HOW MUCH CAN PHOSPHATES IN LUNAR BRECCIAS TELL US ABOUT THE IMPACT HISTORY OF THE MOON?

Constraining the ages of the major lunar impact basins remains a key challenge in understanding the impact history of the Moon. The timing of the Imbrium basin forming impact is particularly important, as it would have likely deposited material at all of the Apollo landing sites. This observation has led to an ongoing debate in the lunar science community about the apparent preponderance of impact-related ages around ~3900 Ma, and whether these ages are indicative of a period of intense meteoritic bombardment (often referred to as the “Late Heavy Bombardment” or LHB), or if the 3900 Ma signal is simply a reflection of sampling bias in the Apollo collection. In this presentation we will summarise the results from recent SIMS analyses of U-Pb systematics in phosphates from breccias collected at the Apollo 12, 14 and 17 landing sites, combined with new data from the Apollo 15 samples 15445 and 15455. These two impact melt breccias have been previously identified as the most likely candidates for samples of impact melt from the Imbrium basin forming event. Collectively, these data define an age of 3922±2 Ma, which is interpreted as the best estimate for the formation of the Imbrium basin. Nonetheless, a number of apparently older ages (between 3930-3940 Ma) were recorded in several Apollo 14 and 17 breccias. Two main interpretations of these older ages will be discussed: either that these data may all represent the same impact event, but simply reflect complexities in the U-Pb systematics that have not been fully accounted for (e.g. effects of partial resetting of the U-Pb system in older phosphates, or mixing variable amounts of very radiogenic initial lunar Pb); or that the data are truly resolving distinct impact events on the Moon. Both interpretations have important implications for the debate surrounding the existence and nature of the LHB, and how much more we can hope to learn about the impact history of the Moon through U-Pb analyses of phosphate grains.