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首页> 外文期刊>Contributions to Mineralogy and Petrology >Constraints on the pre-metamorphic trace element composition of Barberton komatiites from ion probe analyses of preserved clinopyroxene
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Constraints on the pre-metamorphic trace element composition of Barberton komatiites from ion probe analyses of preserved clinopyroxene

机译：离子对保存的斜基吡咯的Barberton komatiite的变质前痕量元素组成的限制

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摘要

The trace element compositions of igneous augite preserved in metamorphosed komatiite samples from the lower Komati Formation, Barberton Mountainland, South Africa (3.49 Ga), were analyzed in order to estimate the pre-metamorphic bulk composition of the samples. The chemical effects of metamorphism are evaluated by calculating the trace element (TE) composition of a hypothetical augite in equilibrium with the bulk sample composition, and comparing it with ion probe analyses of the cores of augite crystals preserved in that sample. Differences between the calculated and measured composition of the augite indicate chemical gain or loss from the bulk sample during metamorphism. The data indicate that the rare earth elements (REE) and Y were immobile during metamorphism. Immobility is also indicated for Ti and Zr, though chromite fractionation makes this conclusion less firm. Sr and Eu were highly mobile (>50% loss) in all samples studied. Because the REE were immobile, the light REE (LREE) enrichments [(La/Sm)n correlate with Ti/Zr in a strikingly similar manner to variations present in modern boninites, which are high Mg, hydrous subduction zone magmas. Plume-based models that appeal to majorite garnet fractionation cannot produce the low Ti/Zr of komatiites or their wide variation in La/Sm. Thus, we propose that a subduction origin for the Barberton komatiites and basaltic komatiites is more consistent with the available trace element data.
机译：分析了南非Barberton Mountainland低层Komati组（3.49 Ga）中变质的Komatiite样品中保存的火成辉石的痕量元素组成，以估计样品的变质前体成分。通过计算假想的辉石的痕量元素（TE）组成与总体样品组成的平衡来评估其变质的化学作用，并将其与该样品中保存的辉石晶体核心的离子探针分析进行比较。计算的和测量的辉石组成之间的差异表明，在变质过程中大块样品的化学增益或损耗。数据表明，稀土元素（REE）和Y在变质过程中不动。 Ti和Zr也显示出固定性，尽管亚铬酸盐分级分离使该结论不太确定。在所有研究的样品中，Sr和Eu具有很高的迁移率（损失> 50％）。由于稀土元素是不可移动的，因此某些巴伯顿科玛替岩样品中存在的轻稀土元素（[LREE] n [sub 1]）是主要的火成特征，而不是置入后变质作用的结果。 （La / Sm）n 的变化与Ti / Zr具有惊人的相似性，与现代boninite中存在的变化非常相似，后者是高Mg，含水俯冲带岩浆。吸引主要矿石石榴石分馏的基于羽状模型不能产生高铁锰矿的低Ti / Zr或La / Sm的宽泛变化。因此，我们提出Barberton科马提岩和玄武岩科马提岩的俯冲成因与可用的痕量元素数据更加一致。

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    《Contributions to Mineralogy and Petrology》 |2003年第4期|383-396|共14页
  • 作者

    Stephen W. Parman; Nobumichu Shimizu; Timothy L. Grove; Jesse C. Dann;

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    Dept. of Earth Atmosphere and Planetary Sciences Massachusetts Institute of Technology Cambridge MA 02140 USA;

    Dept. of Geology and Geophysics Woods Hole Oceanographic Institute Woods Hole MA 02543 USA;

    Dept. of Earth Atmosphere and Planetary Sciences Massachusetts Institute of Technology Cambridge MA 02140 USA;

    Dept. of Earth Atmosphere and Planetary Sciences Massachusetts Institute of Technology Cambridge MA 02140 USA;

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