Experimental and geochemical studies of terrestrial and lunar magmatic processes

Experimental and geochemical studies were performed to understand the formation of certain terrestrial and lunar igneous rocks. Chapter one is a study of convergent margin magmatism at Medicine Lake Volcano, California. The petrogenesis of a suite of variably porphyritic, high-alumina lavas was inferred from field relations, hydrous melting experiments and geochemical modeling. I conclude that phenocryst-poor lavas formed by hydrous differentiation, while phenocryst-rich lavas formed by a combination of hydrous differentiation and crystal accumulation. Chapters two and three discuss the origin of oceanic island tholeiite at Kilauea volcano, Hawaii. Chapter two reports the results of phase equilibria experiments on an estimated tholeiite primary magma. The results show that tholeiite primary magmas are in equilibrium with depleted harzburgite at lithospheric depths beneath Hawaii. I propose that Hawaiian tholeiite forms by melting of garnet lherzolite in deep parts of a mantle plume and that these melts equilibrate with depleted harzburgite in the plume top. Chapter three is an ion-microprobe study of picritic glass grains from Kilauea volcano, Hawaii. These glasses have the highest MgO-contents of any glass found in Hawaii and were used to make the primary magma estimate studied in chapter two. The glasses have trace element abundances similar to other Hawaiian lavas and show variation consistent with derivation by partial melting of garnet lherzolite. Chapter four models the origin of lunar high-Ti magmas using the results of two experimental studies. First, the phase relations of the most Ti-rich lunar ultramafic glass, Apollo 14 black glass, were determined to 2.5-GPa. Second, the dissolution rate of ilmenite was measured in both highand low-Ti lunar magmas. The experimental results support the generation of the high-Ti lunar ultramafic glasses by assimilation of ilmenite and clinopyroxene into low-Ti primary magmas. This model allows the generation of high-Ti lunar magmas without overturn of the lunar mantle. The thesis also contains two appendices that discuss the construction and calibration of the piston-cylinder device used to perform the experiments. Thesis Supervisor: Dr. Timothy L. Grove, Professor of Geology