Master of Science, The Ohio State University, 2020, Earth Sciences

The depth of magma storage beneath volcanoes has been a primary focus of recent geophysical and petrological research. Investigation of magma plumbing systems has important implications for volcanic hazard mitigation and eruption forecasting, and also for our understanding of the origin and evolution of magmas. This work is particularly important at mid-ocean ridges, as they are responsible for the formation of the majority of Earth's crust. Previous petrologic studies of mid-ocean ridges have suggested that olivine-plagioclase-clinopyroxene-liquid cotectic crystallization begins at mantle depths, which has far-reaching implications for our understanding of the mechanisms for crustal accretion. We demonstrate a procedure for processing pressure results using the Kelley & Barton (2008) geobarometer, which significantly changes the interpretation of these results. This process allows for high-resolution interpretation of the pressures, and thus depths, of partial crystallization in mafic systems. Application of this approach to data from the Juan de Fuca Ridge suggests that olivine-plagioclase-clinopyroxene-liquid cotectic crystallization occurs within the crust, and not in the mantle as suggested previously. The results suggest that partial crystallization along the ridge is polybaric. In the southern portion of the ridge, seismically imaged melt lenses are within range of the calculated pressures, however, the average pressures suggest that the majority of olivine-plagioclase-clinopyroxene-liquid cotectic crystallization occurs at greater depths than the imaged melt lenses. This suggests multi-depth magma storage along much of the Juan de Fuca Ridge, with only the shallowest magma reservoirs being imaged by seismic studies.

Committee: Michael Barton (Advisor); Daniel Kelley (Committee Member); Thomas Darrah (Committee Member); Elizabeth Griffith (Committee Member) Subjects: Geology; Petrology