Re-examination of the formation ages of the Apollo 16 regolith breccias

Details Re-examination of the formation ages of the Apollo 16 regolith breccias Re-examination of the formation ages of the Apollo 16 regolith breccias

Nov 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011gecoa..75.7208j&link_type=abstract

Geochimica et Cosmochimica Acta, Volume 75, Issue 22, p. 7208-7225.

Physics

1

Scientific paper

The lunar regolith is exposed to irradiation from the solar wind and to bombardment by asteroids, comets and inter-planetary dust. Fragments of projectiles in the lunar regolith can potentially provide a direct measure of the sources of exogenous material being delivered to the Moon. Constraining the temporal flux of their delivery helps to address key questions about the bombardment history of the inner Solar System. Here, we use a revised antiquity calibration (after Eugster et al., 2001 ) that utilises the ratio of trapped 40 Ar/ 36 Ar ('parentless' 40 Ar derived from radioactive decay of 40 K, against solar wind derived 36 Ar) to semi-quantitatively calculate the timing of the assembly of the Apollo 16 regolith breccias. We use the trapped 40 Ar/ 36 Ar ratios reported by McKay et al. (1986) . Our model indicates that the Apollo 16 ancient regolith breccia population was formed between ˜3.8 and 3.4 Ga, consistent with regoliths developed and assembled after the Imbrium basin-forming event at ˜3.85 Ga, and during a time of declining basin-forming impacts. The material contained within the ancient samples potentially provides evidence of impactors delivered to the Moon in the Late-Imbrian epoch. We also find that a young regolith population was assembled, probably by local impacts in the Apollo 16 area, in the Eratosthenian period between ˜2.5 and 2.2 Ga, providing insights to the sources of post-basin bombardment. The 'soil-like' regolith breccia population, and the majority of local Apollo 16 soils, were likely closed in the last 2 Ga and, therefore, potentially provide an archive of projectile types in the Eratosthenian and Copernican periods.

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