POTASSIUM ISOTOPIC VARIABILITY AND 40K-BASED GEOCHRONOLOGY

Age calculations in ⁴⁰Ar/³⁹Ar geochronology require that ⁴⁰K/K values do not vary between three materials: 1) the sample of interest, 2) the co-irradiated neutron fluence monitor (“standard”), and 3) the material used to measure decay constants. Until recently, these assumptions were reasonably founded in the lack of K isotope variability found in many terrestrial samples by Humayun and Clayton (1995).

The discovery of measurable terrestrial ∂⁴¹K variability (e.g. Morgan et al., 2018) now questions these assumptions. While ∂⁴¹K for some neutron fluence monitors was reported in Morgan et al. (2018), ∂⁴¹K is not routinely measured on samples dated by the ⁴⁰Ar/³⁹Ar method, and a wide range of silicate materials were found to vary by >2.5‰. Further, evaporite materials used for decay constant measurements (summarized in Beckinsale and Gale, 1969) have not been measured for ∂⁴¹K, yet evaporites have been shown to vary by >1‰ from the mean value of silicates (Morgan et al., 2018).

I will explore the potential effects on ⁴⁰Ar/³⁹Ar ages under different ∂⁴¹K scenarios. For example, consider the primary calibration of the ca. 28 Ma Fish Canyon sanidine (FCs) relative to the ca. 99 Ma Mt. Dromedary biotite (GA-1550). If the two standards have identical ∂⁴¹K values that vary from the material used to determine decay constants by -0.55‰ (as suggested by Morgan et al., 2018), the age of FCs is underestimated by ca. 7 ka (0.23‰). This is a small effect that is becoming important as the analytical precision and accuracy of the ⁴⁰Ar/³⁹Ar geochronometer improve.

Beckinsale, R.D., and Gale, N., 1969, A reappraisal of the decay constants and branching ratio of ⁴⁰K: Earth and Planetary Science Letters, v. 6, p. 289–294.

Humayun, M., and Clayton, R.N., 1995, Precise determination of the isotopic composition of potassium: Application to terrestrial rocks and lunar soils: v. 59, p. 2115–2130.

Morgan, L.E., Ramos, D.P.S., Davidheiser-Kroll, B., Faithfull, J., Lloyd, N.S., Ellam, R.M., and Higgins, J.A., 2018, High-precision ⁴¹K/³⁹K measurements by MC-ICP-MS indicate terrestrial variability of ∂⁴¹K: Journal of Analytical Atomic Spectrometry, doi: 10.1039/C7JA00257B.