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

Doctor of Philosophy, The Ohio State University, 2017, Earth Sciences

Plate spreading at mid-ocean ridges (MOR) is responsible for the creation of most of the crust on earth. The ridge system is very complex and many questions remain unresolved. Among these are the controls on the architecture of magma plumbing systems beneath mid-ocean ridges of different spreading rates and in proximity to transform faults. Previous studies have called into question the hypothesis that a decrease in magma flux and increase in conductive cooling along transforms faults promotes higher pressures of partial crystallization, and that this also explains the higher partial pressures of crystallization inferred for magmas erupted along slow spreading ridges compared to magmas erupted along faster spreading ridges. To test these hypothesis, I undertook a detailed analysis of pressures of partial crystallization (PPC) for magmas erupted along the slow spreading Reykjanes Ridge (RR), indeterminate spreading Juan de Fuca Ridge (JdF), 3 transforms along the fast to intermediate spreading East Pacific Rise (Blanco, Clipperton, and Siqueiros), and 5 transforms along the slow spreading Mid Atlantic Ridge (Oceanographer, Famous Transform A & B, Kane, and 15°20'N). PPC were calculated from the compositions of glasses (quenched liquids) lying along the P (and T) dependent olivine, plagioclase, and augite cotectic using the method described by Kelley and Barton (2008). Published analyses of MOR basalt glasses sampled from the ridges and transforms were used as input data. Samples with anomalous chemical compositions and samples that yielded pressures associated with unrealistically large uncertainties were filtered out of the database. The calculated pressures for the remaining 459 samples for the RR, 564 samples for the JdF, and 1056 samples for the transforms were used to calculate the depths of partial crystallization and to identify the likely location of magma chambers. The RR results indicate that the pressure of partial crystallization decreases from 102 ± 37 (open full item for complete abstract)

Committee: Michael Barton Dr. (Advisor); W. Berry Lyons Dr. (Committee Member); Thomas Darrah Dr. (Committee Member); Derek Sawyer Dr. (Committee Member); Daniel Kelley Dr. (Committee Member) Subjects: Geology; Petrology

Master of Science, The Ohio State University, 2011, Geological Sciences

Plate spreading at the mid-ocean ridges is accompanied by intrusion of dikes and eruption of lava along the ridge axis. It has been suggested that the depth of magma chambers that feed the flows and dikes is related to the heat flux – the higher the heat flux the shallower the magma chamber. To examine this hypothesis, I determined the depths of magma chambers beneath the intermediate spreading Juan de Fuca Ridge (JdF) in the northeast Pacific and the slow spreading Reykjanes Ridge (RR) south of Iceland. Pressures of partial crystallization were determined by comparing the compositions of natural liquids (glasses) with those of experimental liquids in equilibrium with olivine, plagioclase, and clinopyroxene at different pressures and temperatures using the method described by Kelley and Barton (2008). Chemical analyses mid-ocean ridge basalts glasses sampled from along the RR and JdF were used as liquid compositions. Samples with anomalous chemical compositions and samples that yielded pressures associated with unrealistically large uncertainties were filtered out of the database. The calculated pressures for the remaining 519 for the RR and 479 samples for the JdF were used to calculate the depths of partial crystallization and to identify the likely location of magma chambers. The RR results indicate that the pressure of partial crystallization decreases from 102 ± 33 MPa at the Charlie Gibbs Fracture Zone to 21 ± 12 MPa at 56°N, then increases to 367 ± 68 MPa as Iceland is approached. Four magma lenses were identified at depths of 2.5±.8km, 5.2±.8km , 5.9±1km, and 6.7±1. The magma lens at 2.46±.83 km agrees very well with seismically imaged sill at 2.5 km (Peirce et al 2007). The JDF results indicate that the pressure of partial crystallization decreases from 200 to100±50 MPa from the Blanco fracture zone to the north along the Cleft segment of the ridge. Calculated pressures remain approximately constant at 87±.53MPa along ridge segments to the north of the Clef (open full item for complete abstract)

Committee: Michael Barton Ph.D. (Advisor); W. Berry Lyons Ph.D. (Committee Member); Wendy Panero Ph.D. (Committee Member) Subjects: Geology; Petrology