Preprints
https://doi.org/10.5194/gchron-2021-7
https://doi.org/10.5194/gchron-2021-7

  26 Feb 2021

26 Feb 2021

Review status: this preprint is currently under review for the journal GChron.

Short communication: Inverse isochron regression for Re–Os, K–Ca and other chronometers

Yang Li1 and Pieter Vermeesch2 Yang Li and Pieter Vermeesch
  • 1State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029
  • 2Department of Earth Sciences, University College London, Gower Street, London WC1E 6BT

Abstract. Conventional Re–Os isochrons are based on mass spectrometric estimates of 187Re / 188Os and 187Os / 188Os. 188Os is usually far less abundant, and is therefore measured less precisely, than 187Os and 187Re. This causes strong error correlations between the two isochron ratios, which may obscure potentially important geological complexity. Using an approach that is widely accepted in 40Ar / 39Ar and U–Pb geochronology, we here show that these error correlations are greatly reduced by applying a simple change of variables, using 187Os as a common denominator. Plotting 188Os / 187Os vs. 187Re / 187Os produces an inverse isochron, defining a binary mixing line between an inherited Os-component whose 188Os / 187Os-ratio is given by the vertical intercept, and the radiogenic 187Re / 187Os-ratio, which corresponds to the horizontal intercept. Inverse isochrons facilitate the identification of outliers and other sources of data dispersion. They can also be applied to other geochronometers such as the K–Ca method and (with less dramatic results) the Rb–Sr, Sm–Nd and Lu–Hf methods. The generalised inverse isochron method has been added to the IsoplotR toolbox for geochronology, which automatically converts conventional isochron ratios into inverse ratios and vice versa.

Yang Li and Pieter Vermeesch

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on gchron-2021-7', Donald Davis, 08 Mar 2021
    • AC1: 'Reply on RC1', Pieter Vermeesch, 07 Apr 2021
  • CC1: 'Comment on gchron-2021-7', Ryan Ickert, 02 Apr 2021
    • AC2: 'Reply on CC1', Pieter Vermeesch, 09 Apr 2021
  • RC2: 'Comment on gchron-2021-7', Anonymous Referee #2, 13 Apr 2021

Yang Li and Pieter Vermeesch

Yang Li and Pieter Vermeesch

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Short summary
A conventional isochron is a straight line fit to two sets of isotopic ratios, D / d and P / d, where P is the radioactive parent, D is the radiogenic daughter and d is a second isotope of the daughter element. The slope of this line is proportional to the age of the system. An inverse isochron is linear fit through d / D and P / D. The horizontal intercept of this line is inversely proportional to the age. The latter approach is preferred when d < D, which is the case in Re–Os and K–Ca geochronology.