Re-Os isotope systematics and Platinum Group Element fractionation during mantle melt extraction : A study of massif and xenolith peridotite suites

Abstract

Re–Os isotope and platinum group elements (PGE) systematics are presented for peridotite xenoliths from N. Lesotho (on-craton), S. Namibia (circum-cratonic), the Vitim volcanic field (Baikhal Rift), plus massif peridotites from Beni Bousera, N. Morocco. Mg–Fe variations indicate that these samples have experienced between 5% (Vitim–Beni Bousera) and 50% (Lesotho) melt extraction, providing the opportunity to examine PGE fractionation over a large melting interval. The Namibian xenoliths and Beni Bousera massif peridotites show no variation of iridium group (Ir, Ru, Os; I-PGE) abundances or inter-element fractionations relative to melt depletion indices such as Mg number or Al2O3. Lesotho peridotites show large variations in I-PGE abundances (Os range 0.2–13 ppb) at relatively constant Al2O3 that are not easily rationalised by melt-extraction models. Despite these abundance variations, there is no significant inter-element fractionation of I-PGE, e.g., (Os/Ir)n, showing that these elements are not fractionated by even very large degrees of melting (up to 50% melt extraction). Lesotho peridotites are amongst the most P-PGE (Pt, Pd)-depleted mantle rocks, with highly fractionated chondrite-normalised PGE. PGE systematics for all these peridotite suites allow a relative order of PGE compatibility to be firmly established for mantle melting: Dsolid/meltOsDsolid/meltIrDsolid/meltRu>Dsolid/meltPt>Dsolid/meltPd. Vitim peridotite xenoliths have very low Os contents and low Os/Ir (<70% chondritic) compared to the kimberlite-borne xenoliths and massif peridotites. The Vitim low Os/Ir is comparable with other suites of alkali-basalt-borne peridotite xenoliths and may result from syn- or post-eruption sulphide breakdown and alteration. Chondritic (Ru/Ir)n and (Os/Ir)n ratios in Lesotho and other cratonic peridotites show no evidence of anomalous PGE fractionations in the Archean subcontinental mantle and support the Late Veneer hypothesis. In most of the peridotite suites, (Pd/Ir)n values show a strong correlation with Os isotopic composition that is likely the result of melt-residue interaction. The positive variation of both (Ru/Ir)n and (Pd/Ir)n with bulk rock Al2O3 and Os isotopic composition for Beni Bousera and global massif peridotites indicates that these PGE ratios were modified by interaction with melts. Hence, we find no support for the intra-element PGE fractionation in the continental lithospheric mantle (CLM) representing primordial mantle heterogeneity. Highly unradiogenic Os isotope compositions appear characteristic of lithospheric peridotites with the lowest (Pd/Ir)n. In these samples, bulk-rock PGE patterns suggest that Os isotope systematics should be dominated by primary, residual, P-PGE-depleted sulphides, and hence, their bulk rock Re-depletion ages should be expected to approximate the melting age of the rock.