New Anomalously Magnetic Martian Meteorites: the Case for Metal as the Remanence Carrier in the Martian Crust ?

Details New Anomalously Magnetic Martian Meteorites: the Case for Metal as the Remanence Carrier in the Martian Crust ? New Anomalously Magnetic Martian Meteorites: the Case for Metal as the Remanence Carrier in the Martian Crust ?

Dec 2006

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmgp11b0075r&link_type=abstract

American Geophysical Union, Fall Meeting 2006, abstract #GP11B-0075

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1595 Planetary Magnetism: All Frequencies And Wavelengths, 5440 Magnetic Fields And Magnetism

Scientific paper

Based on the comprehensive rock magnetic study of martian meteorites, two minerals magnetite (Ti or Cr substituted) and pyrrhotite- have been proposed as the carriers of Martian crustal remanence [1,2]. However, the recently discovered chassignite NWA2737 (the only chassignite besides Chassigny [3]) and lerhzolitic shergottite NWA1950, appear much more magnetic than the other member of their family and bring the case for an alternative mineral. NWA2737, composed essentially by olivine of nearly black color, have been submitted to detailed TEM and low temperature magnetic study to understand its difference with Chassigny, that is is nearly a pure paramagnet, made of green olivine. The olivine crystals appear to be loaded with nanoparticles of FeNi metal, encompassing the superparamagnetic to single domain transition at 30 K (10 nm range). We interprete the formation of these nanoparticles as being shock induced, as other criteria show that NWA2737 has been more strongly shocked than Chassigny. Shock induced reduction of olivine to produce metal has been invoked on surfaces deprived of atmosphere (Moon, asteroids; e.g. [4]) but never observed on Earth or Mars. The occurrence of metallic iron has also been reported in a shock vein of NWA1950 [5]. Therefore, metal, formed by shock in the heavily cratered Noachian crust may be also considered as a possible carrier for the crustal remanence. Severe problems arise with this hypothesis: the remanence should be patchy and limited to the first few km of the crust (except below large craters), two features not fitting with standard views of Mars crustal magnetization. On the other hand the widespread surface formation of metal nanoparticles could provide the best precursor for the oxydised particles (goethite, hematite) in the martian soils, as demonstrated by the experimental weathering of metal in C02+ H20 atmosphere conducted by [6]. References: [1] Rochette, P. et al. Meteoritics & Planetary Science 40, 529-540, 2006. [2] Yu, Y. and J.S. Gee, EPSL, 232,287-294, 2005. [3] Beck, P. et al. Geochemica et Cosmochimica Acta, 2006 [4] Sasaki, S et al.. Nature 410,555-557, 2001. [5] Mikouchi T Meteoritics 40, 1621-1634, 2005. [6] Chevrier, et al. Geology 32, 1033-1036, 2004.

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