File:The Closest Apollo 16 Moon Rock Match — Gadamis 004 (51942948731).jpg

Presenting the largest central slice of Gadamis 004 — the latest “Apollo Lunar” submitted to the Met. Bull., one of the oldest rocks on the moon. It’s 8.5” x 6” on the face. And what a beauty (click on it to zoom in).

This ancient lunar meteorite is the closest match to the rocks collected on the Apollo 16 mission to the lunar highlands. Classification: “Lunar Ferroan Anorthosite (FAN), cataclastic. Olivine, pigeonite and plagioclase compositional values plot within the FAN suite field. The very high anorthite content (98-99%) and cataclastic texture is similar to Apollo 16 cataclastic FANs. Polished sawcut surfaces reveal cm-size clasts of anorthosite bounded by darker, fine-grained, cataclastic zones. Together, olivine and pyroxene make up a total of ~1-2% of this meteorite.”

For a sense of the scientific importance of these samples, here’s a summary from <a href="http://www.psrd.hawaii.edu/April04/lunarAnorthosites.html" rel="noreferrer nofollow">PSRD 2004</a> on the Apollo 16 FANs:

"Anorthosites, rocks composed almost entirely of plagioclase feldspar, are the oldest rocks on the Moon. They appear to have formed when feldspar crystallized and floated to the top of a global magma ocean that surrounded the Moon soon after it formed.

The isotopic compositions of lunar ferroan anorthosites indicates that the primary lunar crust formed about 100 million years after the oldest datable materials found in primitive meteorites precipitated from the solar nebula. As crystallization of a lunar magma ocean is likely to have been relatively fast, this implies that assembly of the Moon was a relatively late event during the formation of the Solar System. Such a scenario is consistent with the planetesimal accretion hypothesis in which the origin of the Moon was intimately linked to the early evolution of the Earth through gigantic collisions between proto-planets.

The internal structure and chemical compositions of the terrestrial planets provide intriguing clues to their origins, but the record of early events on Earth, Venus, and Mars has been obscured or erased by billions of years of geological activity. Processes such as convection, volcanism, weathering, and erosion have largely obliterated the primary signatures that would inform us about the mechanisms and timing of planetary formation in the inner Solar System. Fortunately, nature has provided a keystone that links the record of early nebular events preserved in meteorites with the subsequent geological evolution of the terrestrial planets, and that keystone is the Moon. For example, volcanism on the Earth and Moon overlapped in time for about a billion years, yet the Moon's crust is sufficiently old that it preserves direct evidence for planetary-scale events that occurred before the Earth's surface stabilized. In effect, the surface of the Moon is a time capsule that carries a record of the physical processes that created and modified the terrestrial planets.

An essential step in unraveling some of the early planetary history was the acquisition of samples from the Moon by the Apollo and Luna exploration missions. For example, the first studies of Moon rocks inspired John Wood (Smithsonian Astrophysical Observatory) to boldly imagine the idea that terrestrial planets must have been extensively molten soon after they formed.

This global melting event produced a stratified Moon with a low-density crust formed by accumulation of the mineral plagioclase overlying a higher density mantle of olivine and pyroxene. Meteorite impacts have reworked the lunar crust extensively over the past 4.5 billion years, and most of the rocks returned from the Moon are breccias. Although these breccias preserve important clues to lithologic and compositional diversity in the lunar crust and the impact history of the Earth and Moon, deciphering the primary record of crustal evolution from these rocks is difficult because they are mechanical mixtures of unrelated rocks.

Lunar anorthosites in particular have assumed a key role in our understanding of the early history of the Moon because lunar geochemists think that these rocks crystallized directly from the global magma ocean. The ages and chemical compositions of lunar anorthosites therefore provide ground truth tests for theoretical models of planetary accretion and differentiation.

The pyroxenes and olivines from these rocks defined an age of 4.46 ± 0.04 billion years (see graph below). This may represent a robust estimate for the primary crystallization age of the earliest lunar crust.

If all of these samples crystallized from a common magmatic system, as suggested by their coherent mineralogical, isotopic, and trace element characteristics, this magma must have been at least 20 km deep, and probably >45-60 km deep to account for the lack of complementary mafic and ultramafic cumulates in the lunar crust. Such a deep magmatic system supports the idea that a global magma ocean was present on the Moon soon after it formed."