Figure 5.

Isochrons of completed Zagami measurements from Anderson et al.,29 recalculated using the sum of ⁸⁶Sr and ⁸⁸Sr for normalization and the total Rb as a proxy for ⁸⁷Rb, as described in the text. The ages shown here are similar to those reported by Anderson et al.,29 using traditionally constructed isochrons: 280 ± 100 Ma for the March run (left) and 400 ± 200 Ma for the May–June run (right). They are also consistent at the <1.5σ level with the TIMS result of 166 ± 6 Ma from Borg et al.28 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.