Experimentally Testing Whether Magmatic Processes can Produce Strong Lunar Crustal Magnetism

Abstract:

The Moon generated a core dynamo magnetic field between at least 4.25 Ga and 1.8 Ga, with intensities reaching ~40-110 µT during the period prior to ~3.56 Ga. Lunar crustal rocks that formed while the dynamo was active retain remanent magnetization. While magnetic anomalies observed within impact basins are likely attributable to the presence of impactor-added metal, other anomalies such as those associated with lunar swirls are not as easily linked to exogenic materials. This has led to the hypothesis that some anomalies may be related to magmatic features such as dikes, sills, and laccoliths. However, basalts returned from the Apollo missions are magnetized too weakly to produce the required magnetization intensities (>0.5 A/m). Here we test the hypothesis that subsolidus reduction of ilmenite within or adjacent to slowly cooled mafic intrusive bodies could locally enhance metallic FeNi contents within the lunar crust. We conducted reduction kinetic experiments on pristine sub-mm samples of a natural geikeilite-rich, low ferric, kimberlitic ilmenite megacryst. The starting material contained approximately 11 mole percent hematite (calculated from stoichiometry considerations using electron microprobe data) and had a natural remanent magnetization of 0.186 A/m. Reduction experiments were conducted using different fO₂ conditions (IW-2, IW-1, IW, and QFM) and for different durations (2, 4, 8, and 16 days) at 800 °C (above the Curie temperature of metallic Fe). Our experiments produced 20-30 mm-wide reaction rims around the original crystals as well as within pre-existing cracks. Reaction products consist of pure ilmenite (i.e., no hematite solid solution), Cr-spinel, rutile exsolution, and 1-2 mm nodules of either taenite or martensite in the IW-1 and IW-2 experiments (no FeNi metals formed at IW and QFM). The at% of Fe₂O₃ decreased from ~11 at the centers of crystals to ~0 at reaction rims and crack edges. FeNi formation appears to positively correlate with lower oxygen fugacities, and it may also correlate with duration of heating experiments. We will further test these correlations with rock magnetic experiments that will quantify changes in bulk magnetic properties and remanence carrying capacities produced by the various reduction experiments.