01 Apr 2021

01 Apr 2021

Review status: this preprint is currently under review for the journal GChron.

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

Florian Hofmann1,3, Emily H. G. Cooperdock2, A. Joshua West2, Dominic Hildebrandt3, Kathrin Strößner3, and Kenneth A. Farley1 Florian Hofmann et al.
  • 1California Institute of Technology, Division of Geological and Planetary Sciences, MC 100-23, 1200 E California Blvd, Pasadena, CA 91125, USA
  • 2University of Southern California, Department of Earth Sciences, 3651 Trousdale Parkway, Los Angeles, CA 90089, USA
  • 3Ludwig-Maximilians-Universität München, Department of Earth and Environmental Sciences, Luisenstr. 37, 80333 Munich, Germany

Abstract. 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 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 expected cosmogenic production. We present Li concentrations, major and trace element analysis, and magnetite (U-Th)/He cooling ages of samples in order to model the contribution from radiogenic, nucleogenic, and cosmogenic thermal neutron production of 3He. We show that mineral inclusions in magnetite can produce 3He concentrations of up to four times that of the cosmogenic 3He 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 ka, we calibrate a magnetite 3He SLHL production rate of 116 ± 13 at g−1 a−1. We suggest that the 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.

Florian Hofmann et al.

Status: open (until 13 May 2021)

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Florian Hofmann et al.

Data sets

Elemental analyses and cosmogenic 3He measurements of bulk soil and detrital minerals from a soil near Whitewater, California, USA, Version 1.0. Hofmann, F., Cooperdock, E., West, A., Hildebrandt, D., Strößner, K., Farley, K.

Florian Hofmann et al.


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Short summary
We use X-ray scanning to improve the quality of geologic ages derived from measuring the products of cosmic rays in the mineral magnetite. We show that the presence of other minerals can lead to wrong age estimates. By detecting those minerals with X-rays and analyzing only pure magnetite, this problem can be avoided. We also measure the rate at which helium from cosmic rays is produced in magnetite, which can be used for similar studies in the future.