The Lunar Crust as Viewed Via Lunar Meteorites

The first meteorite recognized to be a rock from the Moon was found in 1982, nine years after the last Apollo mission. Since then, about 55 more have been found, all in deserts. More than half of the lunar meteorites have been recognized since 2003 as government-sponsored meteorite searches continue in Antarctica and modern-day prospectors scour the world's hot deserts. All lunar meteorites were launched since the Miocene (most were launched in the past million years) from unknown locations on the Moon by impacts of asteroidal meteorites. Most lunar meteorites are polymict impact breccias, and most of these are highly feldspathic, but nine are largely-unbrecciated basalts from the maria. As samples from random locations on the Moon, lunar meteorites provide a type of information not available in rocks of the Apollo collection. We can assume, for example, that half of the lunar meteorites originate from the farside. In contrast, the six Apollo landing sites represent only a small portion of the nearside face of the Moon. More importantly, we now know as a result of whole-moon gamma-ray spectrometry from the Lunar Prospector mission (1998-1999), that three of the Apollo missions landed in a geochemically anomalous, high-Th region of the Moon, the “PKT” (Procellarum KREEP Terrane), and that the other three contain Th-rich breccias from the impact that formed the huge Imbrium basin, which coincidentally landed in the PKT. Lunar meteorites extend the range of rock types, compositions, and period of volcanic activity known for the Moon. For example, from the meteorites we know that the early feldspathic crust also contained magnesian (high Mg/Fe) anorthosites as well as the characteristic ferroan anorthosites of the Apollo missions and that the “magnesian suite” norites, troctolites, and dunite of the Apollo collection are products of the PKT, not the feldspathic highlands.