Fractionation of Platinum-group Elements and Gold in the Upper Mantle: a Detailed Study in Pyrenean Orogenic Lherzolites

Abstract

Platinum-group elements (PGE) and gold have been analysed in INTRODUCTION 68 mantle peridotite samples (lherzolites, cpx-poor lherzolites and Platinum-group elements (PGE) are classified, along with harzburgites) from Pyrenean orogenic massifs. These samples preserve Au and Re, as highly siderophile elements. Pioneering PGE and Au distribution patterns mostly inherited from a 2-Gastudies on basalt-hosted mantle xenoliths (see Jagoutz et old low-degree, partial melting event. The light rare earth element al., 1979; Mitchell & Keays, 1981; Morgan et al., 1981) (LREE)-depleted lherzolites display a remarkably homogeneous suggested that these ‘iron-loving elements’ are overPGE and Au concentration range (0·005 to 0·012× CI chonabundant in the Earth’s mantle compared with theoretical drites). The light PGE (Pd, Rh, Ru) are slightly enriched relative values predicted by core–mantle equilibrium partitioning to the heavy PGE and CI chondrites [(Pd/Ir)N = 1·8 ± 0·2; models (e.g. Borisov et al., 1994). Moreover, highly si(Pd/Pt)N = 0·8 ± 0·2]. The harzburgites are Au and Pd derophile elements occur in approximately chondritic depleted [(Pd/Ir)N < 1; (Au/Ir)N < 0·2] in addition to being Cu relative abundances. These two features have been used and S depleted relative to fertile lherzolites. These coupled variations to support the ‘late-veneer hypothesis’ according to which indicate that Pd and Au were removed along with a sulphide melt. highly siderophile elements were reintroduced into the The contents of Ir, Ru, Rh and sometimes Pt are constant over the mantle by the intense meteoritic bombardment that hit whole range of peridotite compositions (0·6 < Al2O3 < 4·2 wt %), thus reflecting the compatible behaviour of these elements in lowthe primitive Earth after core formation (Chou, 1978; degree mantle melting processes. Ruthenium and rhodium show Morgan, 1986; O’Neill, 1991). Although consistent with similar concentrations to Pd in lherzolite-hosted Cu–Fe–Ni sulphides the chondritic Os isotopic composition inferred for the but their whole-rock abundances do not decrease from lherzolites to primitive upper mantle (Meisel et al., 1996), the late sulphide-free harzburgites. To a lesser extent, this is also true for veneer hypothesis has been contradicted by recently Pt and Ir. Such behaviour suggests that, except for Pd and Au, the published PGE data. Pattou et al. (1996) identified higher PGE reside in mantle sulphides as melting-resistant atomic clusters than chondritic Pd/Ir and Rh/Ir ratios in fertile Pyrenean or micro alloys. Pd, Rh and Ru enrichment relative to Ir similar orogenic lherzolites. Likewise, Snow & Schmidt (1998) to that observed in Pyrenean orogenic lherzolites has now been found Ru/Ir, Rh/Ir, Pt/Ir and Pd/Ir ratios about 40% identified in all the orogenic lherzolites analysed so far for PGE, higher than those of the CI chondrites in abyssal periin abyssal peridotites and a few basalt-hosted mantle xenoliths. dotites. This is inferred to be a characteristic feature of the shallow midHigher than chondritic Ru/Ir, Rh/Ir and Pd/Ir ratios ocean ridge basalt source mantle since at least the early Proterozoic. have now been identified in all the orogenic peridotite