Intellectual merit: The PACMANUS hydrothermal field of the eastern Manus Basin, and its neighbor, the DESMOS field, are unique and important deep-sea hydrothermal systems. They represent hydrothermal activity in a metal-rich and silica-rich setting related to back-arc spreading. PACMANUS was the focus of Leg 193 of the Ocean Drilling Program in late 2000 and early 2001. As such, a considerable investment was made in PACMANUS, and as the Leg 193 results are published a good general understanding of the system has emerged. Volcanological studies have defined the spatial arrangement and origin of volcanic facies within the upper several hundred meters of Pual Ridge, the site of PACMANUS. Rare earth element and sulfur isotope studies of secondary anhydrite both point to a periodic, but still somewhat nebulous, magmatic fluid input to the hydrothermal systems. Strontium isotopes indicate the extent of mixing between seawater and rock-dominated hydrothermal fluid. Mineral chemical studies have defined the temperatures and pH of hydrothermal alteration. Fluid inclusion studies have further documented the subsurface temperature profiles, demonstrated that temperatures must fluctuate, showed that fluid phase separation is widespread at all levels, and provided firm evidence that hydrothermal fluids cool conductively and, in some cases, cold entrained seawater heats conductively prior to fluid mixing. PACMANUS is scheduled to be revisited for detailed geophysical, geochemical and sampling studies in early 2005. There is little doubt that PACMANUS must be evaluated, pro or con, as a potential site for future IODP drilling. Therefore, this proposal requests support for additional laboratory studies on Leg 193 core samples, support for participation of the PI and an undergraduate research student on the upcoming cruise to PACMANUS and DESMOS, and support for laboratory studies on the material to be collected from hydrothermal vents on that cruise. The expected results will help constrain the key processes and parameters that control alteration and mineralization in a felsic back-arc spreading environment.

Broader impact: At Towson University, geology majors (there are only undergraduates) are required to conduct a year-long senior research project. One major impact would be to support a Towson undergraduate to participate in the upcoming NSF-funded cruise (Wolfgang Bach, Woods Hole Oceanographic Institute, is the PI and Chief Scientist) and related laboratory studies. Two more students would be involved in laboratory studies both before and after the cruise. More generally, fluid inclusion studies on PACMANUS specimens have had a major impact on our understanding of the subsurface hydrothermal regime. Inclusion results will continue to have a broad impact as they help constrain the interpretations arising form other types of data such as mineral chemistry, stable isotopes and trace elements. Finally, results from this proposal will be shared at American and European fluid inclusion meetings, informing those communities about current deep-sea hydrothermal research. Participation in those meetings also helps the PI keep current in the latest fluid inclusion techniques and applications, for possible use in future deep-sea hydrothermal studies.