UC San Diego

Studies of volatile elements and their isotopes have provided fundamental constraints on the formation and evolutionary history of the many diverse chemical reservoirs that together form the primary building blocks of planet Earth. This largely stems from significant differences in volatile abundances and isotopic characteristics of Earth's dynamically evolving reservoirs, which are prone to modifications following the transfer of volatiles between them. In this dissertation, I discuss how different volatile sources in the solid Earth can be resolved by means of a combined noble gas and stable isotope approach. I investigate different volatile source regions in the silicate Earth, and the transfer of volatiles from source regions towards the exterior of Earth in various tectonic settings. These settings include the western Sunda arc, Indonesia, as an example of a divergent plate boundary, in addition to two prominent plume-influenced divergent plate boundaries; the East African Rift System (EARS) and Iceland.

Chapter I is intended to provide a general introduction to the use of noble gas and stable isotopes as geochemical tracers, with particular emphasis on their utilization in the elucidation of processes, timescales and source-specific features in the solid Earth. Following this overview, we discuss the main objectives of the various studies of which this dissertation consists.

Chapter II examines the combined He-CO2-N2 abundance and isotope systematics of geothermal fluids and gases from volcanic centers along the western Sunda arc, Indonesia. We assess controls on volatile provenance in this major subduction system by resolving volatiles associated with the sub-arc mantle, that includes the subducting slab and mantle wedge, from inputs derived from the over-riding arc crust.

Chapter III describes a combined He-Ne-Ar isotope study of mantle-derived xenoliths and lavas from different segments of the EARS. This coupled approach provides a powerful tool with which to identify volatile provenance from the deep mantle versus shallow lithospheric sources, which allows for a unique investigation of the number of plume sources located in the East African mantle, and the ultimate source of melts being supplied to the different segments of the EARS.

Chapter IV is a detailed study of noble gases and the stable isotopes of carbon and nitrogen trapped in fluid inclusions of mantle-derived xenoliths from the EARS. In considering their coupled systematics, we evaluate a number of mantle/crustal features controlling their elemental and isotope characteristics, and separate the various sources contributing to the volatile components trapped in the fluid inclusions.

Chapter V focuses on the N2 isotope and abundance systematics of subglacially formed basalts from the Iceland hotspot. After characterization of the nitrogen elemental and isotopic signals of the Iceland mantle plume, we investigate relationships with other isotope and relative abundance systematics in order to discern nitrogen characteristics of the mantle source underlying one of Earth's most prominent hotspots.

Finally, Chapter VI provides concluding remarks on these studies and discusses some future prospects and ways of addressing outstanding questions by, for example, further chemical and isotope characterization of the same sample suite discussed in this dissertation.