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 indic ate that the mantle sources of these materials were broadly chondritic (within +/- 2% of projected chondrite compositions) in their long-ter m Re-Os isotopic evolution. This powerful isotopic constraint argues t hat other highly siderophile elements in the upper mantle also probabl y existed in chondritic relative proportions within the upper mantle, prior to the formation of the Outokumpu rocks. Late accretion of highl y siderophile elements is the only process entirely consistent with th is conclusion. While broadly chondritic, the podiform chromitites that were examined show evidence of at least two sources of Os. The primar y magmatic source represented by a laurite grain has an initial gamma( Os) of - 1.6. Fluids that percolated through the rocks during alterati on processes added more radiogenic Os (gamma(Os) > + 0.4). The source of this radiogenic Os was likely either seawater, added during sepenti nization, or continental crust, added during the obduction process. Th is 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.