Spring 2022

 

Speaker:	Florian Hofmann, University of Southern California and UAF Geophysical Institute
Title:	"Using microCT to Improve the Reliability of Cosmogenic 3He Exposure Dating"
Date:	January 14, 2022
Time:	3:30 pm
Location:	Contact instructor for details. jemezger@alaska.edu

 

 
Abstract:
We have tested whether X-ray micro-computed tomography (microCT) imaging can be used as a tool for screening magnetite grains in order to improve cosmogenic 3He exposure dating. We extracted detrital magnetite from a soil developed on a fanglomerate terrace at Whitewater, California, USA. Using microCT scanning, we identified inclusions in over 3000 magnetite grains based on their detected density. MicroCT scans show that 30 % of all grains are free of inclusions, with the largest proportion of suitable grains having diameters between 400 and 800 µm. We observed that inclusions contribute 3He in excess of the expected spallation production up to a factor of 4, thereby significantly increasing the measured bulk 3He concentration and the derived exposure age. We measured elemental compositions of magnetite grains and the matrix in order to model the contributions from Li-produced 3He and correct the measured 3He concentrations. While grains with inclusions have 3He concentrations far in excess of the expected values, magnetite grains without inclusions have lower concentrations which are close to the predicted depth profile, showing the improvement made by pre-selecting grains. We did not find any detectable contribution to the bulk 3He concentration by fluid inclusions. Through high concentrations of Li or U, solid inclusions can contribute significant amounts of 3He when accumulated over long periods of time (10-100 Ma) since the closure of the system. By comparing the corrected 3He concentrations measured in magnetite grains without inclusions to an existing in-situ depth profile of 10Be in quartz, we calibrate a magnetite 3He production rate at sea level and high latitude of 116±13 at/g/a. We suggest that this microCT pre-screening approach can be used to improve the reliability of 3He measurements of magnetite and other opaque minerals, and can make cosmogenic 3He in magnetite a robust tool for in-situ and detrital studies.