Magnetic fidelity of lunar samples and implications for an ancient core dynamo

Abstract

Some lunar rocks have stable magnetizations that provide compelling evidence for an ancient lunar core dynamo. However, a longstanding problem has been interpreting the unstable alternating field (AF) demagnetization behavior observed in the remaining majority of the Apollo lunar sample suite. Similar unstable behavior has also been observed for many meteorites. It is unclear whether the behavior of these samples indicates that they formed in the absence of an ancient magnetizing field or whether they simply have poor magnetic recording properties. It is necessary to distinguish between these two possibilities in order to determine whether paleomagnetic fields were present on a sample's parent body. To address this issue, we analyzed five samples whose rock magnetic properties span the full suite of observed demagnetization behaviors: mare basalts 15556, 15016, 12017, 10020, and troctolite 76535. We demonstrate that the effects of spurious anhysteretic remanent magnetization (ARM) during AF demagnetization, in combination with multidomain magnetic carriers and magnetic remanence anisotropy, are likely responsible for the apparently poor magnetic behavior of many lunar samples. Therefore, the unstable AF demagnetization behavior observed for many lunar samples is not alone sufficient for ruling out the presence of an ancient lunar core dynamo. Nevertheless, spurious ARM may explain the observation of surprisingly high (≥1μT) Apollo-era paleointensity measurements for samples thought to have formed after the cessation of a lunar core dynamo (<1.5Ga).