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

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:

       lnX_Fe = (−1092 − 5.254T + 7.75P)/T,

where X_Fe, 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.