PARTIAL PRESSURES OF CRYSTALLIZATION AND OXYGEN FUGACITIES FOR THE JUAN DE FUCA RIDGE, VOLCAN DE FUEGO AND VOLCAN DE PACAYA (GUATEMALA): A COMPARATIVE STUDY OF THE DEPTHS OF MAGMA STORAGE AND REDOX CONDITIONS FOR MID-OCEAN RIDGES AND ARC VOLCANOES

Basaltic volcanism occurs in a wide range of geologic settings and is associated with both flux melting and melting due to adiabatic decompression. In order to better understand the magma plumbing systems of mafic volcanic systems in various geologic settings, and to characterize the composition and redox conditions of the mantle sources of these mafic melts, we investigate the partial pressures of crystallization (PPCs) and oxygen fugacities (ƒO₂) for basaltic glasses from the Juan de Fuca Ridge (JdFR), an intermediate-rate spreading ridge, and Fuego and Pacaya volcanoes (Guatemala), two highly-active arc volcanoes with basalts as their primary eruptive products. PPCs are determined using the geobarometer described by Kelley and Barton (2008), which quantitatively calculates pressures by comparing the compositions of mafic glasses to those of experimental glasses in equilibrium with olivine, plagioclase, and clinopyroxene at various pressures, as described by Yang et al. (1996). The Olivine-Melt Equilibrium method is used for ƒO₂ calculations. This method was developed by Barton (2003), and uses equilibrium between olivine and glass in order to determine the melt Fe³⁺/Fe²⁺ ratio to calculate the ƒO₂. It can therefore be applied to glass data for which the Fe³⁺/Fe²⁺ ratio has not been determined analytically.

Published glass data are used for these calculations for the JdFR, and a mixture of published glass data analyses of glass in new samples will be used for the calculations for Fuego and Pacaya. For the JdFR, our results show that there is a general decrease in pressure with increasing latitude. In addition, we have determined that virtually all crystallization occurs within the crust. There is no evidence for significant crystallization in the mantle as suggested by previous workers. The average ƒO₂ for the JdFR is FMQ-1, which is similar to ƒO₂ values determined at other MORs using the olivine-melt method. The samples from Fuego and Pacaya will allow for the direct comparison of the PPCs and ƒO₂ for these volcanoes to those for the JdFR through the use of internally consistent methods.