Rb-Sr constraints on the age of Moon formation

Abstract

Determining the age of the Moon, which is commonly considered as the termination of Earth accretion has been a complex challenge for geochronology. A number of methods have been used to delineate the age of the Moon based either on absolute chronology of lunar rocks or have relied on more indirect methods using short-lived nuclides such as 182Hf that was present in the early history of the Solar System. Model ages usually require some assumptions that are sometimes controversial or harder to verify.In this study, new high precision Sr isotope data (2.4 ppm, 2SD) were obtained for a well-dated lunar anorthosite (60025) in order to better constrain the initial 87Sr/86Sr of the bulk silicate Moon. This new data is then used to model the Sr isotope evolution of the Earth-Moon starting from the beginning of the Solar System. To comply with the HfW and stable isotope constraints, we then assume that the Earth and Moon were equilibrated at the time of Moon formation. By investigating systematically all the sources of uncertainties in our model, we show that compared with previous work on anorthosites, one can tighten the constraints on the youngest age of Moon formation to no >79 Ma after the beginning of the Solar System, i.e. the Moon cannot be younger than 4488 Ma.