RE-OS ISOTOPIC CONSTRAINTS ON THE FORMATION OF MANTLE AND CRUSTAL RESERVOIRS

The measured Os-187/Os-186 of uncontaminated oceanic peridotites and basalts ranges between 1.0032 +/- 0.0072 and 1.111 +/- 0.011 (2-sigma(X)). This range reflects chemical fractionation of the incompatible parent element Re from compatible Os in the mantle during crust production and subsequent isolation of the Re depleted regions of the mantle for time periods in excess of 1000 Ma. The Os-187/Os-186 of subcontinental mantle samples also reflects ancient Re depletion events. A material balance for osmium isotopes in the silicate fraction of the earth is made based on the premise that the average silicate earth Os-187/Os-186 is 1.10. The calculated amount of Re depleted mantle that was caused by the extraction of the continental crust is small, probably less than 3%, and is comparable to the mass of the subcontinental lithosphere that has been depleted with respect to major elements. This calculation illustrates that a complementary relationship may exist between the continental crust and the subcontinental lithosphere with respect to Os isotopes, in marked contrast to results that suggest that the subcontinental lithosphere cannot have been the sole source of incompatible lithophile trace elements in the continental crust. The oceanic mantle is interpreted to have an Os-187/Os-186 reflecting long-term Re depletion. This would require the existence of a Re enriched reservoir in addition to the continental crust. Subducted oceanic crust may be an alternate reservoir of Re and radiogenic Os that has not been stored in the upper continental crust. If the level of Re depletion observed in suboceanic mantle samples extends throughout the entire upper mantle mass, then formation and subsequent isolation of about 10 times the continental crustal mass in the form of oceanic crust early in Earth history is required to satisfy the Re-Os mass balance.