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Empirical evidence for cosmogenic ³He production by muons

Larsen, I. J. and Farley, K. A. and Lamb, M. P. and Pritchard, C. J. (2021) Empirical evidence for cosmogenic ³He production by muons. Earth and Planetary Science Letters, 562 . Art. No. 116825. ISSN 0012-821X. doi:10.1016/j.epsl.2021.116825.

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Cosmic ray muons penetrate deeply into rock where they interact with atoms to produce cosmogenic nuclides. Incorporation of the muon contribution to the production rates of cosmogenic nuclides increases the accuracy of exposure dates, burial ages, and erosion rates inferred from measured nuclide concentrations. In the absence of empirical evidence, it is generally assumed that muons do not produce ³He, a cosmogenic nuclide commonly used for exposure dating. Here we assess whether muons produce ³He by measuring He isotope concentrations in pyroxene and ilmenite from a ∼300 m deep drill core and other subsurface samples of the mid-Miocene Columbia River Basalt in Washington, USA. ³He concentrations in our samples exhibit an exponential decline with depth with an e-folding length of 32.4 m, which corresponds to an attenuation length for ³He production of 8780 g cm⁻². The deeply penetrating exponential is diagnostic of ³He production by cosmic ray muons. Assuming no erosion, we constrain the minimum surface muonogenic production rate to be 0.23 atom g⁻¹ pyroxene yr⁻¹, whereas when incorporating erosion the production rate is 0.45 atom g⁻¹ pyroxene yr⁻¹. ³He concentrations in samples deeper than ∼100 meters are consistent with model-based estimates of depth-independent nucleogenic production from the capture by ⁶Li of neutrons produced by alpha particle reactions on light elements. Measurements in other subsurface samples indicate that muon-produced ³He is prevalent across the Columbia Plateau. The penetration depth of muonogenic ³He production is substantially deeper, and the ratio of muon- to spallation-produced ³He is substantially lower, than found for other cosmogenic nuclides. Our results provide the first definitive empirical evidence for ³He production by muons, which has several implications for quantifying the timing and rates of Earth surface change and interpreting He isotope ratios. Importantly, despite the low production rates, landforms in the Channeled Scablands, which were formed by incision of the Columbia River Basalt by the late-Pleistocene Missoula floods, have high concentrations of ³He inherited from post-Miocene muon exposure. Hence ³He production by muons must be considered, particularly when dating rapid erosional events in old bedrock. Our findings indicate samples with less than several tens of meters of shielding by overlying rock will contain cosmogenic ³He that elevates ³He/⁴He ratios. Hence caution should be used when using ³He/⁴He ratios from samples at shallower depths to infer mantle sources of basalt.

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Farley, K. A.0000-0002-7846-7546
Lamb, M. P.0000-0002-5701-0504
Alternate Title:Empirical evidence for cosmogenic 3He production by muons
Additional Information:© 2021 Elsevier B.V. Received 30 June 2020, Revised 8 February 2021, Accepted 11 February 2021, Available online 19 February 2021. We thank Karin Lehnigk, Scott David, and Madison Douglas for assistance in the field, Devon Dunajski, Patrick Scordato, and Alice Hough for helping separate minerals, Johnathan Treffkorn for assistance with ³He measurements, and Stephen Riedel and Graysen Bjork for helpful information pertaining to the Cheney core. We thank Marissa Tremblay for a constructive review. This research was supported by a collaborative NSF award (1529528, 1529110) to I.J.L., K.A.F. and M.P.L. CRediT authorship contribution statement: I.J. Larsen: Conceptualization, Investigation, Writing – original draft. K.A. Farley: Conceptualization, Investigation, Writing – review & editing. M.P. Lamb: Conceptualization, Investigation, Software, Writing – review & editing. C.J. Pritchard: Resources, Writing – review & editing. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Subject Keywords:3He; cosmogenic nuclides; muon; exposure dating; Columbia River Basalt; mantle helium
Record Number:CaltechAUTHORS:20210517-123150941
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Official Citation:I.J. Larsen, K.A. Farley, M.P. Lamb, C.J. Pritchard, Empirical evidence for cosmogenic 3He production by muons, Earth and Planetary Science Letters, Volume 562, 2021, 116825, ISSN 0012-821X,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:109151
Deposited By: Tony Diaz
Deposited On:17 May 2021 19:55
Last Modified:17 May 2021 19:55

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