INFLUENCE OF RADIATION DAMAGE ON TITANITE (U-TH)/HE DATES FROM THE KAAPVAAL CRATON, SOUTHERN AFRICA

Titanite (U-Th)/He (THe) dating is a little-utilized but promising mid-temperature thermochronometer that early diffusion studies showed is sensitive to temperatures of 220-190°C (Reiners and Farley, 1999; Stockli and Farley, 2004). Despite the numerous advances in high (>300 °C) and low (<120 °C) temperature thermochronometric techniques, the community has few well-developed mid-temperature thermochronometers. Titanite is a relatively common accessory phase in igneous and metamorphic rocks, and THe dating provides an opportunity to better constrain thermal histories in the 300 to 120 °C temperature range. Recent work on apatite (e.g., Shuster et al., 2006; Flowers et al., 2009) and zircon (Guenthner et al., 2013) He diffusion kinetics revealed that radiation damage influences He retentivity, however, this effect has not been previously recognized in titanite.

We acquired 50 single-grain THe analyses from 11 Archean basement samples across the northeastern Kaapvaal craton, southern Africa. The THe dates range from 20.2 ± 0.2 Ma to 1140.9 ± 12.9 Ma and are negatively correlated with their ~5.0-490 ppm span of effective U concentration (eU). This pattern is consistent with radiation damage increasing He diffusivity, analogous to zircon (Guenthner et al., 2013). To more fully test this idea we utilized Raman spectroscopy to qualitatively assess radiation damage magnitudes in a subset of 4 samples that encompass the full eU span of our dataset. The pattern of Raman peak broadening associated with increasing eU in the African titanites supports the use of eU as a proxy for radiation damage in this sample suite. Stepwise degassing ⁴He diffusion experiments on the same 4 titanite samples yield effective closure temperatures that vary by ~200°C and are negatively correlated with eU. Together these results document a strong dependence of titanite He diffusivity on radiation damage. The THe dates fall within a nearly 3 b.y. interval of the craton’s history for which almost no thermochronometric constraints exist. They also have intriguing implications for a significant but previously unidentified Proterozoic erosion event across the ‘stable’ craton, thus demonstrating the utility of this tool for providing new insight into poorly understood portions of geologic histories.