A major element, PGE and Re–Os isotope study of Middle Atlas (Morocco) peridotite xenoliths: Evidence for coupled introduction of metasomatic sulphides and clinopyroxene

Abstract

We present major element and PGE (platinum-group-element) abundances in addition to Re–Os isotope data for 11 spinel-facies whole rock peridotites from a single maar from the Middle Atlas Mountains, Morocco. Major element systematics of these xenoliths are generally correlated with indices of depletion. FeO–MgO systematics appear to suggest spinel-facies melting in the range of 5 to 25%. However, Al 2O 3 abundances in these xenoliths appear elevated relative to primitive mantle (P rima). The Al 2O 3 abundances in conjunction with other major elements require distinct re-enrichment of the Middle Atlas continental mantle root due to melt/rock reaction and precipitation of amphibole and/or clinopyroxene from passing silicate melts akin to MORB or OIB that evolved in reverse direction along the melting curves in e.g. FeO–MgO space. Sc and V confirm the range of apparent depletion and also indicate that the currently preserved fO 2 in these peridotites is distinctly different from fO 2 conditions observed in subduction zones. The majority of these xenoliths have low Os and Ir (I-PGEs) concentrations relative to P rima and modelled sulphide- and clinopyroxene-depleted residues of mantle melting under low fO 2, mid-ocean ridge-like conditions. Moreover, Pt and Pd (P-PGE) abundances are elevated when compared to their expected abundances after substantial melt extraction. Importantly, the systematically low Ir abundances in the majority of samples show well-correlated trends with Al 2O 3, MgO and Cu that are inconsistent with established melting trends. Os isotopes in the Middle Atlas xenoliths range from 187Os/ 188Os = 0.11604 to 0.12664 although most samples are close to chondritic. The Os isotope ratios are decoupled from 187Re/ 188Os but, together with Re abundances, also exhibit a good correlation with Al 2O 3, MgO and Cu. The major element, I-PGE and Os isotope correlations suggest that the initial melt depletion led to the exhaustion of sulphide and clinopyroxene (20 to 30%) without significant stabilization of I-PGE-rich alloys. During later modal metasomatism of the refractory Middle Atlas continental mantle root with silicate melts akin to MORB or OIB the introduction of clinopyroxene/amphibole reduced the volume of the melt inducing sulphur saturation in these melts causing precipitation of secondary sulphides. This coupled crystallization of pyroxenes and sulphides (chalcopyrite) resulted in the two-component mixing systematics exhibited by I-PGEs, Os isotopes with major elements and Cu preserved in the Middle Atlas continental mantle root.