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 six 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 (Mariner and Vai Lili), 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 waht is knonw of these systems stems from a single visit or a return visit years later? Preliminary data indicate substantial changes in fluid composition at at one of these vent sites (Vai Lili) over a 16-year span during which vent fluid temperature decreased from 334°C to 120°C in 2005. We supplimented these sparse data with ~80 new discrete samples from 2006, 30 discrete samples from 2009, and several thousand samples from 7 continuous fluid samplers that were deployed from 2006 to 2009. These fluid samples were then aanlysed for their dissolved gas content, ion composition, and selected isotopic compositions. Results suggest that the temporal evolution of hydrothermal systems influenced by degassing of felsic magmas may be relatively rapid and follow a path that is not analogous to typical basalt-hosted mid-ocean-ridge hot-springs. Our data set allowed us to gauge the time scales over which back-arc hydrothermal systems evolve and the role that magmatic volatiles have in the evolution of hydrothermal systems. Using co-registered chemical and thermal data from a well-characterized geologic setting, we were able to asses the effect of this change while providing the geologic context for two concurrent microbiological studies.
