Highly siderophile element geochemistry of the Earth's mantle: new data for the Lanzo (Italy) and Ronda (Spain) orogenic peridotite bodies

Abstract

Nine plagioclase/spinel lherzolites from the Lanzo (Italy) and Ronda (Spain) peridotite bodies were analysed for Re, Os, Ir, Ru, Rh, Pd, and Au using a high-precision instrumental neutron activation (INA) procedure after NiS fire assay. Our data conform with previous observations that orogenic lherzolite massifs sampled domains of the subcontinental lithospheric mantle (SCLM) displaying a characteristic enrichment of Ru, Rh and Pd (i.e. the light platinum group elements; PGEs) over Os and Ir. The Pd/Ir, Rh/Ir and Ru/Ir ratios are superchondritic and very similar in both suites (2.46±0.32 vs. 2.42±0.21; 0.46±0.07 vs. 0.45±0.10; 1.99±0.14 vs. 2.25±0.25, respectively). Rhenium and gold are generally depleted relative to PGEs, in agreement with their moderately incompatible behaviour in low-degree mantle melting events; however, in both suites, these elements do not correlate with melt depletion indices, indicating a small-scale remobilization by hydrothermal fluids. The Os/Ir ranges from chondritic values at Lanzo (1.09±0.13) to higher than chondritic at Ronda (1.37±0.17). The higher Os/Ir of the Ronda lherzolites is interpreted in terms of mobilization of Os by late magmatic fluids. By contrast, the light PGE excess cannot be ascribed to any geological processes that occurred in the lithosphere such as pervasive melt percolation at the asthenosphere–lithosphere boundary, nor to Fe–Ni sulfide precipitation processes during the low-degree melting event that affected both massifs prior to their incorporation into the SCLM. The fact that superchondritic Ru/Ir, Rh/Ir and Pd/Ir ratios have now been documented in a large number of pristine mantle samples from various tectonic settings as well as in ophiolitic and abyssal peridotites argues for primary variations of highly siderophile element (HSE) abundances in the terrestrial upper mantle.