An overview of the platinum-group element nanoparticles in mantle-hosted chromite deposits

Abstract

This paper reviews the occurrence of platinum-group elements nanoparticles (PGE-NPs) in mantle-hosted chromite deposits, showing that PGE-NPs are more frequently found in these deposits than previously thought. A comparison of published experimental data with observations in natural samples reveal that PGE-NPs might form at the pressure (1-0.5 GPa), temperature (similar to 1200 degrees C) and fO(2) (FMQ +/- 1) conditions in which chromite deposits form in the upper mantle. The crystallization of PGE-NPs may take place earlier or simultaneously to the segregation of chromite and/or sulfide liquids from silicate melts. If the PGEs are dissolved in the silicate melt, the segregation of PGE-NPs could be linked to local changes in fO(2) and fS(2) during the early crystallization of chromite and/or olivine. Mobile crystals of chromite and droplets of sulfide melts entrained in the basaltic parent melt may also play an additional role as physical collectors of PGE-NPs, providing a complementary or even an alternative mechanism for the fractionation of PGEs in high temperature silicate melts. Furthermore, hydrothermal alteration of the chromite deposits during seafloor metamorphism or exhumation (i.e., retrograde metamorphism) of upper mantle rocks has also a significant effect on the internal structure of the oxides and sulfides that host the PGE-NPs. Frequently, PGE-NPs are found along the replacement contacts between primary and secondary minerals, suggesting that PGE-NPs can also form during low temperature alteration events (200-600 degrees C). Finally, heating events overimposed on chromite deposits previously affected by hydrous metamorphism may enhance fluid infiltration subsequently triggering chemical, mineralogical, or textural responses in the oxide or sulfide matrices hosting the PGE-NPs, promoting coarsening of metal nanoparticles. The interplay between these processes can explain the wide spectrum of particle sizes of PGE inclusions that are observed in many altered chromite deposits, which can vary from a few nanometers to larger than a micron. These studies provide evidence that PGE-NPs can eventually form under a wide spectrum of thermal (and pressure) conditions, and show that aqueous fluids may play a relevant role in producing PGE-NPs during metamorphism and metasomatism of mantle-hosted chromite deposits.