Laser dating system was used to step-heat the samples and the monitors, using a 6 W argon-ion laser to step-heat small chips of melt rock following the techniques in Layer et al. (1987). During the heating process, a liquid N cold finger and a Zr-Al getter at 400 degree C purified the gas, which was then measured with a VG3600 mass spectrometer. The Ar isotopes measured were corrected from post irradiation decay of 37Ar and 39Ar, system blank, mass discremination, as well as Ca, K, and Cl interference reactions following procedures of outlined in McDougall and Harrison (1988). The weighted mean of the results obtained on the monitor samples was used on the ensuing calculations for their corresponding set of samples. Between 8 and 12 [...]