Thermal metamorphism of mantle chromites and the stability of noble-metal nanoparticles
Por:
González-Jiménez J.M., Reich M., Camprubí A., Gervilla F., Griffin W.L., Colás V., O’Reilly S.Y., Proenza J.A., Pearson N.J., Centeno-García E.
Publicada:
1 ago 2015
Resumen:
The Loma Baya complex in south-western Mexico is a volume of chromitite-bearing oceanic mantle that records a complex metamorphic history, defined by a first stage of hydrous metamorphism overprinted by a short-lived thermal event associated with an Eocene granite intrusion. During the hydrous metamorphism, the primary magmatic chromite–olivine assemblage was replaced by a secondary, porous intergrowth of Fe2+-rich chromite and chlorite. The heat supplied by an Eocene-age granite intrusion reversed the hydration reaction, producing chromite rims with perfectly developed crystal faces. This third-generation chromite is in equilibrium with highly magnesian (neoformed) olivine and defines a chemical trend analogous to the original magmatic one. The preservation of both reactions in the Loma Baya chromitite provides compelling evidence that the hydration of chromite can be reversed by either prograde metamorphism or any heating event, confirming previous thermodynamic predictions. Understanding these complex features is of particular interest due to the fact that changes in temperature and variable degrees of fluid/rock interaction during metamorphism and intrusion have also significantly affected the chromite-hosted IPGE carrier phases. Here, we propose that the metamorphic fluids involved in the hydrous metamorphism have caused the desulphurization of laurite RuS2, releasing minute particles of Ru–Os–Ir alloys <50 nm in diameter. The following short-lived thermal event that promoted dehydration in the chromitite had the opposite effect on nanoparticle stability, producing a significant coarsening of metal nanoparticles to dimensions larger than a micron. Based on such observations, we argue that IPGE nanoparticles can be exsolved and grown (or coarsen) from sulphide matrices during prograde metamorphism or heating and not exclusively upon cooling under magmatic conditions as it has been previously suggested. These results provide new insights on the relevant role of temperature and nanoparticle–host interaction phenomena in natural systems, shedding new light on the kinetic controls of nano- to micron-scale IPGE particle distributions during metamorphism. © 2015, Springer-Verlag Berlin Heidelberg.
Filiaciones:
González-Jiménez J.M.:
Department of Geology and Andean Geothermal Center of Excellence (CEGA), Universidad de Chile, Plaza Ercilla # 803, Santiago de Chile, Chile
ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS) and GEMOC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia
Reich M.:
Department of Geology and Andean Geothermal Center of Excellence (CEGA), Universidad de Chile, Plaza Ercilla # 803, Santiago de Chile, Chile
Camprubí A.:
Univ Nacl Autonoma Mexico, Inst Geol, Mexico City 04510, DF, Mexico
Gervilla F.:
Instituto Andaluz de Ciencias de la Tierra (Universidad de Granada-CSIC) and Departamento de Mineralogía y Petrología, Universidad de Granada, Avda. Fuentenueva s/n, Granada, 18002, Spain
Griffin W.L.:
ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS) and GEMOC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia
Colás V.:
Departamento de Ciencias de la Tierra, Universidad de Zaragoza, Pedro Cerbuna 12, Saragossa, 50009, Spain
O’Reilly S.Y.:
ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS) and GEMOC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia
Proenza J.A.:
Departament de Cristal·lografia, Mineralogia i Dipòsits Minerals, Universitat de Barcelona, Martí i Franquès s/n, Barcelona, 08028, Spain
Pearson N.J.:
ARC Centre of Excellence for Core to Crust Fluid Systems (CCFS) and GEMOC National Key Centre, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia
Centeno-García E.:
Univ Nacl Autonoma Mexico, Inst Geol, Mexico City 04510, DF, Mexico
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