HR: 1330h
AN: V32C-1029    [PDF]
TI: Effects of Wildfires on (U-Th)/He Ages
AU: * Mitchell, S G
EM: sgm1@u.washington.edu
AF: Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, WA 98195 United States
AU: Reiners, P W
EM: reiners@geology.yale.edu
AF: Department of Geology and Geophysics, Yale University, P.O. Box 208109, New Haven, CT 06520 United States
AB: Apatite and zircon (U-Th)/He thermochronometry yield valuable constraints on the timing and rates of bedrock exhumation through shallow crustal depths. However, the low closure temperatures of these systems (about 70 and 180 degrees C, respectively) raise the possibility that high-temperature ($>$300 degrees C) surficial thermal events unrelated to exhumation, such as wildfires, may significantly influence He ages near exposed rock surfaces. To test this possibility, we measured replicate single-crystal apatite and zircon He ages from 1-cm thick intervals in a depth profile of granodiorite bedrock from a recently burned area of the Washington Cascades. We also measured apatite He ages in modern colluvium in the same region. Apatite and zircon He ages have been reset by as much as 90% and 15%, respectively, in the outermost cm of exposed bedrock, and asymptotically increase, over about 3 cm, to regionally consistent ages measured in interior slabs. In addition, apatite He ages from thin ($<$ 3 cm) flakes exfoliating from a nearby boulder show up to 50% He loss compared to ages from nearby interior bedrock. These results agree well with a simple He diffusion model that predicts that "moderate" wildfire heating of rock surfaces (5-10 minutes at 500-600 degrees C) can substantially reset apatite He ages to ~3 cm below rock surfaces, and partially reset zircon He ages in the outermost 1 cm. These results are important for several reasons. 1) This significant loss of He near rock surfaces indicates that the outermost 3-5 cm of bedrock samples should routinely be removed prior to AHe or ZHe analysis. 2) He ages of detrital apatites derived from wildfire-prone regions may be substantially lower than the crustal exhumation ages, particularly if wildfires themselves generate a significant portion of sediment through mechanical weathering processes. 3) Wildfires likely also significantly disturb fission-track and cosmogenic He systems in bedrock rims and some exfoliated (potentially detrital) fragments. 4) Modeling the observed AHe and ZHe profiles through the bedrock surface allows us to constrain the recent thermal history of the sample; the model and data indicate that He loss from our sample likely resulted from the thermal equivalent of a 5-10 minute, 575-650 degrees C heating event. This technique may be useful for delineating wildfire zones or estimating wildfire intensities.
DE: 1035 Geochronology
DE: 1625 Geomorphology and weathering (1824, 1886)
DE: 3210 Modeling
DE: 5134 Thermal properties
DE: 9604 Cenozoic
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting