THE EFFECT OF DIFFERENTIATION ON MAGMATIC OXIDATION STATE: A CASE STUDY FROM SOUTH SISTER VOLCANO, OR, USA

The origin of elevated oxidation states in subduction zone magmas is widely debated and potentially includes mantle source, degassing and/or differentiation. Effects of differentiation on magmatic fO₂ can only be evaluated on natural samples, as the fO₂ in experiments is typically buffered by oxide-metal powders or an apparatus. An additional complexity with using natural samples is that often subduction zone volcanoes erupt intermediate compositions, and thus 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 24 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 (8-17ka) were collected from the southern peripheral region of South Sister. Intermediate (andesites, dacites; 30-37 ka) and rhyolitic (1.2 to 24ka) flows are from vents now buried on the main cone and the periphery. For olivine-bearing samples, we use MELTS and the new olivine + spinel + aSiO₂(melt) barometer of Bell et al. (2024) to determine magmatic temperature and fO₂. For samples with >57 wt% SiO₂, we apply the geobarometer of Ghiorso and Evans (2008) to ilmenite-magnetite pairs obtain temperature and fO₂. We estimate magmatic H₂O contents using 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 (as a function of the Ni-NiO buffer) 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) ∆NNO; H₂O contents range from 1–3 wt%, 2–4 wt%, and >4.1 wt%. Sample fO₂ increases as whole rock composition evolves (i.e., increases in SiO₂). Temperature decreases as SiO₂ increases, which is expected during differentiation. There is a weak positive correlation between H₂O and increasing oxidation state. Trends in fO₂ and SiO₂ are independent of ages determined by Feirstein et al. (2011). It appears that fractionating assemblages required to produce the range of compositions erupted at South Sister increase the ferric iron content in residual liquids, thus increasing the oxidation state during magmatic differentiation from basalt to rhyolite.