HELIUM DIFFUSION AND (U-TH)/HE THERMOCHRONOMETRY OF TITANITE - A POTENTIAL THERMOCHRONOMETER

Abstract

We have explored the diffusivity characteristics of radiogenic He in titanite (sphene) and have developed analytical techniques for (U-Th)/He dating of this mineral. Results of incremental He outgassing experiments performed on titanites from a variety of geological environments suggest a thermally activated volume diffusion mechanism with an activation energy of 44.6 +/- 3.4 (2σ) kcal/mol and a frequency factor of ~60 cm2/s. Diffusivity is highly linearly correlated with the inverse square of the grain size, indicating that the He diffusion domain in titanite is the crystal itself. For typical titanite grains of 200- to 800-μm minimum dimension, the He closure temperature is in the range 191 to 218°C (for a cooling rate of 10°C/Myr). There is no indication in the titanites we studied that radiation damage plays a major role in He diffusion. (U-Th)/He ages of titanites from quickly cooled rocks yield ages (with ~5-8% 1σ reproducibility) that are concordant with known ages, and (U-Th)/He ages of titanites from slowly cooled rocks are consistent with independently established cooling paths, supporting the closure temperature estimates. These experiments suggest that titanite (U-Th)/He ages may be useful for constraining cooling histories at temperatures near the lower limit of those accessed by feldspar 40Ar/39Ar dating but higher than apatite fission track or (U-Th)/He dating.