PII S0016-7037(99)00143-X Behaviour of Platinum-group elements in the subcontinental mantle of eastern Australia during variable metasomatism and melt depletion MONICA R. HANDLER* ,† and VICTORIA C. BENNETT Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia (Received November 24, 1998; accepted in revised form March 24, 1999) Abstract—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 possible causes of PGE abundance variations, a suite of east Australian, mantle-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 abundances. Whole rock abundances for 21 samples, combined with mineral separate analyses of 2 xenoliths, are presented to constrain the distribution of the PGEs and Re, their inherent heterogeneity at difference scales, and their behaviour during both melt extraction and metasomatism. Fertile (2.9 wt% Al 2 O 3 ) xenoliths have broadly chondritic relative PGE abundances, with the significant exception of positive Rh anomalies and variable negative Os anomalies. The high Rh abundances cannot be attributed to melt extraction or metasomatism. Bulk mineral separate PGE-Re analyses of 2 fertile xenoliths indicate less than 6% of the whole rock PGE budget resides in either silicate or oxide (spinel) phases. The remainder of the PGEs, and at least 80% of the whole rock Re budget, are sited in acid-leachable sulfides and less soluble trace phases such as PGE-sulfides or alloys. Individual PGEs partition into different trace phases resulting in small scale heterogeneity of both PGE ratios and concentrations on the order of 8%–20%. Although these trace phases may be present within the mantle, it is more likely at least some exsolved from monosulfide solid solutions at low temperatures. Ir and Rh abundances are consistent with compatible behaviour during melt extraction, whereas Ru, Pt and Pd abundances are consistent with slightly incompatible behaviour and can be modeled by assuming all reside in sulfides within the mantle, with D sulf Ru  D sulf Pt  D sulf Pd . Comparison of PGE abundances between ‘dry’ xenoliths and modally metasomatised xenoliths, suggests the PGEs are not significantly mobilised during interaction with carbonate melts or during metasomatism leading to hydrous mineral growth. Given the problems of various types of secondary alteration processes, including melt extraction and surficial alteration that commonly affect xenoliths, and as within-locality heterogeneity is on a comparable order to any proposed regional heterogeneity, it may be premature to define significant regional differences, or ‘primary’ non-chondritic PGE patterns in lithospheric peridotites. Copyright © 1999 Elsevier Science Ltd 1. INTRODUCTION The abundances of the highly siderophile platinum-group ele- ments (PGE: Ru, Rh, Pd, Os, Ir, Pt) and Re in the Earth’s mantle cannot be predicted a priori owing to the unknown effects of accretion and core formation. PGE and Re abun- dances in the upper mantle that exceed those predicted by core-mantle equilibrium, combined with their apparent rela- tively chondritic mantle abundances (e.g. Kimura et al. 1974; Chou 1978; Morgan 1986), have been used to argue for the addition of the upper mantle PGE complement as a late veneer, post-core formation. CI-chondrites are normally used as a ref- erence for PGE variations, but the exact nature of the material accreted during the late veneer is unknown (e.g. Morgan, 1986; Meisel et al., 1996). Recent studies have documented variations in the relative PGE abundances of mantle samples (Spettel et al., 1991; Pattou et al., 1996; Snow and Schmidt, 1998). This raises the question of whether the non-chondritic PGE patterns reflect primary mantle features, providing new constraints for accretion models, or if they are secondary features reflecting processes such as melt extraction or metasomatism and are unrelated to large-scale differentiation. Primary constraints on upper mantle PGE concentrations are obtained directly from mantle samples, including abyssal peri- dotites, massif peridotites and xenoliths carried in basalts. Subtle variations in the relative abundances of the PGE, if they can be shown to be primary features, have significant implica- tions for deep Earth processes. The difficulty with interpreting the information from xenoliths is that although they are com- monly pristine samples of the mantle, they typically have major, trace element and isotopic compositions that result from complex processes including melt extraction, metasomatism and low temperature equilibration. In order to use xenoliths to infer primary mantle PGE compositions it is necessary to understand the effect of these secondary processes on their behaviour. Here we present data for a suite of well characterised east Australian spinel peridotite xenoliths to ascertain the effects of both melt extraction and metasomatism. As assessing and pre- dicting how processes affect PGE distributions remains hin- *Address reprint requests to M. R. Handler, Carnegie Institution of Washington, 5241 Broad Branch Rd. NW, Washington, D.C. 20015 USA. † Present address: Department of Terrestrial Magnetism, Carnegie In- stitution of Washington, 5241 Broad Branch Rd. NW, Washington, D.C. 20015 USA Pergamon Geochimica et Cosmochimica Acta, Vol. 63, No. 21, pp. 3597–3618, 1999 Copyright © 1999 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/99 $20.00 + .00 3597