The effect of 1.9 and 1.4 Ga impact events on 4.3 Ga zircon and phosphate from an Apollo 15 melt breccia

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 ± 6Ma (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 60GPa 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 ± 10Ma, 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.