HIGH-PRESSURE STABILITY OF THE FLUOR-ENDMEMBERS AND HYDROXY-ENDMEMBERS OF PARGASITE AND K-RICHTERITE

The high pressure and temperature stabilities of fluor-pargasite, fluor-K-richterite, and hydroxy-K-richerite have been determined at pressures of 35 to 50 kbar pressure. Fluor-pargasite is stable to 1300-degrees-C at 35 kbar, and its thermal stability decreases sharply at higher pressures. In comparison with the hydroxypargasite endmember, it is more stable by 10-15 kbar and 250-degrees-C. Both the hydroxy- and fluor-K-richterite endmembers are stable throughout the pressure range studied, and the temperature of breakdown increases towards higher pressures. At 35 kbar fluor-K-richterite is stable to 100-degrees-C higher than hydroxy-K-richterite, and this difference increases to 200-degrees-C at 50 kbar. The differing temperature stabilities of fluor- and hydroxyamphiboles carries important implications for partial melting processes in the Earth's mantle and lower crust: the hydroxy component will enter initial melt fractions but the amphibole will remain present in the residue due to the increasing importance of the fluor-endmember. Similar differences between the melting temperatures of the F- and OH-end-members of mica and apatite are expected. This solid-solution melting behaviour will result in a large temperature range of melting of such F+OH-bearing minerals within the mantle and will increase the pressure-temperature range of hydrous mineral breakdown in subduction zones.