OS ISOTOPES IN OROGENIC LHERZOLITE MASSIFS AND MANTLE HETEROGENEITIES

The study of Re-187-Os-187 systematics in the orogenic lherzolite bodi es of Baldissero, Beni Bousera, Lanzo and Lherz shows the extreme isot opic heterogeneity of these massifs. Peridotites have Os-187/Os-186 ra tios that cluster around the primitive mantle value (1.05-1.06) while pyroxenites have extremely radiogenic Os-187/Os-186 ratios (from 1.78 to 8.9). These isotopic variations recorded by a single pyroxenite dem onstrate that osmium is heterogeneously distributed on a very small sc ale. The Os-187/Os-186 ratios of peridotites are comparable to those m easured on abyssal peridotites, which suggests that they represent a M ORB source type of mantle. The radiogenic Os-187/Os-186 ratios of pyro xenites suggest that they are melts or cumulates formed similar to 1 G a ago and that they they may represent old delaminated oceanic crust. The preferential melting of pyroxenites durign the formation of oceani c basalts may account for some of the radiogenic Os-187/Os-186 ratios encountered in MORBs and OIBs.