Abstract:Titanite is an ideal mineral for (U- Th)/He dating because of its relatively high U and Th. In this paper, diffusion of helium has been characterized in the Fish Canyon Tuff (FCT) titanite based on high- precision laboratory step heating experiments. We present analyses of FCT titanite (U- Th)/He ages, evaluate data uncertainty, and discuss the influences on (U- Th)/He ages. The ln (D/a2) in FCT titanites determined from laboratory step heating experiments was negatively correlated with reciprocal temperature, as expected for thermally activated 〖JP2〗volume diffusion. The titanite (U- Th)/He ages and Th/U〖JP〗 ratio are clustered at 28.3~24.6 Ma and 4.6~5.5, respectively, with a mean value of 26.7±1.2 Ma and 5.2±0.2, respectively, which agree within analytical uncertainty with previously reported data, respectively, suggesting that the experimental procedure we established is reliable. The outer 20 μm of the 15 original titanite crystals had been partially or completely abraded and the abrasion thickness increased with increasing equivalent radius. The measured (U- Th)/He ages from FCT titanite fragment are older than the original bulk crystal ages (uncorrected for ejection), and some are approximately equivalent to the true (U- Th)/He ages of the whole crystals. The (U- Th)/He ages are closer to the true age of the original bulk crystal due to the larger size and thicker outer abrasion thickness of original crystals, suggesting that the uncertainty of FCT titanites (U- Th)/He age is correlated with grain size and abrasion thickness of the original crystal face.
