Mars' Magnetic Lithosphere: Candidate Minerals

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1517 Magnetic Anomaly Modeling, 1519 Magnetic Mineralogy And Petrology, 1540 Rock And Mineral Magnetism, 5440 Magnetic Fields And Magnetism

Scientific paper

Mars' southern-hemisphere magnetic anomalies require a large Martian magnetic field at the time the lithosphere acquired thermoremanent magnetization (TRM), large magnetic mineral concentrations compared to Earth's lithosphere, a mineral or minerals whose grain size and resulting domain structure generate intense TRM, and/or a high Curie temperature and deep Curie-point isotherm. Based on Martian meteorites and spectroscopy of the Martian surface, the magnetic minerals likely to be important in Mars' lithosphere are magnetite, hematite and pyrrhotite. Martian anomalies are likely an integrated effect over a depth interval of 20-30 km. Pyrrhotite has a low Curie point (320oC) and is found only in specialized settings on Earth, although demagnetization around Hellas and Argyre craters may favor pressure-induced cycling of near-surface pyrrhotite through its 2.8 GPa phase transition. More promising as deeper sources are magnetite and hematite, which are ubiquitous on Earth and have high Curie points (580 and 675oC). TRM of magnetite decreases inversely with increasing grain size, while the opposite is true for hematite. The two minerals have the same TRM intensity around 10-20 micrometers, close to both the upper limit for pseudo-single-domain (PSD) behavior in magnetite and the critical single-domain size of hematite. The lack of any substantial self-demagnetizing field permits a TRM in multidomain hematite orders of magnitude larger than the TRM of multidomain magnetite for field strengths like the Earth's. Either single-domain/ PSD magnetite or multidomain hematite could explain strong anomalies. Single-domain magnetite requires less concentration but has restrictively small (submicron) grain sizes. Recently single-domain hematite has been found to have more intense TRM than previously measured, and it too could be viable. Depending on magmatic conditions, fine-grained magnetite and hematite can occur as segregated phases within titanomagnetite and titanohematite. Ultrafine subdivision might give rise to lamellar magnetism, as reported for terrestrial titanohematites. Thermoviscous magnetization is probably negligible for Mars because of the small present field and the minor enhancement of TRM that likely occurred in ancient fields.

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