Reworking of old continental lithosphere: Unradiogenic Os and decoupled Hf-Nd isotopes in sub-arc mantle pyroxenites

Mantle lithologies in orogenic massifs and xenoliths commonly display strikingly different Hf- and Nd-isotope compositions compared to oceanic basalts. While the presence of pyroxenites has long been suggested in the source region of mantle-derived magmas, very few studies have reported their combined Hf-Nd isotope compositions. We here report the first Lu-Hf data along with Re-Os data and S concentrations on the Cabo Ortegal Complex, where the pyroxenite-rich Herbeira massif has been interpreted as remnants of a delaminated arc root. The pyroxenites, chromitites and their host harzburgites show a wide range of whole-rock Re-187/Os-188 and Os-187/Os-188 (0.16-1.44), indicating that Re was strongly mobilized, partly during hydrous retrograde metamorphism but mostly during supergene alteration that preferentially affected low-Mg#, low Cu/S pyroxenites. Samples that escaped this disturbance yield an isochron age of 838 +/- 42 Ma, interpreted as the formation of Cabo Ortegal pyroxenites. Corresponding values of initial Os-187/Os-188 (0.111-0.117) are relatively unradiogenic, suggesting limited contributions of slab-derived Os to primitive arc melts such as those parental to these pyroxenites. This interpretation is consistent with radiogenic Os in arc lavas being mostly related to crustal assimilation. Paleoproterozoic to Archean Os model ages confirm that Cabo Ortegal pyroxenites record incipient volcanic arc magmatism on the continental margin of the Western African Craton, as notably documented by zircon U-Pb ages of 2.1 and 2.7 Ga. Lu-Hf data collected on clinopyroxene and amphibole separates and whole-rock samples are characterized by uncorrelated Lu-176/Hf-177 and Hf-176/Hf-177 (0.2822-0.2855), decoupled from Nd-isotope compositions. This decoupling is ascribed to diffusional disequilibrium during melt-peridotite interaction, in good agreement with the results of percolation-diffusion models simulating the interaction of an arc melt with an ancient melt-depleted residue. These models notably show that Hf-Nd isotopic decoupling such as recorded by Cabo Ortega] pyroxenites and peridotites (Delta epsilon(Hf(i)) up to +97) is enhanced during melt-peridotite interaction by slow diffusional re-equilibration and can be relatively insensitive to chromatographic fractionation. Finally, we discuss the hypothesis that arc-continent interaction may provide preferential conditions for such isotopic decoupling and propose that its ubiquitous recognition in peridotites