Magnetism of a Very Young Regolith Breccia and Implications for the End of the Lunar Dynamo

The Moon today does not have a global magnetic field. However, recent studies of samples collected during the Apollo missions and spacecraft measurements of the lunar crustal field suggest that an intense core dynamo was active during the period between 4.2 and 3.56 billion years ago (Ga) and may have lasted until at least 1-2.5 Ga in a weakened state. The time at which the dynamo finally ceased has not yet been established. One of the most intriguing and mysterious results of the Apollo era was the apparent discovery of stable natural remanent magnetization in the <0.1 mm thick melt glass rind of the young (<200 million-year-old) regolith breccia 70019. If the glass of 70019 is indeed magnetized, this would be the youngest paleomagnetic record from the Moon. Such a field could be that of a very recent core dynamo or perhaps have been generated by plasma from meteoroid impact. However, an alternative possibility is that the glass is not magnetized, such that Apollo-era studies were actually measuring ancient, entrained rock fragments magnetized billions of year ago. To assess this possibility we conducted a new paleomagnetic study of 70019 using two state-of-the-art instruments to measure rock thin sections and isolated pieces of 70019 while achieving fine spatial discrimination: a SQUID microscope (SM) and a quantum diamond microscope (QDM). Our preliminary results obtained from sample 70019,152 show that the glass indeed appears to be much less magnetized than the adjacent clastic material, with a paleointensity of 0.4 μT estimated from bulk magnetic moment measurements of isolated glassy material. This is corroborated by magnetic mapping of a thin section of 70019,152, which also reveals a non-unidirectional low-coercivity overprint predominantly on the edges of the sample, likely due to sample handling prior to thin section preparation.