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Chronology of dimict breccias and the age of South Ray crater at the Apollo 16 site
Author(s) -
EUGSTER Otto
Publication year - 1999
Publication title -
meteoritics and planetary science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.09
H-Index - 100
eISSN - 1945-5100
pISSN - 1086-9379
DOI - 10.1111/j.1945-5100.1999.tb01347.x
Subject(s) - breccia , impact crater , geology , anorthosite , geochemistry , clastic rock , bedrock , meteorite , chronology , ejecta , radiometric dating , impact structure , geomorphology , paleontology , sedimentary rock , astrobiology , plagioclase , astrophysics , physics , quartz , supernova
— We report the noble gas isotopic abundances of five dimict breccias and one cataclastic anorthosite that were collected at the Apollo 16 landing site. Orbital and surface photographs indicate that rays from South Ray crater, an almost 1 km wide young crater in the Cayley plains, extend several kilometers from their source into the area that was sampled by the Apollo 16 mission. Previous studies have shown that South Ray crater formed 2 Ma ago and that a large number of rocks might originate from this cratering event. On the basis of cosmic‐ray produced nuclei, we find that the six rocks investigated in this work yield the same lunar surface exposure age. Using literature data, we recalculate the exposure ages of additional 16 rocks with suspected South Ray crater origin and obtain an average exposure age of 2.01 ± 0.10 Ma. In particular, all nine dimict breccias (a type of rock essentially restricted to the Apollo 16 area consisting of anorthosite and breccia phases) dated until now yield an average ejection age of 2.06 ± 0.17 Ma. We conclude that they must originate from the Cayley formation or from bedrock underlying the Cayley plain. We determined the gas retention ages for the dimict breccias based on the 40 K‐ 40 Ar and U,Th‐ 136 Xe dating methods: rock 64425 yields a 40 K‐ 40 Ar age of 3.96 Ga and rock 61016 a U,Th‐ 136 Xe age of 3.97 Ga. These results, together with 39 Ar‐ 40 Ar ages obtained by other workers for rocks 64535 (3.98 Ga) and 64536 (3.97 Ga), show that the dimict breccias formed 3.97 Ga ago.