Diffusive equilibration of H2O and oxygen fugacity in natural olivine-hosted melt inclusions

Arc basalts are oxidized relative to mid-ocean ridge basalts; however, the mechanism through which arc basalts gain this elevated oxidative state is debated. This observation has been extended to include Fe3+/∑Fe measurements from olivine-hosted melt inclusions enclosed in relatively Fo-rich olivines (i.e., which have presumably trapped relatively unevolved liquid compositions as melt inclusions). Experimental data suggest, however, that Fe3+/∑Fe recorded by melt inclusions can diffusively equilibrate with the host magma on short timescales, perhaps as rapidly as timescales relevant for H+ diffusive equilibration. Here, we present major, minor, and trace element concentrations, H2O concentrations, and Fe3+/∑Fe 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 equilibration in a suite of natural melt inclusions. The H2O concentrations of Basalt of Round Valley Butte melt inclusions decrease with decreasing melt inclusion radius, consistent with diffusive H+ loss. The Fe3+/∑Fe do not correlate with H2O or inclusion diameter, suggesting that Fe3+/∑Fe equilibration does not occur on the same timescale as H+ diffusion. Calculated oxygen fugacities (ƒO2) from the partitioning of vanadium between olivine and melt are ∼1 log unit higher than those calculated from Fe3+/∑Fe. One explanation for these combined observations is that Fe3+/∑Fe of the Basalt of Round Valley Butte melt inclusions diffusively equilibrated with a host magma at low 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 of the melt inclusions and the partitioning of vanadium between olivine and melt. One volcanological process known to result in a decrease in ƒO2 of magmas stored at shallow pressure is the degassing of S, CO2, and H2O. We show that reasonable estimates for the extent of degassing of Basalt of Round Valley Butte magma, coupled with diffusive equilibration of both H+ and Fe3+/∑Fe, can reconcile Fe- and V-proxies for ƒO2. Additionally, we have compared estimates of ƒO2 from vanadium abundances of melt inclusions and their olivine hosts with recently reported measurements of Fe3+/∑Fe of the melt inclusions for several Mariana arc volcanoes and conclude that this process – small decreases in host magma ƒO2 driven by degassing and rapid equilibration of the Fe3+/∑Fe of olivine-hosted melt inclusions with this host magma - may be a widespread phenomenon.