Shock-induced metallic iron nanoparticles in olivine-rich Martian meteorites

Bertrand van de Moortèle (1) , Bruno Reynard (1) , Pierre Rochette (2) , M. Jackson (3) , Pierre Beck (1) , Philippe Gillet (1) , P. Mc Millan (4, 5) , C. A. Mccammon (6)

1 LST - Laboratoire de Sciences de la Terre
2 CEREGE - Centre européen de recherche et d'enseignement des géosciences de l'environnement
3 Institute for Rock Magnetism
4 Department of Chemistry [UCL, London]
5 Davy-Faraday Research Laboratory
6 Bayerisches Geoinstitut - Bavarian Research Institute of Experimental Geochemistry and Geophysics

Magnetic anomalies observed by the Mars Global Surveyor mission are attributed to crustal remanence. SNC (Shergotty-Nakhla-Chassigny) meteorites are likely samples of the Martian crust and are amenable to mineralogical and magnetic measurements essential to the understanding of the origin of magnetic anomalies. The recently discovered chassignite NWA 2737 and lherzolitic shergottite NWA 1950 display unusual magnetic characteristics that argue for a different magnetic carrier than the oxides and sulfides previously invoked in SNC meteorites. NWA 2737, the second member of the chassignite group, is a dunite with unusually dark-brown olivines and large magnetic susceptibility while Chassigny contains green olivines and is nearly a pure paramagnet. Dark olivines are also found in NWA 1950, a lherzolitic shergottite, which has singular magnetic properties when compared with other shergottites. The dark olivine color is due to the presence of Fe and FeNi metal nanoparticles, identified both by TEM and by magnetic measurements. Their size distribution encompasses the superparamagnetic to single domain transition at 30 K (10 nm range) and explains the magnetic properties of the bulk rocks. The formation of these nanoparticles is attributed to heating during the shock events that affected NWA 2737 and NWA 1950. The production of metal particles by shock-induced reduction of olivine has been invoked on surfaces deprived of atmosphere but never observed on Earth or Mars. Therefore, metal formed by shock in the heavily cratered Noachian crust is a possible carrier for crustal magnetic remanence. Widespread surface formation of metal nanoparticles could provide the precursor for the oxidized particles (goethite, hematite) observed in the Martian soils.

Keywords: olivine; Martian meteorites; shock metamorphism; magnetic properties; impact; regolith; electron microscopy

Bertrand van de Moortèle, Bruno Reynard, Pierre Rochette, M. Jackson, Pierre Beck, et al.. Shock-induced metallic iron nanoparticles in olivine-rich Martian meteorites. Earth and Planetary Science Letters, 2007, 262 (1-2), pp.37-49. ⟨10.1016/j.epsl.2007.07.002⟩. ⟨insu-00367366⟩