Fe-Ni METAL AND MAGNETITE NANO-PARTICLES IN “BROWN” COLOR OLIVINES FROM MARTIAN

Introduction: Recent TEM studies revealed that Fe-Ni metal nano-particles are responsible for the dark “brown” color of olivine in the NWA2737 chassignite, which is interpreted to have formed by reduction of olivine due to heavy shock events [1,2]. Color change of olivine is important to understand the remote sensing data as is the case for space weathering [3,4]. Furthermore, nano-phase Fe particles can be a magnetic carrier of the meteorites, and is significant for their magnetic studies [5]. Similar brown color olivine is fairly common among Martian meteorites, especially in shergottites, and thus Fe-rich nano-particles may be widely present in these olivine grains. Indeed, at the last LPSC we reported that olivines in ALH77005 and Y000097 lherzolitic shergottites included Fe-Ni metal nanoparticles, corresponding to their brown color [6]. At the same time, we also found that the LEW88516 lherzolitic shergottite, which has brown olivine, contains magnetite nano-particles instead of Fe-Ni metal nano-particles. The presence of magnetite nano-particles is also probably responsible for darkening of their host olivine grains, similar to Fe-Ni metal nano-particles. Because magnetite nano-particles can be formed during strong shock under oxidizing conditions [7], we expect that magnetite nano-particles can be more widely present in other Martian meteorites instead of Fe nano-particles. In this study we selected three shergottites (Dhofar 019, LAR06319 and NWA1950) including brown olivine grains to examine what kind of nano-particles they contain in olivine. Analytical methods: We selected the above three samples because their thin sections contain brown olivines although the strength of the color is on many levels. In general, both colorless and brown areas are present in a common single crystal, but we found that there was no clear compositional difference when analyzed by electron microprobe. We prepared TEM samples by crashing small rock chips of each sample into powder by hand. The TEM observation was performed by JEOL JEM 2010 TEM equipped with EDS. Results: TEM revealed the presence of nanoparticles in olivines from all samples. Typical sizes of these particles were 5 to 20 nm in diameter. EDS shows that these particles are enriched in Fe, but do not contain Ni. The electron diffraction patterns show that they are magnetite rather than Fe metal. Fig. 1 TEM image of NWA1950 nano-particles (indicated by arrows).