LUN Group - Lunar Meteorites

The LUN group is a most intriguing class of achondrites since its members represent different types of lunar rocks, i.e., genuine pieces of the Moon. Scientists have speculated for centuries about the possibility that some of the meteorites in our collections might actually be of lunar origin, and some researchers even believed the enigmatic tektites were the products of major volcanic eruptions on the Moon. Finally, with the return of lunar samples by the Apollo and Luna missions in the 1960's and 1970's, it was thought that a definitive test for these ideas was now available. However, subsequent comparisons yielded no match to either tektites or any group of achondrites, and it would take an additional decade before the first lunar meteorite would be identified.

In the late 1970's, Japanese and American researchers recognized the ice fields of Antarctica as a promising hunting ground for meteorites, with the subsequent recovery of thousands of new meteorites; among these, several lunar meteorites, or "lunaites", have been identified. These are mostly small stones resembling certain samples returned by the Apollo missions. In 1990, a small stone weighing just 19 grams was found in Calcalong Creek, Australia. It was thereafter recognized as the first non-Antarctic lunaite. During the past four years, professional meteorite hunters have recovered several more lunaites from the hot deserts of Africa and Oman. Today, the LUN group comprises about 25 members, excluding all probable pairings, with a total known weight of about 8.5 kg (have a look at our list of lunar meteorites).

Lunar meteorites are of major scientific importance because they probably originate from areas of the Moon that were not sampled by the Apollo or Luna missions. Most lunaites in our collections obviously have been blasted from the lunar highlands that cover the far side of the Moon. Only a few lunar meteorites have their origin from the smooth lowlands, the maria of the near side, which served as the preferred landing sites for the Apollo missions. Four different types of lunar rocks have been sampled in the form of meteorites thus far: anorthositic highland rocks (LUN A), mare basalts (LUN B), mare gabbros (LUN G), and a unique lunar norite (LUN N), representing the deep, olivine-rich layers of the lunar crust. All of these meteorites share the typical features of lunar rocks, e.g., similar manganese to iron ratios, and oxygen isotopic compositions that plot on the terrestrial fractionation line. We will discuss each group of lunar meteorites below. >> top...

LUN A - Anorthositic Highland Rocks

The members of this group are designated "LUN A" for their mineral compositions. They are anorthosites, consisting primarily of calcium-rich plagioclase with only minor pyroxene and olivine. Originally, they were coarse-grained, plutonic rocks, forming the ancient highlands that dominate both sides of the Moon. Over time, these rocks were thoroughly granulated, heavily brecciated, and partially melted by impacts, leading to the establishment of three subtypes of lunar anorthosites - regolith breccias, impact-melt breccias, and fragmental breccias.

Regolith breccias: The lunar surface is covered by a thick regolith layer, and consequently, most lunaites are polymict regolith breccias. These rocks consist of abundant white clasts of anorthositic rock and minor dark clasts of highland basalts, combined with various mineral and glass fragments, and mixed with a dark matrix of solidified rock powder. This regolith also contains traces of meteoritic material from many impactors, as well as characteristic amounts of solar wind-implanted noble gases. Typical anorthositic regolith breccias are ALH 81005, the first meteorite recognized as a lunar rock, Dar al Gani 262, the first Saharan lunaite, and Dhofar 025, a recent find from Oman. However, some atypical members also contain abundant inclusions of lunar mare lithologies, e.g., the famous Calcalong Creek. It consists of approximately 50% highland anorthosite, 20% KREEP basalt, and 15% low-titanium mare basalt, together with other minerals that are typical for the lunar maria. Calcalong Creek can be regarded as a transitional specimen between anorthositic highland regoliths and mare basalt regoliths. Recent research suggests that it probably formed between the lunar highlands and Oceanus Procellarum, one of the largest basaltic basins on the near side of the Moon.

Impact-melt breccias: The lunaites of this group are polymict breccias displaying characteristics of severe shock-metamorphism, partial melting, and recrystallization, suggesting that they are the products of larger impact events. Compositionally, they are similar to other anorthositic lunaites, and they consist primarily of plagioclase and minor accessory minerals. The members of this subgroup, such as Dar al Gani 400, Dhofar 026, and the beautiful white-colored NWA 482, were all recovered from the hot deserts of Africa and Oman during the last four years.

Fragmental breccias: The rare members of this subgroup are polymict breccias that, at least superficially, resemble anorthositic regolith breccias. They are composed of anorthositic rock fragments and other, mostly felsic, clasts, in a fine-grained matrix of pyroxene and olivine. However, they lack the regolith components and the implanted noble gases characteristic of other regolith breccias. Obviously, they represent the deeper layers of the lunar surface, of which only a few members are known. The only anorthositic fragmental breccia available to the collector is Dhofar 081 and its pairing, Dhofar 280. Both meteorites were found in close proximity to each other, and similarities in structure and composition suggest that they both are part of a single fall. >> top...

LUN B - Mare Basalts

The members of this group are mare basalts, consisting of phenocrysts of olivine and augite, set within a fine-grained matrix of plagioclase and pyroxene. The accessory minerals include chromite, ilmenite, apatite, troilite, and minor nickel-iron metal. Lunar mare basalts are much younger than the anorthositic highland rocks. They were formed during volcanic eruptions within large basins, mostly located on the near side of the Moon, their shapes delineating the imaginary face of the "Man in the Moon". Large portions of this pristine basaltic crust have been transformed into a regolith by ongoing meteorite bombardment. Consequently, most LUN B members are mare basalt regolith breccias, e.g., the Antarctic lunaites Yamato 793274, Yamato 981031, and QUE 94281. Similarly, most other mare basalts are also heavily brecciated, containing notable regolith portions. Still, there is one unbrecciated member known - the pristine mare basalt NWA 032, and its pairing NWA 479. The latter meteorite was recovered by our team near Khter n'Ait Khebbach, Morocco, in November 2000, and is one of our most unusual finds. Recent research suggests that this unique mare basalt crystallized just 2.8 billion years ago, providing evidence for a prolonged lunar volcanism and making this one of the youngest lunar basalts analyzed so far. >> top...

LUN G - Mare Gabbros

Only two meteorites belong to the classification of "LUN G", which stands for lunar mare gabbros. They consist primarily of plagioclase and pyroxene, with minor accessory minerals such as iron-titanium oxides and sulfides. Although they closely resemble low-titanium mare basalts in their bulk compositions, pyroxene compositional trends, and petrography, they display much coarser grain sizes and a cumulate intergrowth. This suggests formation from similar magmas that were trapped in magma chambers or deeper layers of the lunar surface. These magmas experienced lower cooling rates, resulting in a prolonged crystal growth and the formation of cumulate, gabbroic textures. Both members of this group are Antarctic finds. The first member, Yamato 793169, is a polymict breccia that was initially misclassified as a polymict eucrite. Not until it was re-examined in the early 1980's did it become obvious that this meteorite represented a rare type of lunar gabbroic rock. Although this small stone contains no obvious regolith components, the presence of abundant solar wind-implanted noble gases indicates that it was once part of a regolith. It probably represents a single clast that has been preserved during its passage to Earth. The second member, Asuka 881757, is a coarse-grained cumulate; a typical gabbroic rock representing one of the few unbrecciated lunar meteorites. >> top...

LUN N - Lunar Norites

This group represents a previously unsampled type of lunar rock, and it consists of just one single find, the unique NWA 773. Three stones with a total weight of 633 grams, found near Dchira, Western Sahara, have proved to be a scientific sensation. NWA 773 is a polymict breccia, consisting of two distinct lithologies - a cumulate, olivine-rich gabbronorite, and a dark regolith breccia. The cumulate portion consists of olivine (51%), pyroxene (22%), plagioclase (14%), alkali feldspar (1.5%), and accessory minerals, e.g., chromite, ilmenite, troilite, and nickel-iron metal. The regolith breccia contains fragments of the norite portion, basaltic clasts, and accessory mineral fragments set in a fine, granular matrix. Lunar norites have never been sampled by the Luna or Apollo missions, but remote sensing techniques across most parts of the lunar surface have identified several norite-rich sites that might prove to be the source of NWA 773. A possible source is the Aitken basin on the far side of the Moon - a large impact structure near the lunar South Pole that is famous for its noritic composition and secondary impact craters, and for the appropriately named "Olivine Hill". The large impact that excavated the Aitken basin removed the upper crust, revealing the lower crustal layers that would become the source of olivine-rich cumulates such as norites and gabbronorites. Continued research on NWA 773 and other lunar meteorites will certainly complete our picture of the Moon and its formation history - a history that is inseparably linked to the formation history of its nearest neighbor, the planet Earth. >> top...

Complete List of Lunar Meteorites: