26Al–26Mg systematics in D’Orbigny and Sahara 99555 angrites: Implications for high-resolution chronology using extinct chronometers

We report on an investigation of the 26Al–26Mg isotope systematics in the D’Orbigny and Sahara 99555 angrites. High precision Mg isotope compositions and Al/Mg ratios were measured in mineral separates and whole rock samples from D’Orbigny and Sahara 99555 using multiple-collector inductively coupled plasma mass spectrometry (MC-ICPMS). Plagioclase separates from both angrites have resolvable excesses in 26Mg (Δ26Mg) that correlate with their respective Al/Mg ratios. 26Al–26Mg systematics in the mineral separates and whole rocks define precise isochrons that correspond to 26Al/27Al ratios of (5.06 ± 0.92) × 10−7 and (5.13 ± 1.90) × 10−7 and initial Δ26Mg values of −0.006 ± 0.040‰ and −0.016 ± 0.047‰ for D’Orbigny and Sahara 99555, respectively. The slopes and initial Δ26Mg values are identical for these two meteorites within errors and the data for both angrites considered together define an isochron corresponding to a 26Al/27Al ratio of (5.10 ± 0.55) × 10−7 and initial Δ26Mg value of −0.012 ± 0.019. Relative to the Efremovka E60 CAI, the 26Al/27Al values reported here for these angrites imply 26Al–26Mg ages of 4562.42 ± 0.29 Ma and 4562.43 ± 0.53 Ma for D’Orbigny and Sahara 99555, respectively. These 26Al–26Mg ages are concordant with model ages determined using other extinct radionuclide chronometers (e.g., 53Mn–53Cr and 182Hf–182W), but are ∼2 Myr younger than the absolute 207Pb–206Pb ages that have been reported recently for these angrites. The reason for this discrepancy is not presently known, but may imply disturbance of one or more of the isotope systems under consideration or a possible bias in the 207Pb–206Pb ages of the angrites resulting from natural or analytical causes.