Experimental and theoretical constraints on the hydrothermal alteration of ultramafic rocks

This thesis is composed of three separate papers that have as a common theme the application of experimental and theoretical techniques to better understand hydrothermal alteration processes within the earth's oceanic crust in general and ultramafic terrain in particular. Chapter one describes experiments conducted to assess seawater alteration of ultramafic rocks at temperatures near 400°C at 500 bars. This study emphasizes the role of olivine and pyroxene reaction rates in controlling major element chemistry and pH in ultramafic-hosted hydrothermal systems in the subseafloor. Results of this study provide important constraints on the vent fluid chemistry measured in the ultramafic-hosted Rainbow hydrothermal system on the Mid-Atlantic Ridge. Chapter two is a study assessing the role of mineral hydration reactions in providing heat to drive hydrothermal circulation in ultramafic-hosted systems. Results of this study demonstrate that exothermic heat derived during mineral hydration is likely to be ineffective at increasing the temperature of the hydrothermal fluid, especially at high water to rock mass ratios. Chapter three is an experimental study investigating the mobility and distribution of rare earth elements during ultramafic rock alteration. It has long been argued that the similarity in chondrite-normalized REE patterns of mid-ocean ridge hydrothermal fluids to that of plagioclase phenocrysts indicating crystal chemical exchange between the fluid and plagioclase is the dominant control on REE distribution and mobility in subseafloor hydrothermal systems. However, results of this study indicate that development of LREE enrichment and strong positive Eu anomalies occurs independent of primary lithology. These three studies provide constraints on the nature of thermal and chemical exchanges expected during the hydrothermal alteration of ultramafic rocks in the subseafloor.