Portal de Periódicos da CAPES

Apollo 16 regolith breccias: Characterization and evidence for early formation in the mega‐regolith

1986; American Geophysical Union; Volume: 91; Issue: B4 Linguagem: Inglês

10.1029/jb091ib04p0d277

2156-2202

D. S. McKay, D. D. Bogard, R. V. Morris, R. L. Korotev, P. Johnson, S. J. Wentworth,

Geology and Paleoclimatology Research

All of the Apollo 16 regolith breccias (18 specimens) have been characterized in terms of their petrography, grain‐size distribution, porosity, major and trace element composition, noble gas contents, and ferromagnetic resonance properties. These breccias vary significantly with respect to their density and porosity, with the more dense breccias showing significant shock damage. The regolith breccias resemble the soils in grain‐size distribution and in the relative proportions of major petrological components, except agglutinates. Many of the breccias are compositionally different from the Apollo 16 soils in that they lack an important mafic component present in the soils. Although some groupings occur, the petrologic and chemical compositions of the regolith breccias do not correlate with the station location of the samples. All but one of the breccias show some evidence of irradiation at the lunar surface (solar gases, measurable FMR, agglutinates), and analyses made on grain‐size separates from two disaggregated breccias indicate that this irradiation occurred before compaction when the breccia material was finely disseminated on the surface. However, the concentrations of surface irradiation parameters (solar gases, FMR, agglutinates) for most breccias are far less than seen in any lunar soils or in regolith breccias from other Apollo missions. Several breccias also contain unusually high trapped 40 Ar/ 36 Ar ratios of ∼8–12 and a significant fission Xe component in excess of that expected from in situ production. These observations suggest that the surface irradiation of these breccias occurred as early as 4×10 9 years ago. We conclude that most of the Apollo 16 regolith breccias were not formed from any known Apollo 16 soil. They appear to be well‐comminuted material that contains ancient regolith developed during the late stage heavy bombardment of the moon when large impacts were much more common relative to small impacts so that regoliths did not have time to significantly mature before being diluted by fresh ejecta and buried. This ancient megaregolith is significantly different from more recent lunar regolith but may be similar to asteroidal regoliths from which some brecciated meteorites have formed.