Although hydrothermal vent systems have been intensively studied at mid-ocean ridges where ocean crust is being created, few such studies have been carried out in back arc basin spreading systems where crust is being subducted back into the mantle. This research assesses the chemical evolution of hydrothermal activity and its linkages to associated vent biology in one such back arc basin spreading center in the East Lau Basin. A unique series of time series fluid samples that were collected in 2009 will be analyzed for a variety of geochemical and isotopic species, including magmatic volatile species. These data will be combined with temperature measurements to examine the effect of acid volatiles on the evolution of hydrothermal systems overlying silicic magmas and how this influences the mobilization of metals. It will also be used to examine the role of seawater circulation in the ocean crust and its impact on the removal of CO2 via mineral precipitation. Broader impacts of the work include the support of an early career scientist, involvement of faculty and undergraduates from a college that serves economically struggling urban centers in southeastern Massachusetts, and engaging undergraduates involved in local summer sea-going programs. Hands-on learning modules for K-8 students will be created.
The East Lau Spreading Center (ELSC) was named one of three RIDGE 2000 Integrated Study Sites in part because of the range of geologic substrate resulting from systematic variations in the extent of subduction inputs from north to south. Initial expeditions characterized five sites of hydrothermal venting, showing a range of crustal, fluid, and biological characteristics. Unlike the basalt-hosted hydrothermal systems to the north, the southernmost felsic systems, with a greater portion of subduction inputs, have high concentrations of magmatic gases that influence fluid composition, depositional structures, and biological communities. But how do these systems change with time, given that much of what is known of these systems stems from a single visit in 2005? To address these gaps in our knowledge, a comprehensive suite of hydrothermal fluid samples collected from six vent filed located at the Eastern Lau spreading center were analyzed for their chemical and isotopic composition. Results of this study reveal both similarities and differences in the chemical and isotopic composition of vent fluids collected in 2009 relative to samples collected in 2005. For example, the northernmost vent field at Kilo Moana was characterized by decreased temperature, decreased dissolved concentrations of Fe, Mn, and H2S and increased pH (25°C). In contrast, vent field further to the south showed identical compositions and temperatures to those observed in 2005. Our data set documented the time scales over which back-arc hydrothermal systems evolve and the role that magmatic volatiles have in the evolution of hydrothermal systems. Moreover, the results allow quantitative constraints to be placed on chemical fluxes associated with hydrothermal activity in back-arc environments. The composition of fluids determined during this study also provided a context for the assessment of microbial community composition and function across the Eastern Lau spreading center. Results of these efforts demonstrate that microbial communities associated with hydrothermal mineral deposits in back-arc basins are taxonomically similar to those from mid-ocean ridge systems, but differences in geologic processes between vent fields in a back-arc basin can influence microbial community composition. Similarly, variations in the types of symbiotic microbes associated with gastropod hosts correlate with subsurface geothermal processes. Involvement of three graduate students in the research conducted as part of this study provided opportunities for the training and professional development of early career scientists.
