Highly siderophile and chalcophile element systematics of crust-derived Ligurian garnet pyroxenites

There is growing evidence from major, trace elements and isotopic systematics (e.g. Nd, 187Os, 186Os) that pyroxenites are significant constituents of OIB and MORB sources. Although pyroxenites probably play an important role in the recycling of crustal 187Os and 186Os signatures within the mantle, their highly siderophile and chalcophile element signatures are not well contrained. Here, we present platinum group elements (PGE: Os, Ir, Ru, Pt, Pd), Re, Se, Te and 187Os compositions of nine garnet pyroxenites and two peridotites from the External Ligurian ophiolites (Italy). On the basis of their mineralogical, geochemical and Nd-Hf isotopic features, the pyroxenites (HREE-depleted Type A, HREE-enriched Type-B and retrogressed) were correlated with partial melting of an eclogite source derived from gabbroic protoliths [1]. Whereas the host peridotites exhibit typical mantle-like concentrations and a relatively flat CIchondrite- normalised HSE patterns, the pyroxenites show large concentration ranges, reaching up to 46 ppb Pd, 538 ppb Se and 81 ppb Te. Their HSE patterns display a positive slope from Os to Re, but with significant differences between the pyroxenites. Type-A pyroxenites show lower Os, Ir, higher Pt, Pd concentrations and flatter Pd-Re segments than Type-B pyroxenites. The retrogressed pyroxenites show mixed signatures with Os-Ir contents similar to Type-B pyroxenites but Pt, Pd concentrations as well as Pd-Re segments similar to Type-A. Selenium and Te are significantly enriched in all the pyroxenites but independently of the pyroxenite groups. The 187Os/188Os ratios vary from unradiogenic in the peridotites (0.1257) to highly radiogenic in the pyroxenites (0.2649- 2.4748). Here we show that crust-derived mantle pyroxenites show a large range of highly siderophile and chalcophile elements signatures, which could likely contribute to geochemical and isotopic heterogeneities of the Earth’s mantle. [1] Montanini et al. (2012) EPSL 351-352, 171-181.GOLDSCHMIDT 2013