Evaluation of cosmogenic 3He and 21Ne production rates in olivine and pyroxene from two Pleistocene basalt flows, western Grand Canyon, AZ, USA

Combining cosmogenic He-3 and Ne-21 (He-3(c) and Ne-21(c)) measurements on both pyroxene and olivine from the Pleistocene Bar Ten flows (85-107 ka) greatly increases our ability to evaluate the accuracy of He-3(c) and Ne-21(c) production rates and, therefore, He-3(c) and Ne-21(c) surface exposure ages. Comparison of He-3(c) and Ne-21(c) age-pairs yielded by experimentally determined production rates and composition-based model calculations indicates that the former give more accurate surface exposure ages than the latter in this study. However, experimental production rates should be adjusted to the composition of the minerals being analyzed to obtain the best agreement between He-3(c) and Ne-21(c) ages for any given sample. Ne-21(c)/He-3(c) values are 0.400 +/- 0.029 and 0.204 +/- 0.014 for olivine and pyroxene, respectively, in Bar Ten lava flows, in agreement with previously published values, and indicate that Ne-21(c)/He-3(c) in olivine and pyroxene is not affected by erosion and remains constant with latitude, elevation, and time (up to 10 Myr). Samples with Ne-21(c)/He-3(c) that do not agree with these values may indicate the presence of non-cosmogenic helium and/or neon. The neon three-isotope diagram can also indicate whether or not all excess neon in mineral separates comes from cosmogenic sources. An error-weighted regression for olivine defines a spallation line [y = (1.033 +/- 0.031)x + (0.09876 +/- 0.00033)], which is indistinguishable from that for pyroxene (Schafer et al., 1999). We have derived a production rate of 25 +/- 8 at/g/yr for Ne-21(c) in clinopyroxene (En(43-44)) based on the 40Ar/39Ar age of the upper Bar Ten flow. Our study indicates that the production rate of Ne-21(c) in olivine may be slightly higher than previously determined. Cosmogenic He-3 and Ne-21 remain extremely useful, particularly when paired, in determining accurate eruption ages of young olivine- and pyroxene-rich basaltic lava flows. (C) 2009 Elsevier B.V. All rights reserved.