Abstract:
Melting temperatures of biotite-orthopyroxene-quartz assemblages were determined experimentally using biotites with various Fe-contents in the system KAlO2-MgO-FeO-SiCO2-H2O, in the pressure range 100-500 MPa and under the quartz-fayalite-magnetite fO2 buffer. All experiments were performed with synthetic phases. Water contents in the starting mixtures ranged from 11 to 78 wt.%. The compositions of coexisting minerals and melts were determined by electron microprobe. The analysis of glass (quenched liquids) in the experimental products showed them to have about 79 wt.% SiO2, with very low Fe and Mg contents which do not change with changing Fe-content of the system or temperature. The Fe/(Fe+Mg) ratio of the melt is greater than in the biotite and coexisting orthopyroxene. Solidus temperatures of biotite-orthopyroxene-quartz assemblages decrease considerably with increasing Fecontent in biotite. This means that water-saturated as well as fluid-absent (dehydration) melting of biotite-quartz assemblages may occur at temperatures between 700 and 800°C. Natural biotites are probably stabilized up to higher melting temperatures by the presence of titanium and aluminium.