Melt Influx and the Effects on Os-Sr-Nd Isotopic Systems and Platinum Group Elements in Jurassic Oceanic Mantle (Totalp Massif, Swiss Alps)

地幔中上升的熔体以各种方式与橄榄岩围岩相互作用。裂缝中的运动可能导致橄榄岩矿物的溶解，并在裂缝中析出辉石和石榴石或尖晶石。纯橄榄岩通道中的渗流可能溶解辉石和铝相，并导致橄榄石的析出。在其他环境中，可能会发生先前耗尽的橄榄岩的扩散参照。这些相互作用过程对铂族元素(PGE)和Re丰度模式的影响，以及可能对橄榄岩中放射性成因187-Os的影响尚未得到系统的研究。初步研究结果表明，瑞士阿尔卑斯山的侏罗纪Totalp超镁铁质地块是研究这些影响的合适地点。研究Re-Os、Rb-Sr和Sm-ND同位素体系，获得PGE和硫的含量数据，并与主量元素变化进行对比，以证实与超软体187-Os熔融参考尖晶石橄榄岩的假说。全岩数据将由激光烧蚀-电感耦合等离子体质谱(ICPMS)补充，对来自晶界的硫化物中的PGE和硅酸盐中的包裹体进行测试，以测试微观平衡。该地块中的整合辉石岩层为镁铁质熔体的沉淀物，具有软骨状到高度软骨状的Os同位素比值。本研究将对辉石岩中Os同位素的分布进行评价，特别是在最普遍的辉石岩--尖晶石辉橄榄岩中是否以超软体Os同位素比值为主。放射性成因的Os同位素比值可能反映了对流地幔中的高Re储集层，很可能是再循环洋壳。在地幔中存在硅酸盐和硫化物熔体的情况下，层状辉石岩和橄榄岩的同位素和浓度分布将限制Os同位素均一化的规模。这项工作将加深我们对软流圈和岩石圈过渡区上升熔体对周围地幔的化学影响的理解。

EAR 0309810BeckerMelt ascending in the mantle interacts with peridotite wall rock in various ways. Movement in fractures may result in dissolution of peridotite minerals and precipitation of pyroxenes and garnet or spinel in the fractures. Percolative flow in dunite channels appears to dissolve pyroxenes and Al-phases and result in precipitation of olivine. In other environments, diffuse refertilization of previously depleted peridotite may occur. The effects of these interaction processes on abundance patterns of platinum group elements (PGE) and Re, and possibly on radiogenic 187-osmium in peridotites have not been studied in a systematic way.Results of a pilot study suggest that the Jurassic Totalp ultramafic massif in the Swiss Alps, a lithospheric fragment of the Ligurian ocean basin, is a suitable site to study these effects. The Re-Os, Rb-Sr and Sm-Nd isotopic systematics will be studied, and PGE and sulfur concentration data will be obtained and compared to major element variations in order to confirm the hypothesis that melts with suprachondritic 187-Os refertilized the spinel peridotites. Whole rock data will be supplemented by laser ablation ICP-MS work on PGE in sulfides from grain boundaries and inclusions in silicates to test for micro-scale equlibration. Concordant pyroxenite layers in this massif are precipitates of mafic melts and show subchondritic to highly suprachondritic Os isotopic ratios. This study will evaluate the distribution of Os isotopic ratios in the pyroxenites, particularly, whether suprachondritic Os isotopic ratios predominate in spinel websterites, the most ubiquitous pyroxenite. The radiogenic Os isotopic ratios may reflect a high-Re reservoir in the convecting mantle, most likely recycled oceanic crust. Isotopic and concentration profiles across layered pyroxenites and peridotites will yield constraints on the scale of isotopic homogenization for Os in the presence of silicate and sulfide melts in the mantle. This work will improve our understanding of chemical effects of ascending melts on surrounding mantle in the transition region between asthenosphere and lithosphere.