Shan Ke

High-precision copper and iron isotope analysis of igneous rock standards by MC-ICP-MS

Stable isotopic systematics of Cu and Fe are two important tracers for geological and biological processes. Generally, separation of Cu and Fe from a matrix was achieved by two independent, completely different methods. In this study, we report a method for one-step anion-exchange separation of Cu and Fe from a matrix for igneous rocks using strong anion resin AG-MP-1M. Cu and Fe isotopic ratios were measured by multi-collector inductively coupled plasma mass-spectrometry (Neptune plus) using a sample–standard bracketing method. External normalization using Zn to correct for instrumental bias was also adopted for Cu isotopic measurement of some samples. In addition, all parameters that could affect the accuracy and precision of isotopic measurements were examined. Long-term external reproducibility better than ±0.05‰ (2SD) for δ65Cu and ±0.049‰ (2SD) for δ56Fe was routinely obtained. Cu and Fe isotopic compositions of commercially accessible igneous rock standards including basalt, diabase, amphibolite, andesite and granodiorite were measured using this method. δ65Cu values of igneous rock standards vary from −0.01 to +0.39‰ (n = 11) with an overall range (0.40‰) that exceeds about 8 times that of the current analytical precision. The improved precisions of stable Cu isotopic analysis thus demonstrate that igneous rocks are not homogeneous in Cu isotopic composition. The procedure for one-step separation of Cu and Fe and high-precision analysis of Cu and Fe isotopic ratios have an important advantage for economical and efficient study of stable Cu and Fe isotopic systematics in geological and biological fields.

Magnesium isotopic composition of the deep continental crust

Abstract To constrain the behavior of Mg isotopes during deep crustal processes and the Mg isotopic composition of the middle and lower continental crust, 30 composite samples from high-grade metamorphic terranee and 18 granulite xenoliths were investigated. The composites derive from eight different high-grade metamorphic terranee ill the two largest Archean cratons of China, including 13 TTG gneisses, 5 amphibolites, 4 felsic, 4 intermediate, and 4 mafic granulites. They have variable bulk compositions with SiO2 ranging from 45.7 to 72.5%, representative of the middle crust beneath eastern China. The δ26Mg values of these samples vary from −0.40 to +0.12‰, reflecting heterogeneity of their protoliths, which could involve upper crustal sediments. The granulite xenoliths from the Cenozoic Hannuoba basalts also have a diversity of compositions with MgO ranging from 2.95 to 20.2%. These xenoliths equilibrated under high temperatures of 800–950 °C, corresponding to depths of the lower continental crust (>30 km). They yield a large δ26Mg variation of −0.76 to −0.24‰. The light Mg isotopic compositions likely result from interactions with isotopically light metamorphic fluids, probably carbonate fluids. Together with previously reported data, the average δ26Mg values of the middle and lower continental crusts are estimated to be −0.21 ±0.07‰ and −0.26 ±0.06‰, respectively. The bulk continental crust is estimated to have an average δ26Mg of −0.24 ±0.07‰, which is similar to the average of the mantle. The large Mg isotopic variation in the continental crust reflects the combination of several processes, such as continental weathering, involvement of supracrustal materials in the deep crust, and fluid metasomatism.

Interlaboratory comparison of magnesium isotopic compositions of 12 felsic to ultramafic igneous rock standards analyzed by MC-ICPMS

To evaluate the interlaboratory mass bias for high-precision stable Mg isotopic analysis of natural materials, a suite of silicate standards ranging in composition from felsic to ultramafic were analyzed in five laboratories by using three types of multicollector inductively coupled plasma mass spectrometer (MC-ICPMS). Magnesium isotopic compositions from all labs are in agreement for most rocks within quoted uncertainties but are significantly (up to 0.3‰ in 26Mg/24Mg, >4 times of uncertainties) different for some mafic samples. The interlaboratory mass bias does not correlate with matrix element/Mg ratios, and the mechanism for producing it is uncertain but very likely arises from column chemistry. Our results suggest that standards with different matrices are needed to calibrate the efficiency of column chemistry and caution should be taken when dealing with samples with complicated matrices. Well-calibrated standards with matrix elements matching samples should be used to reduce the interlaboratory mass bias.

Chromium isotope signature during continental crust subduction recorded in metamorphic rocks

The chromium isotope compositions of 27 metamorphic mafic rocks with varying metamorphic degrees from eastern China were systematically measured to investigate the Cr isotope behavior during continental crust subduction. The Cr isotope compositions of all samples studied were Bulk Silicate Earth (BSE) like, with δ53CrNIST979 of greenschists, amphibolites, and eclogites ranging from −0.06‰ to −0.17‰, −0.05‰ to −0.27‰, and −0.01‰ to −0.24‰, respectively. The lack of resolvable isotopic variability among the metamorphic rocks from different metamorphic zones indicated that no systematic Cr isotope fractionation was associated with the degree of metamorphism. However, the Cr isotopic variability among homologous samples may have reflected effects induced by metamorphic dehydration with a change of redox state, rather than protolith heterogeneity (i.e., magma differentiation). In addition, the differences in δ53Cr (Δ53CrCpx-Gt) between coexisting clinopyroxene (Cpx) and garnet (Gt) from two garnet pyroxenites were 0.06‰ and 0.34‰, respectively, indicating that significant inter-mineral Cr isotope disequilibria could occur during metamorphism. To provide a basis for comparison with metamorphic rocks and to provide further constraints on the potential Cr isotope heterogeneity in the mantle and in the protolith of some metamorphic rocks, we analyzed mantle-derived chromites and the associated peridotites from Luobusa, and we obtained the following general order: chromite-free peridotites (−0.21‰ to −0.11‰) < chromite-bearing peridotite (−0.07‰) < chromite (−0.06‰). These findings imply potential mantle heterogeneity as a result of partial melting or fractional crystallization associated with chromite.

Heterogeneous Mg isotopic composition of the early Carboniferous limestone: implications for carbonate as a seawater archive

Carbonate precipitation and hydrothermal reaction are the two major processes that remove Mg from seawater. Mg isotopes are significantly (up to 5‰) fractionated during carbonate precipitation by preferential incorporation of 24Mg, while hydrothermal reactions are associated with negligible Mg isotope fractionation by preferential sequestration of 26Mg. Thus, the marine Mg cycle could be reflected by seawater Mg isotopic composition (δ26Mgsw), which might be recorded in marine carbonate. However, carbonates are both texturally and compositionally heterogeneous, and it is unclear which carbonate component is the most reliable for reconstructing δ26Mgsw. In this study, we measured Mg isotopic compositions of limestone samples collected from the early Carboniferous Huangjin Formation in South China. Based on petrographic studies, four carbonate components were recognized: micrite, marine cement, brachiopod shell, and mixture. The four components had distinct δ26Mg: (1) micrite samples ranged from −2.86‰ to −2.97‰; (2) pure marine cements varied from −3.40‰ to −3.54‰, while impure cement samples containing small amount of Rugosa coral skeletons showed a wider range (−3.27‰ to −3.75‰); (3) values for the mixture component were −3.17‰ and −3.49‰; and (4) brachiopod shells ranged from −2.20‰ to −3.07‰, with the thickened hinge area enriched in 24Mg. Due to having multiple carbonate sources, neither the micrite nor the mixture component could be used to reconstruct δ26Mgsw. In addition, the marine cement was homogenous in Mg isotopes, but lacking the fractionation by inorganic carbonate precipitation that is prerequisite for the accurate determination of δ26Mgsw. Furthermore, brachiopod shells had heterogeneous C and Mg isotopes, suggesting a significant vital effect during growth. Overall, the heterogeneous δ26Mg of the Huangjin limestone makes it difficult to reconstruct δ26Mgsw using bulk carbonate/calcareous sediments. Finally, δ26Mgsw was only slightly affected by the faunal composition of carbonate-secreting organisms, even though biogenic carbonate accounts for more than 90% of marine carbonate production in Phanerozoic oceans and there is a wide range (0.2‰–4.8‰) of fractionation during biogenic carbonate formation.