(U-TH)/HE DATING OF DETRITAL APATITE IN HILLSLOPE AND FLUVIAL SEDIMENT

In order to explore the potential of detrital apatite (U-Th)/He dating, we measured apatite He ages in soils and in hillslope and fluvial sediment in catchments in the Washington Cascades and Sierra Nevada. Our largest sample set is from the Icicle Creek drainage in the Cascades, a 540-km² basin with 2.4 km of relief that primarily drains the ~95-Ma Mt. Stuart batholith. Apatite He ages of bedrock in the catchment range from 9 to 31 Ma (with one sample at 54 Ma), multiple single-grain replicates with ages >10 Ma show crystal-size-age correlations, and a 1.5-km vertical transect shows an age-elevation with a slope of ~0.1 km/myr between 18 and 31 Ma. Excluding wildfire-reset crystals, detrital apatite He ages of modern fluvial sediment draining the catchment range from 9 to 45 Ma (with two samples at 75 and 81 Ma). Assuming a constant proportion between areal distribution and apatite yield, as well as flat age contours across topography, these data can be transformed into a model age-elevation trend for the catchment, which yields ages ranging from 11 to 45 Ma (or to 81 Ma) between 1.0 and 2.4 km, indicating a slow (~0.03 km/myr) erosion rate (or PRZ) during this time. Ages in the lowest ~0.5 km in this model transect, however, range from only 9.3 to 11 Ma, consistent with an episode of relatively rapid (~0.3 km/myr) erosion beginning ~10 Ma (or faster modern erosion at low elevations).

Although these results show promise for detrital apatite He dating, several observations from both the Cascades and samples in the Sierra Nevada suggest complications. First, in many locations, apatite He ages from colluvium, soil, fluvial sediment, and the outermost ~3 cm of exposed bedrock, show fission-track-He age inversions (FT age much younger than He age) and anomalously young ages that are diagnostic of wildfire resetting, due to the contrasting kinetics of He diffusion and FT annealing on short timescales. As much as 50% of apatites in soils show wildfire resetting, although a much smaller proportion in fluvial sediment exhibit these characteristics. Second, many detrital apatites in soil and fluvial sediment show dissolution pits parallel to the c-axis, and significant to extreme rounding suggesting bulk dissolution and/or abrasion. Besides the fact that such properties may affect He ages of measured grains, this may also mean that some proportion of apatites from the landscape may not enter the fluvial system.