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

University Of Maryland, College Park, College Park MD

Investigators

Abstract

EAR-0105837 Becker A 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.