Articles | Volume 1, issue 1
https://doi.org/10.5194/gchron-1-17-2019
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/gchron-1-17-2019
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Research article
| 
26 Sep 2019
Research article |
| 26 Sep 2019
| 26 Sep 2019
Resolving the effects of 2-D versus 3-D grain measurements on apatite (U–Th) ∕ He age data and reproducibility
Department of Geological Sciences, University of Texas at Austin,
Austin, 78712, USA
Woods Hole Oceanographic Institution, Woods Hole, 02543, USA
now at: Department of Earth Sciences, University of Southern
California, Los Angeles, CA 90089, USA
Richard A. Ketcham
Department of Geological Sciences, University of Texas at Austin,
Austin, 78712, USA
Daniel F. Stockli
Department of Geological Sciences, University of Texas at Austin,
Austin, 78712, USA
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Emily H. G. Cooperdock, Florian Hofmann, Ryley M. C. Tibbetts, Anahi Carrera, Aya Takase, and Aaron J. Celestian
Geochronology, 4, 501–515, https://doi.org/10.5194/gchron-4-501-2022, https://doi.org/10.5194/gchron-4-501-2022, 2022
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Apatite and zircon are the most widely used minerals for dating rocks, but they can be difficult to identify in some crushed rock samples. Incorrect mineral identification results in wasted analytical resources and inaccurate data. We show how X-ray computed tomography can be used to efficiently and accurately distinguish apatite from zircon based on density variations, and provide non-destructive 3D grain-specific size, shape, and inclusion information for improved data quality.
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We use microCT scanning to improve the quality of 3He exposure ages measured in detrital magnetite. We show that the presence of inclusions can significantly increase the measured amount of 3He and thereby the exposure age. By prescreening magnetite with microCT and analyzing only inclusion-free grains, this problem can be avoided. We also calibrate the cosmogenic 3He production rate in magnetite relative to 10Be in quartz, which can be used for similar studies in the future.
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We present the first new image-based study to reveal how choices made by different analysts affect the results obtained by fission-track analysis. Participants analyzed an identical image data set with varying grain quality. Experienced analysts tend to select lower rates of unsuitable grains and conduct lower rates of invalid length measurements. Fission-track studies need image data repositories, teaching modules, guidelines, an open science culture, and new approaches for calibration.
Alyssa J. McKanna, Isabel Koran, Blair Schoene, and Richard A. Ketcham
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Apatite and zircon are the most widely used minerals for dating rocks, but they can be difficult to identify in some crushed rock samples. Incorrect mineral identification results in wasted analytical resources and inaccurate data. We show how X-ray computed tomography can be used to efficiently and accurately distinguish apatite from zircon based on density variations, and provide non-destructive 3D grain-specific size, shape, and inclusion information for improved data quality.
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The Geometric Correction Method for zircon (U–Th) ∕ He chronology: correcting systematic error and assigning uncertainties to alpha-ejection corrections and eU concentrations
Technical note: In situ U–Th–He dating by 4He ∕ 3He laser microprobe analysis
A practical method for assigning uncertainty and improving the accuracy of alpha-ejection corrections and eU concentrations in apatite (U–Th) ∕ He chronology
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A revised alpha-ejection correction calculation for (U–Th) ∕ He thermochronology dates of broken apatite crystals
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Olga Valentinovna Yakubovich, Natalia Pavlovna Konstantinova, Maria Olegovna Anosova, Mary Markovna Podolskaya, and Elena Valerevna Adamskaya
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Goethite is one of the most common Fe-(oxy)hydroxide minerals that is formed during hydrolyzation of rocks, implying it is a desired mineral for dating various surface and subsurface geological processes. Nowadays (U–Th) / He dating of goethite is widely used in geochronological studies. Here, in the example of goethite from the Chukchi Borderland, we introduce a new, simple methodological approach for accurate (U–Th) / He dating of goethite.
Francis J. Sousa, Stephen E. Cox, E. Troy Rasbury, Sidney R. Hemming, Antonio Lanzirotti, and Matthew Newville
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We have discovered a new way of measuring the three-dimensional distribution of radioactive elements in individual crystals by shining a very bright light on apatite crystals at the Advanced Photon Source at Argonne National Laboratory. This allows us to learn about the rates and timing of geologic processes and to help resolve problems that previously were unsolvable because we had no way to make this type of measurement.
Spencer D. Zeigler, Morgan Baker, James R. Metcalf, and Rebecca M. Flowers
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(U–Th)/He chronology relies on accurate measurements of zircon grain dimensions, but the systematic error and uncertainty associated with those measurements have been unquantified until now. We build on the work of Zeigler et al. (2023) and present the zircon Geometric Correction Method, a simple solution to correcting the error and quantifying the geometric uncertainty in eU and dates. Including this geometric correction and uncertainty matters for data evaluation and interpretation.
Pieter Vermeesch, Yuntao Tian, Jae Schwanethal, and Yannick Buret
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The U–Th–He method is a technique to determine the cooling history of minerals. Traditional approaches to U–Th–He dating are time-consuming and require handling strong acids and radioactive solutions. This paper presents an alternative approach in which samples are irradiated with protons and subsequently analysed by laser ablation mass spectrometry. Unlike previous in situ U–Th–He dating attempts, the new method does not require any absolute concentration measurements of U, Th, or He.
Spencer D. Zeigler, James R. Metcalf, and Rebecca M. Flowers
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(U–Th) / He dating relies on proper characterization of apatite crystal dimensions so that eU concentrations and dates can be calculated accurately and precisely, but there is systematic error and uncertainty in geometric measurements. By comparing 2D microscopy to
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Natacha Gribenski, Marissa M. Tremblay, Pierre G. Valla, Greg Balco, Benny Guralnik, and David L. Shuster
Geochronology, 4, 641–663, https://doi.org/10.5194/gchron-4-641-2022, https://doi.org/10.5194/gchron-4-641-2022, 2022
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We apply quartz 3He paleothermometry along two deglaciation profiles in the European Alps to reconstruct temperature evolution since the Last Glacial Maximum. We observe a 3He thermal signal clearly colder than today in all bedrock surface samples exposed prior the Holocene. Current uncertainties in 3He diffusion kinetics do not permit distinguishing if this signal results from Late Pleistocene ambient temperature changes or from recent ground temperature variation due to permafrost degradation.
John J. Y. He and Peter W. Reiners
Geochronology, 4, 629–640, https://doi.org/10.5194/gchron-4-629-2022, https://doi.org/10.5194/gchron-4-629-2022, 2022
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Apatite helium thermochronology is a method that dates the time at which a rock (and the apatite crystals contained within) cooled below a certain temperature by measuring radioactive parent isotopes (uranium and thorium) and daughter isotopes (helium). This paper proposes a revision to a commonly used calculation that corrects raw data to account for instances when the analyzed apatite crystals are fragmented. It demonstrates the improved accuracy and precision of the proposed revision.
Stephen E. Cox, Hayden B. D. Miller, Florian Hofmann, and Kenneth A. Farley
Geochronology, 4, 551–560, https://doi.org/10.5194/gchron-4-551-2022, https://doi.org/10.5194/gchron-4-551-2022, 2022
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Noble gases are largely excluded from minerals during rock formation, but they are produced by certain radioactive decay schemes and trapped in mineral lattices. However, they are present in the atmosphere, which means that they can be adsorbed or trapped by physical processes. We present details of a troublesome trapping mechanism for helium during sample crushing and show when it can be ignored and how it can be easily avoided during common laboratory procedures.
David M. Whipp, Dawn A. Kellett, Isabelle Coutand, and Richard A. Ketcham
Geochronology, 4, 143–152, https://doi.org/10.5194/gchron-4-143-2022, https://doi.org/10.5194/gchron-4-143-2022, 2022
Short summary
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Multi-thermochronometry, in which methods such as (U-Th)/He dating of zircon and apatite and apatite fission track dating are combined, is used to reconstruct rock thermal histories. Our ability to reconstruct thermal histories and interpret the geological significance of measured ages requires modeling. Here we use forward models to explore effects of grain size and chemistry on cooling ages and closure temperatures for the (U-Th)/He decay systems in apatite and zircon.
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Short summary
(U–Th) / He chronometry relies on accurate grain-specific size and shape measurements. Using > 100 apatite grains to compare
assumed2-D versus
true3-D grain shapes measured by a microscope and X-ray computed tomography, respectively, we find that volume and surface area both differ by ~ 25 % between the two techniques and directly affect mass and concentration measurements. But we found a very small effect on the FT correction (2 %) and no discernible impact on mean sample age or dispersion.
(U–Th) / He chronometry relies on accurate grain-specific size and shape measurements. Using...
