Highly siderophile elements and Sr–Nd isotopes in refertilized mantle peridotites — A case study from the Totalp ultramafic body, Swiss Alps

Abstract

Understanding the redistribution of highly siderophile elements (HSE) in mantle peridotites during melting and melt transport is crucial for interpreting HSE data and Os isotopes in terms of the chemical and isotopic evolution of the mantle, core formation and the late stages of Earth's accretion. Here we present a case study, in which the effects of melt–rock interaction and melt impregnation on HSE are evaluated. In the Totalp ultramafic body (Davos, Swiss Alps), exhumed on the Jurassic Tethys seafloor, serpentinized and moderately depleted to fertile spinel lherzolites host spinel and spinel-garnet pyroxenite layers. This rock association shows petrographic evidence for melt-migration from pyroxenites into peridotite host rocks and precipitation of pyroxenes, spinel and sulfides under spinel lherzolite facies conditions. Clinopyroxene 87Sr/ 86Sr and 143Nd/ 144Nd signatures from peridotites are consistent with an origin from the depleted upper mantle. Most Totalp peridotites show Re enrichment and suprachondritic Re/Ir and Re/Os, while most other HSE abundances and ratios are similar to or lower than other fertile lherzolites and estimates for the primitive upper mantle. Trends of initial 187Os/ 188Os with Re/Os, Pt/Ir and Pd/Ir, extend towards pyroxenite compositions, and weak positive correlations of these ratios with Al 2O 3 abundance can be observed. These data are inconsistent with an origin as simple residues of partial melting, and likely reflect a history of melt extraction, followed by later addition of a mafic or ultramafic melt with a strongly fractionated HSE pattern (high Re/Os, Pd/Ir and Pt/Ir) and radiogenic 187Os/ 188Os, as indicated by the HSE signatures of associated pyroxenites. HSE element and Os isotope covariations in Totalp lherzolites are consistent with the addition of up to 20% of sulfides derived from mafic melt component to the sulfide budget of depleted peridotite, depending on the actual HSE abundances in the melt. According to model calculations, Re, Pd and Pt may have been added in substantial amounts, whereas Ir, Os, and Ru remained largely unaffected. Consequentially, Re/Os, and occasionally Pd/Ir and Pt/Ir were possibly shifted from subchondritic values, as indicated by depleted lherzolites, to chondritic and suprachondritic values in more strongly refertilized samples, effectively overprinting preexisting Pt–Pd–Re signatures. A remarkable coincidence is that refertilization of lithospheric peridotites is capable of producing compositions that are broadly consistent with chondritic model compositions of the silicate Earth.