New results from shock experiments on pyrrhotite and implications for the magnetization of the Martian crust and meteorites

Vast unmagnetized regions within and around giant impact basins on Mars may have been demagnetized due to a shock-induced phase change or magnetic transition. Pyrrhotite (Fe1-xS, x≤0.13) has been identified as the major carrier of magnetic remanence in many Martian shergottite meteorites and may be an important carrier for the crustal magnetization on Mars. We performed planar shock recovery experiments on two natural pyrrhotite samples at shock pressures ranging from 1.0 to 6.9 GPa. The pyrrhotite samples are mixtures of ferrimagnetic monoclinic (Fe7S8) and antiferromagnetic hexagonal pyrrhotite and ranged in size from nearly all single domain to a mixture of single domain, pseudo-single domain and multidomain grains. We find that shocked pyrrhotite is demagnetized up to 90% in a non-monotonic manner after shock. The preferential removal of low coercivity components of the magnetization is evident from the increased resistance to alternating field demagnetization of the remaining post-shock moment. Contrary to static experiments, we do not observe complete demagnetization. Permanent changes to the magnetic properties include; 1) an increase in saturation isothermal remanent magnetization by up to ~155%; 2) an increase in bulk coercivity, indicating that the low coercivity components responsible for demagnetization have been partially removed; 3) changes in squareness of magnetic hysteresis, increasing by 130% in samples containing multidomain grains; 4) an increase in low- temperature memory; and 5) a rotation of the magnetic moment by up to 150 degrees. These changes are consistent with an increase in the volume fraction of single domain crystals. We find that shock demagnetization of pyrrhotite is not analogous to static pressure demagnetization. Even up to 6.9 GPa we do not observe any magnetic transitions or the onset of transformation to a higher density, high pressure phase. The irreversible changes however are analogous to those seen in static pressure experiments, suggesting that similar processes permanently modify the magnetic properties of minerals under both static and shock pressures. Our results suggest that the lack of magnetic anomalies over large Martian impact basins may be due to the combined effects of demagnetization and rotation of magnetic moment. After shock, the remaining magnetization in the crust may be randomized, resulting in no-field detection at spacecraft altitude. Pyrrhotite- bearing meteorites, like shergottites, can retain records of Martian magnetic fields even if shocked to pressures approaching 7 GPa. However, current paleointensity techniques may underestimate the Martian paleofield inferred from shocked meteorites by an order of magnitude, as they do not correct for changes in saturation remanence and (partial) demagnetization. The zones of complete demagnetization around impact craters may have been significantly overestimated.