Abstract
Apollo 16 soil-like regolith breccia 65745,7 contains two zircon-bearing clasts. One of these clasts is a thermally annealed silica-rich rock, which mineralogically has affinities with the High Alkali Suite (Clast 1), and yields zircon dates ranging from 4.08 to 3.38 Ga. The other clast is a KREEP-rich impact melt breccia (Clast 2) and yields zircon dates ranging from 3.97 to 3.91 Ga. The crystalline cores of both grains, which yield dates of ca 3.9 Ga, have undergone shock pressure modification at less than 20 GPa. We interpret that the U-Pb chronometer in these zircon grains has been partially reset by the Imbrium basin-forming event when the clasts were incorporated into the Cayley Plains ejecta blanket deposit. The zircon grains in Clast 1 have been partially decomposed, resulting in a breakdown polymineralic texture, with elevated U, Pb and Th abundances compared with those in the crystalline zircon. These decomposed areas exhibit younger dates around 3.4 Ga, suggesting a secondary high-pressure, high-temperature event, probably caused by an impact in the local Apollo 16 highlands area.
Highlights
The ancient lunar feldspathic crust has been extensively modified by impact cratering and basin formation, as royalsocietypublishing.org/journal/rsos R
Sample 65745 is a 7.76 gram, 2.6 × 2.2 × 1.2 cm, regolith breccia collected to the south of the Apollo 16 landing site at Station 5 Stone Mountain [75]
Impact melt spherules and agglutinates are distributed throughout the sample with a range of zircon decomposed area
Summary
The ancient (greater than 4.35 Ga) lunar feldspathic crust has been extensively modified by impact cratering and basin formation, as royalsocietypublishing.org/journal/rsos R. [1] and references therein) Temporal relationships between these endogenic and exogenic processes are preserved in the isotopic record of mineral grains and rock fragments collected from the lunar surface regolith by the Apollo astronauts and robotic Luna sample return missions. Unravelling this polyphased geological record is challenged by several issues. More widely there is a lack of clear understanding of the relationship of rock fragments collected in the regolith to their parent bedrock lithology or impact crater formation setting(s) The knowledge we have gained through sample studies about the Moon’s crust formation, impact bombardment and magmatic history is extensive (e.g. [12]), there are still many outstanding questions about the contemporaneous age relationship between primary ferroan anorthosite (FAN) crustal rocks and intrusive rocks of the Mg-Suite and High Alkali Suite (HAS), and the timing and duration of impact basin formation (e.g. [1,3,13,14,15,16,17,18,19,20,21,22,23])
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