Date of Award:

12-2020

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Geology

Committee Chair(s)

John W. Shervais

Committee

John W. Shervais

Committee

Jeffrey G. Ryan

Committee

Alexis K. Ault

Committee

Dennis L. Newell

Abstract

Subduction is the geologic process in which one tectonic plate moves beneath another as it sinks into the Earth’s mantle. Subduction initiation in the Izu-Bonin Marianas system is the result of a gravitational failure during which one tectonic plate (the Pacific plate) spontaneously sinks beneath another (the Philippne Sea plate). Fluids released by the sinking plate that caused the overlying mantle to melt by reducing its meltimg temperature, forming the Izu-Bonin Mariana island arc system.

The resulting melts initially have the chemical compositions that are rich in silica and magnesia, and highly depleted in other elements, refered to a boninite or high-Mg andesite. The goal of this thesis is to examine the chemical evolution of boninite lavas as they ascend from the mantle to the surface. This study intergrates observations of crystal chemistry and textures, with models that show how crystallization alters the chemistry of the melt. Melts begin crystallizing near the base of the crust (≈7-10 km below the surface) and continue to crystallize as they ascend through the crust and into a magma chamber. Separate melts interact and mixwithin this magma chamber, forming a single hybridized melt. In some cases, this mixing results in the eruption of boninite lavas.

Our results also indicate that low-silica boninite and high-silica boninite come from mantle sources with different chemical compositions. It is unclear whether there was mantle source or whether the mantle source is just chemically evolving as melting continues. This question, along with questions on the chemical evolution of subduction fluids, will be examined during future chemical studies of these boninite lavas.

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Included in

Geology Commons

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