Rock-magnetic and remanence properties of synthetic Fe-rich basalts: Implications for Mars crustal anomalies

Abstract

We characterized the magnetic mineral assemblage and remanence properties of a set of synthetic samples patterned on the meteorite-derived basalt composition A ⁎, which contains 18.9% total FeO. Basalts were synthesized at conditions that track 4 oxygen fugacity ( fO 2) buffer curves, from 3.4 log units below the quartz–fayalite–magnetite (QFM) buffer to 5 log units above QFM, and 6 cooling rates from 10 5 to 3 °C/h. The resulting array of samples was characterized using magnetic hysteresis loops, temperature dependence of saturation magnetization and saturation remanence (10 to 1000 K), and the acquisition and demagnetization of anhysteretic remanent magnetization (ARM) and thermoremanent magnetization (TRM). The magnetic mineral assemblage characteristics are strongly dependent on fO 2. Samples synthesized at the iron–wüstite (IW) buffer have a very low concentration of remanence-carrying grains, which are likely near the superparamagnetic-stable-single-domain boundary. Samples synthesized at the QFM and nickel–nickel oxide (NNO) buffers contain a slightly higher concentration of remanence-carrying grains, which are stable-single-domain to fine pseudo-single-domain particles, respectively. Samples synthesized at the manganese oxide (MNO) buffer contain the highest concentration of magnetic grains, which are up to 100 μm in diameter. The dominant Fe–Ti oxide produced is an Mg- and Al-bearing titanomagnetite with 2.4–2.7 Fe cations per formula unit. The Curie temperatures of the QFM samples are consistent with their electron-microprobe derived compositions. Those of the NNO sample set are very slightly elevated with respect to their electron microprobe derived compositions. The Curie temperatures of the MNO samples are elevated up to 200 °C above what they should be for their composition. We attribute the Curie temperature elevation to high-temperature nonstoichiometry of the titanomagnetite. The IW sample set acquired very weak TRMs with intensities of 0.02 to 0.5 A/m. This intensity of remanence is a factor of 50–500 too low to generate the observed 1000 nT anomalies detected on Mars by the Mars Global Surveyor MAG–ER experiment. The QFM, NNO, and MNO samples acquired TRMs up to 40 A/m in a 10-μT applied field, and up to 200 A/m in a 50-μT field, with little or no dependence on cooling rate. Our results suggest that Fe-rich melts that crystallize extensive titanomagnetite can generate an intensely magnetized layer in the Martian crust, even if the remanence was acquired in a weak field. The QFM sample set can easily account for the observed 1000-nT Mars magnetic anomalies, even in a magnetized layer as thin as 15–30 km.