Lherzolitic Shergottites


Synonyms: martian peridotites, peridotites, lherzolites


General: The members of this group are named for Shergotty, an achondrite that fell in India, in 1865. Originally grouped with the HED group, it took more than a century to recognize the martian origin for Shergotty, and a few related falls and finds. More recently, it became more than obvious that the shergottites represent a rather heterogenous group, and thus subgroups were designed to comprise members with similar mineralogies, and formation histories. Being igneous cumulates, the members of the subgroup of the lherzolitic shergottites differ from the other two shergottite grouplets, showing affinities to other groups of martian meteorites, such as the rare chassignites.


Description: The lherzolitic shergottites are medium-grained cumulate ultramafic rocks of plutonic origin, resembling terrestrial lherzolites and harzburgites. Their greenish-yellow matrix is usually interesected by more or less abundant black cromite grains, and the crust is dull black to brownish black.


Mineralogy: The members of this group consist primarily of medium-grained olivines and chromites that are poikilitically enclosed by large orthopyroxene crystals. Minor interstitial areas of plagioclase glass (maskelynite) occur, and clinopyroxenes with accessory oxides and phosphates are also present.


Formation history: The lherzolitic shergottites most probably crystallized as cumulates from residual melts in magma chambers, and they share many mineralogical and chemical features with the basaltic shergottites. Their crystallization ages also mirror the respective ages of the basaltic shergottites, and as their basaltic cousins the plagioclase in lherzolitic shergottites has been converted to maskelynite, a glass that is produced when plagioclase is subjected to shock pressures of at least 30 GPa. It is likely that the maskelynite was formed by the impact forces that blasted the shergottites from the martian surface and into space. Calculations show that it requires a major impact event to accelerate any material to a speed high enough to escape the Red Planet's gravity.


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 suggest that most shergottites were probably derived from a few larger impacts in the Tharsis region of Mars, and Olympus Mons, the largest volcano in our solar system.


Members: Of the 6 lherzolitic shergottites known, 5 are finds from the blue-ice fields of Antarctica, such as the famous ALH 77005, Yamato 793605, and LEW 88516. Only one single lherzolite has been recovered so far from the hot deserts of Northwest Africa, and it is the first member of this rare group to be available to private collectors, and museums - NWA 1950, also named "Jules Verne" for the renowned French author who predicted space travels, and Moon landings more than 100 years before they actually took place.

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