SCEC Award Number 20146 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Cosmogenic 3He dating of alluvial surfaces using detrital magnetite enabled by microCT scanning
Name Organization
Kenneth Farley California Institute of Technology Emily Cooperdock University of Southern California Josh West University of Southern California Florian Hofmann California Institute of Technology
Other Participants
SCEC Priorities 1a, 1a, 1a SCEC Groups SAFS, Geology
Report Due Date 03/15/2021 Date Report Submitted 03/15/2021
Project Abstract
We have tested whether X-ray micro computed tomography (microCT) scanning can improve the quality of cosmogenic magnetite 3He data by pre-selecting mineral grains without inclusions. We extracted magnetite from a paleosol of an offset fanglomerate in the San Gorgonio Pass Special Fault Study Area. We show that inclusions in magnetite can lead to a significant overestimation of the 3He concentration and therefore the exposure age. Grains without inclusions have 3He concentrations close to an expected exponential depth profile after accounting for 3He derived from nucleogenic and cosmogenic neutron produced sources. By comparing the results to an existing depth profile of 10Be and 26Al concentrations in quartz, we calibrate a cosmogenic 3He production rate in magnetite. The method described here, together with the production rate calibration, will make it possible to use 3He in magnetite as a robust tool for cosmogenic exposure studies, adding to the toolkit for quantifying rates of fault motion and other active tectonic processes.
Intellectual Merit This research presents a new tool for screening mineral grains prior to 3He measurement, which leads to more accurate and reproducible results. This will improve the quality of cosmogenic nuclide dating of magnetite and possibly other opaque phases. The microCT screening approach and the cosmogenic 3He production rate calibration presented here will make it possible to use magnetite to obtain cosmogenic dates for offset surfaces, such as the one at Whitewater in the San Gorgonio Pass Special Fault Study Area, which can be used to determine fault slip rates.
Broader Impacts This grant has helped to fund the work of several undergraduate students, two of which are co-authors on the paper presenting this work. Data acquired for this study was used for exercises for classes on Geochronology and Cosmogenic Nuclide Dating at the University of Munich.
Exemplary Figure Figure 2: Depth profiles of measured 3He (top row) and 4He (bottom row) concentrations from unscreened magnetite grains without information about inclusions (black), samples with bright (turquoise), dark (blue), as well as bright and dark inclusions (red), and samples without inclusions (green). All uncertainties shown are at the 1σ level and overlapping data points have been slightly vertically offset for clarity. The expected cosmogenic 3He depth profile based on the known exposure age and the average 4He concentration of aliquots without inclusions are shown as grey lines. Samples without inclusions have 3He concentrations close to the predicted depth profile and all but two have low, nearly constant 4He concentrations (around 0.2 nmol/g). Grains with inclusions have significantly higher 3He and 4He concentrations, showing the effects of radiogenic and thermal neutron produced 3He added by these inclusions. The 3He concentration decreases with depth even in grains with inclusions, indicating a cosmogenic thermal neutron component to nucleogenic production.