Short communication: Inverse isochron regression for Re–Os, K–Caand other chronometers
Yang Li, Pieter Vermeesch
Abstract
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.