GSA Connects 2021 in Portland, Oregon

Paper No. 167-11
Presentation Time: 4:20 PM


HUMPHREYS, Jennifer, Earth and Planetary Sciences, University of California Riverside, Riverside, CA 92521, BROUNCE, M., Earth and Planetary Sciences, University of California Riverside, 1224b Geology Building, 900 University Avenue, Riverside, CA 92125 and WALOWSKI, Kristina, Geology, Western Washington University, Bellingham, WA 98225

Arc basalts are more oxidized relative to mid-ocean ridge basalts. This observation has been extended to include Fe3+/∑Fe ratio measurements from olivine-hosted melt inclusions enclosed in relatively Fo-rich olivines. Experimental data suggest, however, that the Fe3+/∑Fe ratios recorded by melt inclusions can diffusively re-equilibrate with the host magma on timescales as rapid as those of H+ diffusive re-equilibration. Here, we present major, minor, and trace element concentrations, water concentrations, and Fe3+/∑Fe ratios of naturally glassy olivine-hosted melt inclusions from the Basalt of Round Valley Butte, a cinder cone in the Lassen Region of northern California. These melt inclusions record H2O loss through diffusion of H+ and can be used to constrain the extent of Fe3+/∑Fe ratio re-equilibration in a suite of natural melt inclusions. Basalt of Round Valley Butte melt inclusions are contained within Fo84-86 olivines, have 6.1 – 6.8 wt% MgO, 0.3 –1.8 wt% H2O, 0.18 – 0.12 Fe3+/ΣFe, and 0.02-0.04 DVolv/melt inclusion. The H2O concentrations of BRVB melt inclusions decrease with decreasing melt inclusion radius, consistent with diffusive H+ loss. The Fe3+/∑Fe ratios do not correlate with H2O or inclusion diameter, suggesting that Fe3+/∑Fe re-equilibration does not occur on the same timescale as H+ diffusion. Calculated oxygen fugacities from DVolv/melt inclusion are ~1 log unit higher than those calculated from Fe3+/∑Fe ratios. One explanation for these combined observations is that Fe3+/∑Fe ratios of these melt inclusions diffusively re-equilibrated with a host magma at shallow pressure on shorter timescales than H+ diffusion. This scenario requires that the host magma decreased in ƒO2 after the time of melt inclusion entrapment to explain the offset between ƒO2 constraints from Fe3+/∑Fe ratios of the melt inclusions and DVolv/melt inclusion. One volcanological process shown to decrease ƒO2 of magmas stored at shallow pressure is the degassing of S, CO2, H2O. We show that reasonable estimates for the extent of degassing of Basalt of Round Valley Butte magma, coupled with diffusive re-equilibration of both H+ and Fe3+/∑Fe ratios, can reconcile Fe- and V-proxies for fO2. The rapid timescale of these processes indicates that this may be a more widespread phenomenon, including for Mariana arc volcanoes.