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Chassignite / Mars Meteorite
Synonyms: olivine achondrites, martian dunites
General: This group is named for Chassigny, a meteorite that fell in France in 1815. In the early days of meteoritics nobody suspected that it would become one of the key samples in the future, and thus most of it was subsequently lost. Only a bit more than 600 grams of the original 4 kilos of Chassigny have been preserved up to this day, and until more recently, when a second chassignite, NWA 2737, was recovered from the deserts of Africa, only small crumbs of this ultra-rare class of martian meteorites were available to institutions, museums, and private collectors.
Description: The chassignites are medium- to fine-grained ultra-mafic rocks, resembling terrestrial dunites and peridotites. The color of their interiors varies: the original Chassigny is a light-green rock with only minor black spots in its matrix while NWA 2737 has a more dark appearance, with blueish glas veins quenched in a shock-blackened matrix. The fusion crust on Chassigny is black while NWA 2737 doesn't show remnant fusion crust due to terrestrial weathering.
Mineralogy: Chassignites are cumulate rocks, resembling terrestrial dunites, and peridotites. They consist of about 90% Fe-rich olivine, minor clinopyroxene, plagioclase, chromite, melt inclusions, and other accessory minerals and phases. Melt inclusions in Chassigny contain rare amphiboles, and cracks within the rock are often filled with carbonate and sulfate salts, indicating a mild pre-terrestrial aqueous alteration. The new chassignite, NWA 2737 has just been officially classified, and thus we don't have too much data on it, so far. It resembles the original Chassigny in many aspects, but it also differs in others, e.g., in its shock level. NWA 2737 displays shock-blackened olivines that haven't been witnessed in any terrestrial or extra-terresrial rock type, thus far.
Formation history: Crystallization ages of about 1.36 billion years, and compositional and elemental trends indicate a relationship between the chassignites and the nakhlites, suggesting an origin from the same parent magma on Mars. However, the chassignites show noble gas values different from those found in other SNC members or in the martian atmosphere. It is suspected that these gases might originate from the martian mantle, suggesting a formation within magma plutons deep inside the martian crust for the chassignites.
Origin: Planetary. Comparisons between various characteristics of the members of the SNC group, and data obtained about Mars by space probes and landers, such as Viking, Pathfinder, and the new Mars rovers Spirit and Opportinity, have provided strong proof for the martian origin of the SNCs, and today it is widely accepted that these achondrites actually represent genuine Mars rocks that have been blasted off of the surface of the Red Planet by major impacts. Recent studies, based on data provided by the Mars Odyssey Orbiter, and THEMIS (Thermal Emission Imaging System), suggest that both, the nakhlites, and the chassignites, were ejected from a large impact crater in the northeast region of the Syrtis Major volcanoes.
Members: For nearly 200 years Chassigny remained the only representative of its own class, and only more recently a second chassignite has been recovered from Northwest Africa by a French team: NWA 2737, also named "Diderot" in homage to the 18th century encyclopaedist of Langres, France.