PGE-Ni-Cu sulphide segregation by interaction of basaltic melt and peridotite xenoliths of the Catalan Volcanic Zone (Spain)

Abstract

Spinel lherzolite xenoliths from the Sant Corneli volcano (Catalan Volcanic Zone, NE Spain) carry the geochemical imprint of melt/rock reaction events that have affected the subcontinental lithospheric mantle (SCLM) beneath the northeastern Iberian margin. Trace element signatures of clinopyroxene indicate that this volume of the SCLM initially experienced low degrees (F = 8 %) of partial melting, followed by extensive refertilization by alkaline silicate melts undergoing chromatographic fractionation while percolating through the mantle peridotites. Furthermore, the presence of interstitial sulphide-bearing silicate glass, as well as secondary coronitic rims around mantle minerals, records the melt/rock reaction product associated with the infiltration of the host alkaline basalts while erupting to the surface. Abundant irregular/blocky sulphides located within the interstitial glass patches are comprised of myrmekitic intergrowths of pentlandite ± bornite ± chalcopyrite, suggesting their derivation from immiscible droplets of Fe-Ni-Cu sulphide melt transported by the host alkaline basalts. The variable chondrite-normalized platinum-group element (PGE) systematics and chalcogenes (Se, Te, As, Bi and Sb) abundances of these sulphides track two distinct transport mechanisms for their parental sulphide melts: 1) by unmixing of Ni-Cu-rich sulphide liquid in alkaline basalts attaining sulphide-saturation while interacting with the peridotite xenoliths, and 2) by mechanical transport of immiscible droplets of Ni-Cu-rich sulphide liquid originally extracted by residual monosulphide solid solution undergoing incongruent melting in their mantle source. In addition, many sulphides have PGE abundances that cannot be explained solely by solid-melt chemical partition coefficients but that were likely influenced by the mechanical entrapment, or early-magmatic segregation, of pre-existing PGE-rich nanoparticles or nanomelts. The geochemical signal of these mineral nanoparticles may significantly influence sulphides PGE distribution, sometimes resulting in pronounced positive anomalies in IrRh, Au, or RuRh, along with negative anomalies in Pt.