Platinum-group elements and melt percolation processes in Sidamo spinel peridotite xenoliths, Ethiopia, East African Rift

Abstract

Platinum-group elements (Os, Ir, Ru, Rh, Pt, Pd) and gold contents have been determined along with S and modal sulfide abundances in 15 well-characterized spinel peridotite xenoliths from the Sidamo area (Gregory Rift, Ethiopia). In this xenolith suite, a group of deformed peridotites (refractory lherzolites to harzburgites) has been metasomatized by volatile-rich small melt-fractions while a group of granular peridotites (harzburgites to cpx-rich, fertile lherzolites) indicate extensive re-equilibration with high fractions of OIB-like melts at the bottom of the sub-continental lithosphere. The PGE data can be explained by a model combining a recent porous flow melt percolation event at variable melt–rock ratios superimposed on a protolith variably depleted by an old partial melting event. All PGE spectra have sub-chondritic Pt/Ir and Pt/Ru, decreasing with decreasing bulk-rock Al 2O 3 contents, and thus inherited from the partial melting event; likewise, whether deformed or granular, all the harzburgites have PGE and S abundances characteristic of residues from high degree partial melting, i.e. almost no base metal sulfides (especially Mss) and Pd- and Au-depleted CI-normalized PGE patterns. The two textural groups of lherzolites display contrasting PGE abundances and sulfide modal compositions, both reflecting the differing recent magmatic histories of the two groups. In the deformed lherzolites, Pd and Au abundances increase relative to Pt and Ir in parallel with the amount of Al-poor metasomatic diopside and intergranular Cu–Ni-rich sulfides, also of indisputable metasomatic origin. For this textural group, transport and precipitation of S, Cu, Pd and Au from the volatile-rich small-melt fraction is likely. Despite their fertile bulk-rock compositions (up to 3.65 wt.% Al 2O 3 contents) and evidence of metasomatic Fe–Ni-rich sulfides precipitated along with cpx, the granular lherzolites show surprisingly low total PGE contents (11.9–13.3 ppb) as well as sub-chondritic Pd/Ir ratios. Such a PGE budget can be explained by extensive re-equilibration between the OIB-like melts and Mss of residual origin, coupled with a mobility of sulfide melts in high-porosity zones of the lithospheric mantle. Our data indicate that porous flow melt percolation events may significantly alter the PGE budget of the sub-continental lithospheric mantle with respect to PUM estimates.