PHASE-RELATIONS OF OSUMILITE AND DEHYDRATION MELTING IN PELITIC ROCKS- A SIMPLE THERMODYNAMIC MODEL FOR THE KFMASH SYSTEM

Thermodynamic modelling of (1) osumilite solid solutions and (2) dehyd ration melting in pelitic compositions within the KFMASH system is qui te successful in reproducing the invariant and univariant reactions de termined in experimental studies. Even though rather preliminary, such melt thermodynamic models may be very useful in interpolating and ext rapolating the limited information available from a small number of ex perimental runs. These methods allow the compositions of all phases to be monitored as a function of pressure, temperature and equilibrium p hase assemblage for any desired bulk composition. Locating the higher variance phase fields (e.g. quadrivariant, quinivariant) is often diff icult or impossible by inspection, but is made relatively easy using t hermodynamic software such as thermocalc. In the KFMASH system the cal culated partition of Fe and Mg between osumilite, garnet, cordierite, orthopyroxene and biotite are shown to be in good agreement with exper imental and natural data and allow reliable calculation of mineral com positions coexisting with quartz-saturated and H2O-undersaturated melt s for a variety of bulk compositions. These phase diagram calculations allow quite tight limits to be placed on the pressure, temperature an d water activity conditions which accompanied metamorphism of natural osumilite occurrences in Nain, Namaqualand, and Rogaland. At fixed bul k composition, the initial melting of pelites by dehydration of biotit e can occur via univariant, divariant or trivariant equilibria dependi ng upon pressure of metamorphism. Of particular interest is that, for low pressures or more magnesian bulk compositions, fluid-absent meltin g begins by generating liquid from the high- variance assemblage bioti te + cordierite + K-feldspar + quartz. This type of modelling allows i nvestigation, at least qualitatively, of the fine scale details of mel t production as a function of changes in pressure, temperature and bul k composition.