Evidence from correlated Ir/Os and Cu/S for late-stage Os mobility in peridotite xenoliths: Implications for Re-Os systematics

Combined Ir, Os, Cu, and S concentrations from 18 eastern Australian peridotite xenoliths demonstrate systematic variations in Os concentrations with S loss and require a late-stage process capable of fractionating Os from the geochemically similar Ir. This effect accounts for the greater variability and lower mean Os concentrations observed in peridotite xenoliths as compared with massif peridotites and abyssal peridotites. Late-stage mobility of Os in xenoliths has important implications for interpreting Os isotopic data, and strong correlations between Cu/S and Ir/Os may provide a method for screening Re-Os data to provide more reliable Os isotopic model ages. The ability of cryptic secondary processes to fractionate some platinum-group elements (PGEs), in this case Ir and Os, must be taken into account before attributing fractionated PGE patterns in xenoliths to primary mantle processes.