Table T4. Apatite fission track analytical results.

Hole, core	Lithology	Number of grains	Standard track density (x 106/cm2)	Fossil track density (x 104/cm2)	Induced track density (x 105/cm2)	2 probability (%)	Central age ±1 (Ma)	Mean track length* (µm)	Standard deviation (µm)
180-1108B-6R	Microgranite clast	22	1.43 (4919)	4.97 (27)	4.652 (2527)	25	2.8 ± 0.6	14.6 ± 0.4 (5)	1.0

Notes: Parentheses enclose number of tracks counted (density) or measured (track length). Standard and induced track densities were measured on mica external detectors (geometry factor = 0.5), and fossil track densities were measured on internal mineral surfaces. Apatites were mounted in epoxy resin on glass slides, ground and polished to reveal an internal surface, and then etched for 20 s at room temperature in 5-N HNO₃ to reveal spontaneous fission tracks. Apatite ages were determined using the external detector method. Samples were irradiated at the Oregon State University Nuclear reactor in the slow soaker position B-3 (thermal column number 5), which has a Cd for Au ratio of 13.6 at the column face. The mounts were counted at a magnification of 1250x. Ages were calculated using the zeta calibration method (zeta = 361 ± 10 for dosimeter glass CN5) following the procedures of Hurford and Green (1983) and Green (1985). Analytical errors were calculated using the "conventional method" (Green, 1981). The ² test performed on single-grain data (Galbraith, 1981) determines the probability that the counted grains belong to a single age population (within Poissonian variation). If ² < 5%, it is likely that the grains counted represent a mixed-age population with real age differences between single grains. Track lengths were measured using "confined" fossil fission tracks using only those that were horizontal (Laslett et al., 1984). Tracks were measured under a 100x dry objective using a projection tube and a digitizing tablet attached to a microcomputer.