RESULTS

Table 1 lists the analytical details of the FT data measured for samples from Hole 976B together with the calculated ages, and the means and standard deviations of the track-length distributions. Sample 161-77R-2 (Piece 3B), failed to yield adequate apatite grains for analysis, whereas high numbers of inclusions that were found in apatite from Sample 83R-1 (Piece 11A), precluded confident identification of fission tracks.

Ages are central ages, modal ages weighted to account for the difference in precision of the individual crystal ages within a sample analysis. Two uncertainties attach to the central age: (1) the ± indicates analytical precision (derived from the numbers of tracks counted); and (2) the age dispersion signifies the spread of the individual crystal ages and tests if a single population is present. The significance of the age data is clearer on a radial plot (Galbraith, 1990), which is a graphical method for comparing crystals of different ages and precisions. Figure 1 shows radial plots of this type, as well as histograms of the single-crystal age data from Site 976, and confined track-length distributions for each sample analyzed in this study. The position on the x-scale in the radial plots records the uncertainty of individual age estimates, whereas each point has the same standard error on the y-scale (illustrated as ± 2). In essence, the further the data point plots from the origin, the more precise the measurement. The age of each crystal may be determined by extrapolating a line from the origin at the left of the plot through the crystal's x and y coordinates to intercept the age scale. The spread of single-crystal ages in each sample is very small: the indices of age dispersion show, in each case, relative errors <10% and P(²) values >5% (Table 1), which indicate that all analyzed grains are derived from a single population and have a common age with only Poissonian variation (Galbraith, 1981; Galbraith and Laslett, 1993). Such homogeneity of age suggests a relatively simple thermal history. Measured apatite central ages range between 15.2 ± 1.3 Ma and 21.4 ± 2.3 Ma (± 1) for samples drilled over a vertical distance of ~115 m. Plots of relative sample depth vs. measured ages (Fig. 2A) and mean track lengths (Fig. 2B) show overlapping results, which, at 95% confidence limits, are indistinguishable among samples.