A multi-technique study of platinum group element systematic in some Ligurian ophiolitic peridotites, Italy

Fe-Ni-Cu sulfide mineralogy has been investigated along with bulk-rock and in-situ PGE analyses by ICP MS and LA-ICPMS in eight lherzolites from the Internal (IL) and External Liguride (EL) ophiolites (Italy). The two EL lherzolites are fertile (24% partial melting) and slightly serpentinized while the six IL cpx-poor lherzolites have experienced 5-10% of partial melting, impregnation by instantaneous melt fractions [Geochim. Cosmochim. Acta 61 (1997) 4557] and have been highly serpentinized.The EL lherzolites show broadly chondritic PGE relative abundances with a slight to pronounced enhancement of the light PGE (Ru, Rh and Pd) relative to the heavy PGE (Os, Ir and Pt) (Ru-N/Ir-N=1.13; RhN/IrN=1.08-1.10; Pd-N/Ir-N=1.24-1.62; N=C-chondrite normalized). Their magmatic sulfide modal abundances and S contents, similar to the crogenic peridotites values, are consistent with their very low degree of partial melting. The occurrence of Cu-Rh-Pd-rich pentlandite, however, demonstrates that, even for low degree of partial melting, a Cu-Ni-rich sulfide liquid can segregate, leaving the residual monosulfide solid solution (Mss) (now transformed into Cu-poor pentlandite) depleted in Rh and Pd (Rh-N/Ir-N and Pd-N/Ir-N < 1).The IL cpx-poor lherzolites display a broadly flat PGE patterns from Os to Pt with a slight enhancement of Ru and Rh (Ru-N/Ir-N=1.05-1.38; Rh-N/Ir-N=1.01-1.31). Pd-N/Ir-N ratios range from chondritic to superchondritic (1.02-2.99) and cannot be interpreted in terms of partial melting models. Rh-Pd-Cu-Ni-rich pentlandite grains are associated with large corroded cpx crystals ascribed to exotic melt percolation by Rampone et al. [Geochim. Cosmochim. Acta 61 (1597) 4557]. It is concluded that precipitation of Cu-Ni-Rh-Pd-rich sulfides has significantly enhanced the Pd concentrations well as the magmatic sulfide modal abundances. Such processes, previously documented in abyssal peridotites from slow-spreading mid-oceanic ridges, characterize residues from low to moderate melting degrees (5-10%) of the oceanic mantle as a whole, either from mature ocean or from short-lived oceanic basins.