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

Research Article| December 01, 2003 Influence of wildfires on apatite and zircon (U-Th)/He ages Sara Gran Mitchell; Sara Gran Mitchell 1Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, Washington 98195, USA Search for other works by this author on: GSW Google Scholar Peter W. Reiners Peter W. Reiners 2Department of Geology and Geophysics, Yale University, New Haven, Connecticut 06520, USA Search for other works by this author on: GSW Google Scholar Geology (2003) 31 (12): 1025–1028. https://doi.org/10.1130/G19758.1 Article history received: 17 Apr 2003 rev-recd: 05 Sep 2003 accepted: 07 Sep 2003 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Sara Gran Mitchell, Peter W. Reiners; Influence of wildfires on apatite and zircon (U-Th)/He ages. Geology 2003;; 31 (12): 1025–1028. doi: https://doi.org/10.1130/G19758.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search 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. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.