Description of new shock-induced phases in the Shergotty, Zagami, Nakhla and Chassigny meteorites

The Shergottite, Nakhlite and Chassignite (SNC) meteorites, Shergotty, Zaga mi, Nakhla and Chassigny, have been studied by analytical transmission elec tron microscopy. New phases, characteristic of strong shock conditions, hav e been discovered: calcium-rich majorites in Shergottty, wadsleyite with an omalously elevated iron content in Chassigny, and impact melts in Shergotty , Zagami and Nakhla. Cristobalites (alpha and beta polymorphs) observed in Shergotty and Zagami may also be related to shock and are interpreted as ba ck transformation products of post-stishovite silica polymorphs. Shocks cor responding to pressure and temperature conditions characteristic of the Ear th's transition zone and lower mantle have occurred in those meteorites. Mo reover, impact melts indicate high-temperature conditions in localized area s. On the other hand, no massive impact melting is observed in those meteor ites, consistent with previous descriptions. These observations provide evi dence of highly heterogeneous shock conditions at the scale of few micromet ers in these meteorites. Strongly heterogeneous conditions such as those su ggested by the present study may help to explain the preservation in martia n meteorites of phases practically unaffected by shock being very close to strongly shock-metamorphized minerals.