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

准确估计对流上地幔的Os同位素组成是限制该储集层和大块地幔中再循环洋壳可能比例的先决条件之一。以前在大洋中脊玄武岩和深海橄榄岩上进行的Re-Os研究遇到了各种问题，这些材料通常用于限制对流上地幔的成分。在某些情况下，人们怀疑这两种岩石类型的187Os/188Os可能是由于海水的污染而增加的，对于块状岩石来说，产生了球粒状到超软骨状的187Os/188Os。侏罗纪大洋橄榄岩地块(瑞士阿尔卑斯山Totalp超镁铁质地块)橄榄岩全岩的Re-Os数据介于软骨岩和超软骨岩之间，类似于深海橄榄岩。该地块含有整合的辉石岩层，在160 Ma，即上复的远洋沉积物的年龄，显示出亚软骨状到高放射性的初始187Os/188Os。辉石岩可能是对流上地幔中的次要成分。这项初步研究将试图通过获得辉石岩和橄榄岩中分离出的矿物的Re-Os数据来证实整个岩石的结果，这些数据显示出超软体的187Os/188Os。对于辉石岩，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，而对于放射性成因橄榄岩，全岩的初始187Os/188Os应与个别阶段的初始187Os/188Os重合。辉石岩的侏罗纪Re-Os年龄支持晶体沉淀和熔融大约在此时发生的观点，辉石岩及其放射性成因Os来自对流上地幔。

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.