Figure 4.

Isochron of the Duluth Gabbro using the sum of ⁸⁶Sr and ⁸⁸Sr for normalization and the total Rb as a proxy for ⁸⁷Rb, as detailed in the text. This minimizes the contribution of counting statistics to our uncertainties. Although our analyses are not primarily count‐limited, this reanalysis improves our precision to the 200 Ma specified by Barney et al.,44 and our result coincides with the TIMS measurement of 1096 ± 14 Ma.37, 45 Ellipses represent 1σ confidence contours. As in Fig. 1, all spot analyses in which ⁸⁵Rb, ⁸⁶Sr, ⁸⁷Sr, ⁸⁷Rb, and ⁸⁸Sr were detected at twice the background level are shown and included in the fit.