Shock and thermal history of equilibrated eucrites from Antarctica

We report petrologic observations of seven equilibrated basaltic eucrites from Antarctica. These eucrites are metamorphic rocks (type 4 to 7), as observed for non-Antarctic monomict eucrites. A-881388 and -881467 are unusual; they are granulitic breccias, but portions still preserve vestiges of igneous textures. A-87272 is a coarse-grained rock, containing pyroxenes with remnant Ca-zoning and inversion textures (type 7) and is one of the most shocked eucrites. A-881747 is a typical type 4 and Y-86763 a type 5 eucrite. RKPA80204 has very fine-grained, basaltic clasts in which some pigeonites are partially inverted to orthopyroxene due to slow subsolidus cooling (type 6). LEW 86002 is a moderately shocked type 5 eucrite. These rocks cooled rapidly at their liquidi, but orders of magnitude slower at subsolidus temperatures. The most likely heat source for metamorphism is simple burial of a succession of lava flows as the crust grew by extrusive volcanism and intrusions. Heat diffusing from the hot interior of the parent body caused the temperature to rise in the crust, leading to widespread metamorphism. The variety of shock textures observed suggests that impact events were active before, during, and after thermal metamorphism on the asteroid 4 Vesta, the likely eucrite parent body.