Magnetic Mineralogy in Lunar Mare Basalts and Implications for Paleointensity Retrieval

Lunar paleomagnetic studies have identified multidomain metallic Fe–Ni alloys as the dominant magnetic contributors in mare basalts. Here, we explore the low-temperature magnetic behavior of standard samples for a suite of opaque minerals that occur within mare basalts (single-domain and multidomain Fe, wüstite, ulvöspinel, iron chromite, ilmenite, and troilite). We compare the observed low-temperature behaviors to those of several Apollo mare basalt samples (10003, 10044, 10020, 10069, 10071, 12009, 12022, 15597). Notable magnetic transitions were detected at 30 K (ilmenite), 60–80 K (chromite, troilite), and 100–125 K (ulvöspinel, chromite). We also investigated the effects of low-temperature cycling on mare basalt remanence and observed that only grains with coercivities 20–40 mT were cleaned. This suggests a minimal impact of diurnal temperature cycling at the lunar surface on the retrieved lunar paleointensity values. Using comprehensive electron microscopy techniques, including scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), wavelength dispersive spectroscopy (WDS), x-ray diffraction, and transmission electron microscopy (TEM), we further examined magnetic phases within four Apollo 11 mare basalt samples. Our findings revealed the presence of Fe grains (one to 10 μm in diameter) associated with troilite contain sub-grains ranging in size from tens to hundreds of nanometers in some samples. These grains, which fall within the single-domain to multi-domain range as observed in their first-order reversal curves, might have the potential to retain high coercivity components and thereby effectively record an ancient dynamo field.