From crustal protoliths to mantle pyroxenites: a highly siderophile elements and Os isotope perspective from the External Liguride mantle section (N Apennine, Italy).

Pyroxenites derived from crustal recycling are a major form of mantle heterogeneity. They are believed to play a key role in the magma genesis, contributing to the radiogenic Os signatures within the sources of oceanic basalts. However, natural examples are rare. Here, we present HSE (Os, Ir, Pt, Pd, Re) and 187Os/188Os isotopic systematics of pyroxenites enclosed in fertile mantle sequences of the Jurassic northern Apennine ophiolites. Two case studies are considered: garnet clinopyroxenites and websterites from the Rio Strega-Mt. Prinzera (RS-MP) bodies and spinel pyroxenites from Monte Gavi (MG). We will show that relics of ancient subducted crust are heterogeneous as a consequence of geochemical variability of the protoliths, modification during recycling processes and interaction with the host peridotites, producing centimetre- to metre-scale 187Os isotopic heterogeneity. The RS-MP garnet clinopyroxenites have heterogeneous mafic crustal precursors that experienced a longlived evolution of recycling into the mantle (1.5-1.0 Ga) as inferred from Lu-Hf isotope systematics. They originated by crystallization of eclogite-derived melts, whereas the websterites were interpreted as secondary pyroxenites with a crustal geochemical fingerprint and a peridotite wall rock contribution. All the pyroxenites are variably depleted in Os and Ir and enriched in Pt, Pd and Re with respect to the host peridotites. Bulk rock HSE compositions of the garnet clinopyroxenites are consistent with sulphur saturation and sulfide crystallization from partial melts of gabbro-derived eclogites. 187Os/188Os ratios recalculated for the age of a Mesozoic melting event inferred from Nd-Hf isotopes are unradiogenic to slightly radiogenic in the peridotites (0.124-0.134) and moderately to highly radiogenic in the pyroxenites (0.149-2.190). Decoupling between Re/Os (TMa = 2.0-2.8 Ga) and Lu-Hf isotope systematics may be due to fractionation of Re/Os ratios with no Os isotopic homogenization of the sulfide melt fraction. At Monte Gavi, a undeformed large body (up to 10 m-thick) of Al-Fe-rich spinel pyroxenites is associated with cm-thick layers of Mg-rich pyroxenites locally containing olivine relics from the host lherzolite. The Fe-rich pyroxenite body has melt-like HSE patterns, whereas the Mg-rich pyroxenites are enriched in Os and Ir. Bulk rock Os isotopes show increasingly radiogenic composition from Mg- to Fe-rich pyroxenites (187Os/188Os165Ma = 0.185-0.518). The MG pyroxenites may have formed by crystallization of melts derived from “aged” eclogite-rich sources. The thick pyroxenite body represents a melt-dominated system, whereas the thin pyroxenite layers derived by melt/peridotite hybridization. In both occurrences, the host peridotites show nearly flat chondrite-normalized HSE patterns, possibly suggesting a history of depletion in incompatible HSE such as Pd and Re followed by refertilization through interaction with the pyroxenite-forming melts.