On the significance of ultra-magnesian olivines in basaltic rocks
On the significance of ultra-magnesian olivines in basaltic rocks
复制标题
论玄武岩中超镁橄榄石的意义
DOI：
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发表时间：
2011
期刊：
影响因子：
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通讯作者：
D. Jerram
中科院分区：
文献类型：
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作者：
J. Keiding;R. Trumbull;I. Veksler;D. Jerram
南极的Karoo LIP （Heinonen and Luttinen, 2010）和纳米比亚西北部的Parana-Etendeka LIP （Thompson and Gibson, 2000）是本文的主题。这些省超镁质橄榄石的共同特征是它们罕见，只在少数地方出现，并且它们的镁含量太高，不能被认为是玄武岩寄主岩中的平衡斑晶。值得注意的是，这些橄榄石与丰富得多的镁质晶体（f85 - 90）共存，镁质晶体的组成与寄主岩石的mg#接近平衡，因此可以被认为是斑晶。由此可见，超镁质橄榄石在目前的寄主中不是斑晶，而是异晶。这些橄榄石的高CaO含量明显区别于地幔橄榄石的夹带晶体，大多数研究得出结论，这些超镁质晶体起源于未喷发的科马长岩熔体，并被合并到第二种更进化的岩浆中。我们强调，这一结论仅基于橄榄石和块状岩石组成。然而，橄榄石组成只限制了平衡液的Mg# [MgO/(MgO + FeO)摩尔百分比]，而不限制其MgO浓度，这是推断母熔体的科马长岩亲和力和高温的关键依据。为了计算熔体的氧化镁，需要得到其FeO含量的信息。本研究提出了一个通过分析超镁橄榄石中包裹体直接回收原始熔体成分的例子。这消除了结果对岩石组成和体积的依赖
Karoo LIP in Antarctica (Heinonen and Luttinen, 2010), and the Parana-Etendeka LIP in northwest Namibia (Thompson and Gibson, 2000), the subject of this paper. Shared features of ultra-magnesian olivines in these provinces are that they are rare, occurring in just a few localities, and they are too Mg rich to be considered equilibrium phenocrysts in the basaltic host rocks. It is signifi cant that these olivines coexist in the same hand specimens with much more abundant, less magnesian crystals (Fo 85‐90 ), the compositions of which are close to equilibrium with the Mg# of the host rocks, and can therefore be considered as phenocrysts. It follows that the ultra-magnesian olivines are not phenocrysts in the present host, but represent xenocrysts. The high CaO contents in these olivines clearly distinguish them from entrained crystals of mantle olivine, and most studies have concluded that the ultra-magnesian crystals originated from an unerupted komatiitic melt and were incorporated into a second, more evolved magma. We emphasize that this conclusion is based only on olivine and bulk-rock compositions. However, olivine compositions constrain only the Mg# [MgO/(MgO + FeO) in molar percent] of the equilibrium liquid and not its MgO concentration, on which the inference of a komatiitic affi nity and high temperature of the parental melts critically depend. To calculate the melt MgO requires information on its FeO content. This study presents an example where original melt compositions are recovered directly by analysis of inclusions trapped in the ultra-magnesian olivines. This removes the dependence of the results on the bulk rock composition and