Osmium isotopic characteristics of mantle-derived rocks

Osmium isotopic compositions and Re and Os concentrations have been determined on ultramafic and mafic rocks derived from the contemporary mantle. These samples have a large range in measured 187Os186Os (1.0032 to 1.173). Peridotites from the Southwest Indian Ridge and Zabargad Island (Red Sea) and cumulate chromitites from Zambales ophiolite (Philippines) range in 187Os186Os from 1.083 to 1.0032. Ocean island basalts (OIBs) from Hawaii and Iceland considered to be representative of a distinct plume-source mantle have 187Os186Os of about 1.10. Other OIBs which have high 187Os186Os of ≥ 1.151 require involvement of at least one additional component with a different Os isotopic composition in their formation. The observed range in Os isotopic composition is consistent with an average silicate earth 187Os186Os of 1.10. This value is higher than the value determined by Allègre and Luck (1980) but is within the range observed in meteorites. In this case, all the analyzed suboceanic peridotites as well as subcontinental samples have been affected to varying degrees by Re depletion related to ancient melt extraction events which occurred in some regions of the mantle.