Abstract
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.
| Original language | English (US) |
|---|---|
| Pages (from-to) | 37-49 |
| Number of pages | 13 |
| Journal | Earth and Planetary Science Letters |
| Volume | 262 |
| Issue number | 1-2 |
| DOIs | |
| State | Published - Oct 15 2007 |
Bibliographical note
Funding Information:We wish to thank the Museum National d’Histoire Naturelle for supplying the Chassigny sample, C. Cayron and P.-H. Jouneau (CEA-Grenoble) for their experimental help on the LEO 1530 Gemini SEM, and H. Gnägi from DIATOME. The CLYM (Centre LYonnais des Microscopies) is gratefully acknowledged for the access of the Jeol 2010F TEM facilities and the MATEIS laboratory for granting access to the JEOL 840 SEM. Raman spectroscopy measurements were made on an INSU (Institut National des Sciences de l’Univers) national instrument. The Institute for Rock Magnetism is supported by grants from the Instruments and Facilities Program, Earth Science Division, National Science Foundation. This work was financially supported by the ANR (ECSS project) and the Région Rhône-Alpes (Emergence program).
Keywords
- Martian meteorites
- electron microscopy
- impact
- magnetic properties
- olivine
- regolith
- shock metamorphism