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

Abstract

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 40Ar/36Ar 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 4×109 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.

“New” lunar meteorites: Implications for composition of the global lunar surface, lunar crust, and the bulk Moon

“…Their compositions tend to closely follow the average composition of the underlying crust (albeit always dampened, in a statistical way, by contributions from more distant sources), which makes them good indicators of regional variations in crustal composition. The variability at any given site is greater for regolith breccias than for evolving soils (lunaite regolith breccias attest to the occasional displacement of regolith breccias over great distances), but the vast majority of regolith breccias at a given locale are similar in bulk composition to local soils (McKay et al 1986(McKay et al , 1989.…”

Chronology, geochemistry, and petrology of a ferroan noritic anorthosite clast from Descartes breccia 67215: Clues to the age, origin, structure, and impact history of the lunar crust

“…The trapped 40 Ar/ 36 Ar ratio in 67215 is unknown, and the Ar-Ar "plateau" age of 3.93 Gyr for the clast was determined by correcting for trapped 40 Ar/ 36 Ar = 5.0. Trapped 40 Ar/ 36 Ar ratios for several Apollo 16 regolith breccias lie in the range of <1 to ~12, and this ratio tends to be significantly larger for lunar samples that acquired their trapped gases early in lunar history (McKay et al 1986). An isochron plot (R 2 = 0.994) of 40 Ar/ 36 Ar versus 39 Ar/ 36 Ar for 9 extractions of the clast releasing ~17-100% of the 39 Ar gives an age of 4.06 Gyr and a trapped 40 Ar/ 36 Ar intercept of -5.2 ± 2.8.…”

The Monahans chondrite and halite: Argon‐39/argon‐40 age, solar gases, cosmic‐ray exposure ages, and parent body regolith neutron flux and thickness

“…Mare surfaces in existence for the past -3.8 Ga have regoliths that are a few meters deep but very mature regoliths. Older highland surfaces, however, have deeper and much less mature mega-regoliths, produced by the higher rate of large impacts >3.8 Ga ago (McKay et al, 1986).…”