MAC88105-A regolith breccia from the lunar highlands: Miner~o~c~ petrological, and geochemical studies*

The new large Iunar 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 MAC88 105 have been subjected to a shock pressure of about 40-45 GPa as evident from melt pockets in a clast. The ~pulation of lithic clasts in MAC88105 is similar to other lunar highland breccias. Vitric breccias are more abundant than gram&tic 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 homfelsic to granulitic metabreccias are also present. Lithic clast compositions are predominantly anorthositic noritic (or no&c anorthositic), and anorthositic troctolitic. Spinel-bearing rocks are present, but do not belong to the spinel-tr~olite group. A spinel-bearing clast in MAC88105 consists of anorthite + pigeon&e + spine1 and indicates a different heritage, possibly similar to spine1 cataclasites described from Apollo 17. The pyroxenes in MAC88 10.5 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 m&c 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 ano~ositi~ meteorites, with a smaller positive Eu anomaly. This is in agreement with a possible KJ%EEP contribution, possibly introduced from glasses. MAC88105 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 MAC88 105 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. MAC88 105 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. INTRODUCI’ION and are of anorthositic composition (e.g., PALME et al., 1983; THE STUDY OF LUNAR meteorites contributes essential details towards a better understanding of the composition and history of the lunar surface. After the discovery of the first piece from the moon about eight years ago in the Allan Hills/Victoria Land, Antarctica (MARVIN, 1983), several other samples have been found at different geographical locations throughout Antarctica. At this time, eleven lunar meteorites have been recovered by Japanese and American expeditions to Antarctica. Seven samples from the Antarctic meteorite collection have been identified as lunar highland rocks (ALHA 1005, Y791197, Y82192, Y82193, Y86032, MAC88104, MAC8810.5). It is somewhat peculiar that all the first lunar meteorites identified have originated from the lunar highlands * This paper is part of a consortium study of the largest lunar meteorite MAC88104/5. WARREN and KALLEMEYN, 1986, 1987; OSTERTAG et al., 1986; BISCHOFF et al., 1987; YANAI et al., 1987; TAKEDA et al., 1987, 1989; KOEBERL, 1988; KOEBERL et al., 1989). Recently, EET87521 was identified as the first sample with lunar mare composition (WARREN and KALLEMEYN, 1989; DELANEY, 1989; DELANEY et al., 1990). Y793274, a very small sample (8.66 g), was first thought to be of anorthositic (highlands) provenance as well (YANAI and KOJIMA, 1987), but more detailed consortium studies (TAKEDA et al., 1990b; LINDSTROM and MARTINEZ, 1990; WARREN and KALLEMEYN, 1990; KURAT et al., 1990; KOEBERL et al., 1991; LINDSTROM et al., 199 1 a) found that it is a mixture of mare and highlands components at a ratio of about 2:l. It may have originated from an impact that occurred at a mare/ highlands boundary, or penetrated through a thin mare layer to underlying highlands. Y793 169 and Asuka3 1 have been