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Abstract 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. 

The suevite (polymict melt rock-bearing breccia) composing the upper peak ring of the Chicxulub impact crater is extremely heterogeneous, containing a combination of relict clasts and secondary minerals. Using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDS) and electron probe microanalysis (EPMA), we investigated the nature and occurrence of primary and secondary Fe-oxide and Fe-sulfide minerals to better understand hydrothermal trends such as mineral precipitation and dissolution, and to document the remobilization of Fe and associated siderophile elements within suevites. Large primary Fe-oxides (~20–100 µm) reveal decomposition and dissolution patterns, forming sub-micrometer to micrometer Fe-oxide phases. Secondary sub-micrometer Fe-oxide crystals are also visibly concentrated within clay. The occurrence of Fe-oxide crystals within clay suggests that these likely formed at temperatures ≤100 °C, near the formation temperature of smectite. The formation of Fe-oxide minerals on clay surfaces is of interest as it may form a micro-setting, where free electrons (from the oxidation of Fe2+) and the adsorption of simple organic molecules on the surface of clay could generate reactive conditions favorable to microbial communities. Primary and secondary Fe-sulfide minerals exhibiting a variety of morphologies are present within samples, representing different formation mechanisms. Secondary Fe-sulfide minerals occur within rims of clasts and vesicles and in fractures and voids. Some secondary Fe-sulfide grains are associated with Ni- and Co-rich phases, potentially reflecting the post-impact migration of siderophile elements within the suevite of the Chicxulub crater.