THE EFFECT OF F(O-2) ON PHASE-RELATIONS AND MELT COMPOSITION IN A PARTIALLY MOLTEN AMPHIBOLITE

The influence of f(O2) on phase relationships and melt compositions in a partially molten synthetic amphibolite (65 wt% natural hornblende, 32 wt% natural plagioclase (An(57)), 3 wt% natural biotite) was invest igated at P = 10 kbar and T = 1000 degrees C in a piston-cylinder appa ratus. C-H-O fluids in equilibrium with graphite were used as redox se nsors. f(O2) Was varied over an interval of 4 orders of magnitude (4 l og units). Different redox conditions are due to different pressure tr ansmitting materials and different starting compositions in the sensor capsule. The effect of f(O2) on the phase compositions is small in th e investigated redox interval. The recrystallized amphiboles show a sl ight increase of X-Mg with decreasing f(O2) (i.e. increasing f(H2)). T he composition of the partial melts does not change systematically wit h f(O2) except for FeOtotal. FeOtotal decreases from approximately 6.9 wt% at log f(O2) = -9.5 to 5.5 wt% at log f(O2) = -13.6. In some runs orthopyroxene was observed as additional new phase. The composition o f partial melts coexisting with orthopyroxene is very different from t he runs without orthopyroxene. The most remarkable difference is the h igh X-Mg of melts in orthopyroxene bearing samples. The composition of these melts approaches the composition of parental magmas of anorthos ites.