Geochemical comparison of impact glasses from lunar meteorites ALHA81005 and MAC88105 and Apollo 16 regolith 64001
Abstract
Most glasses that occur in lunar highland regolith are quenched droplets of impact melt. The chemical compositions of these glasses are equivalent, in the absence of volatile losses, to the original target materials. The compositional range of impact glasses in a regolith reflects the chemical diversity that existed throughout the region up to the time of system closure (e.g., breccia formation). Since these glasses are a product of widespread and random sampling, both in terms of space and time, they can be used for geochemical exploration of the Moon. The major-element compositions of impact glasses occurring in three samples of lunar feldspathic regolith (ALHA81005; MAC88105; Apollo 16 64001) have been determined by electron microprobe. The glass populations among these three unrelated samples are compositionally distinct. While most of the impact glasses within each of these three samples are compositionally similar to the regolith in which they are found, up to 40% of the impact glasses are different. Some of the compositionally exotic glasses were ballistically transported from other areas of the Moon and thereby provide information about the compositional range of regoliths that exist elsewhere. Since the geological setting of the Apollo 16 region is well known compared to the source areas of the lunar meteorites, the Apollo 16 glasses provide a ground truth for interpretations. The thirty-four impact glasses in meteorite ALHA81005 overlap the compositional range of feldspathic regoliths represented by all of the lunar highland meteorites (i.e., Y86031; Y82192; Y791197; MAC88105; ALHA81005) and are compositionally distinct from highland regoliths sampled by Apollo 14, Apollo 16, Apollo 17, and Luna 20. A small proportion of ALHA81005 glasses contain a mare component. No KREEP glasses were observed. Due to their scarcity, only ten impact glasses in lunar meteorite MAC88105 were analyzed. The spherules occur in two, broadly defined, compositional groups. One group of glasses is similar to the bulk composition of MAC88105, whereas the other group is more mafic. All of these glasses are distinct from highland regoliths sampled by Apollo 14, Apollo 16, and Apollo 17, and Luna 20. A low-Ti mare glass was also analyzed in MAC88105. No KREEP glasses were observed. The two hundred and fifty-three aluminous impact glasses analyzed in Apollo 16 regolith 64001 display a prominent grouping of compositions equivalent to the local regolith. In addition, about 10% of the glasses in 64001 are distinct from the local regolith and chemically resemble the lunar highland meteorites (ALHA81005; MAC88105). These glasses may record the chemical composition of an ancient regolith that occurred in the Apollo 16 region prior to the arrival of KREEP and mare components. Twentythree percent of the glasses in 64001 have high-Ti mare affinities. These mare glasses are clearly exotic to the Apollo 16 site and have been transported from distances of at least 300 km.
- Publication:
-
Geochimica et Cosmochimica Acta
- Pub Date:
- November 1991
- DOI:
- 10.1016/0016-7037(91)90470-P
- Bibcode:
- 1991GeCoA..55.3019D
- Keywords:
-
- Breccia;
- Glass;
- Impact Melts;
- Lunar Crust;
- Meteoritic Composition;
- Regolith;
- Apollo 16 Flight;
- Lunar Composition;
- Magnesium Compounds;
- Phase Diagrams