Stable isotopes as tracers in hydrogeologic processes: Case studies from the Connecticut River of New England and the Heart Mountain fault of Wyoming and Montana.

摘要

Recently, it has been proposed that detachment of the 3400 km 2 low angle (3°) Heart Mountain detachment fault of northwestern Wyoming and southwestern Montana was facilitated by the presence of lubricating fluids. A recent stable isotopic study (Templeton et al., 1995) suggests that fluids along the Heart Mountain fault detachment surface originated from hydrothermal systems associated with extensive Eocene intrusive centers. Herein, we present results from a combined stable isotopic, fluid inclusion and 40Ar/ 39Ar geochronologic investigation of the relationship between shallow crustal fluids at the Eocene intrusive center of the New World Mining district and fluids associated with the Heart Mountain fault. Our results suggest that Eocene intrusives at the New World Mine focused magmatic-hydrothermal fluids along specific units and structures of similar composition to those at the Heart Mountain fault at the same time as emplacement of the Eocene intrusives, between 50.1 and 48.1 Ma. These fluids were focused along the Heart Mountain fault from the breakaway region near Silvergate, Montana toward Heart Mountain in Cody, Wyoming. These data suggest that the hydrothermal activity generated by intrusion of Eocene rhyodacite and dacite magmas provided the source of fluids associated with Heart Mountain faulting.; In a separate study, we studied the relationship between land use, geology and water chemistry in the Connecticut River watershed of northeastern USA by combining Geographic Information Systems (GIS) with hydrogen and nitrogen stable isotopic analyses, 87Sr/86Sr analyses and major cation and anion geochemistry. Nitrate concentrations and hydrogen and nitrogen isotopic ratios suggest non-point source nitrate inputs to the Connecticut River are directly related to land use practices in the pristine forested ecosystem and in areas of urban development. Results from 87Sr/86Sr values and major element concentrations from river water suggest that major cations in the Connecticut River water are dominated by carbonate weathering despite the limited presence of calcareous metasedimentary rocks (∼10% of watershed bedrock). In addition, the relative contribution of carbonate to silicate weathering increases down river as a result of input from tributaries draining Paleozoic carbonate-bearing rocks.