Title: Origins of platinum-group minerals in upper mantle rocks

Lecturer: Dr. José María González Jiménez (Geochemical Evolution and Metallogeny of Continents (GEMOC); Dep. Earth and Planetary Sciences; Macquarie University, Australia).

Date: Tuesday January the 07th, 2014 at 16:00h

Place: Room 11


In the upper mantle, the platinum-group elements reside mainly in specific minerals known as the platinum-group minerals (PGMs), and to a lesser extent dissolved in base-metal minerals (including Fe-Ni-Cu sulfides, arsenides) and alloys. They vary widely in distribution, morphology, and chemistry and Os-isotope compositions, suggesting different possible origins. The remarkable abundance of these minerals in chromite-rich rocks may reflect the role of chromite as a physical “collector” of the platinum-group elements during crystallisation of both phases from melts. However, despite a great body of work, the links between the PGMs (and their associated base-metal minerals) and the mechanism of formation of their host chromitites are still unclear and hotly debated. Platinum-group minerals found in the upper mantle chromite-rich rocks show significant Os-isotope heterogeneity on the scale of single micrometric grains enclosed within individual chromite grains. This micro-scale isotopic heterogeneity, and their random spatial distribution within the chromites, argues strongly against an origin of the platinum-group minerals via exsolution from the chromite lattice during post-magmatic cooling. A re-interpretation of their microstructures and Os isotopes analysed in situ had led us to review the traditional models, and to propose several origin(s) for the platinum-group minerals in chromitites. (1) The suites of platinum-group minerals mainly record crystallisation during mixing of multiple different melts and physical entrapment in chromite. (2) Some platinum-group mineral sin chromitites may have been scavenged from wall-rock peridotite during migration of the parental melts of the chromitite. (3) Other platinum-group minerals may have precipitated from metasomatic fluid/melts that infiltrated existing chromitites. The recrystallization of pre-existing platinum-group minerals during polyphase metamorphism or “recycling” of the chromitite into deeper mantle levels can explain the presence of micrometric platinum-group minerals with distinct Os-isotope compositions, associated with minerals such as chlorite.