Cosmogenic ^3He production rates in apatite, zircon and pyroxene inferred from Bonneville flood erosional surfaces

Abstract

^3He concentrations were measured in zircon, apatite and pyroxene separates from rhyolite surfaces along the Snake River near Twin Falls, Idaho. These fluted and potholed surfaces were presumably scoured by the Bonneville outburst flood ~17.5 ka. Several of the samples contain inherited ^3He from prior exposure, reflecting the complex incision history of the Snake River canyon and suggesting earlier flood events. Each individual mineral from the remaining surfaces yielded ^3He concentrations that are within error of each other. ^3He produced by neutron capture on ^6Li was established from shielded samples and a simple neutron production model. The remaining ^3He is spallogenic and was used to calibrate the production rate of cosmogenic ^3He in zircon, apatite and pyroxene by assuming minimal erosion since the Bonneville flood. In all three phases the resulting production rates are ~7–12% lower than previous ^3He production rate estimates obtained by reference to various production rates for ^(10)Be in quartz. This disagreement is partially reconciled here by recalculating previous results with a ^(10)Be production rate of 4.51 at g^(−1) a^(−1) in quartz. Adopting this revised rate brings three independent ^3He production rate estimates for zircon and apatite into agreement within 5%, with grand means of 103 ± 3 at g^(−1) a^(−1) in zircon and 133 ± 6 at g^(−1) a^(−1) in apatite. The major source of uncertainty in cosmogenic ^3He dating of these phases is now correction for ^3He produced by neutron capture on ^6Li. Calculations are presented for assessing the amount of uncertainty introduced by this correction as a function of Li concentration, cosmic-ray exposure age, and He closure age.

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