Lunar zircons did not record the cataclysm

Lunar zircons crystallized from the trace-element-enriched residum at the end of the lunar magma ocean, and in gabbroic intrusions in the lunar crust up until the end of the late bombardment. Most zircons are broken free from their parental mineral assemblage and are now found as rounded (detrital) fragments in trace-element-rich breccias and soils. They range of shape, size, internal structure, chemical composition, and age – many U-Pb compositions being concordant. However, they generally lack the oscillatory zoning, or multigenerational features, characteristic of terrestrial zircon.

Lunar zircons did not record the lunar cataclysm, nor the impact event that produced the breccias they are found in. There is the possibility that they may record the age of earlier impact-generated magmatism, but it is more likely that the magmatism is of endogenous origin.

A small number of lunar zircons have attached plagioclase, pyroxene or ilmenite giving clues that their original parental rock assemblage was noritic or gabbroic and relatively mafic in nature. Some zircons are found within silica-K-feldspar intergrowths – sometimes referred to as “lunar granophyre”.

Zircons are found in relative abundance in the Apollo 14 Fra Mauro breccias (Imbrium ejecta) and in the aphanitic breccias from the South Massif at Apollo 17 (Serenitatis ejecta?). While zircons from the Apollo 14 breccias range in age from 4.4 – 3.9 Ga., the zircons from Apollo 17 group tightly around 4.35 Ga.

The oldest (and largest) lunar zircon we have studied has an original crystallization age of 4417 ± 6 Ma with evidence of shock event at 4333 ± 7 Ma. Another large fragmented zircon is dated at 4315 ± 15 Ma with secondary age of 4187 ± 11 Ma. We interpret the older ages as evidence that the lunar magma ocean had already differentiated and solidified by ~ 4.4 Ga.