An experimental study of phase equilibria and Fe oxy-component in kaersutitic amphibole: implications for the fH2 and alphaH2O in the upper mantle

The variation of Fe oxy-component content in a titanian pargasite megacryst amphibole was determined at temperatures from 500 to 1200 degrees celsius, pressures from 1 atmosphere to 10 kbar, and fH2 from that of the IQF buffer to air. It showed a linear variation of log fH2 as a function of log (Fe3+ /Fe2+) at fixed T and P of the form log fH2 = a + b log(Fe3+ /Fe2+). The values of a and b are presented in a tabular form. The results indicate that the relatively high Fe3+/Fe(sub tot) of many natural kaersutitic amphibole occurs due to P-T-fH2 conditions of the upper mantle.