LOCALIZED SHOCK VERSUS BULK SHOCK IN MARTIAN METEORITES

Shock effects in the form of veins and pockets are described from selected martian basaltic shergottites, including Northwest Africa 4468, Dar al Gani 476 and Los Angeles. The localized development of various high-pressure polymorphs is contrasted with lower pressure effects witnessed by the bulk of the meteorites. This contrast is attributed to shock wave amplification associated with microfaulting, mineral phase transformations, and resulting melting and crystallization under HP and/or HT conditions. The relationship between the phosphate polymorphs apatite, merrillite and tuite are explored via Raman spectroscopy and field emission scanning electron microscopy, as well as other polymorphic associations/state transformations: plagioclase-maskelynite, olivine-ringwoodite, quartz-stishovite and pyroxene-majorite. The HP-HT polymorph distribution is assessed in the light of bulk shock modification. Peak conditions are estimated to be in the range of 2000 degrees celsius and 23 GPa, versus bulk effects being significantly less. The HP-HT excursions are attributed to shock focussing processes that accompanied sample lofting off the planet, an event that is predominantly responsible for the shock history of these basaltic materials. These effects are linked to likely launch zones within source impact structures, with release to space being facilitated by surface spall as a result of shock rarefaction.