DIFFUSIVE RE-EQUILIBRATION OF WATER AND OXYGEN FUGACITY IN NATURAL OLIVINE HOSTED MELT INCLUSIONS

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 Fo_84-86 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 D_V^{olv/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 D_V^{olv/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 D_V^{olv/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.