Behaviour of Platinum-group elements in the subcontinental mantle of eastern Australia during variable metasomatism and melt depletion

Increasing recognition of complexities in the Platinum-group element (PGE) and Re concentration patterns in mantle samples are challenging the view of chondritic relative abundances in the upper mantle. To investigate the pos sible causes of PGE abundance variations, a suite of east Australian, mantl e-derived, spinel peridotite xenoliths, ranging from fertile lherzolites to depleted harzburgites, and including apatite +/- phlogopite +/- amphibole bearing samples, have been analysed for their whole rock PGE and Re abundan ces. Whole rock abundances for 21 samples, combined with mineral separate a nalyses of 2 xenoliths, are presented to constrain the distribution of the PGEs and Re, their inherent heterogeneity at difference scales, and their b ehaviour during both melt extraction and metasomatism. Fertile (>2.9 wt% Al2O3) xenoliths have broadly chondritic relative PGE abu ndances, with the significant exception of positive Rh anomalies and variab le negative Os anomalies. The high Rh abundances cannot be attributed to me lt extraction or metasomatism. Bulk mineral separate PGE-Re analyses of 2 f ertile xenoliths indicate less than 6% of the whole rock PGE budget resides in either silicate or oxide (spinel) phases. The remainder of the PGEs, an d at least 80% of the whole rock Re budget, are sited in acid-leachable sul fides and less soluble trace phases such as POE-sulfides or alloys. Individ ual PGEs partition into different trace phases resulting in small scale het erogeneity of both PGE ratios and concentrations on the order of 8%-20%. Al though these trace phases may be present within the mantle, it is more like ly at least some exsolved from monosulfide solid solutions at low temperatu res. Ir and Rh abundances are consistent with compatible behaviour during m elt extraction, whereas Ru, Pt and Pd abundances are consistent with slight ly incompatible behaviour and can be modeled by assuming all reside in sulf ides within the mantle, with D-sulf(Ru) similar to D-sulf(Pt) > D-sulf(Pd). Comparison of PGE abundances between 'dry' xenoliths and modally metasomat ised xenoliths, suggests the PGEs are not significantly mobilised during in teraction with carbonate melts or during metasomatism leading to hydrous mi neral growth. Given the problems of various types of secondary alteration p rocesses, including melt extraction and surficial alteration that commonly affect xenoliths, and as within-locality heterogeneity is on a comparable o rder to any proposed regional heterogeneity, it may be premature to define significant regional differences, or 'primary' non-chondritic PGE patterns in lithospheric peridotites. Copyright (C) 1999 Elsevier Science Ltd.