Geochemistry of xenolithic eclogites from West Africa, Part I: A link between low MgO eclogites and Archean crust formation
Geochemistry of xenolithic eclogites from West Africa, Part I: A link between low MgO eclogites and Archean crust formation
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DOI：
10.1016/s0016-7037(00)00626-8
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发表时间：
2001-05
影响因子：
5
通讯作者：
M. Barth;R. Rudnick;I. Horn;W. McDonough;M. Spicuzza;J. Valley;S. Haggerty
中科院分区：
文献类型：
--
作者：
M. Barth;R. Rudnick;I. Horn;W. McDonough;M. Spicuzza;J. Valley;S. Haggerty
- 报告了低MgO（6-13重量%）的氧同位素、矿物微量元素以及测量和重建的全岩成分塞拉利昂科伊杜金伯利岩杂岩中的榴辉岩捕虏体。在干净矿物上用激光衍射法测得石榴石的δ ~（18）O值（4.7-6.8‰），超出了地幔橄榄岩的δ ~（18）O值。所有低MgO榴辉岩都具有重建的微量元素模式，其贫Ba、Th、U和轻稀土元素（LREE），富硬玉样品具有比贫硬玉样品更多变的微量元素模式。这些意见，再加上低SiO2含量，富Nb，但LREE贫化重建的全岩成分，表明低MgO榴辉岩是残余的蚀变洋壳，在俯冲过程中部分熔融。高压下镁铁质原岩的部分熔融（留下含石榴子石的残留物）是解释太古代英云闪长岩-奥长花岗岩-花岗闪长岩（TTG）系列起源的首选模型，该系列构成太古代英云闪长岩中地壳的大部分。因此，我们认为Koidu低MgO榴辉岩可能是太古代陆壳形成的残留物。
—Oxygen isotope, mineral trace element, and measured and reconstructed whole rock compositions are reported for low MgO (6–13 wt.% MgO in the whole rock) eclogite xenoliths from the Koidu kimberlite complex, Sierra Leone. The δ18O values of garnet (4.7–6.8‰), determined by laser fluorination on clean mineral separates, extend beyond the range for mantle peridotites. All low MgO eclogites have reconstructed trace element patterns that are depleted in Ba, Th, U, and light rare earth element (LREE), with jadeite-rich samples having more variable trace element patterns than jadeite-poor samples. These observations, coupled with low SiO2contents, and Nb-rich but LREE-depleted reconstructed whole rock compositions, suggest the low MgO eclogites are remnants of altered oceanic crust that was partially melted during subduction. Partial melting of a mafic protolith at high pressure (leaving a garnet-bearing residue) is the preferred model to explain the origin of Archean tonalite-trondhjemite-granodiorite (TTG) suites, which make up large portions of the crust in Archean cratons. We therefore suggest that the Koidu low MgO eclogites may be residues from Archean continental crust formation.