Rationale
Zinc isotopes are becoming increasingly applicable in high‐temperature geochemistry, for example in crust–mantle interaction and volatilization‐related processes. The published zinc isotope ...data for some commonly used reference materials, however, show large interlaboratory offsets. In addition, there is still limited data for zinc isotope compositions of many widely used geological reference materials.
Methods
For precise and accurate zinc isotopic ratio analysis of chemically diverse geostandards, including ultramafic to felsic igneous rocks, carbonatites, sediments and soils, an improved procedure for chemical purification of zinc was introduced in this study. The factors potentially affecting zinc isotopic ratio measurement were assessed. The accuracy and long‐term reproducibility were obtained by measurements on both synthetic solutions and well‐characterized geostandards.
Results
Purification of geologic samples with different zinc concentrations and matrix compositions yields consistent elution curves with nearly 100% recovery. Acidity and concentration mismatches and the presence of some matrix elements (e.g., Mg, Ti and Cr) have significant impacts on the precision and accuracy of zinc isotopic ratio measurement. The zinc isotope compositions of a suite of reference materials were measured using this method.
Conclusions
The present study describes methods for the chemical purification of zinc and high‐precision and accurate zinc isotopic ratio measurements using multicollector inductively coupled plasma mass spectrometer (MC‐ICP‐MS). The long‐term external reproducibility for δ66Zn values is ±0.04‰ (2SD). High‐quality zinc isotope data of chemically different geostandards were reported to stimulate future interlaboratory calibrations.
Rationale
Laser ablation multicollector inductively coupled plasma mass spectrometry (LA‐MC‐ICP‐MS) has become a powerful technique for in situ Cu isotopic analysis in natural geological samples. Cu ...isotopic compositions in natural chalcopyrites have been used to reveal aspects of the mineralization processes directly. However, internationally or commercially available matrix‐matched chalcopyrite reference materials for mass fractionation correction or quality control purposes are still lacking for in situ Cu isotopic analysis using LA‐MC‐ICP‐MS.
Methods
Three natural chalcopyrites 14ZJ12‐1, JGZ‐29 and JGZ‐78, and one copper metal GBW02141 with different Cu isotopic compositions were investigated as potential microanalytical reference materials by LA‐MC‐ICP‐MS. Ga element was used as an internal standard to correct the mass fractionation of Cu isotopes during LA‐MC‐ICP‐MS analysis.
Results
A large number of Cu isotope ratio measurements using femtosecond LA‐MC‐ICP‐MS were conducted and produced good intermediate precision of δ65CuNIST976 (0.07–0.08‰, 2 standard deviations), demonstrating the homogeneous Cu isotopic distribution in the recommended samples. The mean δ65CuNIST976 values of −0.21 ± 0.04‰, 0.46 ± 0.04‰, −0.06 ± 0.04‰ and 0.11 ± 0.05‰ (2 standard deviations) in 14ZJ12‐1, JGZ‐29, JGZ‐78 and GBW02141, respectively, were obtained using solution‐MC‐ICP‐MS in the four recommended samples.
Conclusions
Here, we describe three natural chalcopyrites 14ZJ12‐1, JGZ‐29, JGZ‐78, and one copper metal GBW02141 as the potential Cu isotopic reference materials for LA‐MC‐ICP‐MS analysis. Our analyses demonstrate that these recommended materials have a high degree of elemental and isotopic homogeneity, indicating that they are suitable for microanalysis techniques for data quality assurance or interlaboratory calibration.
Adakites are commonly associated with porphyry Cu–Au ore deposits worldwide. Two groups of early Cretaceous adakites occur widely in central-eastern China but their association with mineralization ...contrasts sharply: adakites from the Lower Yangtze River Belt (LYRB) host one of the largest porphyry Cu–Au deposit belts in China, whereas those from the South Tan-Lu Fault (STLF), which is adjacent to the LYRB, are all ore-barren. These adakites, thus, provide a rare opportunity to explore the main factor that controls the genetic links between adakites and Cu–Au mineralization. Here we report new chronological, elemental and Sr–Nd–Pb isotopic data and present a comprehensive geochemical comparison for these two groups of adakites. At a given SiO
2, the STLF adakites show lower Al
2O
3 and higher K
2O, K
2O/Na
2O, MgO, Cr, Ni and Mg# than the LYRB adakites. These systematic differences may indicate a dry basaltic source for the STLF adakites and a water-enriched basaltic source for the LYRB adakites. The STLF adakites have high Sr/Y and (La/Yb)
N, which are positively correlated, and low Sr/La and Ce/Pb, while the LYRB adakites show lower (La/Yb)
N but higher Sr/Y, Sr/La and Ce/Pb than the STLF adakites. Furthermore, the LYRB adakites are characterized by highly radiogenic Pb isotopic compositions with
206Pb/
204Pb(t) up to 18.8, which are clearly distinct from the STLF adakites with low radiogenic Pb (
206Pb/
204Pb(t)
=
15.8–16.4). Although the high Mg# of the two groups of adakites suggest reaction with mantle peridotites during magma ascent, the geochemical comparisons indicate that the STLF adakites were derived from partial melting of the delaminated eclogitic lower continental crust, while the LYRB adakites were derived from partial melting of the seawater-altered oceanic crust that was being subducted towards the LYRB during the early Cretaceous. The petrogenetic contrasts between these two groups of high-Mg adakites, therefore, indicate that the large-scale Cu–Au mineralization is associated with oceanic slab melting, not delamination or recycling of the ancient lower continental crust, as previously proposed.
New geochronological and geochemical data on magmatic activity from the India-Asia collision zone enables recognition of a distinct magmatic flare-up event that we ascribe to slab breakoff. This ...tie-point in the collisional record can be used to back-date to the time of initial impingement of the Indian continent with the Asian margin. Continental arc magmatism in southern Tibet during 80-40 Ma migrated from south to north and then back to south with significant mantle input at 70-43 Ma. A pronounced flare up in magmatic intensity (including ignimbrite and mafic rock) at ca. 52-51 Ma corresponds to a sudden decrease in the India-Asia convergence rate. Geological and geochemical data are consistent with mantle input controlled by slab rollback from ca. 70 Ma and slab breakoff at ca. 53 Ma. We propose that the slowdown of the Indian plate at ca. 51 Ma is largely the consequence of slab breakoff of the subducting Neo-Tethyan oceanic lithosphere, rather than the onset of the India-Asia collision as traditionally interpreted, implying that the initial India-Asia collision commenced earlier, likely at ca. 55 Ma.
Abstract
Intraplate basaltic volcanism commonly exhibits wide compositional ranges from silica-undersaturated alkaline basalts to silica-saturated tholeiitic basalts. Possible mechanisms for the ...compositional transition involve variable degrees of partial melting of a same source, decompression melting at different mantle depths (so-called ‘lid effect’), and melt-peridotite interaction. To discriminate between these mechanisms, here we investigated major-trace elemental and Sr–Nd–Mg–Zn isotopic compositions of a suite of intraplate alkaline and tholeiitic basalts from the Datong volcanic field in eastern China. Specifically, we employed Mg and Zn isotope systematics to assess whether the silica-undersaturated melts originated from a carbonated mantle source. The alkaline basalts have young HIMU-like Sr and Nd isotopic compositions, lower δ26Mg (-0·42‰ to -0·38‰) and higher δ66Zn (0·40‰ to 0·46‰) values relative to the mantle. These characteristics were attributable to an asthenospheric mantle source hybridized by carbonated melts derived from the stagnant Pacific slab in the mantle transition zone. From alkaline to tholeiitic basalts, δ26Mg gradually increases from -0·42‰ to -0·28‰ and δ66Zn decreases from 0·46‰ to 0·28‰ with decreasing alkalinity and incompatible trace element abundances (e.g. Rb, Nb, Th and Zr). The Mg and Zn isotopic variations are significantly beyond the magnitude (<0·1‰) induced by different degrees of fractional crystallization and partial melting of a same mantle source, excluding magmatic differentiation, different degrees of partial melting and the ‘lid effect’ as possible mechanisms accounting for the compositional variations in the Datong basalts. There are strong, near-linear correlations of δ26Mg and δ66Zn with 87Sr/86Sr (R2=0·75 − 0·81) and 143Nd/144Nd (R2=0·83 − 0·90), suggesting an additional source for the Datong basalts. This source is characterized by pristine mantle-like δ26Mg and δ66Zn values as well as EM1-like Sr–Nd isotopic ratios, pointing towards a metasomatized subcontinental lithospheric mantle (SCLM). Isotope mixing models show that mingling between alkaline basaltic melts and partial melts from the SCLM imparts all the above correlations, which means that the SCLM must have been partially melted during melt-SCLM reaction. Our results underline that interaction between carbonated silica-undersaturated basaltic melts and the SCLM acts as one of major processes leading to the compositional diversity in intracontinental basaltic volcanism.
Many high silica granites (>70 wt% SiO2) are closely linked to ore deposits. There has been an ongoing debate about whether they are formed through extensive fractional crystallization or partial ...melting, as traditional major-trace element and radiogenic isotopes make it challenging to distinguish among these mechanisms. Recent studies suggest that zinc isotopes have the potential to trace crustal differentiation and monitor fluid evolution during the late stage of magma evolution. In this study, we investigated the Lenglongling leucogranites in the North Qilian Orogenic belt, characterized by high SiO2 contents ranging from 72 % to 78 wt%, extremely low Zr/Hf ratios (15.8–21), low Nb/Ta ratios (3.8–9.3), and strongly negative Eu/Eu* anomalies (0.04–0.32). The invariant and basalt-like zinc isotopic compositions of the Lenglongling high silica leucogranites (0.27 ‰ ±0.03 ‰ to 0.38 ‰ ± 0.03 ‰) reveal that they did not undergo extensive fractional crystallization or fluid exsolution. Instead, their homogeneous zinc isotope compositions and distinct geochemical characteristics were inherited from their protoliths during partial melting.
A large set of zinc (Zn) stable isotope data for continental basalts from eastern China were reported to investigate the application of Zn isotopes as a new tracer of deep carbonate cycling. All of ...the basalts with ages of <110 Ma have systematically heavy δ66Zn (relative to JMC 3-0749L) ranging from 0.30‰ to 0.63‰ (n=44) compared to the mantle (0.28±0.05‰; 2sd) and >120 Ma basalts from eastern China (0.27±0.06‰; 2sd). Given that Zn isotope fractionation during magmatic differentiation is limited (≤0.1‰), the elevated δ66Zn values reflect the involvement of isotopically heavy crustal materials (e.g., carbonates with an average δ66Zn of ∼0.91‰) in the mantle sources. SiO2 contents of the <110 Ma basalts negatively correlate with parameters that are sensitive to the degree of partial melting (e.g., Sm/Yb, Nb/Y, Nb) and with the concentration of Zn, which also behaves incompatibly during mantle melting. This is inconsistent with a volatile-poor peridotite source and instead suggests partial melting of carbonated peridotites which, at lower degree of melting, generates more Si-depleted (and more Ca-rich) melts. Zinc isotopic compositions are positively correlated with Sm/Yb, Nb/Y, Nb and Zn, indicating that melts produced by lower degrees of melting have heavier Zn isotopic compositions. Carbonated peridotites have a lower solidus than volatile-poor peridotites and therefore at lower melting extents, contribute more to the melts, which will have heavier Zn isotopic compositions. Together with the positive relationships of δ66Zn with CaO and CaO/Al2O3, we propose that the heavy Zn isotopic compositions of the <110 Ma basalts were generated by incongruent partial melting of carbonated peridotites. Combined with previously reported Mg and Sr isotope data, we suggest that the large-scale Zn isotope anomaly indicates the widespread presence of recycled Mg (Zn)-rich carbonates in the mantle beneath eastern China since the Late Mesozoic. Since Zn is a trace element in the mantle and Zn isotopic compositions of marine carbonates and the mantle differ markedly, we highlight Zn isotopes as a new and useful tool of tracing deep carbonate cycling in the Earth's mantle.
•A systematic study of Zn isotopes on continental basalts from eastern China.•The <110 Ma basalts have heavy δ66Zn compared with the normal mantle.•δ66Zn positively correlates with Sm/Yb, Nb/Y, Zn, Nb, CaO and CaO/Al2O3.•At low melting extents, carbonated peridotites contributed heavy δ66Zn to melts.•Zn isotopes are a new tracer of deep carbonate cycling in the Earth's mantle.
The geological implications of granitoid magmas with high Sr/Y and La/Yb are debated because these signatures can be produced by multiple processes. This study presents comprehensive major and trace ...element compositions and zircon SHRIMP U–Pb age data of 81 early Cretaceous granitoids and 4 mafic enclaves from the Dabie orogen to investigate partial melting of the thickened lower continental crust (LCC). On the basis of Sr/Y ratios, granitoids can be grouped into two magma series: (i) high Sr/Y granitoids (HSG) and (ii) normal granitoids with low Sr/Y. Relative to normal granitoids, HSG display the following distinct chemical features: (1) at given SiO
2 and CaO contents, the HSG have significantly higher Sr than normal granitoids, defining two different trends in Sr versus SiO
2, CaO diagrams; (2) highly depleted heavy rare earth element (REE) relative to middle and light REE with (Dy/Yb)
N and (La/Yb)
N up to 3.2 and 151, respectively; (3) variable and higher Nb/Ta; and (4) positive correlations among Sr/Y, (Dy/Yb)
N, (La/Yb)
N, and Nb/Ta.
High Sr/Y, (La/Yb)
N, (Dy/Yb)
N, and Sr/CaO of HSG do not correlate with major elements (e.g., SiO
2). Large variations in these ratios at a given SiO
2 content indicate that these features do not reflect magma mixing or fractionation. HSG have higher Sr at a given CaO content and larger variation of (Dy/Yb)
N than old crustal rocks (including exposed basement, global mafic LCC xenoliths, high Sr/Y TTG suites, and adakites in modern arcs). This precludes inheritance of the HSG chemical features from these source rocks. Instead, the chemical features of the HSG are best explained by partial melting of the thickened LCC with garnet-dominant, plagioclase-poor, and rutile-present residual lithologies. The coupled chemical features of the HSG are not observed in post-collisional granitoids younger than ca.130
Ma, indicating removal of the eclogitic source and/or residuum of HSG underneath the orogen. These characteristic chemical relationships in the Dabie HSG may be applied to distinguish partial melts of thickened LCC from high Sr/Y intermediate-felsic magmatic rocks which do not show clear indications for melting depth.
The zinc (Zn) stable isotope system has great potential for tracing planetary formation and differentiation processes due to its chalcophile, lithophile and moderately volatile character. As an ...initial approach, the terrestrial mantle, and by inference, the bulk silicate Earth (BSE), have previously been suggested to have an average δ66Zn value of ∼+0.28‰ (relative to JMC 3-0749L) primarily based on oceanic basalts. Nevertheless, data for mantle peridotites are relatively scarce and it remains unclear whether Zn isotopes are fractionated during mantle melting. To address this issue, we report high-precision (±0.04‰; 2SD) Zn isotope data for well-characterized peridotites (n=47) from cratonic and orogenic settings, as well as their mineral separates. Basalts including mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) were also measured to avoid inter-laboratory bias. The MORB analyzed have homogeneous δ66Zn values of +0.28±0.03‰ (here and throughout the text, errors are given as 2SD), similar to those of OIB obtained in this study and in the literature (+0.31±0.09‰). Excluding the metasomatized peridotites that exhibit a wide δ66Zn range of −0.44‰ to +0.42‰, the non-metasomatized peridotites have relatively uniform δ66Zn value of +0.18±0.06‰, which is lighter than both MORB and OIB. This difference suggests a small but detectable Zn isotope fractionation (∼0.1‰) during mantle partial melting. The magnitude of inter-mineral fractionation between olivine and pyroxene is, on average, close to zero, but spinels are always isotopically heavier than coexisting olivines (Δ66ZnSpl-Ol=+0.12±0.07‰) due to the stiffer Zn-O bonds in spinel than silicate minerals (Ol, Opx and Cpx). Zinc concentrations in spinels are 11–88 times higher than those in silicate minerals, and our modelling suggests that spinel consumption during mantle melting plays a key role in generating high Zn concentrations and heavy Zn isotopic compositions of MORB. Therefore, preferential melting of spinel in the peridotites may account for the Zn isotopic difference between spinel peridotites and basalts. By contrast, the absence of Zn isotope fractionation between silicate minerals suggests that Zn isotopes are not significantly fractionated during partial melting of spinel-free garnet-facies mantle. If the studied non-metasomatized peridotites represent the refractory upper mantle, mass balance calculation shows that the depleted MORB mantle (DMM) has a δ66Zn value of +0.20±0.05‰ (2SD), which is lighter than the primitive upper mantle (PUM) estimated in previous studies (+0.28±0.05‰, 2SD, Chen et al., 2013b; +0.30±0.07‰, 2SD, Doucet et al., 2016). This indicates that the Earth’s upper mantle has a heterogeneous Zn isotopic composition vertically, which is probably due to shallow mantle melting processes.
Tracking the final fate of subducting carbon is crucial to understanding global carbon cycles and climate changes in the history of the Earth. Available geochemical tracers such as carbon isotopes ...are apt to identify recycled organic carbon but usually insufficient to discriminate between primordial carbon in the mantle and carbon derived from recycled carbonate sediments. In the past decade, magnesium and zinc isotope systematics have been proposed as novel proxies for subducting carbon owing to the noticeable isotopic offsets between carbonate sediments and the mantle (i.e., δ26Mgcarbonate < δ26Mgmantle; δ66Zncarbonate > δ66Znmantle). Nonetheless, isotopic effects induced by subduction-zone processes and crystal-melt differentiation may obscure the information of Mg and Zn isotopic compositions of mantle-derived magmas. In this paper we firstly discuss how these processes modify the Mg and Zn isotopic systematics of mantle-derived magmas. Based on the fact that different carbonate species (calcite, dolomite, and magnesite) possess distinct Mg and Zn contents and their stabilities in subduction zones vary with pressure, we then develop the two isotope systematics as tools to track the final storage depth of subducting carbon. We test this application by collating available Mg and Zn isotopic compositions of ultramafic xenoliths and basaltic lavas sourced from various mantle depths. The lack of light Mg and heavy Zn isotopic anomalies of global arc lavas supports experimental and theoretical prediction that the dissolved carbonate species in the sub-arc mantle−if any−is dominated by calcium-rich carbonate. The findings of pervasive low-δ26Mg and high-δ66Zn ultramafic xenoliths and basaltic lavas sourced from the sub-continental lithospheric mantle (SCLM) suggest that the SCLM is an important storage of subducting carbon via metasomatism by dolomite that can be substantially dissolved by supercritical fluids at depths of >160 km. Intraplate alkali basalts with low δ26Mg and high δ66Zn are commonly restricted to the regions with stagnant slabs at depths of ~410–660 km, suggesting that the mantle transition zone is another global storage of subducting carbon composed mainly of Mg-rich carbonates. Overall, observations on mantle-derived rocks, with Mg and Zn isotopes as the tracers, indicate that a significant flux of Earth's surface carbon has survived the arc regime and been recycled into the deeper mantle. Future studies that explore a quantitative relationship between Mg-Zn isotopic ratios and the flux of subducting carbon will further promote the application of the paired isotopic proxies.
•Factors obscuring the interpretation of Mg and Zn isotopic data of mantle-derived rocks or magmas were reviewed.•Developing Mg and Zn isotopes as novel proxies for the storage depths of subducting carbonates in the mantle.•Test this application using available Mg and Zn isotope data of natural samples from various mantle depths.•A significant flux of Earth's surface carbon have survived the arc regime and been recycled into the deeper mantle.
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