Estimating erosion rates and exposure ages with 36Cl produced by neutron activation

The isotopic composition of water-soluble Cl, extracted from mineral grains in granitic rocks, appears to reflect the relative age of debris-flow fan surfaces and the erosion rate of bedrock outcrops. Our data indicate that some granitic landforms are stable landscape elements eroding on the order of meters to tens of meters per million years. Samples collected from debris-flow fan surfaces of three distinct relative ages show the expected increase of model age with time but the 36Cl-exposure ages for boulders on any particular surface vary by a factor of more than five. The method we used for extracting Cl was designed to isolate the 35Cl (n, γ) 36Cl production pathway and to simplify the chemical preparation of samples. Such isolation from fluid inclusions allows direct interpretation of measured 36Cl/Cl ratios but requires a model to predict the thermal-neutron production and absorption profile below the rock surface. Using such a model, we find good agreement between measured and predicted 36Cl/Cl for eight samples collected below the ground surface as well as consistency between several groups of samples collected near one another. Using elemental abundance data for 129 granitic rocks, we modeled 36Cl/Cl supported by radiogenic neutrons and find that it varies from 4 to 125 × 10 −15 (mean = 21 ± 18, 1 σ). However, from the surface to a depth of 10 m, thermal neutrons resulting from muon interactions have the potential to generate more 36Cl than do radiogenic neutrons. The production of 36Cl in the subsurface has important implications for surface-exposure dating of young, low-altitude samples collected from moraines and debris-flow or alluvial fans, especially at ages where erosion of the boulder surface is significant or where the source area for boulders is at altitudes much higher than the landform to be dated.