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

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 2O 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.