Alteration or Mantle Origin of Radiogenic Osmium in Pyroxenites and Peridotites From Jurassic Oceanic Crust (Totalp Massif, Swiss Alps)?

对对流上地幔锇同位素组成的精确估计是限制该储层和整体地幔中海洋地壳可能再循环比例的先决条件之一。以前对洋中脊玄武岩和深海橄榄岩的Re-Os研究遇到了各种各样的问题，这些物质通常用于约束对流上地幔的组成。在某些情况下，人们怀疑这两种岩石类型的1870 / 188o可能由于海水污染而增加，使块状岩石产生球粒质或超球粒质1870 / 188o。侏罗系海相橄榄岩地块（瑞士阿尔卑斯山脉Totalp超镁铁质地块）整体岩石的Re-Os数据介于亚和超球粒岩之间，与深海橄榄岩相似。该地块包含一致的辉生岩层，显示出亚球粒岩至高放射成因的初始1870 / 1880年代，其上覆的上层沉积物年龄为160 Ma。有人认为，辉石岩可能是对流上地幔的一个次要成分。这项初步研究将试图通过获得辉石岩和橄榄岩矿物分离物的Re-Os数据来证实整个岩石的结果，这些矿物分离物显示超软质187Os/188Os。在辉石岩中，一致的内部矿物等时线应证实高放射性成因的初始187Os/188Os的有效性，而对于放射性成因的橄榄岩，整个岩石的初始187Os/188Os应与单个相的初始187Os/188Os一致。辉石岩的侏罗纪Re-Os年龄支持晶体沉淀和熔融大约发生在这一时期的观点，辉石岩及其放射性成因Os来自于对流的上地幔。

EAR-0105837BeckerA precise estimate of the osmium isotopic composition of the convecting upper mantle is one of the prerequisites to obtain constraints on the likely proportion of recycled oceanic crust in this reservoir as well as in the bulk mantle. Previous Re-Os work on mid-ocean-ridge basalts and abyssal peridotites, materials commonly used to constrain the composition of the convecting upper mantle, encountered various problems. In some instances, it has been suspected that 187Os/188Os of both rock types may have been increased by contamination with seawater, yielding chondritic to suprachondritic 187Os/188Os for bulk rocks. Re-Os data on peridotite whole rocks from a Jurassic oceanic peridotite massif (Totalp ultramafic massif, Swiss Alps) range between sub- and suprachondritic, similar to abyssal peridotites. This massif contains concordant pyroxenitic layers that show subchondritic to highly radiogenic initial 187Os/188Os at 160 Ma, the age of the overlying pelagic sediments. It has been suggested that pyroxenites may be a minor component in the convecting upper mantle. This pilot study will attempt to confirm the results on whole rocks by obtaining Re-Os data on mineral separates from pyroxenites and peridotites that show suprachondritic 187Os/188Os. In the case of the pyroxenites, consistent internal mineral isochrones should confirm the validity of the highly radiogenic initial 187Os/188Os, whereas for the radiogenic peridotites, initial 187Os/188Os for whole rocks should coincide with initial 187Os/188Os of individual phases. Jurassic Re-Os ages for the pyroxenites would support the notion that crystal precipitation and melting occurred at about this time, and that the pyroxenites and their radiogenic Os were derived from the convecting upper mantle.