Rock Magnetic and Remanence Properties of Synthetic Martian Basaltic Intrusions: Implications for Mars crustal anomalies

Two basalts deemed relevant to the crust of Mars were synthesized to examine contrasts in rock magnetic and remanence properties following identical thermal histories and oxygen fugacity conditions. The composition denoted T-type is rich in Al and poor in Fe, reflecting constraints provided by thermal emission spectroscopy that the Martian crust is somewhat terrestrial in character. The M-type composition is poor in Al and rich in Fe, reflecting the composition of basaltic liquid in equilibrium with Martian meteorite phase assemblages. The two compositions are identical with respect to MgO, SiO2, and TiO2. Batches of each composition were cooled from > 1200 °C to 1070 °C at 4 °C/h and annealed at 1070°C for 100 h, then quenched. Samples were then held at 650°C for periods ranging from 21 to 158 days under quartz-fayalite-magnetite (QFM) fO2 buffer conditions, then quenched. The experimental conditions are germane to shallow igneous intrusions, which might be a significant volumetric fraction of the Martian crust and potential carriers of crustal magnetic anomalies, and provide an important contrast to a previous set of fast-cooled (3-230 °C/h) basalts our group performed on the same two compositions. M-type samples contain Fe-Ti-Al-Mg oxide grains 40-50 μm in diameter with skeletal morphologies. T-type samples contain equant euhedral Fe-Ti-Al-Mg oxides with grain diameters ranging from 15-30 μm as well as elongated anhedral ilmenite grains. For M-type samples both the starting material and the samples annealed at 650 °C have narrow multidomain hysteresis loops and similar hysteresis parameters. T-type starting materials and samples annealed at 650 °C have pseudo single domain (PSD) hysteresis loops, but the annealed samples plot lower and to the right within the PSD field on a Day plot, indicating coarser magnetic grains. Alternating field demagnetization of anhysteretic remanent magnetization (ARM) shows median destructive fields < 10 mT. M-type samples exhibited higher magnetic susceptibility and intensity of remanence than T-type samples. Both M-type and T-type samples carry an intense natural remanent magnetization (NRM). The NRM is inferred to be a thermoremanent magnetization (TRM) acquired during quenching and air-cooling after the 650 °C anneal. NRM values range from 7.0 to 61.7 mAm2/kg for M-type samples and 1.3 to 22.8 mAm2/kg for T-type samples, values comparable to those observed in rapidly cooled synthetic basalts of the same chemical composition. However, the slow-cooled samples have a much “softer” coercivity spectrum. The multi-domain magnetic mineral assemblage suggests that while intrusions generated by slow-cooled basaltic melts are capable of carrying intense TRMs they may be less stable over geologic time.