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Cosmogenic ^3He production rate in ilmenite and the redistribution of spallation ^3He in fine-grained minerals

Larsen, Isaac J. and Farley, Kenneth A. and Lamb, Michael P. (2019) Cosmogenic ^3He production rate in ilmenite and the redistribution of spallation ^3He in fine-grained minerals. Geochimica et Cosmochimica Acta, 265 . pp. 19-31. ISSN 0016-7037. doi:10.1016/j.gca.2019.08.025.

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Cosmogenic nuclide surface exposure dating and erosion rate measurements in basaltic landscapes rely primarily on measurement of ^3He in olivine or pyroxene. However, geochemical investigations using ^3He have been impossible in the substantial fraction of basalts that lack separable olivine or pyroxene crystals, or where such crystals were present, but have been chemically weathered. Fine-textured basalts often contain small grains of ilmenite, a weathering-resistant mineral that is a target for cosmogenic ^3He production with good He retention and straightforward mineral separation, but with a poorly constrained production rate. Here we empirically calibrate the cosmogenic ^3He production rate in ilmenite by measuring ^3He concentrations in basalts with fine-grained (∼20 μm cross-section) ilmenite and co-existing pyroxene or olivine from the Columbia River and Snake River Plain basalt provinces in the western United States. The concentration ratio of ilmenite to pyroxene and olivine is 0.78 ± 0.02, yielding an apparent cosmogenic ^3He production rate of 93.6 ± 7.7 atom g^(−1) yr^(−1) that is 20–30% greater than expected from prior theoretical and empirical estimates for compositionally similar minerals. The production rate discrepancy arises from the high energy with which cosmic ray spallation reactions emit tritium and ^3He and the associated long stopping distances that cause them to redistribute within a rock. Fine-grained phases with low cosmogenic ^3He production rates, like ilmenite, will have anomalously high production rates owing to net implantation of ^3He from the surrounding, higher ^3He production rate, matrix. Semi-quantitative modeling indicates implantation of spallation ^3He increases with decreasing ilmenite grain size, leading to production rates that exceed those in a large grain by ∼10% when grain radii are <150 μm. The modeling predicts that for the ilmenite grain size in our samples, implantation causes production rates to be ∼20% greater than expected for a large grain, and within uncertainty resolves the discrepancy between our calibrated production rate, theory, and rates from previous work. The redistribution effect is maximized when the host rock and crystals differ substantially in mean atomic number, as they do between whole-rock basalt and ilmenite.

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Larsen, Isaac J.0000-0003-3094-4830
Farley, Kenneth A.0000-0002-7846-7546
Lamb, Michael P.0000-0002-5701-0504
Alternate Title:Cosmogenic 3He production rate in ilmenite and the redistribution of spallation 3He in fine-grained minerals
Additional Information:© 2019 Elsevier Ltd. Received 22 January 2019, Accepted 19 August 2019, Available online 26 August 2019. We thank George Rossman, Chi Ma, and Michael Jercinovic for assistance with mineral characterization and Lindsey Hedges, Ryan McKeon, Elliot Simon, and Derek Berman for laboratory assistance. We thank Ben Mackey for making his SRP samples and his analyses of He in olivine available and Willy Amidon for sharing and providing guidance in using his code to model ^3He production via neutron capture. We thank Pieter Vermeesch and two anonymous reviewers for constructive comments. This research was supported by a collaborative NSF award (1529528, 1529110) to I.J.L. K.A.F., and M.P.L. and NASA grant NNX13AM83G to MPL.
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Official Citation:Isaac J. Larsen, Kenneth A. Farley, Michael P. Lamb, Cosmogenic 3He production rate in ilmenite and the redistribution of spallation 3He in fine-grained minerals, Geochimica et Cosmochimica Acta, Volume 265, 2019, Pages 19-31, ISSN 0016-7037, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:98227
Deposited By: Tony Diaz
Deposited On:26 Aug 2019 17:23
Last Modified:16 Nov 2021 17:37

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