Dating lunar rocks
Fortunately, the primary record of the early crustal genesis and evolution on the Moon has not been completely destroyed.
Lunar scientists have developed criteria such as low abundances of siderophile elements (which are present in high concentrations in most meteorites relative to common igneous rocks) and other chemical and petrographic data, to identify a suite of rocks thought to represent primary igneous cumulates from the lunar highlands.
Not all ages determined for anorthosites, however, are as old as we expected--one appeared to be only 4.29 billion years old.
Mineral compositions and trace element characteristics of this clast show that it is genetically related to the main group of lunar ferroan anorthosites.
These cumulate rocks are rich in plagioclase, and most are classified as anorthosites (90% plagioclase), norites (plagioclase plus low-Ca pyroxene) and troctolites (plagioclase plus olivine).