The Os isotopic composition of Proterozoic upper mantle: Evidence for chondritic upper mantle from the Outokumpu ophiolite, Finland

Osmium isotopic data for chromite, gersdorffite and laurite separates from the upper mantle section of the 1.97 Ca Outokumpu ophiolite indicate that the mantle sources of these materials were broadly chondritic (within +/- 2% of projected chondrite compositions) in their long-term Re-Os isotopic evolution. This powerful isotopic constraint argues that other highly siderophile elements in the upper mantle also probably existed in chondritic relative proportions within the upper mantle, prior to the formation of the Outokumpu rocks. Late accretion of highly siderophile elements is the only process entirely consistent with this conclusion. While broadly chondritic, the podiform chromitites that were examined show evidence of at least two sources of Os. The primary magmatic source represented by a laurite grain has an initial gamma(Os) of - 1.6. Fluids that percolated through the rocks during alteration processes added more radiogenic Os (gamma(Os) > + 0.4). The source of this radiogenic Os was likely either seawater, added during sepentinization, or continental crust, added during the obduction process. This study highlights the need for exercising great care in choosing the types of samples analyzed to reveal the isotopic composition of Os in ancient mantle reservoirs.