AN EXPERIMENTAL-STUDY OF THE FE-MN EXCHANGE BETWEEN GARNET AND ILMENITE

The exchange equilibrium 1/3Fe(3)Al(2)Si(3)O(12) + MnTiO3 = 1/3Mn(3)Al (2)Si(3)O(12) + FeTiO3 almandine pyrophanite spessartine ilmenite was studied by reversal experiments as a function of temperature (650 less than or equal to T less than or equal to 1000 degrees C), pressure (1 0 less than or equal to P less than or equal to 20 kbar), and chemical composition. Experiments were performed in a piston-cylinder apparatu s using starting mixtures consisting of 95% garnet and 5% ilmenite. At the lower temperatures, 3-5% PbO flux was added to the reactants. The PbO was reduced to metallic lead by the graphite of the capsules. The EMP analysis shows that ilmenite is essentially a solid solution of F eTiO3 and MnTiO3 with up to 4.5 mol% Fe2O3 (for Fe-rich compositions). Garnet is compositionally close to (Fe,Mn)(3) Al2Si3O12 but apparentl y contains up to 1.0 wt% TiO2. As garnet was usually analyzed within 5 -15 mu m distance from ilmenite grains, the Ti measured in garnet appe ars to be largely an analytical artifact (due to secondary fluorescenc e). This was confirmed by analyzing profiles across a couple construct ed from ilmenite and Ti-free garnet. The more than 100 exchange runs i ndicate that the distribution coefficient K-D [= (X-Mn(gnt) . X-Fe(ilm ))/(X-Fe(gnt) . X-Mn(ilm))] is essentially independent of P and decrea ses with T. With a few exceptions at Mn-rich compositions, the present results are consistent with previous studies on the Fe-Mn partitionin g between garnet and ilmenite. Contrary to previous studies, however, the narrow experimental brackets obtained during the present calibrati on constrain that, at constant T, K-D is larger for Mn-rich compositio ns than for Fe-rich ones. This compositional dependence of K-D will co mplicate garnet-ilmenite geothermometry. Mutually consistent activity models for Fe-Mn garnet and ilmenite, based on a thermodynamic analysi s of the present results and other phase equilibria studies in the sys tem Fe-MnO-Al2O3-TiO2-SiO2-O-2, Will be presented in a following contr ibution (M. Engi and A. Feenstra, in preparation).