An Experimental Study Investigating the Role of Chromite on Affecting the Behaviour of the Platinum Group Elements in Igneous Systems PDF Download

Author: Craig Stewart Finnigan
Publisher:
ISBN: 9780494218693
Category :
Languages : en
Pages : 306

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Book Description
Experiments were performed to document the role of chromite in affecting the behaviour of the platinum group elements (PGEs) during melting and crystallization. Initial experiments involving diffusive exchange of natural chromite with PGE-saturated basaltic melt revealed platinum group minerals (PGMs; including metal alloys and laurite) as precipitates along the chromite-melt interface. X-ray photoelectron spectroscopy of the glass near the chromite showed lower Fe3+/Fe2+ relative to measurements far removed from the interface, indicating a local zone of reduction The reduction is likely caused by preferential uptake of Fe3+ and Cr 3+ by the chromite relative to the melt. The effect causes the precipitation of PGMs due to their locally lowered solubility. PGM precipitation was also found to occur during spinel growth experiments, which we attribute to the same effect. This process could explain the commonly observed inclusion of PGM in chromite in ophiolites and layered intrusions, and may contribute to the fractionation of the PGEs due to their differential solubilities in silicate melt. Chromite may therefore concentrate and fractionate the PGEs by two different mechansims, one involving physical entrapment of PGMs following local reduction, and the other involving direct solid solution into the chromite lattice. The diffusive exchange technique was also used to measure chromite-melt partitioning of the PGEs, which avoids the problem of PGE contamination during heterogeneous nucleation of chromite in metal-saturated experiments. Run product chromite crystals and glass were analysed by laser ablation---inductively coupled plasma---mass spectrometry (LA-ICP-MS). Partition coefficients were calculated using the composition of chromite margins and coexisting glass. In sulfur-free experiments, chromite-melt partition coefficients (D) for Rh and Ru are large (> 10), and show a progressive decrease with lower fO 2, but still remain> 1. Partition coefficients for Pt are & sim;2 or less, whereas values for Pd are “1 under all conditions studied. Partition coefficients for Os range from 9 to 213, and minimum partition coefficients for Ir are all> 1. Partition coefficients for Ru, Rh and Pt measured at log fS2 of -0.23 are all lower than those measured in S-free experiments at the same or similar fO2. Of Significance, Os and Ru retain their compatibility in chromite in S-bearing experiments at FMQ. Measured partition coefficients confirm that chromite can fractionate Os, Ir, Ru and Rh from Pt and Pd during crystallization, provided the system is sulfur-poor.