Intellectual merit. The Grand Canyon of the Yellowstone River, Yellowstone National Park, Wyoming, exposes 300-400 vertical meters of hydrothermally altered volcanic rocks that are very poorly known. The outcrops along the canyon contain the uppermost parts of the older (fossil) Yellowstone hydrothermal system. No other locations in the park provide access to cross-sections of the hydrothermal system, and the potential exists in the Canyon to learn about the deeper system that is elsewhere inaccessible. The active thermal springs at Yellowstone are well known, and as the surface expression of the vapor-dominated system, provide a model against which the results of the research proposed here can be tested. Preliminary field and laboratory work has found both acid-sulfate and alkali chloride epithermal mineral assemblages in the Canyon walls. These data suggest the presence of two fluids at some time in the past, analogous to the two fluids found in the active system today. The research proposed here will endeavor to provide a reconstruction of the upper 300-400 meters of the older hydrothermal system based on boiling in a fluid rising along the hydrostatic boiling curve, will evaluate the longevity ofthe Yellowstone hydrothermal system, will examine the older fluids and their relationships, and will assess hydrothermal mass and energy flux through the upper crust over time in the Yellowstone system.
Broader Impacts. Yellowstone Park is visited by millions of people from around the world every year. They come with a great interest in and curiosity about the park's geologic wonders. The Grand Canyon of the Yellowstone River is one of the most popular sites in the park, yet very little scientific information about the alteration in the canyon is available. This alteration, in fact, is the source of the park's name. It is hoped that the results of the research proposed here will be made available to the park's visitors to enhance their understanding and appreciation of the geologic processes that have shaped the park, and the world around them. Additional broader impacts include applications to volcanic hazards, geothermal resources, and ore-forming processes.
