Influence of wildfires on apatite and zircon (U-Th)/He ages

Abstract

Low closure temperatures of the apatite and zircon (U-Th)/He thermochronometers allow valuable constraints on timing and rates of bedrock exhumation through shallow crustal depths, but raise the possibility that shallow-level processes other than exhumation-related cooling may also influence He ages. A simple He diffusion model predicts that wildfires can completely or partially reset apatite He ages as much as 3 cm below rock surfaces and partially reset zircon He ages in the outermost 1 cm. Measured He ages in bedrock and sediments from the Washington Cascades that were exposed to extensive wildfires in 2001 show strong agreement with these model predictions. Apatite He ages decrease from a regionally consistent age of 19.5 ′ 1.2 Ma at a distance >3 cm from the rock surface to as low as 1.9 Ma in the outermost 1 cm, whereas zircon He ages decrease from 65 to 55 Ma over the same distance. Thin (<3 cm) flakes shed from a nearby boulder during or after the most recent fire have apatite He ages ranging from 9.7 ′ 0.6 to 17.2 ′ 1.0 Ma. The partial-resetting profiles are best explained by model thermal histories involving at least one short-duration (∼5-10 min), high-temperature (575-650 °C) event and at least one longer (30-40 min), lower-temperature (350-450 °C) event. Age-depth profile data may be useful in determining wildfire intensities or locations and also suggest that He ages of detrital apatites from some environments may be subject to bias from the thermal effects of wildfires.