UC San Diego

Volatile studies in various tectonic settings have revealed important information about interactions between different mantle reservoirs and the Earth's surface. By employing a combined noble gas and stable isotope approach, we are able to discern surficial processes from intrinsic mantle characteristics. This dissertation discusses laboratory and field based instrumentation developments, which enable high precision measurements of volatile species in the laboratory and improved sampling techniques of volatiles in the field. In addition, the origin(s), transport and behavior of volatiles are discussed in three plume-rift related geological settings : a) Rungwe Volcanic Province (RVP), b) Iceland, and c) the Central Indian Ridge (CIR). Following a short introduction to the utility of noble gas and stable isotopes as geochemical tracers in plume-rift systems in Chapter I, we discuss the main objectives of the various studies that comprise this dissertation. Chapter II is a description of syringe pump apparatus for the retrieval and temporal analysis of helium (SPARTAH), a recently developed, semi-autonomous instrument, which enables continuous collection of fluid samples in the field. Fluids can subsequently be sectioned, time-stamped and targeted for volatile characteristics (e.g., He-isotopes), thus creating a time-series record of chemical perturbations in the groundwater system. Chapter III describes a N₂ extraction and purification system recently constructed in the Fluids and Volatiles Laboratory at SIO. We describe the configuration of the system as well as initial test results that demonstrate the utility of our technique. Chapter IV examines He-CO₂ and abundance systematics of RVP geothermal fluids and gases. We evaluate the origin of cold CO₂ mazuku-like features, estimate CO₂ fluxes, and address the apparent He -isotope disparity observed between gas, fluid, rock and mineral phases in the region. Chapter V focuses on the CO₂ isotope and abundance systematics of Iceland geothermal fluids, gases and basalts. We consider various processes such as hydrothermal phase separation, calcite precipitation and degassing which may act to alter intrinsic mantle- source features and attempt to reconstruct source characteristics of the Icelandic mantle prior to modification. In addition, CO₂ fluxes are estimated for Iceland and compared with previous estimates from Iceland and analogous geological settings. Chapter VI investigates the combined Ne-N-Ar-CO₂ systematics of the CIR as well as Ne-Ar systematics of Réunion xenoliths. We demonstrate that a Réunion-like plumecomponent is evident in the coupled Ne-N-Ar systematics and that both solar and recycled crustal components are detectable in the Réunion- plume source. Finally, we adopt a coupled assimilation and fractional equilibrium degassing model to explain and reconstruct initial volatile ratios and CO₂ source characteristics