Nature and Scale of Mantle Heterogeneities in Upper Mantle Reservoirs: Evidence from trace element abundances and Os isotopic compositions in the Oman ophiolite

地球化学和同位素证据表明，作为大洋中脊玄武岩(MORB)来源的上地幔，在地质时期的熔体提取过程中已经枯竭。有人认为，MORB提取后的残馀地幔代表了一种地球化学均质储集体。最近，MORB的Re-Os系统学被用来假设与古代洋壳再循环过程有关的全球亏损大洋上地幔的化学不均一性。这些由再循环过程引入的地幔不均质性的性质和规模尚不清楚。通过直接分析地幔橄榄岩来研究地幔不均质性是困难的，因为不相容的亲石元素极度贫化。Re-Os法是研究超镁铁质(地幔)岩和探测上地幔再循环古洋壳的理想方法，因为Os是一种与地幔矿物相容的元素，而Re则是中等不相容的。可育地幔的少量部分熔融将从源头移除一些Re，减少残留物中放射性成因187Os的生长。我们对阿曼蛇绿岩地幔岩石的初步分析表明，该蛇绿岩的Os同位素组成具有很大的展布，表明古海脊具有沿走向的同位素变异性，这表明存在一个高度不均匀的上地幔。拟议研究的目的是确定大洋岩石圈不同岩性的微量元素特征和Os同位素组成，并监测地球化学和同位素遗传的性质和规模。

Geochemical and isotopic evidence indicates that the upper mantle, the source of mid-ocean ridge basalts (MORB), has been depleted by melt extraction processes over geological periods. It has been suggested that the residual mantle, after extraction of MORB, represents a geochemically homogeneous reservoir. Recently, Re-Os systematics of MORB have been used to postulate global chemical heterogeneities of the depleted oceanic upper mantle that is related to the recycling process of ancient oceanic crust. The nature and scale of these mantle heterogeneities, introduced by the recycling processes, is unknown. The study of mantle heterogeneities by the direct analyses of mantle peridotites is difficult because of the extreme depletion of incompatible, lithophile elements. The Re-Os method is ideally suited for studying ultramafic (mantle) rocks and detecting recycled ancient oceanic crust in the upper mantle, as Os an element compatible with mantle minerals, while Re is moderately incompatible. Small degrees of partial melting of a fertile mantle will remove some Re from the source reducing the growth of the radiogenic 187Os in the residue. Our preliminary analyses on mantle rocks of the Oman ophiolite reveal a large spread in Os isotopic compositions suggesting an along-strike isotope variability of the paleo-ridge which suggest the presence of a highly heterogenous upper mantle. The purpose of the proposed study is, to determine the trace element signature and the Os isotopic composition of the different lithologies of the oceanic lithosphere and, in addition, to monitor the nature and scale of the geochemical and isotopic herteogeneities.