203 Old and the New, Long and the Short: Perspectives on Integration of Timescales of Magmatic Processes: Special Session Related to MGPV Awards to Madison Myers and Anita Grunder

We encourage papers that challenge and give insights into the interplay between timescales, methodologies, and processes to build a cohesive understanding of the lifespan, evolution, storage, and eruption of magma, from plutonic to volcanic systems.

Tuesday, 21 October 2025
1:30 PM - 5:30 PM
HBGCC, 216AB

Magmatic Differentiation as the Source of Oxidation in Earth’s Continents: Perspectives from South Sister Volcano, OR, USA

Paper number: 203-8
Program: Technical Sessions
Time: 3:30 PM – 3:45 PM

Authors

Abstract

The origin of elevated oxidation states in subduction zone magmas could potentially be due to mantle source, degassing and/or differentiation. Effects of differentiation on magmatic fO2 can only be evaluated using natural samples, as the fO2 in experiments is typically buffered by the experimental technique. A complexity with using samples from stratovolcanoes to test the origin of the elevated oxidation states in subduction zones is that these volcanoes generally erupt a limited range of intermediate compositions, and thus provide a narrow compositional record of magmatic fO2. However, South Sister volcano, in the central Cascades, features eruptions ranging from basalt through rhyolite. We use mineral assemblages in 34 crystal-poor volcanic rocks (52.5 to 73.4 wt% SiO2) from South Sister to assess changes in magmatic fO2 as a function of composition. Mafic rocks were collected from the southern peripheral region of South Sister. Intermediate (andesite, dacite) and rhyolite flows are from vents on the main cone and the periphery. We use a new olivine + spinel + aSiO2(melt) barometer to determine fO2 for olivine(+spinel)–bearing samples. We use ilmenite-magnetite geothemobarometry to obtain temperature and fO2 for intermediate and rhyolitic samples. Magmatic H2O contents are determined via plagioclase hygrometry. For basalts, intermediates and rhyolites: magmatic temperatures range from 1106­–1195(±13)°C, 911(±25)–997(±24)°C and 842(±13)–868(±22)°C. Oxidation states range from –0.3(±0.4) to 0(±0.2), –0.1(±0.1) to +0.5(±0.1), and +0.6(±0.2) to +0.8(±0.05) log units relative to the NNO buffer. Sample fO2 increases as whole rock composition increases in SiO2; this trend is independent of age. Temperature decreases as SiO2 increases, which is expected during differentiation. There is a positive correlation between H2O and oxidation state; however, previous work suggests H2O degassing does not significantly impact Fe3+/ FeT (the oxidation state). Average oxygen isotopes ratios (as d18O; ±2s) for basalts, intermediates and rhyolites range from 6.2±0.5, to 6.1±0.3, to 6.6±0.6, suggesting that assimilation of hydrothermally altered crust is not the source of the increase in oxidation state. It appears that fractionating assemblages required to produce the range of compositions erupted at South Sister also increase the ferric iron content in residual liquids, thus increasing the oxidation state during magmatic differentiation from basalt to rhyolite.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414146

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