HR: 08:30h
AN: GP21C-03    [Abstracts]
TI: Shock-Induced Demagnetization of Pyrrhotite and Implications for the Martian Crust
AU: Stewart, S T
EM: sstewart@eps.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University, 20 Oxford Street, Cambridge, Ma 02138
AU: Weiss, B P
EM: bpweiss@mit.edu
AF: Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue 54-724, Cambridge, Ma 02139
AB: Maps of the remanent magnetic field of Mars show demagnetized zones within and around giant impact basins. It is possible that vast regions of the Martian crust were demagnetized due to a phase transition of the magnetic carriers induced by a shock wave. This hypothesis is supported by the fact that around the Hellas and Argyre basins, the magnetized and unmagnetized zones are separated by a peak shock pressure contour line between 1 and 3 GPa. Static pressure experiments at room temperature have indicated that pyrrhotite (Fe$_{7}$S$_{8}$) undergoes a phase transition from ferrimagnetic to paramagnetic at $\sim$2.8 GPa, with rapid loss of magnetization above 1 GPa. Although pyrrhotite is not a major magnetic phase on Earth, it is a common carrier of magnetization in Martian meteorites. No previous experiments have demonstrated that shocks below 3 GPa can induce this phase change and demagnetize pyrrhotite. To investigate this possibility, we performed shock recovery experiments on pyrrhotites (Mrs/Ms$\sim$0.7) using a gas gun to simulate natural impact events. The experiments were preceded and followed by a suite of material and magnetic characterization measurements (including X-ray diffraction, magnetic hysteresis and low temperature magnetism) to assess the effects of the shock on the crystallographic and magnetic properties of the pyrrhotite. We will present results from experiments achieving shock pressures in the range of 1 to 4 GPa. These experiments serve as an analogue for the demagnetization of crustal rocks on Mars.
DE: 5440 Magnetic fields and magnetism
DE: 6022 Impact phenomena
DE: 6225 Mars
DE: 1540 Rock and mineral magnetism
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting