Zinc isotope fractionation during magmatic differentiation and the isotopic composition of the bulk Earth
Zinc isotope fractionation during magmatic differentiation and the isotopic composition of the bulk Earth
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DOI：
10.1016/j.epsl.2013.02.037
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发表时间：
2013-05-01
影响因子：
5.3
通讯作者：
Moynier, Frederic
中科院分区：
文献类型：
--
作者：
Chen, Heng;Savage, Paul S.;Moynier, Frederic
锌稳定同位素体系已成功地应用于地球化学的许多领域，但迄今为止，这一同位素体系如何受到岩浆作用的影响仍不完全清楚。为了评估Zn同位素作为行星分异和挥发历史的替代物的潜在应用，重要的是要限制岩浆分异引起的Zn同位素分馏的幅度。在这项研究中，我们提出了高精度的锌同位素分析的两套化学多样性，同源样品基拉韦厄伊基熔岩湖，夏威夷，和Hekla火山，冰岛，基拉韦厄伊基样品的锌同位素组成显示出微小但可分辨的变化（0.26份/千份<Δ Zn-66 < 0.36份/千份; Δ Zn-66定义为样品的Zn-66/Zn-64组成比与JMC-Lyon标准的每摩尔偏差），其中最分化的岩性表现出更正的Δ Zn-66值。这种分馏可能是橄榄石和/或Fe-Ti氧化物结晶的结果，它们都可以在其晶体结构中容纳Zn。来自Hekla的样品具有相似的同位素变化范围（0.22 ppm < delta Zn-66 < 0.33 ppm），然而，岩浆分异引起的分馏程度不太显著（仅0.07 ppm），并且没有看到同位素组成与分异程度之间的相关性。我们的结论是，高温岩浆分异可以导致锌同位素分馏，在目前的精度水平是可分辨的，但只有在成分演变的岩性。关于原始（超镁铁质和玄武质）物质，这意味着陆地地幔基本上是均匀的锌同位素。利用玄武岩和超镁铁质样品分析，从不同的地质背景，我们估计，平均锌同位素组成的散装硅酸盐地球是三角洲Zn-66=0.28 +/- 0.05千分之（2s.d）。(C)2013爱思唯尔有限公司版权所有。
The zinc stable isotope system has been successfully applied to many and varied fields in geochemistry, but to date it is still not completely clear how this isotope system is affected by igneous processes. In order to evaluate the potential application of Zn isotopes as a proxy for planetary differentiation and volatile history, it is important to constrain the magnitude of Zn isotopic fractionation induced by magmatic differentiation. In this study we present high-precision Zn isotope analyses of two sets of chemically diverse, cogenetic samples from Kilauea Iki lava lake, Hawaii, and Hekla volcano, Iceland, which both show clear evidence of having undergone variable and significant degrees of magmatic differentiation.The Kilauea Iki samples display small but resolvable variations in Zn isotope composition (0.26 parts per thousand < delta Zn-66 < 0.36 parts per thousand; delta Zn-66 defined as the per mule deviation of a sample's Zn-66/Zn-64 compositional ratio from the JMC-Lyon standard), with the most differentiated lithologies exhibiting more positive delta Zn-66 values. This fractionation is likely a result of the crystallization of olivine and/or Fe-Ti oxides, which can both host Zn in their crystal structures. Samples from Hekla have a similar range of isotopic variation (0.22 parts per thousand < delta Zn-66 < 0.33 parts per thousand), however, the degree of fractionation caused by magmatic differentiation is less significant (only 0.07 parts per thousand) and no correlation between isotope composition and degree of differentiation is seen. We conclude that high temperature magmatic differentiation can cause Zn isotope fractionation that is resolvable at current levels of precision, but only in compositionally-evolved lithologies. With regards to primitive (ultramafic and basaltic) material, this signifies that the terrestrial mantle is essentially homogeneous with respect to Zn isotopes. Utilizing basaltic and ultramafic sample analyses, from different geologic settings, we estimate that the average Zn isotopic composition of Bulk Silicate Earth is delta Zn-66=0.28 +/- 0.05 parts per thousand (2s.d.). (C) 2013 Elsevier B.V. All rights reserved.