Constraining the Flux of Impactors Postdating Heavy Bombardment Using U-Pb Ages of Impact Glasses

Introduction: Spherules of glass varying in size from a few micrometres to a few millimetres are common in the lunar regolith. While some of these glass beads are products of pyroclastic fire fountains others originate as impact melt ejected from the target that breaks into small droplets and solidifies as spherical particles while raining back to the lunar surface. These glasses preserve information about the chemical composition of the target and often contain sufficient amount of radioactive nuclides such as 40 K to enable 40 Ar39 Ar dating of individual beads. Studies measuring the age of glass beads have been used in attempts to establish variations in the flux of impactors hitting the Moon, particularly during the period that postdates the formation of major impact basins [1,2]. These studies proposed a possibility of spike in the impact flux about 800 Ma [2] and over the last 400 Ma [1]. More recently U-Th-Pb isotopic systems have been also utilized to determine the age of impact glasses from the Apollo 17 regolith [3]. Our aim is to extend the application of the U-Pb system in impact glasses to spherules isolated from Apollo 14 soil 14163 in an attempt to further investigate the applicability of this isotopic system to the chronology of impact glass beads and gain additional information on the impact flux in the immer Solar system. Analytical techniques: 145 impact glasses have been selected, using criteria described by Delano et al. [4], from the population of spherules present in the Apollo 14 soil sample 14163. Major elements compositions of these beads have been determined using a wavelength-dispersive electron microprobe at the Centre for Microscopy, Characterization and Analysis (CMCA) at the University of Western Australia. U-Pb ages of the glasses containing more than 0.025 ppm of total Pb have been analysed using a Cameca IMS1280 ion probe located at the Swedish Museum of Natural History in Stockholm (NordSIMS facility), while trace elements and U-Th-Pb chemical ages of all beads have been determined using laser ablation ICPMS with a Lamda Physik Compex 110i excimer laser coupled to an Agilent 7500 quadrupole ICPMS at the Australian National University. Results: Impact glasses commonly show a broad and continuous range of chemical compositions that are consistent with mixing of diverse components either within the lunar regolith or during the impact process. Age (Ma) 0 2 4 6 8 10 12 14 16 18 40 39 ArAr ages