EFFECTS OF DEFORMATION ON APATITE (U-TH)/HE SINGLE-GRAIN AGES

Apatite (U-Th)/He thermochronometry is routinely used to evaluate exhumation processes and landscape evolution. It is a powerful tool for constraining rates of such upper crustal and surficial processes. However, in some cases the technique yields dispersed single-grain ages from a single sample. Previous investigations have shed light on factors affecting the internal consistency of single-grain ages. These factors include grain size, U concentration and zoning, the presence of inclusions, He implantation from high-U phases, and grain chemistry. Another possible explanation is the presence of crystal defects (dislocations and subgrain boundaries) in which age dispersion increases with increasing defect density. We present results from a first-order investigation into the effects of crystal defects on (U-Th)/He ages in apatite.

Preliminary (U-Th)/He apatite data from a small-scale shear zone sampled within the Round Valley Peak granodiorite between Big McGee Lake and Hopkins Pass in the Sierra Nevada show increasing single-grain age dispersion with increasing deformation. Single-grain ages from the most deformed sample, 1989S24, range from 87 to 36 Ma and yield an average of 57.9 ± 19.5 Ma. The moderately deformed sample from the shear zone margin, 1989S23, yields an average age of 51.8 ± 6.1 Ma, with ages ranging from 62 to 48 Ma. The undeformed country rock yields an age of 40.4 ± 1.7 Ma, with a much less dispersed age range of 42 to 37 Ma. We interpret these data to suggest dislocations and increased dislocation density affects the diffusion kinetics of He in apatite and hence the internal consistency of single-grain ages.

We approximate the dislocation density in apatite by first determining the stress recorded by apatite. From an experimentally derived flow law for apatite, we determined apatite experiences stresses similar to quartz at moderate temperatures (300 to 500 °C) and strain rates (10^-12 to 10^-14 sec^-1). Quartz microstructural analysis can therefore provide information on stress experienced by apatite. Since dislocation density scales with stress squared, we can approximate dislocation density in apatite for each sample, and therefore assess whether or not the variations in single-grain ages are a function of dislocation density.