Bidong ZHANG^1,2, Yangting LIN³, Jialong HAO³, Devin L. SCHRADER^4,5,Meenakshi WADHWA⁵, Randy L. KOROTEV⁶, William K. HARTMANN⁷, andAudrey BOUVIER^2,8
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.14078]
¹Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, Los Angeles, California, USA
²Department of Earth Sciences, The University of Western Ontario, Ontario, London, Canada
³Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing,China
⁴Buseck Center for Meteorite Studies, Arizona State University, Arizona,Tempe, USA
⁵School of Earth and Space Exploration, Arizona State University, Arizona,Tempe, USA
⁶Department of Earth and Planetary Sciences and McDonnell Center for the Space Sciences, Washington University, St. Louis,Missouri, USA
⁷Planetary Science Institute, Arizona,Tucson, USA
⁸Bayerisches Geoinstitut, Universität Bayreuth, Bayreuth, Germany
Published by arrangement with John Wiley & Sons

About half of the lunar meteorites in our collections are feldspathic breccias. Acquiring geochronologic information from these breccias is challenging due to their low radioactive-element contents and their often polymict nature. We used high-spatial-resolution (5 μm) NanoSIMS (nanoscale secondary ion mass spectrometry) U-Pb dating technique to date micro-zircons in the lunar feldspathic meteorites Dhofar 1528 and Dhofar 1627. Three NanoSIMS dating spots of two zircon grains from Dhofar 1528 show a discordia with an upper intercept at 4354 ± 76 Ma and a lower intercept at 332 ± 1407 Ma (2σ, MSWD = 0.01, p = 0.91). Three spots of two zircon grains in Dhofar 1627 define a discordia with an upper intercept at 3948 ± 30 Ma and a lower intercept at 691 ± 831 Ma (2σ, MSWD = 0.40, p = 0.53). Both samples likely experienced shock metamorphism caused by impacts. Based on the clastic nature, lack of recrystallization and the consistent U-Pb and Pb-Pb dates of the zircons in Dhofar 1528, the U-Pb date of 4354 Ma is interpreted as the crystallization age of its Mg-suite igneous precursor. Some of the Dhofar 1627 zircons show poikilitic texture, a crystallization from the matrix impact melt, so the U-Pb date of 3948 Ma corresponds to an impact event, likely the Imbrium basin-forming event. These data are the first radiometric ages for these two meteorites and demonstrate that in situ (high spatial resolution) U-Pb dating has potential for extracting geochronological information about igneous activities and impact events from lunar feldspathic and polymict breccias.