See more from this Division: Topical Sessions
See more from this Session: Planetary Pummeling: Cataclysmic Bombardment of the Solar System as Catastrophe, Catalyst, Cauldron, and Crucible
Abstract:
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.
See more from this Division: Topical Sessions
See more from this Session: Planetary Pummeling: Cataclysmic Bombardment of the Solar System as Catastrophe, Catalyst, Cauldron, and Crucible