AN EVALUATION OF LOW-TEMPERATURE APATITE U-TH HE THERMOCHRONOMETRY/

Three studies have been conducted to test the usefulness of apatite U- Th/He thermochronometry. The first study compared U-Th/He and fission track ages from quickly cooled plutonic rocks having concordant zircon and apatite fission track ages. Several samples analyzed show good he lium retention. The second experiment assessed the effect of compositi onal variations in apatite on closure temperature. Calculated closure temperatures agree with existing data and show no statistical dependen ce on composition. Arrhenius plots for all of the samples show linear trends at low temperatures (representing volume diffusion) that ''roll over'' to more shallowly sloping non-linear paths at higher temperatu res. The ''roll over'' temperature appears to increase with increasing Ci content in the apatite. The third test focused on fluorapatites fr om the KTB borehole in Germany to compare laboratory estimates with di ffusion behavior in a natural setting. The KTB apatites show a fair am ount of scatter and record U-Th/He ages that are younger than or equal to apatite fission track ages, indicating a closure temperature less than or equal to that for the apatite fission track method. Closure te mperatures predicted from the scattered data range from similar to 80- 120 degrees C (1 degrees C/Ma cooling rate) or similar to 95-135 degre es C (10 degrees C/Ma cooling rate). The low end of this range agrees with our laboratory diffusion estimates for fluorapatite. Copyright (C ) 1997 Elsevier Science Ltd.