Abstract
Zircon and phosphate grains from matrix and quartz‐monzodiorite (QMD) clasts in two thin sections of Apollo 15 impact melt breccia 15405 were investigated using optical microscopy, scanning electron microscopy, Raman spectroscopy, and ion microprobe U‐Pb analyses. U‐Pb results for zircon grains with well‐defined cathodoluminescence zoning define the primary (i.e., magmatic) crystallization age as 4330 ± 6 Ma (2σ). One zircon consists of a preserved inner part surrounded by a porous polycrystalline (“granular”) mixture of zircon and baddeleyite, indicating incomplete reaction of the zircon with melt. Previous work showed that this microstructure could form at pressures above 60 GPa and a temperature close to ~1700°C and is evidence of an impact‐related melting event. The U‐Pb system of this grain indicates a resetting event at 1940 ± 10 Ma, interpreted as the age of this impact (impact #1). Other zircon and phosphate grains also have disturbed U‐Pb systems, showing an even younger reset event (impact #2) at 1407 ± 57 Ma. Evidence of impact is supported by microstructures of zircon and baddeleyite such as secondary rims. These impacts are tentatively identified as those having formed Autolycus and Aristillius craters.
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