PETROLOGY OF MT. BAKER: VARIABLE REDOX STATES OF POST-KULSHAN CALDERA ANDESITES

The redox state of magmas is a factor in controlling volatile speciation, phenocryst compositions, and crystallization path. Arc magma oxygen fugacities span 6 log units, with a range of at least 4 log units at Mt. Baker from Fe-Ti oxide oxybarometry on andesites. Coeval (~300 ka) andesites provide the means to study the origins of redox state variations in simultaneously erupted andesites at the same volcanic field. While these andesites possess similar SiO₂, phenocryst assemblages and redox states are markedly different. Fe-Ti oxide oxybarometry on Coleman Pinnacle (CP) and the oldest Table Mountain flow (TM) shows non-overlapping fO₂ values, with a boundary at QFM+1.8. The TM flow ranges from QFM+0.8 to 1.8, while the CP flow ranges from QFM+1.8 to 3, one of the highest fO₂ values recorded at an arc volcano. Because Fe-Ti oxides do not crystallize over the entire history of an evolving magma, they capture only part of the redox state fluctuations. A more complete history is preserved in fine-scale redox state fluctuations in other phenocrysts. The CP flow contains a unique assemblage of pseudobrookite, magnetite and ilmenite, thermodynamically stabilized by substitution of Mg for Fe. Experiments on compositions of coexisting psb + ilm indicate an average fO₂ ~ QFM + 3.1, consistent with mt + ilm oxybarometry (fO₂ ~ QFM + 2.8). Pyrrhotite-magnetite assemblages in the CP flow also show a concordant fO₂. Zoned amphibole phenocrysts of the CP and TM flows provide additional information on fO₂ fluctuations, with microXANES measurements showing Fe³⁺/Fe_T zoning from 18 to 81% and 59 to 77% Fe³⁺/Fe_T, resp. Mg-rich zones in all amphiboles contain higher Fe³⁺/Fe_T than Fe-rich zones, with variations of up to 30% and 20% Fe³⁺/Fe_T, resp. This zoning preserves a record of fluctuations in fO₂ during amphibole growth. Redox states in amphibole are controlled by a dehydrogenation reaction and data from Popp et al. (2006) indicate fO₂ ranges of NNO + 4.75-8.75 (CP), and NNO + 6.5-8 (TM). These unrealistically high fO₂ values imply that dehydrogenation of the amphiboles during eruption overprinted the amphiboles with additional Fe³⁺. The H₂O content of the lavas correlates with fO₂, as the H₂O-poor TM magma is consistently less oxidized than the H₂O-rich CP magma. Water-soluble trace element and LREE abundances are also the lowest in the TM samples.