TY - JOUR
T1 - The Pb-Pb age of Angrite SAH99555 revisited
AU - Connelly, J.N.
AU - Bizzarro, M.
AU - Thrane, K.
AU - Baker, J.A.
N1 - Funding Information:
Fernando Corfu generously provided 202 Pb– 205 Pb double spike and access to his mass spectrometry facilities at the University of Oslo. R. Zartman, G. Lugmair and L. Nyquist are gratefully acknowledged for their thoughtful reviews. Financial support for this project was provided by the Danish National Science Foundation (MB), Carlsberg Foundation (MB and JNC), Danish National Science Research Council (KT) and The Jackson School of Geosciences (JNC). Discussions with Yuri Amelin throughout this study have been very helpful.
PY - 2008/10/1
Y1 - 2008/10/1
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=51349163041&partnerID=8YFLogxK
U2 - 10.1016/j.gca.2008.06.007
DO - 10.1016/j.gca.2008.06.007
M3 - Article
AN - SCOPUS:51349163041
SN - 0016-7037
VL - 72
SP - 4813
EP - 4824
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
IS - 19
ER -