Helium diffusion and (U-Th)/He thermochronometry of titanite

We have explored the diffusivity characteristics of radiogenic He in titani te (sphene)and have developed analytical techniques for (U-Th)/He dating of this mineral. Results of incremental He outgassing experiments performed o n titanites from a variety of geological environments suggest a thermally a ctivated volume diffusion mechanism with an activation energy of 44.6 +/- 3 .4 (2 sigma) kcal/mol and a frequency factor of similar to 60 cm(2)/s. Diff usivity is highly linearly correlated with the inverse square of the grain size, indicating that the He diffusion domain in titanite is the crystal it self. For typical titanite grains of 200- to 800-mu m minimum dimension, th e He closure temperature is in the range 191 to 218 degrees C (for a coolin g rate of 10 degrees C/Myr). There is no indication in the titanites we stu died that radiation damage plays a major role in He diffusion. (U-Tn)/He ag es of titanites from quickly cooled rocks yield ages (with similar to 5-8% l sigma reproducibility) that are concordant with known ages, and (U-Th)/He ages of titanites from slowly cooled rocks are consistent with independent ly established cooling paths: supporting the closure temperature estimates. These experiments suggest that titanite (U-Th)/He ages may be useful for c onstraining cooling histories at temperatures near the lower limit of those accessed by feldspar Ar-40/Ar-39 dating but higher than apatite fission tr ack or (U-Th)/He dating. Copyright (C) 1999 Elsevier Science Ltd.