At the University of Wisconsin Rare Gas Geochronology Laboratory, the groundmass packets and mini-cores were ncrementally heated in a double-vacuum resistance furnace attached to a 300 cm3 gas clean-up line. Prior to each incremental-heating experiment, samples were heated to 500-675 �C and pumped to remove potentially large amounts of atmospheric argon and water (Baksi, 1974). Fully automated experiments consisted of 4-10 steps from 675-1450 �C; each step included a 2 min increase to the desired temperature that was maintained for 10-15 min. During the heating time, and for an additional 5-8 min afterward, the sample gas was exposed to three SAES C50 Zr-Al getters. Isotopic measurements and data reduction followed the procedures of [...]
