Evidence for multiple 4.0–3.7 Ga impact events within the Apollo 16 collection

1,2,3,4Takafumi Niihara,4,5Sky P. Beard,4,5Timothy D. Swindle,6Lillian A. Schaffer,1,2Hideaki Miyamoto, 3,4David A. Kring
Meteoritics & Planetary Science (in Press) Link to Article [https://doi.org/10.1111/maps.13237]
1Department of Systems Innovation, University of Tokyo, , Tokyo, 113‐8656 Japan
2University Museum, University of Tokyo, , Tokyo, 113‐0033 Japan
3Center for Lunar Science and Exploration, Lunar and Planetary Institute, Universities Space Research Association, , Houston, Texas, 77058 USA
4SSERVI, NASA Ames Research Center, , Mountain View, California, 94035 USA
5Lunar and Planetary Laboratory, University of Arizona, , Tucson, Arizona, 85721–0092 USA
6Department of Earth and Atmospheric Sciences, University of Houston, , Houston, Texas, 77204–5007 USA
Published by arrangement with John Wiley & Sons

In a histogram of lunar impact ages from the Apollo 16 site, there is a spike circa 3.9 Ga that has been interpreted to represent either a large number of nearly synchronous events or an abundance of samples that were affected slightly differently by the event that produced the Imbrium basin. To further scrutinize those age relationships, we extracted six centimeter‐sized clasts of impact melt from ancient regolith breccia 60016 and performed petrological and geochronological (40Ar‐39Ar) analyses. Three clasts have similar poikilitic textures, while others have porphyritic, aphanitic, or intergranular textures. Compositions and abundances of relict minerals are different in all six clasts and variously imply Mg‐suite and ferroan anorthosite target sequences. Estimated bulk compositions of four clasts are similar to previously defined group 1 Apollo 16 impact melt rocks, while the other two have higher Al2O3 and lower FeO+MgO compositions. All six clasts have similar K2O and P2O5 concentrations, which could have been derived from a KREEP‐bearing component among target sequences. Eighteen 40Ar/39Ar analyses of the six clasts produced an age range from 3823 ± 75 to 4000 ± 23 Ma, consistent with estimates for the proposed late heavy bombardment. Four clasts have multiple temperature steps that define plateau ages. These ages are distinct, so they cannot be explained by a single impact event, such as the one that produced the Imbrium impact basin. The conclusion that these represent distinct ages remains after considering the possibility of artifacts in defining plateaus.

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