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Cosmogenic and nucleogenic ^3He in apatite, titanite, and zircon

Farley, K. A. and Libarkin, J. and Mukhopadhyay, S. and Amidon, W. (2006) Cosmogenic and nucleogenic ^3He in apatite, titanite, and zircon. Earth and Planetary Science Letters, 248 (1-2). pp. 451-461. ISSN 0012-821X. doi:10.1016/j.epsl.2006.06.008.

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Cosmogenic ^3He was measured in apatite, titanite, and zircon and cosmogenic ^(21)Ne in quartz at 13 depth intervals in a 2.7-m long drill core in a Miocene ignimbrite from the Altiplano of Bolivia. All three ^3He depth profiles as well as the ^(21)Ne profile attenuate exponentially with depth, indicating that both of these isotopes are cosmogenic in origin with no significant contribution from other sources. The attenuation lengthscale for ^3He production of Λ = 180 ± 11 g/cm^2 is consistent with expectations for neutron spallation, and is identical to that found for the cosmogenic ^(21)Ne in quartz. By normalizing the measured ^3He concentrations to ^(21)Ne and using the independently known cosmogenic ^(21)Ne production rate, the apparent cosmogenic ^3He production rates in apatite, titanite, and zircon were respectively found to be 112, 97, and 87 atoms/g/yr at sea-level and high latitude. The formal uncertainty on these estimates is ~20% (2σ), and arises in equal parts from uncertainties in the measured ^3He/^(21)Ne ratios and the uncertainty in the ^(21)Ne production rate. However an additional factor affecting the apparent ^3He production rate in these phases arises from the long stopping range of spalled ^3He and tritium (which decays to ^3He). Because all three accessory phases have higher mean atomic number than major rock-forming minerals, they will have lower 3He production rates than their surroundings. As a consequence the long stopping ranges will cause a net implantation of ^3He and therefore higher apparent production rates than would apply for purely in-situ production. Thus these apparent production rates apply only to the specific grain sizes analyzed. Analysis of sieved zircon aliquots suggests that a factor of 2 increase in grain size (from ~50 to ~100 μm cross-section) yields a 10% decrease in apparent production rate. While this effect warrants further study, the grain sizes analyzed here are typical of the accessory phases commonly encountered, so the apparent rates provide an appropriate starting place for surface exposure dating using ^3He in these minerals.

Item Type:Article
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Farley, K. A.0000-0002-7846-7546
Additional Information:© 2006 Elsevier. Received 7 April 2006; revised 6 June 2006; accepted 6 June 2006. Editor: R.W. Carlson. Available online 17 July 2006. We thank C. Garzione for field assistance and thoughtful discussion and Don Burnett for numerous helpful conversations regarding nuclear processes. We also acknowledge two anonymous reviewers. This research was supported by the National Science Foundation EAR (0350396, Libarkin); EAR (0408526, Farley); and by funds provided to SM by Harvard University.
Errata:Correction to “Cosmogenic and nucleogenic ^3He in apatite, titanite, and zircon" and "Anomalous cosmogenic ^3He production and elevation scaling in the high Himalaya" [Earth Planet. Sci. Lett. 248 (2006) 451–461 and 265 (2008) 287–301].
Group:Caltech Tectonics Observatory
Funding AgencyGrant Number
NSFEAR 0350396
NSFEAR 0408526
Harvard UniversityUNSPECIFIED
Subject Keywords:cosmogenic helium; apatite; zircon; titanite; production rate; Altiplano
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Caltech Tectonics Observatory46
Issue or Number:1-2
Record Number:CaltechAUTHORS:20090901-133907935
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Official Citation:K.A. Farley, J. Libarkin, S. Mukhopadhyay, W. Amidon, Cosmogenic and nucleogenic 3He in apatite, titanite, and zircon, Earth and Planetary Science Letters, Volume 248, Issues 1-2, 15 August 2006, Pages 451-461, ISSN 0012-821X, DOI: 10.1016/j.epsl.2006.06.008.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:15528
Deposited By: Katherine Johnson
Deposited On:14 Sep 2009 23:37
Last Modified:08 Nov 2021 23:20

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