An empirical test of helium diffusion in apatite: borehole data from the Otway basin, Australia

We have analyzed helium ages of apatites from several boreholes in the Otwa y basin, Australia, to evaluate whether laboratory helium diffusivity can b e accurately extrapolated to conditions relevant in nature. Downhole apatit e helium ages define a broad swath of values from 78-71 Ma at the surface ( 15 degrees C) to nearly zero at depths corresponding to ambient temperature s of similar to 80 degrees C, The width of the swath results from uncertain ties in corrected borehole temperatures, differences in the thermal history experienced by the various boreholes, and possibly from slightly different helium diffusivities among the detrital apatite grains studied. In the eas tern Otway basin, the shape and position of the helium age profile is in go od agreement with predictions based on the extrapolation of laboratory diff usivity data for Durango apatite coupled with published thermal histories f or this part of the basin. In contrast, helium ages are much younger than p redicted in the western Otway basin. Based on measured ages from Otway sedi ments, which have been essentially isothermal over the last few million yea rs, an empirical diffusivity (D/a(2)) of 2 x 10(-15) s(-1) can be assigned to apatites residing at downhole temperatures of 67-97 degrees C. This empi rical diffusivity is consistent with laboratory diffusion measurements, dem onstrating that such measurements are reasonably accurate and can be applie d with confidence to natural geologic settings. Given this confirmation of the laboratory diffusivity data, the discrepancy between the observed and m odeled helium age profiles in the western Otway basin suggests that these s ediments recently experienced higher temperatures than presently supposed. (C) 1999 Elsevier Science B.V. All rights reserved.