This project will: (a) Investigate the effects of weathering and hydrothermal circulation on Re-OS isotope systematics; (b) Test the potential for high-precision geochronology on young rocks using the Re-Os dating system; and (c) Investigate the degree of involvement of the continental crust during the formation of flood basalt. Results from this investigation should bear significantly on the use of the Re-Os isotopic system as a chronometer and offer new information for the Os isotopic composition of the mantle and crust.