OS ISOTOPES IN OROGENIC LHERZOLITE MASSIFS AND MANTLE HETEROGENEITIES

Abstract

The study of 187 Re-187OS systematics in the orogenic lherzolite bodies of Baldissero, Beni Bousera, Lanzo and Lherz shows the extreme isotopic heterogeneity of these massifs. Peridotites have 187Os/186Os ratios that cluster around the primitive mantle value (1.05–1.06) while pyroxenites have extremely radiogenic 187Os/186Os ratios (from 1.78 to 8.9). The isotopic variations recorded by a single pyroxenite demonstrate that osmium is heterogeneously distributed on a very small scale. The 187Os/1s6Os ratios of peridotites are comparable to those measured on abyssal peridotites, which suggests that they represent a MORB source type of mantle. The radiogenic 187Os/ 1860s ratios of pyroxenites suggest that they are melts or cumulates formed ∼ 1 Ga ago and that they may represent old delaminated oceanic crust. The preferential melting of pyroxenites during the formation of oceanic basalts may account for some of the radiogenic 187Os/ 186Os ratios encountered in MORBs and OIBs.