Trace element proxies and mineral indicators of hydrothermal fluid composition and seafloor massive sulfide deposit formation processes

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution June 2017This thesis analyzes compositions of seafloor massive sulfide (SMS) deposits and related hydrothermal vent fluids to identify proxies of reaction zone conditions (host-rock lithology, hydrothermal fluid temperature and chemistry). Chapter 2 investigates the morphology, mineralogy, and geochemistry of SMS deposits from six vent fields along the Eastern Lau Spreading Center (ELSC), demonstrating that ELSC SMS deposits record differences in hydrothermal fluid temperature, pH, sulfur fugacity and host-rock lithology related to proximity to the nearby Tonga Subduction Zone. Chapters 3 and 4 focus on partitioning of Co, Ni, Ga, Ag, and In between hydrothermal vent fluids and chalcopyrite lining fluid conduits in black smoker chimneys. Chapter 3 develops secondary ion mass spectrometry (SIMS) as a technique to measure Co, Ni, Ga, Ag, and In in chalcopyrite and identifies a correlation between Ga and In in chalcopyrite and hydrothermal fluid pH. Chapter 4 presents new data on these elements in ELSC hydrothermal fluids that, combined with SIMS analyses of chalcopyrite chimney linings and previously published data on vent fluids from the Manus Basin, provide evidence that supports partitioning of Ag a lattice substitution for Cu. Together, concentrations of Ga, In, and Ag in chalcopyrite provide proxies of hydrothermal fluid pH and metal (i.e., Ag and Cu) contents.Work presented in Chapter 2 of this thesis was supported by the National Science Foundation through grants OCE-1038135 to GNE and MKT, OCE-1038124, OCE-0241796, OCE-1233037 to JSS, and OCE-0242088 to CGW. Chapters 3 and 4 were supported by the National Science Foundation through grants OCE-1038135 and NSF Graduate Research Fellowship to GNE and funding from grant NSF OCE-1536480 to MKT