The Pb-Pb age of Angrite SAH99555 revisited

Representing a suite of well-preserved basaltic meteorites with reported ages from 4566.18±0.14 Ma to 4557.65±0.13 Ma, angrites have been recurring targets for cross-calibrating extinct and absolute chronometers. However, inconsistencies exist in the available chronological data set, including a 4566.18±0.14 Ma Pb-Pb age reported by Baker et al. [Baker J., Bizzarro M., Wittig N., Connelly J. and Haack H. (2005) Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites. Nature 436, 1127-1131] for Sahara 99555 (herein SAH99555) that is significantly older than a Pb-Pb age for D'Orbigny, despite the two meteorites yielding indistinguishable Hf-W and Mn-Cr ages. We re-evaluate the Pb-Pb age of SAH99555 using a stepwise dissolution procedure on a whole rock fragment and a pyroxene separate. The combined data set yields a linear array that reflects a mixture of radiogenic Pb and terrestrial contamination and corresponds to an age of 4564.58±0.14 Ma, which is 1.60±0.20 Ma younger than that reported by Baker et al. [Baker J., Bizzarro M., Wittig N., Connelly J. and Haack H. (2005) Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites. Nature 436, 1127-1131]. Our conclusion that SAH99555 crystallized at 4564.58±0.14 Ma requires that all initial Pb was removed in the first progressive dissolution steps, an assertion supported by linearity of data generated by stepwise dissolution of a single fragment and the removal of an obvious highly-radiogenic component early in the dissolution process. We infer that the linear array defined by Baker et al. [Baker J., Bizzarro M., Wittig N., Connelly J. and Haack H. (2005) Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites. Nature 436, 1127-1131] and their older age reflects a ternary mixture of Pb with constant relative proportions of highly-radiogenic initial Pb and radiogenic Pb with varying amounts of a terrestrial contamination. This requires that the phase harboring the initial Pb is insoluble in 2 M HCl, the only acid applied to the samples by Baker et al. [Baker J., Bizzarro M., Wittig N., Connelly J. and Haack H. (2005) Early planetesimal melting from an age of 4.5662 Gyr for differentiated meteorites. Nature 436, 1127-1131] prior to dissolution.