloor subduction and resultant subduction of continental materials, and the representativeness of studied HP–UHP metamorphic rocks.
... the MMEs dispersed in the granitoid host magmas eventually solidified such as the AKAZ pluton. We can model the Sr-Nd-Hf isotope compositions of the AKAZ pluton in terms of melting-induced source mixing (see Niu and Batiza, 1997 for concept) by using subducted basaltic ocean crust and subducted terrigenous sediments. There is no published isotope data on the ophiolite in MKS, but MKS is suggested to be related to the southern belt of the East Kunlun and extended to the A'nyemaqen suture zone (e.g., Bian et al., 2001a,b; Matte et al., 1996; Pei, 2001), where abundant ophiolites of the Paleo-Tethys Ocean regime exist (Bian et al., 2001a,b; Pei, 2001), like the N-MORB type basalt from the Zongwulong and Buqingshan ophiolite (~350 Ma) of the A'nyemaqen suture zone (Guo et al., 2007aGuo et al., ,b, 2009). ...
... Zircon U–Pb dating of a dolerite dike has yielded a concordant age of 125.7 ± 0.9 Ma; all of the Zhongba mafic rocks show OIB affinity (Dai et al., 2012), and OIB-type rocks have also been reported from the Dongbo, Purang (near the Lhanag-tso area) and Xiugugabu massifs (Xia, 1991; Bezard et al., 2011; Liu et al., 2013a Liu et al., , 2014; Yang and Dilek, 2015). Liu et al. (2014) obtained igneous ages of 141 Ma from the OIB rocks in Dongbo and 137 Ma from the E-MORB-like basalts Cheng et al., 1987); Davidson Seamount (Castillo et al., 2010); EPR (Eastern Pacific Rise) (Niu and Batiza, 1997; Niu et al., 2002); Indian ocean hotspot are from modern Indian Ocean hotspot islands of Reunion, Mauritius, Crozet, and Amsterdam (Zhang et al., 2005; Mahoney et al., 1998 Mahoney et al., , 2002 Sheth et al., 2003); Turkey alkaline basalts (Gökten and Floyd, 2007); Armenian alkaline basalt (Rolland et al., 2009); YZSZ MORB (Zhang et al., 2005; Liu et al., 2013a Liu et al., , 2013b Xu et al., 2008; Niu et al., 2006; Xu and Castillo, 2004); Purang E-MORB (Liu et al., 2014). Other data from the YZSZ are the same as in Fig. 6. occurring as interlayers in the radiolarian cherts in the Purang massif. ...
The >2000 km-long Yarlung Zangbo Suture Zone (YZSZ) in southern Tibet includes the remnants of the Mesozoic Neotethyan oceanic lithosphere, and is divided by the Zhada-Zhongba microcontinent into northern and southern branches in its western segment. Zircon U–Pb dating of a doleritic rock from the northern branch has revealed a concordant age of 160.5 ± 1.3 Ma. All of the doleritic samples from the northern branch and the pillow basalt and gabbro samples from the southern branch display consistent REE and trace element patterns similar to those of modern OIB-type rocks. The geochemical and Sr-Nd-Pb isotopic signatures of these OIB-type rocks from the western segment are identicial with those of OIB-type and alkaline rocks from other ophiolite massifs along the central and eastern segments of the YZSZ, suggesting a common mantle plume source for their melt evolution. The enriched Sr-Nd-Pb isotopic character of the gabbroic dike rocks from the southern branch points to a mantle plume source, contaminated by subducted oceanic crust or pelagic sediments. We infer that the mafic rock associations exposed along the YZSZ represent the remnants of a Neotethyan oceanic lithosphere, which was developed as part of a plume-proximal seafloor-spreading system, reminiscent of the seamount chains along-across the modern mid-ocean ridges in the Pacific Ocean.
... For example at least two different, ultradepleted compositions were analyzed from dredge SM D13 (Fig S2.6). Ultradepleted basalts erupt most commonly but not exclusively in " leaky " transform faults (Perfit et al., 1996 ) and at near-ridge seamounts (Niu and Batiza, 1997 ). At least one sample is from a propagating ridge segment (EN112 from the Juan Fernandez Microplate), and several are from normal spreading segments. ...
... These regions all underwent intense magmatic activity8. Plot of Th vs. Th/La (MORB data (Niu and Batiza, 1997)). Dagze basaltic rocks (Gao et al., 2008), ultrapotassic rocks (Zhao et al., 2009; L.S. Guo et al., 2013; Z.F. ...
The Zhunuo Cu-bearing porphyries are located in the westernmost part of the Miocene Gangdese porphyry Cu (Mo-Au) deposit belt. Zircon U-Pb dating of the diorite porphyry, K-feldspar granite porphyry, and monzonitic granite porphyry in Zhunuo yielded crystallization ages of 12.5±0.4Ma, 12.3±0.3Ma, and 12.4±0.3Ma, respectively. The diorite porphyry is characterized by low SiO2 (58.61-61.14wt.%) and Th (0.30-0.76ppm) concentrations, low Th/La (0.05-0.1) ratios, and high Mg# (>49) values coupled with low (87Sr/86Sr)i (0.703777-0.703783) and high εNd(t) (+4.07 to +4.90) values. They also have adakite-like affinities, such as low Y (10.5-12.0ppm), and high Sr/Y ratios (61-65). They were probably derived from a thickened juvenile lower continental crust. The K-feldspar granite porphyry probably originated in the middle-upper continental crust because of their high SiO2 (73.59-74.98wt.%) and Th (50.1-52.1ppm) concentrations, high Th/La (1.67-2.10), and low Sr/Y (20.2-20.7) ratios and Mg# (32-38) values, combined with high (87Sr/86Sr)i (0.710921-0.712008), low εNd(t) (-8.47 to -9.26) isotopic compositions and old Nd model ages (1.16-1.25Ga). Their magmas were most likely partial melts of the preserved ancient crust similar to the central Lhasa subterrane. The geochemical characteristics and Sr-Nd isotopic compositions of the monzonitic granite porphyry display trends that lie between those of the diorite porphyry and K-feldspar granite porphyry, and they are therefore likely to be production of hybridization between the above two melts. The ore-bearing diorite porphyry and monzonitic granite porphyry have higher zircon Ce4+/Ce3+ ratios than the ore-barren K-feldspar granite porphyry, indicating a higher oxygen fugacity in the ore-bearing magmas. We suggest that metals were released from the re-melting of arc-related cumulates which formed during lower crustal growth and thickening. This mechanism provides a reasonable explanation for the significant flare-up of mineralization during the Miocene in the Gangdese region. The lower continental crust beneath southern Lhasa subterrane probably was uniformly juvenile but the region to the west of Zhunuo was not mineralized due to input of large ancient crustal materials in the source of these ore-barren adakite-like rocks.
... For example at least two different, ultradepleted compositions were analyzed from dredge SM D13 (Fig S2.6). Ultradepleted basalts erupt most commonly but not exclusively in " leaky " transform faults (Perfit et al., 1996 ) and at near-ridge seamounts (Niu and Batiza, 1997 ). At least one sample is from a propagating ridge segment (EN112 from the Juan Fernandez Microplate), and several are from normal spreading segments. ...
In order to better determine the behavior of CO2 relative to incompatible elements, and improve the accuracy of mantle CO2 concentration and flux estimates, we determined CO2 glass and vesicle concentrations, plus trace element contents for fifty-one ultradepleted mid-ocean ridge basalt (MORB) glasses from the global mid-ocean ridge system. Sixteen contained no vesicles and were volatile undersaturated for their depth of eruption. Thirty-five contained vesicles and were slightly oversaturated, and so may not have retained all of their CO2. If this latter group lost some bubbles during emplacement, then CO2/Ba calculated for the undersaturated group alone is the most reliable and uniform ratio at 98 ± 10, and CO2/Nb is 283 ± 32. If the oversaturated MORBs did not lose bubbles, then CO2/Nb is the most uniform ratio within the entire suite of ultradepleted MORBs at 291 ± 132, while CO2/Ba decreases with increasing incompatible element enrichment.
... Un certain nombre d'études ont porté dans les années 1980 et 1990 sur la géochimie des MORB de la partie nord de l'EPR située entre Clipperton FZ (10˚N) et Orozco FZ (15˚N), et du nord de la zone de fracture de Rivera (21˚N) (Hekinian et Walker, 1987). Alors que certains auteurs ont tenté de relier géochimie des éléments majeurs/traces et morphologie de la dorsale (e.g Hekinian et Fouquet, 1985 ;Hekinian et al., 1989), d'autres se sont plutôt focalisés sur les compositions isotopiques et le traçage des sources mantelliques (e.g Hamelin et al., 1984 ;White et al., 1987 ;Ito et al., 1987 ;Prinzhofer et al., 1989 ;Reynolds, 1992 ;Niu et Batiza, 1997), mais il existe très peu d'études qui ont essayé de corréler cet ensembl à l'échelle adéquate l'ensemble de ces données. Ces travaux ont néanmoins permis de montrer que ces basaltes de la partie nord recouvrent la moitié de la gamme de composition géochimique déjà rencontrée dans les MORB de l'EPR. ...
120 Mid-Ocean-Ridge basaltic (MORB) glasses were collected on discrete lava flow (-200m sampling interval) during submersible dives along the East-Pacific-Rise (EPR), between 15°37’N and 14°47’N, precisely where the ridge intersects the Mathematicians hotspot track. The data display a geochemical variability that has never been observed along a ridge at such a small spatial scale. The range of isotopic compositions along this 15 km segment is commensurable to that of the entire EPR. It can be accounted or by a mixture of three main components, representative of the hotspot heterogeneity. The dense sampling, along and across the ridge segment, matches the resolution of the micro-bathymetric data, which made the spatio temporal reconstruction of the geochemical and morphological evolution of the EPR/Mathematician hotspot system possible. The latest starts 600 kya with the segment inflation and a global change in the ambient mantle composition, followed by two successive jumps of the ridge axis (250 and 150 kya) towards the seamounts chain. During this phase as the two systems are getting closer, new geochemical signatures emerge in MORB. Among them, relics of regional depleted mantle, small enriched local heterogeneities, and two types of heterogeneities belonging to the hotspot source. The last one become apparent only during the last 100 years around 15°44’N, and constitutes a novel geochemical signature for MORB. This new component’s most noticeable property is its very unradiogenic Pb (“Unradiogenic Lead Component”, ULC) associated with mostly enriched Sr, Nd and Hf isotopic signatures. Putted together, major, trace elements and isotopes (Sr, Nd, Hf, Pb and He) suggest an ancient (>2Ga) lower continental metagabbroic origin for this material, while the involvement of sulfides is considered in order to explain the unradiogenic lead compositions. Overall, the preferred model for the formation of ULC is the recycling within the upper mantle of sulfide bearing pyroxenites coming from continental arc roots. ULC-influenced basalts represent magmatic witnesses of the melting of this cryptic reservoir that can contribute to solve the Pb paradox.
... To account for this observation, the authors suggested that this enrichment is likely related to smaller degrees of partial melting and/or greater extends of fractional crystallization due to smaller spreading rates, even though the presence of prominent recycled source component or variable proportion of pyroxenite in the Atlantic mantle source is also envisaged. Nevertheless, for the most enriched MORB, it is now commonly accepted that the genesis of E-MORB does not only occur because of smaller degrees of partial melting but requires an enriched source contribution [Dosso et al., 1991[Dosso et al., , 1993 Michael et al., 1994; Niu and Batiza, 1997; Donnelly et al., 2004; Hémond et al., 2006; Nauret et al., 2006] . Recent studies on MAR basalts propose that Atlantic MORB heterogeneities have two origins: (1) a long-wavelength character revealing the influence of hot spot mantle (e.g., Iceland), ...
[1] This paper deals with the origin of enriched MORB independent from any hot spot activity. Indeed, MORB enrichment was readily attributed to a ridge/hot spot interaction and in absence of identified neighboring hot spot, to more questionable processes (e.g., incipient plume or plume activity residue). More recently, the existence of enriched MORB away from any identifiable hot spot was attributed to different origins (i.e., recycled oceanic crust and/or enriched mantle after subduction metasomatism). Within this frame, we present here a new set of geochemical analyses of major and trace elements and Sr, Nd and Pb isotopes on samples collected by submersible on both intersections of the 15 20′N fracture zone with the spreading axis of the Mid-Atlantic Ridge. This area is characterized by an increasing enrichment of the lava compositions from north to south through the fracture zone. Results show that the geochemical enrichment observed with a different intensity on both sides of the fracture zone is linked to the 14 N topo-graphic and geochemical anomaly. Our modeling shows that both trace element and isotopic compositions are consistent with a binary mixing between the regional depleted MORB mantle source and a recycled OIB/seamount, as previously proposed to explain the observed enrichment at 14 N. This model can also account for other enriched MORB i.e., the 18 –20 S region of the Central Indian Ridge, illustrating that it does not represent an isolated and local process. On the basis of our results and on the DMM isotopic evolution, the age of the recycled OIB/seamount is estimated to be $250 Ma, suggesting a recycling within the upper mantle. Considering the huge number of ocean islands and seamounts upon the ocean floor, their recycling into the upper mantle is a plausible process to produce enriched MORB.
... 9. (a) Th/La, (b) Ba/La, and (c) Zr/Y vs Sm/La for melt inclusions , submarine glasses from Pagan and NW Rota-1 volcanoes, and Mariana Trough submarine glasses. The grey field is Pacific MORB data from Niu & Batiza (1997) , shown for comparison. Data for discrete sedimentary materials from the Pacific plate from ODP sites 800 and 801 are shown as small black and white stars, respectively, and their calculated bulk compositions are shown as large black and white stars (Plank & Langmuir, 1998). ...
Arc basalts are more oxidized than mid-ocean ridge basalts, but it is unclear whether this difference is due to differentiation
processes in the Earth’s crust or to a fundamental difference in the oxygen fugacity of their mantle sources. Distinguishing
between these two hypotheses is important for understanding redox-sensitive processes related to arc magmatism, and thus more
broadly how Earth materials cycle globally. We present major, volatile, and trace element concentrations in combination with
Fe3+/∑Fe ratios determined in olivine-hosted glass inclusions and submarine glasses from five Mariana arc volcanoes and two regions
of the Mariana Trough. For single eruptions, Fe3+/∑Fe ratios vary along liquid lines of descent that are either slightly oxidizing (olivine + clinopyroxene + plagioclase fractionation,
CO2 ± H2O degassing) or reducing (olivine + clinopyroxene + plagioclase ± magnetite fractionation, CO2 + H2O + S degassing). Mariana samples are consistent with a global relationship between calc-alkaline affinity and both magmatic
H2O and magmatic oxygen fugacity, where wetter, higher oxygen fugacity magmas display greater affinity for calc-alkaline differentiation.
We find, however, that low-pressure differentiation cannot explain the majority of variations observed in Fe3+/∑Fe ratios for Mariana arc basalts, requiring primary differences in magmatic oxygen fugacity. Calculated oxygen fugacities
of primary mantle melts at the pressures and temperatures of melt segregation are significantly oxidized relative to mid-ocean
ridge basalts (∼QFM, where QFM is quartz–fayalite–magnetite buffer), ranging from QFM + 1·0 to QFM + 1·6 for Mariana arc basalts,
whereas back-arc related samples record primary oxygen fugacities that range from QFM + 0·1 to QFM + 0·5. This Mariana arc
sample suite includes a diversity of subduction influences, from lesser influence of a homogeneous H2O-rich component in the back-arc, to sediment melt- and fluid-dominated influences along the arc. Primary melt oxygen fugacity
does not correlate significantly with sediment melt contributions (e.g. Th/La), nor can it be attributed to previous melt
extraction in the back-arc. Primary melt oxygen fugacity correlates strongly with indices of slab fluids (e.g. Ba/La) from
the Mariana Trough through the Mariana arc, increasing by 1·5 orders of magnitude as Ba/La increases by a factor of 10 relative
to mid-ocean ridge basalts. These results suggest that contributions from the slab to the mantle wedge may be responsible
for the elevated oxygen fugacity recorded by Mariana arc basalts and that slab fluids are potentially very oxidized.
... the MMEs dispersed in the granitoid host magmas eventually solidified such as the AKAZ pluton. We can model the Sr-Nd-Hf isotope compositions of the AKAZ pluton in terms of melting-induced source mixing (see Niu and Batiza, 1997 for concept) by using subducted basaltic ocean crust and subducted terrigenous sediments. There is no published isotope data on the ophiolite in MKS, but MKS is suggested to be related to the southern belt of the East Kunlun and extended to the A'nyemaqen suture zone (e.g., Bian et al., 2001a,b; Matte et al., 1996; Pei, 2001), where abundant ophiolites of the Paleo-Tethys Ocean regime exist (Bian et al., 2001a,b; Pei, 2001), like the N-MORB type basalt from the Zongwulong and Buqingshan ophiolite (~350 Ma) of the A'nyemaqen suture zone (Guo et al., 2007aGuo et al., ,b, 2009). ...
The West Kunlun orogenic belt (WKOB) at the northwest margin of the Greater Tibetan Plateau records seafloor subduction, ocean basin closing and continental collision with abundant syncollisional granitoids in response to the evolution of the Proto- and Paleo-Tethys Oceans from the early-Paleozoic to the Triassic. Here we present a combined study of detailed zircon U-Pb geochronology, whole-rock major and trace elements and Sr-Nd-Hf isotopic geochemistry on the syncollisional Arkarz (AKAZ) pluton with mafic magmatic enclaves (MMEs) exposed north of the Mazha-Kangxiwa suture (MKS) zone. The granitoid host rocks and MMEs of the AKAZ pluton give the same late Triassic age of ~ 225 Ma. The granitoid host rocks are metaluminous granodiorite and monzogranite. They have initial 87Sr/86Sr of 0.70818 to 0.70930, εNd(225 Ma) = -4.61 to -3.91 and εHf(225 Ma) = -3.01 to 0.74. The MMEs are more mafic than the host with varying SiO2 (51.00-63.24 wt.%) and relatively low K2O (1.24-3.02 wt.%), but have similar Sr-Nd-Hf isotope compositions to the host ((87Sr/86Sr)i = 0.70830-0.70955, εNd(225 Ma) = -4.88 to -4.29, εHf(225 Ma) = -2.57 to 0.25). Both the host and MMEs have rare earth element (REE) and trace element patterns resembling those of bulk continental crust (BCC). The MMEs most likely represent cumulate formed from common magmas parental to the granitoid host. The granitoid magmatism is best explained as resulting from melting of amphibolite of MORB protolith during continental collision, which produces andesitic melts with a remarkable compositional similarity to the BCC and the inherited mantle-like isotopic compositions. Simple isotopic mixing calculations suggest that ~ 80% ocean crust and ~ 20% continental materials contribute to the source of the AKAZ pluton. Thus, the hypothesis “continental collision zones as primary sites for net continental crust growth” is applicable in the WKOB as shown by studies in southern Tibet, East Kunlun and Qilian orogens. In addition, we also propose a new view for the tectonic evolution of the Paleo-Tethys Ocean in geological regions recorded and represented by the MKS.
... Such mantle sampling leads to smoothing of its geochemical heterogeneity (Rubin et al., 2009). In contrast, off-axis magmas are formed far enough from the axis to bypass the crustal chambers system beneath the ridge and record larger mantle heterogeneity (Batiza and Vanko, 1984; Niu and Batiza, 1997; Niu et al., 2002; Brandl et al., 2012). Among the 38 PAR studied samples, 30 have been dredged at the ridge axis, the 8 others from off-axis volcanic seamounts located 10 to 300 km away from the ridge (Fig. 1 and table 1). ...
To better address how Mid-Ocean Ridge Basalt (MORB) sulfur isotope composition can be modified by assimilation and/or by immiscible sulfide fractionation, we report sulfur (S), chlorine (Cl) and copper (Cu) abundances together with multiple sulfur isotope composition for 38 fresh basaltic glasses collected on the Pacific-Antarctic ridge. All the studied glasses - with the exception of 8 off-axis samples - exhibit relatively high Cl/K, as the result of pervasive Cl-rich fluid assimilation. This sample set hence offers an opportunity to document both the upper mantle S isotope composition and the effect of hydrothermal fluids assimilation on the S isotope composition of erupted basalts along segments that are devoid of plume influence.
... As several studies concluded that the Oman ophiolite was formed at a fast spreading ridge (e.g., Boudier and Nicolas, 1985; MacLeod and Rothery, 1992; Tilton et al., 1981), and the only near-ridge-off-axis magmatic activity reported so far in this environment seems to be the production of seamounts, the Mansah diapir is an obvious candidate for the root of a type of seamount. Bathymetric (e.g., Sheirer and Macdonald, 1995; Shen et al., 1995; Smith and Jordan, 1988) and geochemical studies (e.g., Fornari et al., 1988; Batiza et al., 1990; Niu et al., 1996; Niu and Batiza, 1997) in the Pacific ocean report a variety of seamounts that may have had different origins. Some of the seamounts may arise from tensional cracks in the oceanic plate (Winterer and Sandwell, 1987 ), some may be associated with hot spot volcanism (Desonie and Duncan, 1990 ) or have other types of deep and wide plumelike sources (Shen et al., 1995). ...
Large scale structural mapping of the Oman ophiolite indicates that the Mansah area (Sumail massif) was a ridge off-axis region at the time the ophiolite was detached. This paper presents a detailed structural mapping of the region. We show that, as opposed to other off-axis areas, it contains plunging lineations, correlated with a thick Moho transition zone and chromite pods, indicative of a mantle diapir. However this diapir has a discontinuous structure, it is bounded by shear zones and types of diabases that are not found elsewhere in Oman; it also has a broken crust, strongly affected by hydrothermal alteration. This suggests that Mansah fossilized an off-axis diapir intruding a cooling lithosphere. It may be a good candidate to be the root of an off-axis seamount such as those found in the East Pacific, and may bring new insights into this particular volcanism which we are only beginning to explore.
We conducted a systematic geochemical study of the Mt. Changbai basalts (MCBs) in NE China to better constrain their mantle sources and petrogenesis. The MCBs display ocean island basalt-like trace element patterns (e.g., positive Nb–Ta anomalies) and variable MgO contents (3.2–10.0 wt.%), trace element ratios (e.g., Ba/Nb and Rb/Nb), and Sr–Nd–Pb–Hf isotopic ratios (87Sr/86Sr(i) = 0.704497–0.705629; εNd(i) = –2.8 to +3.3; 206Pb/204Pb(i) = 16.9–18.1; εHf = −1.2 to +6.2), which indicate derivation from a heterogeneous mantle source. The least evolved samples (MgO > 7 wt.%), with moderate Fe/Mn and FC3MS (defined as FeOT/CaO – 3MgO/SiO2) values, require a hybrid pyroxenite–peridotite mantle source. The isotopic compositions indicate the mantle source is a mixture between depleted mid-ocean ridge basalt mantle and two enriched mantle components. Our geochemical and published geophysical data indicate that these two enriched components are recycled ancient sediments and Pacific oceanic crust, which were derived from the mantle transition zone. An upwelling wet plume from the mantle transition zone transported these two enriched components into the asthenospheric mantle. These enriched components were then transformed into pyroxenite and/or eclogite through melt–rock reaction or mechanical mixing with asthenospheric mantle peridotite. We further propose that partial melting of this heterogeneous asthenospheric mantle containing fusible pyroxenite and/or eclogite (i.e., the enriched components) and refractory peridotite produced the MCBs.
Based on the analysis of satellite altimetry and deep-water geological and geophysical surveys, it was established that there is a close connection between underwater volcanoes and tectonics on the crest of the East Pacific Rise, on its western flank and in the deep-water basin. Underwater volcanoes are characterized by different sizes; their basalts are characterized by variable degree of differentiation. The aggregated results denote the existence and relevance of studying the major problem of the volcanic-tectonic activity of the oceanic lithosphere at different stages of its evolution.
Late Cretaceous (ca. 100−80 Ma) magmatism in southern Lhasa subterrane records critical geological events, which can provide important insights into the regional tectonic evolution and geodynamic process of South Tibet. This study presents new zircon U-Pb ages, whole-rock geochemistry and Sr-Nd-Fe and zircon U-Pb-O isotopic data for two dioritic plutons in the southern Lhasa subterrane. Secondary ion mass spectrometry U-Pb dating on magmatic zircons from these rocks yielded a consistent age at ca. 90 Ma. The rocks exhibit variable SiO2 contents (52−59 wt%), high Fe2O3T contents (7.1−10.0 wt%), and low K2O/Na2O ratios (0.18−0.48). Most samples have high Al2O3 (17.0−19.5 wt%) and Sr (493−678 ppm), but low Yb (0.9−2.4 ppm) and Y (9−25.2 ppm) concentrations, and thus high Sr/Y (23−74) ratios, typical of adakite-like geochemical features. The adakitic rocks have relatively uniform initial 87Sr/86Sr isotopic ratios (0.7043−0.7046) and εNd(t) values (+3.67 to +4.16), indicating derivation from similar parental magmas. The δ56Fe values of whole-rock samples vary from 0.011 to 0.091‰ with an average of 0.045 ± 0.046‰ (two standard deviations), reflecting a homogeneous Fe isotopic composition, which is associated with melt-mantle interaction. In addition, the rocks are characterized by relatively high zircon δ18O values of 5.72−7.19‰, indicating the involvement of an 18O-enriched component during magma formation. The calculation of Al-in-hornblende barometer indicates that the adakitic rocks were emplaced at pressures of 6.4−9.8 kbar. Therefore, it is proposed that the adakitic rocks were most likely generated by partial melting of mantle wedge that had been previously modified by slab-melts at a relatively shallow depth, followed by minor fractional crystallization of hornblende. Taking into account previously published data in the southern Lhasa subterrane, we suggest that the ca. 90 Ma magmatism could be related to a period of Neo-Tethyan oceanic slab roll-back, which can provide new insights into the revolution process of the Neo-Tethyan ocean realm and the accretion of the Himalaya-Tibetan Plateau.
The genesis of high-K calc-alkaline rocks are important in evaluating subduction-related magmatism particularly with regard to melts derived from recycled subducted sediments that require unusually steep geothermal gradients. Here we present high precision LA-ICP-MS zircon U-Pb, bulk-rock geochemical, Sr and Nd isotopic, mineral chemical, and in-situ zircon Hf isotope data for high-K calc-alkaline granodiorites and associated enclaves in the SE Lhasa Block, Tibet. The LA-ICP-MS zircon U-Pb data show that the granodiorites were emplaced at ca. 124.1-123.8 Ma and the associated enclaves formed at ca. 132.7-129.6 Ma. The granodiorites are high-K, calc-alkaline and slightly peraluminous with A/CNK values in the range of 0.96-1.11. The high Th/La and La/Sm ratios, Th and Nb content, and low Nb/Ta, Ba/Th, U/Th, and Zr/Nb ratios are similar to those in rocks formed from sediment-derived melts. The uniformly high initial 87Sr/86Sr values (0.710277-0.712581), low εNd(t) (-8.3 to -11.2), and negative and variable zircon εHf(t) values (-11.71 to -0.96) imply continental crustal sources. The enclaves are basaltic to andesitic and have high MgO (3.82–5.48wt.%), and their zircon Hf isotopic compositions and presence of magnesian-hornblende and plagioclase with labradorite-bytownite composition indicate a possible mantle connection. In conjunction with regional geological features, we infer that these rocks were derived from melting of recycled subducted sedimentary materials in the mantle wedge, during the southward subduction of Bangong-Nujiang Tethyan ocean. Our new data, together with those from the Bangong-Nujiang suture zone and its flanks suggest the commencement of magmatic activity associated with the subduction of Bangong-Nujiang Tethyan oceanic lithosphere from Late Jurassic to Early Cretaceous.
We analysed whole-rock major and trace elements and Sr-Nd-Pb-Hf isotopes of the late Cenozoic volcanic rocks in the Leizhou Peninsula, South China to investigate their mantle source characteristics. These volcanic rocks, collected from Jiujiang, Tianyang and Huoju areas of the Leizhou Peninsula, are characterized by incompatible element enrichment but variable isotopic depletion. The volcanic rocks from Jiujiang and Tianyang show prominent primitive-mantle-normalized positive Nb, Ta and Sr anomalies and depleted Sr-Nd-Pb-Hf isotope compositions, whereas those from Huoju show slight positive to negative Nb and Ta anomalies, a prominent positive Pb anomaly, and more enriched Sr-Nd-Pb-Hf isotope compositions. Two types of mantle metasomatism are required to explain the geochemical characteristics of these rocks. The Jiujiang and Tianyang samples were largely derived from a mantle source metasomatized recently by a low-F melt. Such low-F melt is generated within the asthenospheric mantle, which is enriched in volatiles and incompatible elements with positive Sr anomaly and depleted Sr-Nd-Pb-Hf isotope compositions. The Huoju samples were largely derived from a mantle source metasomatized by recycled upper continental crust material. These two types of mantle metasomatism beneath the Leizhou Peninsula are consistent with trace element characteristics of mantle mineralogy (e.g. clinopyroxene vs. amphibole), which reflects source evolution in space and time (e.g. tectonic setting change).
The East Kunlun Orogenic Belt (EKOB) in northern Tibet provides an important record of the amalgamation of the Wanbaogou oceanic basalt plateau and the Qaidam Block. Here we report geochemical, geochronological, and Hf isotopic data for newly identified late Silurian–Early Devonian mafic–ultramafic igneous complexes from the EKOB at the northern margin of the Tibetan Plateau. These complexes are dominantly composed of gabbro and pyroxenite rocks. Three complexes yield zircon U–Pb ages of 398.8 ± 1.8, 420.2 ± 1.2, and 413.4 ± 0.78 Ma. The εHf(t) values of zircons range from +0.8 to +3.3 with TDM1 ages of 897 to 998 Ma. Modelling of the geochemical data indicates that these igneous complexes have a hybrid origin, involving depleted mantle fluids derived from a previous subduction event and crustal materials. The geochemical and geochronological data suggest that these complexes formed in a post-collisional setting linked to break-off of a subducted oceanic slab, which occurred after the Wanbaogou oceanic basalt plateau amalgamated with the Qaidam Block in the late Silurian–Early Devonian.
We present major and trace element as well as Sr, Nd, and Hf isotope data on a suite of 87 plutonic rock samples from 27 felsic crustal intrusions in seven blocks of the Oman ophiolite. The rock compositions of the sample suite including associated more mafic rocks range from 48 to 79 wt% SiO2, i.e. from gabbros to tonalites. The samples are grouped into a Ti-rich and relatively light rare earth element (LREE)-enriched P1 group [(Ce/Yb)N
> 0.7] resembling the early V1 lavas, and a Ti-poor and LREE-depleted P2 group [(Ce/Yb)N
< 0.7] resembling the late-stage V2 lavas. Based on the geochemical differences and in agreement with previous structural and petrographic models, we define phase 1 (P1) and phase 2 (P2) plutonic rocks. Felsic magmas in both groups formed by extensive fractional crystallization of olivine, clinopyroxene, plagioclase, apatite, and Ti-magnetite from mafic melts. The incompatible element compositions of P1 rocks overlap with those from mid-ocean ridges but have higher Ba/Nb and Th/Nb trending towards the P2 rock compositions and indicating an influence of a subducting slab. The P2 rocks formed from a more depleted mantle source but show a more pronounced slab signature. These rocks also occur in the southern blocks (with the exception of the Tayin block) of the Oman ophiolite implying that the entire ophiolite formed above a subducting slab. Initial Nd and Hf isotope compositions suggest an Indian-MORB-type mantle source for the Oman ophiolite magmas. Isotope compositions and high Th/Nb in some P2 rocks indicate mixing of a melt from subducted sediment into this mantle.
Phanerozoic growth of continental crust has widely been considered as an important geological phenomenon and mainly occurs in an arc setting. However, the crustal growth models (mantle-derived basalt underplating or accretion of island or intra-oceanic arc complexes or oceanic plateau) have been disputed. Here we present new zircon LA-ICPMS U-Pb age, whole-rock major and trace element, Sr-Nd and zircon Hf isotopic data for Late Mesozoic intermediate-felsic intrusive rocks in the Rena Co area in southern Qiangtang, central Tibet. LA-ICP-MS zircon U-Pb dating for two granodiorite and three diorite samples and one granodiorite porphyry sample gives ages of ca. 150 Ma, ca. 112 Ma, respectively, indicating they were generated in the Late Jurassic-Early Cretaceous. All rocks are sub-alkaline in composition and belong to the high-cal-alkaline series. The ~150 Ma diorites (SiO2 = 57.9-61.2 wt.%) exhibit relatively high MgO (3.13-3.88 wt.%) and Cr (52.4-282 ppm) contents and Mg# (47-51) values, similar to magnesian diorites. They are geochemically characterized by uniformly low εNd(t) (-5.5 to -5.2), high (87Sr/86Sr)i (0.7071 to 0.7078) and Th/La (0.22-0.32), and variable zircon εHf(t) (-8.7 to +4.8) values. They were probably generated by melting of oceanic sediment diapirs, followed by interaction with the surrounding mantle during the northward subduction of Bangong-Nujiang Oceanic lithosphere. The ~150 Ma granodiorites and ~112 Ma granodiorite porphyries are characterized by low MgO (<3 wt.%) contents and Mg# (<45) values, high Al2O3 (>15% wt.%) and Sr (>400 ppm) and low Y (<18 ppm) and Yb (<1.9 ppm) contents, and high Sr/Y and La/Yb ratios, which are similar to those of typical adakites. The granodiorites have low εNd(t) (-7.6 to -3.7) and zircon εHf(t) (-9.8 to +0.2) and high (87Sr/86Sr)i (0.7069 to 0.7086) values, and were likely produced by partial melting of a thickened and heterogeneous ancient lower continental crust. The relatively depleted isotope compositions [(87Sr/86Sr)i = 0.7054-0.7065; εNd(t) = -0.61 to +0.25; zircon εHf(t) = +4.7 to +9.7] of the granodiorite porphyries indicate that they were most probably generated by partial melting of newly underplated and thickened basaltic lower crust. Taking into account ophiolites in the Bangong-Nujiang Suture and Late Mesozoic magmatic rocks in the southern Qiangtang sub-block, we suggest that this area was located in a continental arc setting. Moreover, from the Late Jurassic to Early Cretaceous, the ancient lower crust in the southern Qiangtang sub-block was gradually replaced by mantle-derived juvenile materials. The crustal evolution indicates that, in a continental arc, basaltic magma underplating plays a key role in vertical crustal growth.
The Xing'an-Inner Mongolia accretionary belt (XIMAB) in the southeastern segment of the Central Asian Orogenic Belt (CAOB) was produced by the long-lived subduction and eventual closure of the Paleo-Asian Ocean, and by the convergence between the North China Craton and the Mongolian micro-continent. Two ophiolite belts have been recognized: the northern Erenhot-Hegenshan-Xi-Ujimqin ophiolite belt and the southern Solonker-Linxi ophiolite belt. Most basalts in the northern ophiolite belt exhibit characteristics of N-type to E-type MORB affinities with depleted Nd isotopic composition (εNd(t) > +5), comparable to modern Eastern Pacific mid-ocean ridge basalts. Most basaltic rocks in the southern belt show clear geochemical features of supra-subduction-zone type (SSZ-type) oceanic crust, probably formed in an arc/back-arc environment. The inferred back-arc extension along the Solonker-Linxi belt started at ca. 280 Ma. Statistics of all the available age data for the ophiolites indicates two cycles of seafloor spreading/subduction, which gave rise to two main epochs of magmatic activity at 500-410 Ma and 360-220 Ma, respectively, with a gap of ~50 million years (Myrs). The spatial and temporal distribution of the ophiolites and concurrent igneous rocks favor bilateral subduction towards the two continental margins in the convergence history, with final collision at ~230-220 Ma. In the whole belt, signals of continental collision and Himalayan-style mountain building are lacking. We thus conclude that the Xing'an-Inner Mongolia segment of the CAOB experienced two cycles of seafloor subduction, back-arc extension and final “Appalachian-type” soft collision.
The West Kunlun orogenic belt (WKOB) at the northwest margin of the Greater Tibetan Plateau records seafloor subduction, ocean basin closing and continental collision with abundant syncollisional granitoids in response to the evolution of the Proto- and Paleo-Tethys Oceans from the early-Paleozoic to the Triassic. Here we present a combined study of detailed zircon U-Pb geochronology, whole-rock major and trace elements and Sr-Nd-Hf isotopic geochemistry on the syncollisional Arkarz (AKAZ) pluton with mafic magmatic enclaves (MMEs) exposed north of the Mazha-Kangxiwa suture (MKS) zone. The granitoid host rocks and MMEs of the AKAZ pluton give the same late Triassic age of ~225 Ma. The granitoid host rocks are metaluminous granodiorite and monzogranite. They have initial 87Sr/86Sr of 0.70818 to 0.70930, εNd(225 Ma) = −4.61 to −3.91 and εHf(225 Ma) = −3.01 to 0.74. The MMEs are more mafic than the host with varying SiO2 (51.00–63.24 wt.%) and relatively low K2O (1.24–3.02 wt.%), but have similar Sr-Nd-Hf isotope compositions to the host ((87Sr/86Sr)i = 0.70830–0.70955, εNd(225 Ma) = −4.88 to −4.29, εHf(225 Ma) = −2.57 to 0.25). Both the host and MMEs have rare earth element (REE) and trace element patterns resembling those of bulk continental crust (BCC). The MMEs most likely represent cumulate formed from common magmas parental to the granitoid host. The granitoid magmatism is best explained as resulting from melting of amphibolite of MORB protolith during continental collision, which produces andesitic melts with a remarkable compositional similarity to the BCC and the inherited mantle-like isotopic compositions. Simple isotopic mixing calculations suggest that ~80% ocean crust and ~20% continental materials contribute to the source of the AKAZ pluton. Thus, the hypothesis “continental collision zones as primary sites for net continental crust growth” is applicable in the WKOB as shown by studies in southern Tibet, East Kunlun and Qilian orogens. In addition, we also propose a new view for the tectonic evolution of the Paleo-Tethys Ocean in geological regions recorded and represented by the MKS.
A fundamental question in the formation of orogenic andesites is whether their high melt SiO 2 reflects the recycling of silicic melts from the subducted slab or the processing of basaltic mantle melts in the overlying crust. The latter model is widely favoured, because most arc magmas lack the ‘garnet’ signature of partial slab melts. Here we present new trace element data from Holocene high-Mg# >64–72 calc-alkaline basalts to andesites (50–62 wt% SiO 2 ) from the central Mexican Volcanic Belt that crystallize high-Ni olivines with the high ³ He/ ⁴ He=7–8 of the upper mantle. These magmas have been proposed to be partial melts from ‘reaction pyroxenites’, which formed by hybridization of mantle peridotite ( c. 82–85%) and heavy rare earth element-depleted silicic slab melt (>15–18%). Forward and inverse models suggest that the absence of a garnet signature in these melts reflects the efficient buffering of the heavy rare earth elements (Ho to Lu) in the subarc mantle. In contrast, all elements more incompatible than Ho – excepting TiO 2 – are more or less strongly controlled by the silicic slab flux that also directly contributes to the silicic arc magma formation. Our study emphasizes the strong link between slab recycling and the genesis of orogenic andesites.
Supplementary material
Methods, additional data and modelling parameters are available at http://www.geolsoc.org.uk/SUP18686
Ultramafic–intermediate rocks exposed on the South Island of the Percy Isles have been previously
grouped into the ophiolitic Marlborough terrane of the northern New England Fold Belt. However,
petrological, geochemical and geochronological data all suggest a different origin for the South
Island rocks and a new terrane, the South Island terrane, is proposed. The South Island terrane rocks
differ from ultramafic–mafic rocks of the Marlborough terrane not only in lithological association, but
also in geochemical features and age. These data demonstrate that the South Island terrane is genetically
unrelated to the Marlborough terrane but developed in a supra-subduction zone environment
probably associated with an Early Permian oceanic arc. There is, however, a correlation between
the South Island terrane rocks and intrusive units of the Marlborough ophiolite. This indicates that the
two terranes were in relative proximity to one another during Early Permian times. A K/Ar age of
277 6 7 Ma on a cumulative amphibole-rich diorite from the South Island terrane suggests possible
affinities with the Gympie and Berserker terranes of the northern New England Fold Belt.
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