Oceanic borehole fluid chemistry and analysis of chlorine stable isotopes in silicate rocks

In order to understand the chemical exchange between seawater and the oceanic crust, it is desirable to collect fluids circulating in the crust from different geological settings and crustal ages. In principle, formation waters can be collected by sampling oceanic borehole fluids, in a manner similar to the use of groundwater wells on land. Part I of this thesis describes recent effort to use open oceanic boreholes for collecting formation waters. Several processes may effect the composition of borehole fluids including those occurring within the borehole environment. The chemical composition of borehole fluids from DSDP Site 504B indicates mixing of unaltered seawater with a reacted fluid containing elevated concentrations of Ca and Si and decreased Mg, Na, and K concentrations. While these chemical changes are consistent with seawater/basalt reaction at the prevailing temperatures (160{dollar}spcirc{dollar}C), evidence is presented which suggests that the reacted fluid originates by reaction in the rubble accumulated at the borehole bottom. Fluids collected from DSDP Site 395A are chemically indistinguishable from seawater, suggesting that downhole mixing dominates borehole fluids from this Site. The chemical composition of fluids sampled from Site 534A suggests that formation water from the surrounding sediments is flowing into the open borehole.; The development and application of techniques for the analysis of chlorine stable isotopes in silicate rocks is the subject of Part II. Chlorine is extracted from rocks by pyrohydrolysis, purified, and analyzed as the Cs{dollar}sb2{dollar}C1{dollar}sp{lcub}+{rcub}{dollar} ion for the ratio of {dollar}sp{lcub}37{rcub}{dollar}Cl to {dollar}sp{lcub}35{rcub}{dollar}Cl by thermal ionization mass spectrometry. This method allows analysis of microgram quantities of Cl with a precision capable of resolving natural variations in Cl isotope ratios. Chlorine isotope analyses were conducted on samples of meteorites, fresh mid-ocean ridge and back-arc basin basalt glasses, and altered basalts. The results suggest that Cl isotopes are fractionated between the oceanic and interior Cl reservoirs on Earth. Some implications of this data are that Cl isotopes can be used to further understand the chemical evolution of the of the oceans and to investigate the cycling of volatile elements between the exogenous and interior Earth reservoirs.