Fractionation of platinum-group elements and gold in the upper mantle: a detailed study in Pyrenean orogenic lherzolites

Platinum-group elements (PGE) and gold have been analysed in 68 mantle peridotite samples (lherzolites, cpx-poor lherzolites and harzburgites) from Pyrenean orogenic massifs. These samples preserve PGE and Au distribution patterns mostly inherited from a 2-Ga-old low-degree, partial melting event. The light rare earth element (LREE)-depleted lherzolites display a remarkably homogeneous PGE and Au concentration range (0.005 to 0.012 x CI chondrites). The light PGE (Pd, Rh, Ru) are slightly enriched relative to the heavy PGE and CI chondrites [(Pd/Ir)(N) = 1.8 +/- 0.2; (Pd/Pt)(N) = 0.8 +/- 0.2]. The harzburgites are Au and Pd depleted [(Pd/Ir)(N) < 1; (Au/Ir)(N) < 0.2] in addition to being Cu and S depleted relative to fertile lherzolites. These coupled variations indicate that Pd and au were removed along with a sulphide melt. The contents of Ir, Ru, Rh and sometimes Pt are constant over the whole range of peridotite compositions (0.6 < Al2O3 < 4.2 wt %), thus reflecting the compatible behaviour of these elements in low-degree mantle melting processes. Ruthenium and rhodium show similar concentrations to Pd in lherzolite-hosted Cu-Fe-Ni sulphides but their whole-rock abundances do not decrease from lherzolites to sulphide-free harzburgites. To a lesser extent, this is also true for Pt and Ir. Such behaviour suggests that, except for Pd and Au, the PGE reside in mantle sulphides as melting-resistant atomic clusters or micro alloys. Pd, Rh and Ru enrichment relative to Ir similar to that observed in Pyrenean orogenic lherzolites has now been identified in all the orogenic lherzolites analysed so far for PGE, in abyssal peridotites and a few basalt-hosted mantle xenoliths. This is inferred to be a characteristic feature of the shallow mid-ocean ridge basalt source mantle since at least the early Proterozoic.