Sheng-Ao Liu

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.

Erratum: Corrigendum: Magmatic record of India-Asia collision

Scientific Reports 5 Article number: 14289; 10.1038/srep14289 published online: September232015; updated: December182015. In Supplementary Figure 1a, the Linzizong volcanic rock samples ‘12LZ14-1’ and ‘12LZ13-1’ should read ‘13LZ14-1’ and ‘13LZ13-1’ respectively. The correct Supplementary Figure 1a appears below as Fig. 1. Figure 1 In Table S1, samples ‘12LZ13-1@02’ and ‘12LZ14-1@02’ should read ‘13LZ13-1@02’ and ‘13LZ14-1@02’ respectively.

Zinc isotope evidence for intensive magmatism immediately before the end-Permian mass extinction

The end-Permian extinction is typically ascribed to massive volcanic eruptions, but direct geochemical evidence linking the two independent events is generally lacking. Zinc is an important micronutrient of marine phytoplanktons, and Zn isotope (δ66Zn) ratios of seawater are markedly higher than those of volcanic rocks and riverine waters. We conducted high-resolution Zn concentration and Zn isotope analyses on carbonate rocks across the Permian-Triassic boundary (PTB) in the Meishan section of south China. An abrupt increase of Zn concentration and a concomitant 0.5‰ decrease in δ66Zn occur ∼35 k.y. before the mass extinction and carbon isotope (δ13C) minima. Mass balance calculation demonstrates that a 0.5‰ negative shift in δ66Zn within thousands of years requires rapid and massive input of isotopically light Zn from volcanic ashes, hydrothermal inputs, and/or extremely fast weathering of large igneous provinces. A positive δ66Zn shift of as much as 1.0‰ following the mass extinction demonstrates that primary productivity recovered and reached a maximum in fewer than 360 k.y. Our finding provides insights into the marine Zn cycling across the PTB and clarifies the temporal relationship and duration of events, including intensive volcanism, carbon isotope excursion, mass extinction, and widespread ocean anoxia.

Copper and zinc isotope systematics of altered oceanic crust at IODP Site 1256 in the eastern equatorial Pacific

This paper presents the first combined Cu and Zn isotopic study of altered oceanic crust (AOC) at IODP Hole 1256D that penetrates a volcanic section, a lava-dyke transition zone, a sheeted dyke complex, and a plutonic complex. In the volcanic section, all but one rocks have Cu and Zn isotopic compositions similar to fresh mid-ocean ridge basalt (MORB), reflecting restricted seawater circulation and low oxygen fugacity. Rocks in the transition zone have MORB-like δ65Cu and δ66Zn, indicating the dominant influence of basalt-derived Cu and Zn during alteration. Rocks in the dyke complex have more variable δ65Cu (-0.50 – 0.90‰) and δ66Zn (0.19 – 0.55‰) and those in the plutonic complex have δ65Cu of -0.43 to 0.20‰ and δ66Zn of 0.21 to 0.41‰. The rocks with heavier δ66Zn and heavier or lighter δ65Cu relative to MORB are characterized by Cu-Zn depletions, low Li/Yb (<1.0) and low δ18O (<5‰), suggesting that hydrothermal extraction during high temperature alteration of oceanic crust can result in significant Cu and Zn isotope fractionation. Such large Cu and Zn isotopic variations are the results of redox transformation of Cu as well as Cu and Zn isotope fractionation between altered basaltic rocks and dissolved Cu- and Zn-species in hydrothermal fluids (e.g., [CuCl3]1-, Zn(HS)42-). This work is the first to define the distribution of Cu and Zn isotopes in an intact oceanic crust with concentration-weighted averages of δ65Cu (0.05 ± 0.03‰) and δ66Zn (0.27 ± 0.01‰). The potential implications of these new observations are discussed.

Deep carbon cycle recorded by calcium‐silicate rocks (rodingites) in a subduction‐related ophiolite

Carbon cycling in subduction zones remains poorly constrained due to the lack of relevant geological records. Here we report magnesium isotope data (δ26MgDSM3) from calcium-silicate rocks (rodingites) from the Xigaze ophiolite, southern Tibet, which is thought to represent remnants of Neo-Tethyan oceanic lithosphere. Behaviors of immobile trace elements in rodingites resemble those of their mafic dike protoliths, showing subduction-related signatures. The majority of rodingites exhibits low δ26Mg values of −0.72‰ to −0.33‰ with a weighted average of −0.47 ± 0.11‰ (2 SD), significantly lighter than that of their protoliths (−0.31 ± 0.03‰). This difference likely reflects the interaction of the protolith with isotopically light carbonate fluids. Modeling indicates that this hypothesis requires the input of 5 to 15 wt % carbonates during rodingitization. Our study suggests that rodingite may represent a previously unrecognized reservoir of dissolved Ca from subducted carbonates.