Constraining the Decline of the Lunar Dynamo Field at ≈3.1 Ga Through Paleomagnetic Analyses of Apollo 12 Mare Basalts

Abstract

Recent paleomagnetic studies of lunar rocks have suggested that the magnetic field of the Moon reached peak intensities on the order of ≈77 μT between 3.85 billion and 3.56 billion years ago (Ga) and subsequently declined to surface intensities of ≈4 μT by 3.19 Ga. However, this decline in the intensity of the lunar field has only been shown in a small number of samples, presenting challenges for constraint of its timing and thus the dynamo generation mechanisms that could be responsible. We present microscopic and magnetic analyses of Apollo samples 12008, 12009, and 12015, three fine-grained mare vitrophyre basalts with high magnetic fidelity, indicating that these samples were not magnetized in conditions consistent with a planetary magnetic field exceeding 4–7 μT during their formation. We further report updated radiometric ages for samples 12008 and 12009 and the first-ever radiometric age for sample 12015, dating this lunar field intensity constraint to ≈3.1 Ga. These data are consistent with results of previous work on the initial decline of the Moon's magnetic field and confirm that the mechanism of lunar dynamo generation changed dramatically between 3.6 and 3.1 Ga.