MAC88105—A regolith breccia from the lunar highlands: Mineralogical, petrological, and geochemical studies

Abstract

The new large lunar meteorite MAC88105 is a dense breccia, with lithic and mineral clasts and fragments set into a welded matrix. It is a regolith breccia which shows some recrystallization and evidence for a late shock event during which anorthositic glass veins were formed. Shock effects (most probably due to the impact ejection from the moon) are present throughout the sample and require a shock pressure of about 25–30 GPa, in agreement with observations made on other lunar meteorites. Some components of MAC88105 have been subjected to a shock pressure of about 40–45 GPa as evident from melt pockets in a clast. The population of lithic clasts in MAC88105 is similar to other lunar highland breccias. Vitric breccias are more abundant than granulitic breccias and plutonic rock fragments. The presence of devitrified glass (spheres and shards) supports a regolith origin. Most common are metameltbreccias consisting of abundant anorthitic plagioclase clasts and a dense, fine-grained matrix. Some fine-grained hornfelsic to granulitic metabreccias are also present. Lithic clast compositions are predominantly anorthositic noritic (or noritic anorthositic), and anorthositic troctolitic. Spinel-bearing rocks are present, but do not belong to the spinel-troctolite group. A spinel-bearing clast in MAC88105 consists of anorthite + pigeonite + spinel and indicates a different heritage, possibly similar to spinel cataclasites described from Apollo 17. The pyroxenes in MAC88105 have somewhat unusual compositions; orthopyroxenes and augites appear to be rare. Most lithologies (breccias and igneous rocks) contain solely pigeonite. We have found one igneous mafic rock, a gabbro, which has a low mg-number and is possibly of mare origin. A metal grain of a composition similar to metal in H chondrites supports the interpretation of MAC88105 as a regolith breccia. The bulk composition of MAC88105 is similar to the other lunar highland meteorites. The REE contents are slightly higher than for the other anorthositic meteorites, with a smaller positive Eu anomaly. This is in agreement with a possible KREEP contribution, possibly introduced from glasses. MACS 8105 is a mixture of rocks with a predominant contribution from the anorthosite suite and small admixtures from other rock types (mare basalt, Mg suite), which becomes obvious in plots of molar mg or Sm content vs. the Ti Sm ratio. The siderophile element abundances in MAC88105 are similar to other highland meteorites and show the characteristic Co excess and a Au Ir ratio similar to the other anorthositic meteorites and lower than the hyperchondritic ratio in Apollo 16 rocks, thus support the notion that the lunar meteorites are a more representative sample of the lunar highlands than the Apollo samples. The chemistry and mineralogy of MAC88105 is different from that of the other lunar meteorites and suggests a different source, which is supported by cosmic-ray and noble gas data. At this time it seems likely that about four individual impact events have been responsible for delivering the seven highland meteorites. MAC88105 and other lunar meteorites are important as they are probably random samples from the lunar surface and may thus be more representative than the Apollo and Luna rocks which come from only a small area of the moon.