Short communication: Mechanism and prevention of irreversible trapping of atmospheric He during mineral crushing
Abstract
A pervasive challenge in noble gas geochemistry is to ensure that analytical techniques do not modify the composition of the noble gases in the samples. Noble gases are present in the atmosphere and are used in a number of manufacturing procedures and by laboratory equipment. Of particular concern is the introduction of atmospheric or laboratory noble gases to samples during preparation before samples are placed in a vacuum chamber for analysis. Recent work has shown the potential for contamination of crushed samples with air-derived He that is not released by placing the samples under vacuum at room temperature. Using pure He gas as a tracer, we show that the act of crushing samples to a fine powder itself can introduce He contamination but that this is easily avoided by crushing under liquid or in an inert atmosphere. Because the He is trapped during crushing, the same concern does not extend to samples that are naturally fine-grained when collected. We also show model results that demonstrate when this effect might significantly impact samples for (U–Th) He geochronology or 3He cosmogenic nuclide dating. The degree of He contamination from crushing samples to sizes smaller than the > 63 µm range typically used for geochronology is insignificant for samples with a date of at least 1 Ma and 1 ppm U, and the degree of He contamination from crushing samples to sizes smaller than the 100–500 µm range typically used for cosmogenic nuclide dating is also insignificant for samples with a date of at least 10 ka with typical 3He production rates.
Copyright and License
© Author(s) 2022. This work is distributed under the Creative Commons Attribution 4.0 License.
Published by Copernicus Publications on behalf of the European Geosciences Union.
Acknowledgement
We thank Greg Balco and the Geochronology staff for editorial handling and Jonathan Tucker and Pierre-Henri Blard for constructive reviews.
Contributions
Data Availability
The code and data used in this study are included in the Supplement and as a permanent repository item (Cox, 2022).
Supplemental Material
The supplement related to this article is available online at: https://doi.org/10.5194/gchron-4-551-2022-supplement.
Additional Information
This paper was edited by Greg Balco and reviewed by Pierre-Henri Blard and Jonathan Tucker.
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Additional details
- Accepted
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2022-06-22Accepted
- Caltech groups
- Division of Geological and Planetary Sciences
- Publication Status
- Published