Experiments and models bearing on the role of chromite as a collector of platinum group minerals by local reduction

Chromite is widely recognized to act as a collector for platinum group elements (PGE), which tend to be observed as discrete grains of platinum group minerals included within magmatic chromite grains. In the course of experiments involving the re-equilibration or growth of chromite and Cr-spinel in molten silicate, we observe that platinum group minerals (PGM; including metal alloys and laurite) form at the mineral-melt interface. The formation of PGM to the extent observed requires a mechanism involving sustained transport of PGE from a source within the experiment to the site of deposition. We propose that the driving force for this process is a redox gradient developed in response to mineral growth or re-equilibration with the surrounding melt. The mechanism is local reduction within the mineral-melt interfacial region as a consequence of the selective uptake of trivalent Cr and Fe from the melt by spinel relative to the divalent species. We have modeled the transient perturbation of fO₂ in a compositional boundary layer melt around spinel for both crystal growth and diffusive re-equilibration of mineral and melt. We find that metal solubilities decrease by several per cent in the silicate melt at the melt-crystal interface during crystal growth, providing the driving force for PGM formation. In magmas that are saturated with PGM, as a result of falling temperature and oxygen fugacity during spinel crystallization, nucleation of PGM will be impeded by interfacial tension everywhere except in the reduced boundary layer around spinel crystals. The resulting concentration and trapping of alloy particles in the growing chromite crystals can produce apparent bulk chromite/melt partition coefficients exceeding 20 even if there is no solid solution of PGE in the chromite. The introduction of spinel grains, initially equilibrated with a mafic magma, into a more primitive magma, with higher Cr/Al, would lead to disequilibrium between chromite and melt. The perturbation of f O₂ in the compositional boundary layer surrounding a chromite xenocryst would exceed 0.1 log unit, leading to local reduction of alloy solubility of the order of 13-18%. A small number of spinel xenocrysts could serve as collection sites for all of the excess PGE in the magma, leading to the eventual observation that a few chromite crystals contain many PGM inclusions, whereas the rest of the chromite population may be relatively free of PGM.