^1,2Chitrangada Datta,^1,3Yuri Amelin,^1,4,5Evgenii Krestianinov,⁶Anthony J. Irving,¹Ian S. Williams
Icarus (in Press) Open Access Link to Article [https://doi.org/10.1016/j.icarus.2024.115979]
¹Research School of Earth Sciences, The Australian National University, Canberra, ACT 2601, Australia
²School of Earth, Atmosphere & Environment, Monash University, Clayton, VIC 3168, Australia
³Korea Basic Science Institute, Ochang, Cheongwon, Cheongju, Chungbuk 28119, Republic of Korea
⁴Department of Geological Sciences, Universidad Catolica del Norte, UCN, Antofagasta, Chile
⁵Center for Excellence in Astrophysics and Associated Technologies, CATA, Santiago, Chile
⁶Department of Earth & Space Sciences, University of Washington, Seattle, WA 98195, USA
Copyright Elsevier

Pb-isotopic dating of the recently discovered diabasic angrite Northwest Africa (NWA) 12,320 yielded ages of 4564.17 ± 0.50 Ma and 4562.82 ± 0.37 Ma for acid-insoluble and soluble minerals respectively. These ages are calculated using measured 238U/235U ratios of soluble and insoluble minerals, thereby excluding the possibility that the apparent age difference is caused by internal U-isotopic variations. The age of the insoluble minerals is indistinguishable from published Pb isotopic ages of other medium-grained angrites SAH 99555 and D’Orbigny. The age of acid-soluble minerals is distinctly younger, with a 1.35 ± 0.62 Ma age gap between the Pbsingle bondPb ages of the two mineral fractions. The mineral hosts of U, Th and radiogenic Pb in the meteorite were identified through in-situ SIMS measurements of these elements in all minerals. The principal carriers of U, Th and radiogenic Pb are acid insoluble Al-Ti-rich augite and soluble Si-rich phosphate. Closure temperatures for diffusion of radiogenic Pb in these minerals are calculated to be respectively 820 ± 90 °C and 450 ± 80 °C using the observed range of crystal dimensions and the published experimental data for Pb diffusion in augite and apatite. The age difference between the pyroxene and phosphates has two potential explanations: (1) a late reheating event >450 °C which may have re-set the Usingle bondPb systematics of the phosphates but not the pyroxenes or (2) slow cooling of the rock from ~450–820 °C. If slow cooling is the reason for the age difference, we can estimate a model cooling rate of 270 ± 150 °C/Ma over the estimated closure temperature range between pyroxenes and phosphates. This rate is ~11 orders of magnitude slower than petrologic cooling rates of quenched angrites at temperatures above 1000 °C determined experimentally using the composition of Asuka 881,371 groundmass (Keil, 2012), which is similar to NWA 12320 groundmass. If there was no late reheating event, the discrepancy between fast cooling rates at high temperatures near the solidus, followed by much slower cooling at lower temperatures, might have been caused by impact melting. It could also be attributed to the initial eruption of the source magma of NWA 12320 near the surface of the parent body, followed by burial due to subsequent eruptions.