Abstract
Abstract. Conventional Re–Os isochrons are based on mass spectrometric estimates of 187Re/188Os and 187Os/188Os, which often exhibit strong error correlations that 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. Conventional and inverse isochron ages are similar for precise datasets but may significantly diverge for imprecise ones. A semi-synthetic data simulation indicates that, in the latter case, the inverse isochron age is more accurate. 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.
Highlights
The [187Os/188Os] budget of a 187Re-bearing rock or mineral can be divided into an inherited component and a radiogenic component:
Strong error correlations are commonly observed in other conventional isochron systems, where they may arise from a number of mechanisms including poor counting statistics, blank correction (e.g. Vermeesch, 2015; Connelly et al, 2017) or fractionation (e.g. Ludwig, 1980)
Conventional isochrons are straight-line regressions between two ratios D/d and P /d, where P and D are the parent and daughter nuclides, and d is a non-radiogenic isotope of the daughter element
Summary
The [187Os/188Os] budget of a 187Re-bearing rock or mineral can be divided into an inherited component and a radiogenic component: 187Os. One drawback of the conventional isochron definition of Eq (1) is that the rarest isotope, 188Os, which is associated with the largest mass spectrometer uncertainties, appears in the denominator of both x and y This has the potential to produce strong error correlations (Stein et al, 2000). 3. The isochron fit exhibits a mean squared weighted deviation (MSWD) of 2.5, which indicates the presence of a moderate amount of overdispersion of the data with respect to the formal analytical uncertainties. The isochron fit exhibits a mean squared weighted deviation (MSWD) of 2.5, which indicates the presence of a moderate amount of overdispersion of the data with respect to the formal analytical uncertainties It is not immediately clear which aliquots are responsible for the poor goodness of fit
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