Fluid-absent melting of F-rich phlogopite+rutile+quartz

Abstract

. Fluid-absent melting is believed to be an important process in the generation of melts m the lower crust and upper mantle. Breakdown of phlogopite makes H 0 available and thus controls the conditions at which fluid-absent melting occurs. Both F2andTi in biotite ha:e been shown.to affect strongly the thermal stability of biotite. To model better the flUId-absentI?eltmg of a.ssemblagescontaining phlogopite, the reaction F-phlogopite + quartz + rutIle = enstatIte + melt has been studied experimentally. Experiments were pe~o~med at 7, 10, and 15 kbar using a natural F-rich phlogopite with a starting com~os~tIOn ofF/(F + OH) = 0.43 and Mg/(Mg + Fe) (in molar proportions) = 0.94. Results mdlcate that the ° therma.l stability of F-rich phlogopite + quartz + rutile is extended by as much as 450 C relatIve to the KMASH system and by 300°C relative to the Ti-free F-KMASH system. Approach to equilibrium in the experiments was assessed by convergence.of results of melting and crystallization experiments. Phlogopite compositions from e~penmental products show that, although F-rich phlogopite incorporates relatively little TI (~-3 wt% ~i02), the combination of F and Ti increases the stability of phlogopite to consIderably hIgher temperatures (~300 0C)than that of either component alone. Melts formed by the fluid-absent melting ofF-rich phlogopite + quartz + rutile at temperatures > 1000°Care ~anitic and stro~g1yperaluminous. The compositions of these melts suggest that the formatIOnofmetalummous to peralkaline A-type granites by fluid-absent melting of halogen-enriched sources is unlikely.