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U-loss associated with laser-heating of hematite and goethite in vacuum during (U-Th)/He dating and prevention using high O₂ partial pressure

Hofmann, Florian and Treffkorn, Jonathan and Farley, Kenneth A. (2020) U-loss associated with laser-heating of hematite and goethite in vacuum during (U-Th)/He dating and prevention using high O₂ partial pressure. Chemical Geology, 532 . Art. No. 119350. ISSN 0009-2541. https://resolver.caltech.edu/CaltechAUTHORS:20191111-071935108

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Abstract

Single-aliquot (U-Th)/He dating of hematite has been used to study iron-oxide precipitation in various environments, but we show there is an important challenge to the method: highly retentive hematite samples require temperatures of > 1000° C to be completely degassed, whereas the temperature for major U-loss is ∼980° C. This leads to erroneously high (U-Th)/He ages. Through the analysis of U, Th, and Sm of hematite and goethite samples, we show the degree of U-loss at this temperature and demonstrate that prolonged heating at temperatures of 950° C can lead to U-loss. We show that loss of U in goethite and hematite samples is associated with phase change from hematite to magnetite as Fe is reduced. The onset temperature of vacuum reduction of hematite can be increased from about 800-900° C in vacuum to approximately 1250° C in an oxygen partial pressure of 100 mbar. We show that samples can be outgassed to extract helium at 1150° C without U-loss in an O₂-rich atmosphere during heating, which does not increase the analytical blanks. We describe our implementation and automation of the procedure. An average age calculated on a reference hematite sample from replicate aliquots (n=12), which were analyzed using this procedure, has a relative uncertainty of 2% (1σ), and is within uncertainty of the previously measured two-aliquot age. We suggest this O₂ degassing procedure as a way to precisely and reproducibly determine single-aliquot hematite and goethite (U-Th)/He ages.


Item Type:Article
Related URLs:
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https://doi.org/10.1016/j.chemgeo.2019.119350DOIArticle
ORCID:
AuthorORCID
Farley, Kenneth A.0000-0002-7846-7546
Additional Information:© 2019 Elsevier B.V. Received 23 June 2019, Revised 20 October 2019, Accepted 25 October 2019, Available online 9 November 2019. We thank Hayden Miller for providing samples, sharing data, and constructive discussions. Assistance with ATR-FTIR spectroscopy from George Rossman is greatly appreciated. We thank Nathan Dalleska for his help with ICP-MS at the Environmental Analysis Center at Caltech. Peter Reiners and two anonymous reviewers are thanked for their insightful comments, which helped to improve this manuscript.
Subject Keywords:Hematite; Goethite; (U-Th)/He dating; single-aliquot; geochronology
Record Number:CaltechAUTHORS:20191111-071935108
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20191111-071935108
Official Citation:Florian Hofmann, Jonathan Treffkorn, Kenneth A. Farley, U-loss associated with laser-heating of hematite and goethite in vacuum during (U–Th)/He dating and prevention using high O2 partial pressure, Chemical Geology, Volume 532, 2020, 119350, ISSN 0009-2541, https://doi.org/10.1016/j.chemgeo.2019.119350. (http://www.sciencedirect.com/science/article/pii/S0009254119304796)
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:99760
Collection:CaltechAUTHORS
Deposited By: Tony Diaz
Deposited On:12 Nov 2019 22:03
Last Modified:03 Dec 2019 19:34

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