12 Tectonics and Magmatism in the Modern and Ancestral Cascades Volcanic Arc II

Wednesday, 2 April 2025
1:30 PM - 5:30 PM
Balboa/Calaveras

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

Paper number: 12-4
Program: Theme Sessions
Time: 2:30 PM – 2:50 PM

Authors

  • Laura Waters Presenting Author
    New Mexico Institute of Mining and Technology, Socorro, NM
  • Noël Skocko
    New Mexico Institute of Mining and Technology, Socorro, NM
  • Benjamin Andrews
    New Mexico Institute of Mining and Technology, Socorro, NM
  • Aaron Bell
    University of Colorado Boulder, Boulder, CO

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 57, No. 4

doi: 10.1130/abs/2025CD-410064

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