UC San Diego

The study of volatiles in both subaerial and submarine terrestrial samples is central to understanding the compositions of, and interactions between, the mantle- crust-atmosphere system. This dissertation focuses on the origin, transport and behavior of noble gases at four different geologic settings - a) the San Andreas Fault Zone (SAFZ), a transform plate boundary, b) the Costa Rica margin, a convergent plate boundary, c) Iceland, a ridge- centered hotspot, and d) the Central Indian Ridge, a mid- ocean ridge segment near an off-axis hotspot. Following a brief introduction to the utility of noble gases as geochemical tracers (Chapter I), the helium characteristics of submarine fluid seepage in a strike- slip setting associated with the SAFZ are discussed in Chapter II. Cold seep sites at Extrovert Cliff (Monterey Bay) were chosen for deployment of submarine flux meters and continuous sampling of fluids over several weeks. We assess the origin of dissolved noble gases in the seep fluids, and determine the cause of any temporal variations of the volatile characteristics. Chapter III focuses on the He-CO₂ isotope and abundance systematics of submarine cold seep fluids emanating at the Costa Rica fore-arc. We evaluate the origin of dissolved helium and carbon in the fluids, and we estimate the total flux of carbon through fluid venting at mound structures at the fore-arc in order to better constrain the carbon mass balance for the Central America convergent margin. Chapter IV examines the He-Ne systematics of geothermal fluids and the He-Ne-Ar isotope and relative abundance characteristics of subglacial glasses from the neovolcanic zones and older parts of the crust in Iceland. We investigate the processes contributing to the apparent decoupling of the He and Ne isotope systematics observed previously on the Reykjanes Peninsula and question whether this phenomenon is common to other parts of the neovolcanic zones. Chapter V investigates the postulated interaction between the Central Indian Ridge (CIR) and the Réunion hotspot, located 1̃100 km off-axis to the west. The CIR passed over the hotspot 34 Ma ago, and the goal of this study is to test if CIR basalts still record a Réunion-like helium isotope signature