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 for 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: lnXFe = (−1092 − 5.254T + 7.75P)/T,where XFe, T and 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.
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