Zinc isotope fractionation during mantle melting and constraints on the Zn isotope composition of Earth’s upper mantle
Zinc isotope fractionation during mantle melting and constraints on the Zn isotope composition of Earth’s upper mantle
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DOI：
10.1016/j.gca.2016.11.014
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发表时间：
2017-02
影响因子：
5
通讯作者：
Ze‐Zhou Wang;Sheng‐Ao Liu;Jingao Liu;Jian Huang;Yan Xiao;Zhuyin Chu;Xinmiao Zhao;Limei Tang
中科院分区：
文献类型：
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作者：
Ze‐Zhou Wang;Sheng‐Ao Liu;Jingao Liu;Jian Huang;Yan Xiao;Zhuyin Chu;Xinmiao Zhao;Limei Tang
锌（Zn）稳定同位素系统由于其亲铜、亲石和中等挥发性的特性，在追踪行星形成和分化过程方面具有巨大潜力。作为初步方法，陆地幔以及由此推断的块状硅酸盐地球 (BSE) 先前被认为具有~+0.28‰的平均 δ66Zn 值（相对于 JMC 3-0749L），主要基于海洋玄武岩。然而，地幔橄榄岩的数据相对匮乏，目前尚不清楚锌同位素是否在地幔熔融过程中发生分馏。为了解决这个问题，我们报告了克拉通和造山环境中特征明确的橄榄岩 (n= 47) 及其矿物分离物的高精度 (±0.04‰；2SD) 锌同位素数据。还测量了玄武岩，包括洋中脊玄武岩（MORB）和洋岛玄武岩（OIB），以避免实验室间偏差。 MORB 分析的同质 δ66Zn 值为 +0.28 ± 0.03‰（此处和全文，误差以 2SD 形式给出），与本研究和文献中获得的 OIB 值相似（+0.31 ± 0.09‰）。排除交代橄榄岩的 δ66Zn 范围为-0.44‰至+0.42‰，非交代橄榄岩具有相对均匀的 δ66Zn 值为+0.18±0.06‰，比 MORB 和 OIB 轻。这种差异表明，在地幔部分熔融过程中存在微小但可检测的锌同位素分馏（~0.1‰）。平均而言，橄榄石和辉石之间的矿物间分馏程度接近于零，但由于尖晶石中的 Zn-O 键比硅酸盐矿物（Ol、Opx 和 Cpx）更硬，因此尖晶石的同位素总是比共存橄榄石重 (Δ66ZnSpl-Ol= +0.12 ± 0.07‰)。尖晶石中的锌浓度比硅酸盐矿物中的锌浓度高 11-88 倍，我们的模型表明，地幔熔化过程中尖晶石的消耗在生成 MORB 的高锌浓度和重锌同位素组成中起着关键作用。因此，橄榄岩中尖晶石的优先熔融可能是尖晶石橄榄岩和玄武岩之间锌同位素差异的原因。相比之下，硅酸盐矿物之间不存在锌同位素分馏，这表明在无尖晶石石榴石相地幔的部分熔融过程中，锌同位素没有显着分馏。如果所研究的非交代橄榄岩代表难熔上地幔，则质量平衡计算表明，贫化MORB地幔（DMM）的δ66Zn值为+0.20±0.05‰（2SD），比先前研究估计的原始上地幔（PUM）轻（+0.28±0.05‰，2SD，Chen等，2013b； +0.30 ± 0.07‰，2SD，Doucet 等人，2016）。这表明地球上地幔在垂直方向上具有不均匀的锌同位素组成，这可能是由于浅地幔熔融过程造成的。
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.