Group , Poster #057, Earthquake Geology

Cosmogenic 3He exposure dating of alluvial surfaces using detrital magnetite enabled by microCT scanning

Florian Hofmann, Emily Cooperdock, A. Joshua West, Dominic Hildebrandt, Kathrin Strößner, & Kenneth Farley
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Poster Presentation

2021 SCEC Annual Meeting, Poster #057, SCEC Contribution #11151 VIEW PDF
We have tested whether X-ray micro-computed tomography (microCT) scanning can be used to improve the quality of cosmogenic 3He exposure dating of magnetite by pre-selecting mineral grains without inclusions. We extracted detrital magnetite from a paleosol of a fanglomerate terrace offset by the Banning Strand in the San Gorgonio Pass Special Fault Study Area. We show that the presence of inclusions in magnetite can lead to a significant overestimation of the bulk 3He concentration and therefore the exposure age. Grains without inclusions have lower 3He concentrations, which are close to an expected exponential depth profile after correcting for Li-produced 3He, showing the improvement made b...y avoiding mineral inclusions in the target phase. By comparing corrected 3He concentrations of magnetite grains without inclusions to an existing profile of 10Be in quartz, we calibrate a spallogenic 3He production rate in magnetite at sea level and high latitude of 116±13 at/g/a (2σ). We suggest that this microCT pre-screening method can be used to improve the quality of cosmogenic 3He measurements in magnetite and other opaque mineral phases for in-situ and detrital studies. This technique makes magnetite 3He a more robust chronological tool for dating fault offsets, especially in areas in which quartz is sparse.
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