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 plat-inum 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 fO