OXIDE EQUILIBRIA IN CALC-ALKALINE MAGMAS AT HIGH OXYGEN FUGACITY

Two-oxide thermometry/oxybarometry is one of the most useful methods for constraining T-fO conditions in many types of rocks under reducing conditions. However, recent experimental and field-based studies on dacites and rhyolites have shown this method produces systematic errors in T and fO under more oxidizing conditions using current models. We have analyzed oxides from several experiments of Luhr (1990) to evaluate the effects of fO on oxide composition in samples with andesite bulk composition. El Chichon trachyandesite doped with anhydrite was equilibrated under vapor saturated conditions at 850^oC and 2 kbar. Charge 139 was equilibrated at fO of magnetite-hematite (MTH) and charge 165 at manganosite-hausmanite (MNH). Both charges contain large, euhedral ilmenite as phenocrysts and smaller, euhedral magnetite as groundmass grains and inclusions in plagioclase and clinopyroxene phenocrysts. The ilmenite is (Fe²⁺_0.17Mg_0.06Mn_0.01Fe³⁺_1.50Ti_0.24)O₃ and the magnetite is (Fe²⁺_0.70Mg_0.24Mn_0.07Fe³⁺_1.84Al_0.11Ti_0.02)O₄ in charge 139. In charge 165 the ilmenite is (Fe²⁺_0.25Mg_0.06Mn_0.01Fe³⁺_1.30Ti_0.33)O₃ and the magnetite is (Fe²⁺_0.82Mg_0.20Mn_0.06Fe³⁺_1.73Al_0.12Ti_0.08)O₄. All the phenocrysts are chemically homogeneous, suggesting a close approach to equilibrium during the experiments.

Temperature and fO were calculated for each of the charges using the mixing models of QUILF (Andersen et al., 1991) and MELTS (Ghiorso & Sack, 1991). For charges 139 and 165, the QUILF model gives 173^o and 668^oC, compared with 907^o and 1215^oC respectively for the MELTS model. At 850^oC QUILF produces DlogfO of MTH -1.1 and MNH +0.2, whereas MELTS yields DlogfO of MTH -1.6 and MNH -0.2 for experiments conducted on those buffers. The thermometry is rather uncertain due to the acute intersection angle of the isopleths and probably the effect of errors in the mixing models. The fO₂ calculations for the MNH runs are more accurate than similar calculations in more felsic systems and are within predicted errors. This suggests that two oxides can be used for fO₂ calculation up to MNH in andesites provided there is an independent estimate of temperature. The fO calculations for the more oxidized MTH runs have unacceptably large and systematic errors, indicating that those systems need modification.