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

在地幔上升的beckermelt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，并在裂缝中沉淀辉石、石榴石或尖晶石。白云岩通道中的渗透水流溶解辉石相和铝相，导致橄榄石的沉淀。在其他环境中，可能发生先前耗尽的橄榄岩的弥漫性再成矿。这些相互作用过程对橄榄岩中铂族元素（PGE）和稀土（Re）丰度模式的影响，以及可能对放射性成因187-锇的影响尚未得到系统的研究。初步研究结果表明，瑞士阿尔卑斯山脉的侏罗纪Totalp超镁铁质地块，利古里亚海洋盆地的岩石圈碎片，是研究这些影响的合适地点。研究Re-Os、Rb-Sr和Sm-Nd同位素系统，获得PGE和硫浓度数据，并与主要元素变化进行比较，以证实尖晶石橄榄岩与超软质187-Os熔体融合的假设。整个岩石的数据将由激光烧蚀ICP-MS对晶界硫化物和硅酸盐包裹体中的PGE进行补充，以测试微观尺度的平衡。该地块内和谐辉石岩层为基性熔体的沉淀，其Os同位素比值为亚球粒质至高度超球粒质。本研究将评估辉石岩中Os同位素比值的分布，特别是在最普遍存在的尖晶石韦氏岩中，超球粒型Os同位素比值是否占主导地位。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储层，很可能是再循环的海洋地壳。层状辉石岩和橄榄岩的同位素和浓度分布将限制地幔中硅酸盐和硫化物熔体存在时Os同位素均质化的尺度。这项工作将提高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作用的认识。

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.