特提斯洋对流上地幔Os同位素组成的重新估算

The hypothesis of “late veneer” assumes the concentrations of highly siderophile elements in the mantle, which are substantially higher than would have resulted from metal-silicate equilibration, could be attributable to the accretion of small bodies to the Earth following core formation. Determining the 187Os/188Os of the convecting upper mantle has been previously attempted in many ways to explore the composition of the late veneer, however, all studies yielded no consensus isotopic composition. In comparison with other studies focused on mantle peridotites, ophiolite chromitites or MORB, early-crystallizing cumulate complex and minerals in ophiolites could record more precise Os isotopic characteristics of the convecting upper mantle. For this study, we choose the early-crystallizing cumulate dunite and troctolite from several MOR-type ophiolites in Tibet for Re-Os analysis and separate zircon from troctolite and gabbro in the same cumulate section for U-Pb dating. Then, we can combine the cumulate rock Os isotopes with accurate zircon U-Pb ages to constrain the geochemical characteristics of convecting mantle sources for ophiolites. The present day Os isotopic composition of Tethyan oceanic upper mantle could provide constraints on the composition of late veneer and contribute to the interpretation and calculation of Re depletion ages for mantle peridotites.

依据“后增薄层”假说，地幔中过高含量的强亲铁元素是地核形成后陨石加入地幔中的结果。前人尝试了多种方法来估算上地幔的Os同位素组成，探讨地球后期加积物质性质，但至今仍未得到统一的结果。与其它用于估算地幔Os同位素组成的研究对象相比，蛇绿岩中早期结晶分异的堆晶岩及其矿物的Os同位素特征，可以更准确地反映对流上地幔源区的性质。本申请以青藏高原MOR型蛇绿岩堆晶序列中早期结晶的纯橄岩和橄长岩为主要研究对象，在详细的野外观察和岩石成因认识基础上，以Re-Os同位素体系为主要研究手段，结合同一堆晶序列中橄长岩和辉长岩中锆石U-Pb定年，反演地幔源区Os同位素特征，探讨对流上地幔性质，为探究地球后增薄层的物质组成提供证据，并有助于地幔橄榄岩Re亏损年龄的理解和计算。