Olivin-phyric Shergottites: SAU 090, SAU 125, NWA 1775, SAU 130, NWA 1068 Louise Michele, SAU 005-008, DAG 476, Dhofar 019.


Olivine-phyric Shergottites


Synonyms: picritic 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 olivine-phyric shergottites represents the youngest of these three groups, and maybe one of the most interesting classes of martian meteorites.


Description: Typically light to dark greenish rocks with more or less dull black fusion crusts. Most desert finds are devoid of remnant crust, and they are hard to distinguish from terrestrial igneous rocks. As the name implies, olivine-phyric shergottites all exhibit porphyritic textures of large olivine-crystals (phenocrysts) set in a fine-grained basaltic groundmass.


Mineralogy: Olivine-phyric shergottites are primarily composed of olivine phenocrysts set in a basaltic groundmass of pigeonite, plagioclase shock-converted to maskelynite, minor augite, and olivine. Accessory minerals are chromite, merrillite, ilmenite, ulvöspinel, and pyrrhotite. Due to some differences in mineralogy, the olivine-phyric shergottites are subdivided into the original olivine-phyric group, and the olivine-orthopyroxene-phyric group. The members of the latter group exhibit - often preferentially oriented - pyroxene phenocrysts besides the typical olivine phenocrysts.


Formation history: Considering the links between the basaltic and lherzolitic shergottites, older theories suggested that the olivine-phyric shergottites represent intermediate forms between these two groups. It was thought that they were formed through partial melting of lherzolitic and other source rocks, resulting in a magma that crystallized within an extruded lava flow near the martian surface. However, more recent studies suggest that the olivine-phyric shergottites formed from independent, olivine-saturated magmas on Mars that might have been parental to basaltic shergottites, making the olivine-phyric shergottites precursors of their younger basaltic cousins.


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: Today, 9 distinct olivine-phyric shergottites are known, most of them having been recovered from the hot deserts of Africa, and Oman. Typical examples include Dar al Gani 476, the first Mars meteorite from the hot deserts, Dhofar 019, NWA 1068 (and its pairings, e.g. NWA 1775), as well as SaU 005/008, the largest Mars meteorite, with its pairings SaU 090, SaU 125, and SaU 130.

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