Multiple events in the Neo-Tethyan oceanic upper mantle: Evidence from Ru–Os–Ir alloys in the Luobusa and Dongqiao ophiolitic podiform chromitites, Tibet
Multiple events in the Neo-Tethyan oceanic upper mantle: Evidence from Ru–Os–Ir alloys in the Luobusa and Dongqiao ophiolitic podiform chromitites, Tibet
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DOI：
10.1016/j.epsl.2007.05.044
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发表时间：
2007-09
影响因子：
5.3
通讯作者：
R. Shi;O. Alard;X. Zhi;S. O’Reilly;N. Pearson;W. Griffin;Ming Zhang;Xiaoming Chen
中科院分区：
文献类型：
--
作者：
R. Shi;O. Alard;X. Zhi;S. O’Reilly;N. Pearson;W. Griffin;Ming Zhang;Xiaoming Chen
通过对西藏罗布莎和东桥蛇绿岩豆荚状铬铁矿中170个Ru-Os-Ir合金颗粒的主量元素、铂族元素和Os同位素组成的原位分析，对合金颗粒的来源、新特提斯洋壳上地幔的Os同位素组成以及寄主蛇绿岩超镁铁橄榄岩的熔体-岩石反应时间提供了新的制约。大多数合金颗粒为锇铱和铱锇矿。根据元素和同位素组成，可以确定两个种群。第Ⅰ组，在两个蛇绿岩体中均有发现，具有典型的合金分馏PGE模式，187Os/188Os比值为0.12645 ± 4（2s; n = 145）。东桥蛇绿岩中的Ⅱ族合金显示出平滑的PGE模式，187Os/188Os值为0.12003~0.12194（Re贫化年龄≥ 1.1Ga）。PGE模式和第II组的Os同位素组成类似于熔融后残留的硫化物。I组合金代表了早期累积阶段的岩浆结晶过程中，被解释为来自软流圈熔体。与此相反，第二组合金被推断为来自残留硫化物的蛇绿岩，这可能是一个次大陆岩石圈地幔的遗迹地幔部分的故障。东桥样品中两组合金的共存反映了软流圈熔体与古老的亏损岩石圈地幔的相互作用，表明铬铁矿是通过熔体-岩石反应形成的。第一组合金的数据支持的假设，Re-Os的对流海洋地幔的演化类似于顽火辉石。
In situ analysis of major elements, platinum-group elements and Os isotope composition data for 170 grains of Ru–Os–Ir alloys from podiform chromitites in the Luobusa and Dongqiao ophiolites, Tibet, provide new constraints on the sources of the alloy grains, the Os-isotope composition of the Neo-Tethyan oceanic upper mantle, and the timing of melt–rock reactions in the ultramafic peridotites of the host ophiolites. Most alloy grains are osmiridium and iridosmine. Based on elemental and isotopic compositions, two populations can be identified. Group I, found in both ophiolite bodies, has fractionated PGE patterns typical of alloys and homogeneous187Os/188Os 0.12645±4 (2s; n=145). Group II alloys from the Dongqiao ophiolites show smooth PGE patterns and187Os/188Os ranges from 0.12003 to 0.12194 (yielding Re-depletion ages ≥1.1 Ga). Both the PGE patterns and the Os isotopic compositions of Group II resemble those of sulfides residual after melting. Group I alloys represent an early cumulate phase during the crystallization of magmas and are interpreted as derived from an asthenospheric melt. In contrast, Group II alloys are inferred to derive from the breakdown of residual sulfides from the mantle section of the ophiolite, which may represent relics of a sub-continental lithospheric mantle. The coexistence of the two groups of alloys in Dongqiao samples reflects interaction between asthenosphere-derived melts and old depleted lithospheric mantle and suggests that the chromitites originated through melt–rock reactions. The data on the Group I alloys support the hypothesis that the Re–Os evolution of the convective oceanic mantle resembles that of enstatite chondrites.