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Exposure dating of detrital magnetite using ³He enabled by microCT and calibration of the cosmogenic ³He production rate in magnetite

Hofmann, Florian and Cooperdock, Emily H. G. and West, A. Joshua and Hildebrandt, Dominic and Strößner, Kathrin and Farley, Kenneth A. (2021) Exposure dating of detrital magnetite using ³He enabled by microCT and calibration of the cosmogenic ³He production rate in magnetite. Geochronology, 3 (2). pp. 395-414. ISSN 2628-3719. doi:10.5194/gchron-3-395-2021.

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We test whether X-ray micro-computed tomography (microCT) imaging can be used as a tool for screening magnetite grains to improve the accuracy and precision of cosmogenic ³He exposure dating. We extracted detrital magnetite from a soil developed on a fanglomerate at Whitewater, California, which was offset by the Banning strand of the San Andreas Fault. This study shows that microCT screening can distinguish between inclusion-free magnetite and magnetite with fluid or common solid inclusions. Such inclusions can produce bulk 3He concentrations that are significantly in excess of the expected spallation production. We present Li concentrations, major and trace element analyses, and estimated magnetite (U–Th) / He cooling ages of samples in order to model the contribution from fissiogenic, nucleogenic, and cosmogenic thermal neutron production of ³He. We show that mineral inclusions in magnetite can produce ³He concentrations of up to 4 times that of the spallation component, leading to erroneous exposure ages. Therefore, grains with inclusions must be avoided in order to facilitate accurate and precise magnetite ³He exposure dating. Around 30 % of all grains were found to be without inclusions, as detectable by microCT, with the largest proportion of suitable grains in the grain size range of 400–800 µm. While grains with inclusions have ³He concentrations far in excess of the values expected from existing ¹⁰Be and ²⁶Al data in quartz at the Whitewater site, magnetite grains without inclusions have concentrations close to the predicted depth profile. We measured ³He concentrations in aliquots without inclusions and corrected them for Li-produced components. By comparing these data to the known exposure age of 53.5 ± 2.2 ka, we calibrate a production rate for magnetite 3He at sea level and high latitude (SLHL) of 116 ± 13 at g⁻¹ a⁻¹. We suggest that this microCT screening approach can be used to improve the quality of cosmogenic ³He measurements of magnetite and other opaque mineral phases for exposure age and detrital studies.

Item Type:Article
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URLURL TypeDescription availabllity: Elemental analyses and helium isotope data (accessed at the EarthChem data repository) availabllity: MicroCT scans (accessed at the Digital Rocks Portal)
Hofmann, Florian0000-0002-9836-2338
Cooperdock, Emily H. G.0000-0002-0154-8719
West, A. Joshua0000-0001-6909-1471
Hildebrandt, Dominic0000-0002-9891-0664
Farley, Kenneth A.0000-0002-7846-7546
Alternate Title:Exposure dating of detrital magnetite using 3He enabled by microCT and calibration of the cosmogenic 3He production rate in magnetite
Additional Information:© Author(s) 2021. This work is distributed under the Creative Commons Attribution 4.0 License. Received: 22 Mar 2021 – Discussion started: 01 Apr 2021 – Accepted: 16 Jun 2021 – Published: 15 Jul 2021. We thank Tautis Skorka at the Molecular Imaging Center at the University of Southern California for microCT scanning and Jonathan Treffkorn for helium mass spectrometry at Caltech. Reviews by Samuel Niedermann and Pierre-Henri Blard as well as editorial handling by Hella Wittmann-Oelze and Greg Balco helped to improve the paper. Funding was provided by Southern California Earthquake Center 2020 award no. 20146 to Kenneth A. Farley, Emily H. G. Cooperdock, A. Joshua West, and Florian Hofmann. Student contributions at Ludwig-Maximilians-Universität München were funded by Studi_forscht@GEO grant S20-F17 awarded to Florian Hofmann. This research has been supported by the Southern California Earthquake Center (grant no. 20146) and the Ludwig-Maximilians-Universität München (Studi_forscht@GEO, grant no. S20-F17). Author contributions. FH, EHGC, AJW, and KAF conceptualized the study and acquired funding. FH, DH, and KS carried out mineral separation, sample processing, and data analysis. FH, EHGC, AJW, and KAF contributed to the interpretation of the data. The paper and figures were prepared by FH and edited and reviewed by all co-authors. The authors declare that they have no conflict of interest. Review statement. This paper was edited by Hella Wittmann-Oelze and reviewed by Samuel Niedermann and Pierre-Henri Blard.
Funding AgencyGrant Number
Southern California Earthquake Center (SCEC)20146
Ludwig-Maximilians-Universität MünchenStudi_forscht@GEO, S20-F17
Issue or Number:2
Record Number:CaltechAUTHORS:20211208-560311000
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Official Citation:Hofmann, F., Cooperdock, E. H. G., West, A. J., Hildebrandt, D., Strößner, K., and Farley, K. A.: Exposure dating of detrital magnetite using 3He enabled by microCT and calibration of the cosmogenic 3He production rate in magnetite, Geochronology, 3, 395–414,, 2021.
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:112286
Deposited By: George Porter
Deposited On:10 Dec 2021 22:54
Last Modified:10 Dec 2021 22:54

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