Magmatic platinum nanoparticles in metasomatic silicate glasses and sulfides from Patagonian mantle xenoliths

• DOI: 10.1007/s00410-019-1583-5
• Publication Date: 2019-05-14T01:31:26Z
• Authors: José María Gonzál...; Josep Roqué-Rosell; Abigail Jiménez-F...; Santiago Tassara; Fernando Nieto; Fernando Gervilla; Sandra Baurier; Joaquín A. Proenza; Edward Saunders; Artur P. Deditius; Manuel Schilling; Alexandre Corgne
• Keywords: Metasomatism; Mineral redox buffer; Peridotite; Pentlandite

The authors are grateful to D. Kamenetsky, Dave Holwell and an anonymous reviewer for their criticism that greatly improved our manuscript. We also acknowledge Associate Editor Chris Ballhaus for careful editorial handling. This research was supported by spanish projects: RTI2018-099157-A-I00 and CGL2015-65824-P granted by the “Ministerio de Ciencia, Innovación y Universidades” and Ministerio de Economía y Competitividad” (MINECO), respectively. Additional funding for LA–ICP–MS analysis was provided by the Ramón y Cajal Fellowship RYC-2015-17596 to JMGJ. This study also contributes to the DID-UACh project #S-2015-52 to M. Schilling and the PhD project (S. Tassara) at University of Chile supported by the CONICYT Award scholarship #21170857. A. Jiménez is supported with a postdoctoral grant from the National Council on Science and Technology (CONACYT) of Mexico. Jesús Montes is acknowledged for preparation of thin sections. Trifon Trifonov (Polytechnic University of Catalonia), Rocío Márquez and María del Mar Abad (CIC of the University of Granada) and Macolm P. Roberts (CMCA of the University of Western Australia) and Rosanna Murphy (GAU of Macquarie University) are acknowledge for their assistance with FIB, FESEM, HRTEM and EMPA, respectively.<br/>Platinum-rich nanonuggets (s.l., nanoparticles) are commonly produced in experiments attempting to quantify the solubility or partitioning of noble metals in silicate and sulfide melts. However, it has been thought that these represent artifacts produced during quenching of the experimental runs. Here, we document nanoparticles (~ 20–80 nm) of Pt-rich alloys and arsenides dispersed in high-temperature metasomatic silicate glasses and in base-metal sulfides (BMS) entrained in them, found interstitially between minerals of mantle peridotite xenoliths from southern Patagonia. Pt-rich nanoparticles found in the interstitial silicate glasses are frequently attached to, or in the proximities of, oxides (ilmenite or Cr-spinel) suggesting a close link between the formation of the oxides and the Pt-rich nanoparticles. The interstitial glasses in the studied xenoliths correspond to quenched alkaline basaltic melts that infiltrated the subcontinental lithospheric mantle (SCLM) at > 1000 °C at an oxygen fugacity (fO2) near the fayalite–magnetite–quartz (FMQ) buffer. Experimental works indicate that at these conditions the crystallization of oxides such as ilmenite or Cr-spinel may lower fO2 to promote the precipitation of Pt-rich nanoparticles. The investigation of four Pt-rich nanoparticles hosted in two different pentlandite grains using a combination of focused ion beam and high-resolution transmission electron microscopy (FIB/HRTEM) show that these nanoparticles consists of polycrystalline aggregates < 10 nm that are randomly oriented relative to their sulfide host matrices. These observations suggest that these nanoparticles could be segregated either directly from the infiltrating alkaline basaltic melt prior to sulfur saturation in the silicate melt, or from droplets of immiscible sulfide melt once sulfur saturation was achieved. The formation of Pt-rich nanoparticles in high-temperature melts, either silicate or sulfide, provides new clues on the processes of fractionation, transport and concentration of Pt in the mantle.<br/>