Exposure dating of detrital magnetite using <sup>3</sup>He enabled by microCT and calibration of the cosmogenic <sup>3</sup>He production rate in magnetite
<p>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 <span class="inline-formula"><sup>3</sup></span>He exposure dating. We extracted detri...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2021-07-01
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| Series: | Geochronology |
| Online Access: | https://gchron.copernicus.org/articles/3/395/2021/gchron-3-395-2021.pdf |
| _version_ | 1797231034273628160 |
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| author | F. Hofmann F. Hofmann E. H. G. Cooperdock A. J. West D. Hildebrandt K. Strößner K. A. Farley |
| author_facet | F. Hofmann F. Hofmann E. H. G. Cooperdock A. J. West D. Hildebrandt K. Strößner K. A. Farley |
| author_sort | F. Hofmann |
| collection | DOAJ |
| description | <p>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 <span class="inline-formula"><sup>3</sup></span>He 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 <span class="inline-formula"><sup>3</sup></span>He 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) <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="93e47eb16cb371fe6916d3191efc4f1d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-3-395-2021-ie00001.svg" width="8pt" height="14pt" src="gchron-3-395-2021-ie00001.png"/></svg:svg></span></span> He cooling ages of samples in order to model the
contribution from fissiogenic, nucleogenic, and cosmogenic thermal neutron
production of <span class="inline-formula"><sup>3</sup></span>He. We show that mineral inclusions in magnetite can
produce <span class="inline-formula"><sup>3</sup></span>He 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 <span class="inline-formula"><sup>3</sup></span>He 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 <span class="inline-formula">µ</span>m.
While grains with inclusions have <span class="inline-formula"><sup>3</sup></span>He concentrations far in excess of
the values expected from existing <span class="inline-formula"><sup>10</sup></span>Be and <span class="inline-formula"><sup>26</sup></span>Al data in quartz at
the Whitewater site, magnetite grains without inclusions have concentrations
close to the predicted depth profile. We measured <span class="inline-formula"><sup>3</sup></span>He concentrations in
aliquots without inclusions and corrected them for Li-produced components.
By comparing these data to the known exposure age of 53.5 <span class="inline-formula">±</span> 2.2 ka, we
calibrate a production rate for magnetite <span class="inline-formula"><sup>3</sup></span>He at sea level and high latitude (SLHL) of 116 <span class="inline-formula">±</span> 13 at g<span class="inline-formula"><sup>−1</sup></span> a<span class="inline-formula"><sup>−1</sup></span>. We suggest that this microCT screening approach can be
used to improve the quality of cosmogenic <span class="inline-formula"><sup>3</sup></span>He measurements of magnetite
and other opaque mineral phases for exposure age and detrital studies.</p> |
| first_indexed | 2024-04-24T15:37:58Z |
| format | Article |
| id | doaj.art-f6cc45509c634571b5f5eb3bbd88b131 |
| institution | Directory Open Access Journal |
| issn | 2628-3719 |
| language | English |
| last_indexed | 2024-04-24T15:37:58Z |
| publishDate | 2021-07-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Geochronology |
| spelling | doaj.art-f6cc45509c634571b5f5eb3bbd88b1312024-04-02T01:16:37ZengCopernicus PublicationsGeochronology2628-37192021-07-01339541410.5194/gchron-3-395-2021Exposure dating of detrital magnetite using <sup>3</sup>He enabled by microCT and calibration of the cosmogenic <sup>3</sup>He production rate in magnetiteF. Hofmann0F. Hofmann1E. H. G. Cooperdock2A. J. West3D. Hildebrandt4K. Strößner5K. A. Farley6Division of Geological and Planetary Sciences, California Institute of Technology, MC 100-23, 1200 E California Blvd, Pasadena, CA 91125, USADepartment of Earth and Environmental Sciences, Ludwig-Maximilians-Universität München, Luisenstr. 37, 80333 Munich, GermanyDepartment of Earth Sciences, University of Southern California, 3651 Trousdale Parkway, Los Angeles, CA 90089, USADepartment of Earth Sciences, University of Southern California, 3651 Trousdale Parkway, Los Angeles, CA 90089, USADepartment of Earth and Environmental Sciences, Ludwig-Maximilians-Universität München, Luisenstr. 37, 80333 Munich, GermanyDepartment of Earth and Environmental Sciences, Ludwig-Maximilians-Universität München, Luisenstr. 37, 80333 Munich, GermanyDivision of Geological and Planetary Sciences, California Institute of Technology, MC 100-23, 1200 E California Blvd, Pasadena, CA 91125, USA<p>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 <span class="inline-formula"><sup>3</sup></span>He 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 <span class="inline-formula"><sup>3</sup></span>He 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) <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mo>/</mo></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="8pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="93e47eb16cb371fe6916d3191efc4f1d"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="gchron-3-395-2021-ie00001.svg" width="8pt" height="14pt" src="gchron-3-395-2021-ie00001.png"/></svg:svg></span></span> He cooling ages of samples in order to model the contribution from fissiogenic, nucleogenic, and cosmogenic thermal neutron production of <span class="inline-formula"><sup>3</sup></span>He. We show that mineral inclusions in magnetite can produce <span class="inline-formula"><sup>3</sup></span>He 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 <span class="inline-formula"><sup>3</sup></span>He 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 <span class="inline-formula">µ</span>m. While grains with inclusions have <span class="inline-formula"><sup>3</sup></span>He concentrations far in excess of the values expected from existing <span class="inline-formula"><sup>10</sup></span>Be and <span class="inline-formula"><sup>26</sup></span>Al data in quartz at the Whitewater site, magnetite grains without inclusions have concentrations close to the predicted depth profile. We measured <span class="inline-formula"><sup>3</sup></span>He concentrations in aliquots without inclusions and corrected them for Li-produced components. By comparing these data to the known exposure age of 53.5 <span class="inline-formula">±</span> 2.2 ka, we calibrate a production rate for magnetite <span class="inline-formula"><sup>3</sup></span>He at sea level and high latitude (SLHL) of 116 <span class="inline-formula">±</span> 13 at g<span class="inline-formula"><sup>−1</sup></span> a<span class="inline-formula"><sup>−1</sup></span>. We suggest that this microCT screening approach can be used to improve the quality of cosmogenic <span class="inline-formula"><sup>3</sup></span>He measurements of magnetite and other opaque mineral phases for exposure age and detrital studies.</p>https://gchron.copernicus.org/articles/3/395/2021/gchron-3-395-2021.pdf |
| spellingShingle | F. Hofmann F. Hofmann E. H. G. Cooperdock A. J. West D. Hildebrandt K. Strößner K. A. Farley Exposure dating of detrital magnetite using <sup>3</sup>He enabled by microCT and calibration of the cosmogenic <sup>3</sup>He production rate in magnetite Geochronology |
| title | Exposure dating of detrital magnetite using <sup>3</sup>He enabled by microCT and calibration of the cosmogenic <sup>3</sup>He production rate in magnetite |
| title_full | Exposure dating of detrital magnetite using <sup>3</sup>He enabled by microCT and calibration of the cosmogenic <sup>3</sup>He production rate in magnetite |
| title_fullStr | Exposure dating of detrital magnetite using <sup>3</sup>He enabled by microCT and calibration of the cosmogenic <sup>3</sup>He production rate in magnetite |
| title_full_unstemmed | Exposure dating of detrital magnetite using <sup>3</sup>He enabled by microCT and calibration of the cosmogenic <sup>3</sup>He production rate in magnetite |
| title_short | Exposure dating of detrital magnetite using <sup>3</sup>He enabled by microCT and calibration of the cosmogenic <sup>3</sup>He production rate in magnetite |
| title_sort | exposure dating of detrital magnetite using sup 3 sup he enabled by microct and calibration of the cosmogenic sup 3 sup he production rate in magnetite |
| url | https://gchron.copernicus.org/articles/3/395/2021/gchron-3-395-2021.pdf |
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