THE EXPANDING TEMPERATURE SENSITIVITY RANGE OF (U-TH)/HE THERMOCHRONOLOGY FROM IMPROVED UNDERSTANDING OF THE “BIG THREE” (APATITE, ZIRCON AND TITANITE): APPROACHES AND EXAMPLES (Invited Presentation)

(U-Th)/He thermochronology is a rapidly changing field, with much research focused both on developing non-traditional He thermochronometers as well as on improving our understanding of the three most widely utilized minerals used in (U-Th)/He dating – apatite, zircon, and titanite. Over the last decade, the recognition that the accumulation and annealing of radiation damage in these three phases can strongly influence their He diffusivity has led to the deciphering of thermal histories over a wide temperature range (up to a 200 °C window) using only a single mineral. Here we give two examples of how radiation damage effects on the “big three” He thermochronometers can be exploited to obtain new constraints on thermal histories and the geologic events that they record.

First, we present a zircon and apatite (U-Th)/He (ZHe, AHe) dataset from Proterozoic basement rocks of the Colorado Front Range in which the He dates for high-damage zircon (~20 Ma) are reproducibly younger than AHe dates (~55 Ma) for the same and nearby samples. The “inverted” ZHe and AHe dates are consistent with He diffusion kinetic models, provide insight into a previously undetected thermal event localized along the range front, and demonstrate the promise of using He data for high-damage zircons to detect low-temperature (<50 °C) events within and below the temperature sensitivity of the AHe system.

Second, we show a titanite and zircon (U-Th)/He dataset from the Kaapvaal craton of southern Africa. Our work here demonstrates that titanite undergoes a decrease in He diffusivity with accumulated damage, similar to zircon. However, because of its lower U-Th concentrations than zircon, titanite commonly will access temperatures higher than zircon in the same sample. Our (U-Th)/He dates for these phases span ca. 1 Gyr and yield distinct but overlapping data patterns that allow for improved constraints on cryptic portions of the Kaapvaal craton’s history. The data unexpectedly imply that a Mesoproterozoic exhumation event was followed by regional reheating, possibly associated with Namaqua-Natal orogenesis. Together our examples show how thoughtful consideration and modeling of somewhat complex (U-Th)/He datasets can tap valuable new information about geologic histories.