Mullen, E. and McCallum, I.S., 2007, Petrology of Mount Baker: variable redox states of post-Kulshan caldera andesites: Geological Society of America Abstracts with Programs, v. 39, n. 4, p. 66.
Petrology of Mount 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 (ü300 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 ü QFM + 3.1, consistent with mt + ilm oxybarometry (fO2 ü 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+/FeT zoning from 18 to 81% and 59 to 77% Fe3+/FeT, resp. Mg-rich zones in all amphiboles contain higher Fe3+/FeT than Fe-rich zones, with variations of up to 30% and 20% Fe3+/FeT, 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.