Empirical constraints on the titanite (U–Th)/He partial retention zone from the KTB drill hole

We analyzed (U–Th)/He ages of titanite from both the initial pilot hole (4 km depth) and the main drill-hole (9 km depth) of the German Continental Deep Drilling Project (KTB), to empirically evaluate the titanite (U–Th)/He partial retention zone (HePRZ) and laboratory helium diffusion data. Laser (U–Th)/He ages on single titanite fragments from the KTB drill hole, uncorrected for α emission, range from ∼94 to ∼2 Ma with reproducibility better than ±9%. Nearly invariant titanite (U–Th)/He ages, concordant with reported apatite fission track data, suggest that rocks from the upper ∼3 km of the KTB drill hole rapidly cooled from temperatures >200 to <110 °C between ∼75 and 85 Ma, followed by protracted cooling. Below ∼3 km, mean titanite (U–Th)/He ages decrease systematically with increasing borehole depth and down-hole temperature in a pattern similar to the titanite HePRZ predicted on the basis of laboratory-determined He diffusivities. Our results demonstrate that experimental titanite He diffusion data are applicable in the natural setting and that titanite (U–Th)/He thermochronometry provides a means to reliably reconstruct and quantify thermal histories between ∼100 and 180 °C, bridging the temperature ranges constrained by other techniques such as 40Ar/39Ar K-feldspar multi-diffusion domain modeling (350–150 °C) and apatite fission track dating (<110 °C).