Experimental study of Fe3+ solubility in cristobalite and its application to a metamorhosed quartz-magnetite rock from Mt. Riiser-Larsen area, Napier Complex, East Antartica

Abstract

This report concerns the Fe3+ solubility in cristobalite coexisting with hematite obtained from heating experiments in air at 990-1460°C in the SiO2-Fe2O3 system. The ferric iron substitutes for the tetrahedral silicon in cristobalite and Si4+ also substitutes ror the octahedral Fe3+ in hematite. Combining the chemical and P-T data of the quartz in the high-pressure eclogite from Sanbagawa belt, central Shikoku, Japan with the present experimental data, we found the content of ferric iron in quartz increases with increasing pressure and temperature: InXFe = (-1092-5.254T+ 7.75P)/T, where XFe, Tand P are the number of Fe atoms in quartz per formula unit based on a 2-oxygen atom normalization (cationic mole fraction of Fe), temperature in Kelvin and pressure in kbar, respectively. This new geothermometer is applied to the ultrahigh-temperature metamorphosed quartz-magnetite rock from Mt. Riiser-Larsen area, Napier Complex, East Antarctica, resulting that the metamorphic temperatures are estimated as about 994-1095°C for pressures 5-15 kbar.