Preprints
https://doi.org/10.5194/gchron-2020-18
https://doi.org/10.5194/gchron-2020-18

  19 Jun 2020

19 Jun 2020

Review status: this preprint was under review for the journal GChron. A revision for further review has not been submitted.

Potassium isotopic variability and implications for 40K-based geochronology

Leah E. Morgan Leah E. Morgan
  • U.S. Geological Survey, Denver, CO 80225, USA

Abstract. 40Ar/39Ar and K-Ar geochronology assume that 40K/K values are invariant among the sample of interest, the co-irradiated neutron fluence monitor (standard), and the material used to measure decay constants. Until recently, this assumption was reasonable due to the small K isotope (41K, 40K, 39K) variability found in many terrestrial samples and the negligible effect of any variation relative to the precision of the determined age. The recent discovery of measurable δ41K variability in terrestrial samples now questions this assumption. Although δ41K values for some neutron fluence monitors have now been reported, potassium isotopes are not routinely measured on samples dated by the 40Ar/39Ar method even though a wide range of silicate materials were found to vary by > 2.5 ‰. Further, the 40K decay constants used in 40Ar/39Ar geochronology are based on activity counting of radioactive decay in K-rich salts. These salts have not been measured for δ41K, yet evaporites have been shown to vary by > 1 ‰ from the mean value of silicates. The potential effects of δ41K variability on 40Ar/39Ar ages are illustrated using the case of the ca. 28.2 Ma Fish Canyon sanidine (FCs) and the ca. 99 Ma Mt. Dromedary Biotite (GA-1550). If the two standards have δ41K values as measured and the material used to determine decay constants is appropriately represented by δ41K of evaporites, the age of FCs is underestimated by ca. 7 ka (0.25 ‰). Although this is a small effect, such bias is becoming important as the analytical precision and accuracy of isotopic measurements and calculation of 40Ar/39Ar ages continue to improve.

Leah E. Morgan

 
Status: closed (peer review stopped)
Status: closed (peer review stopped)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
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Status: closed (peer review stopped)
Status: closed (peer review stopped)
AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment
Printer-friendly Version - Printer-friendly version Supplement - Supplement

Leah E. Morgan

Leah E. Morgan

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Short summary
Potassium-based geochronology, including the 40Ar/39Ar and K-Ar methods, provides critical constraints on the timing and duration of many events and processes in earth history. These methods assume that a key parameter, the fraction of potassium with a mass of 40, does not vary in some relevant materials. Some recent work has identified some variability in these materials. This work examines the effects of this variability on calculated K-Ar and 40Ar/39Ar ages.