Noble gas exposure ages of samples from Cone and North Ray craters: Implications for the recent lunar cratering chronology

Abstract

Cosmic ray exposure (CRE) ages of rocks that were ejected by the impacts that created Cone and North Ray craters provide two crucial calibration points at <100 Ma for the lunar cratering chronology function, which relates the crater density of geological units on the Moon to their absolute age. To reassess the formation ages of these two craters, we determine here the accumulated abundances of “cosmogenic” noble gas nuclides (3Hecosm, 21Necosm, 38Arcosm), as well as the corresponding CRE ages, in six Apollo 14 rocks (i.e., one breccia and five basalts) and two Apollo 16 anorthosites that were collected near the rims of Cone and North Ray craters, respectively. Although noble gas concentrations allow CRE ages to be derived, the calculated 21Ne and 38Ar exposure ages of a given sample cover a significant range of values because published empirical or theoretical production rates of cosmogenic nuclides are highly variable. Nonetheless, it is evident that mare basalts 14053 and 14072 as well as breccia 14068, which were collected near the rim of Cone crater, were exposed at the lunar surface more recently than the three KREEP basalts (14073, 14077, 14078) collected farther away. The 38Ar exposure ages of anorthosites 67075 and 67955 from North Ray crater slightly exceed those of samples 14053, 14068, and 14072. These results confirm that Cone crater is younger than North Ray crater. However, the formation ages of Cone and North Ray craters have larger uncertainties than previously acknowledged. This implies that the uncertainties of noble gas exposure ages should be taken into account when remotely dating young surfaces on the Moon and on other planetary bodies in the solar system.

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“…Nitrogen was purified in a Pyrex and quartz glass line using a copper oxide (CuO) furnace cycled between 450° and 900°C and a U-shaped cold trap held at −180°C. The three isotopes of Ne and Ar and the three isotopologues of N 2 were analyzed sequentially using multicollection ( 38 , 39 ).…”