Anne Feenstra á Martin Engi An experimental study of the Fe-Mn exchange between garnet and ilmenite Received: 12 September 1996 / Accepted: 11 December 1997 Abstract The exchange equilibrium 1=3Fe 3 Al 2 Si 3 O 12 MnTiO 3 1=3Mn 3 Al 2 Si 3 O 12 FeTiO 3 almandine pyrophanite spessartine ilmenite was studied by reversal experiments as a function of temperature (650 £ T £ 1000 °C), pressure (10 £ P £ 20 kbar), and chemical composition. Experiments were performed in a piston-cylinder apparatus using starting mixtures consisting of 95% garnet and 5% ilmenite. At the lower temperatures, 3±5% PbO ¯ux 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 FeTiO 3 and MnTiO 3 with up to 4.5 mol% Fe 2 O 3 (for Fe-rich com- positions). Garnet is compositionally close to (Fe,Mn) 3 Al 2 Si 3 O 12 but apparently contains up to 1.0 wt% TiO 2 . As garnet was usually analyzed within 5±15 lm distance from ilmenite grains, the Ti measured in garnet appears to be largely an analytical artifact (due to secondary ¯uorescence). This was con®rmed by analyzing pro®les across a couple constructed from ilmenite and Ti-free garnet. The more than 100 exchange runs indicate that the distribution coecient K D X gnt Mn X ilm Fe =X gnt Fe X ilm Mn is essentially independent of P and decreases with T. With a few exceptions at Mn- rich compositions, the present results are consistent with previous studies on the Fe-Mn partitioning between garnet and ilmenite. Contrary to previous studies, however, the narrow experimental brackets obtained during the present calibration constrain that, at constant T,K D is larger for Mn-rich compositions than for Fe- rich ones. This compositional dependence of K D will complicate garnet-ilmenite geothermometry. Mutually consistent activity models for Fe-Mn garnet and ilme- nite, based on a thermodynamic analysis of the present results and other phase equilibria studies in the system Fe-MnO-Al 2 O 3 -TiO 2 -SiO 2 -O 2 , will be presented in a following contribution (M. Engi and A. Feenstra, in preparation). Introduction Garnet occurs as a rock-forming mineral over a wide range of metamorphic grades and magmatic conditions and therefore is one of the most important phases used in petrogenetic calculations (see e.g., reviews of Essene, 1989, and Spear, 1993). In a vast majority of bulk compositions, the chemical composition of garnet can be approximated in the quaternary system Fe 3 Al 2 Si 3 O 12 - Mg 2 Al 2 Si 3 O 12 -Mn 3 Al 2 Si 3 O 12 -Ca 3 Al 2 Si 3 O 12 (almandine± pyrope±spessartine±grossular). To obtain accurate geothermobarometric results from phase equilibria in- volving multicomponent garnet, solution properties de- rived from experimental work are a prerequisite. Most of these experimental activity-composition (a-X) studies have been restricted to binary garnet joins (e.g., Fe-Mg: Geiger et al. 1987; Hackler and Wood 1989; Koziol and Bohlen 1992; Fe-Mn: Pownceby et al. 1987; Fe-Ca: Geiger et al. 1987; Koziol 1990; Ca-Mn: Koziol 1990; Gavrieli et al. 1996; Mn-Mg: Wood et al. 1994). Some experiments also involved ternary solutions of Fe-Mg- Ca garnets (Koziol and Newton 1989; Berman and Koziol 1991; Ganguly et al. 1996), Fe-Mn-Ca garnets (Pownceby et al. 1991) and Mg-Mn-Ca garnets (Gang- uly et al. 1996). Recently, limited experimental work on Fe-Mg-Mn-Ca garnets has been conducted by Koziol (1996) and Ganguly et al. (1996). These ternary and quarternary experimental data provide an important test for the current solution models for Fe-Mg-Mn-Ca gar- net (e.g., Ganguly and Saxena 1984; Berman 1990), which are mainly based on the binary experimental data. Contrib Mineral Petrol (1998) 131: 379±392 Ó Springer-Verlag 1998 A. Feenstra 1 (&) á M. Engi Mineralogisch-petrographisches Institut, University of Berne, Baltzerstrasse 1, CH-3012 Berne, Switzerland Present address: 1 Geo Forschungs Zentrum Potsdam, Telegrafenberg, D-14473 Potsdam, Germany Editorial responsibility: V. Trommsdor