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 helium retention. The second experiment assessed the effect of compositional variations in apatite on closure temperature. Calculated closure temperatures agree with existing data and show no statistical dependence 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 temperatures. The "roll over" temperature appears to increase with increasing Ci content in the apatite. The third test focused on fluorapatites from the KTB borehole in Germany to compare laboratory estimates with diffusion behavior in a natural setting. The KTB apatites show a fair amount 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 temperatures predicted from the scattered data range from similar to 80-120 degrees C (1 degrees C/Ma cooling rate) or similar to 95-135 degrees 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.