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

• Main Authors: F. Hofmann, E. H. G. Cooperdock, A. J. West, D. Hildebrandt, K. Strößner, K. A. Farley
• Format: Article
• Language: English
• Published: Copernicus Publications 2021-07-01
• Series: Geochronology
• Online Access: https://gchron.copernicus.org/articles/3/395/2021/gchron-3-395-2021.pdf

<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">³</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">³</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">³</span>He. We show that mineral inclusions in magnetite can produce <span class="inline-formula">³</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">³</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">³</span>He concentrations far in excess of the values expected from existing <span class="inline-formula">¹⁰</span>Be and <span class="inline-formula">²⁶</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">³</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">³</span>He at sea level and high latitude (SLHL) of 116 <span class="inline-formula">±</span> 13 at g<span class="inline-formula">⁻¹</span> a<span class="inline-formula">⁻¹</span>. We suggest that this microCT screening approach can be used to improve the quality of cosmogenic <span class="inline-formula">³</span>He measurements of magnetite and other opaque mineral phases for exposure age and detrital studies.</p>