Basaltic Shergottites

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. The subgroup of the basaltic shergottites can be considered as the original group of shergottites, as the namesake, Shergotty, is a rather typical basaltic shergottite.


Description: Basaltic shergottites are grey basaltic rocks with shiny black fusion crusts. Their igneous textures range from medium- to coarse-grained, and some members display multiple lithologies, e.g. the Antarctic shergottite EETA 79001. While lithology B represents a typical basaltic texture, lithology A represents an olivine-phyric rock, showing the close relationship between these two shergottite subgroups.


Mineralogy: The members of this group consist primarily of the clinopyroxenes pigeonite and augite, minor plagioclase converted to maskelynite, and accessory phases such as fine-grained mesostasis, impact-melt, olivine, orthopyroxene, and various oxides, sulfides and phosphates. Several basaltic shergottites contain evidence of interaction with martian water, either in the form of hydrated silicates, or in the form of carbonate and sulfate salts.


Formation history: Basaltic shergottites are igneous rocks of volcanic origin, resembling terrestrial rocks more closely than any other achondrite group. They show exceptionally young crystallization ages of about 150 to 300 million years, and they usually exhibit signs of severe shock-metamorphism. Typically, the plagioclase in 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: Comprising about 10 distinct members, the basaltic shergottites represent the most abundant type of martian meteorite. Typical members are Shergotty and Zagami - both witnessed falls - as well as Los Angeles, Dhofar 378, and a few recent finds from the deserts of Northwest Africa, such as NWA 480, NWA 856, NWA 1669, and NWA 3171.

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