After irradiation, the mineral separates were analyzed by step heating, using a 6-watt argon-ion laser, as described in Layer and others (1987). On average, 12-16 fractions were collected for each sample. Standards were included with each set of unknowns to nomitor the neutron flux. Each gas fraction was analyzed for Ar isotopic ratios in a VG-3600 mass spectrometer. The argon isotopes measured were corrected fro system blank and mass discrimination, as well as Ca, K, and Cl interference. Where 3 or more consecutive heating steps yielded the same age (+ or (-) 1 sigma), which accounted for more than 50% of the released Ar, the weighted average of these steps was calculaated as the