Impact melting in the Cumberland Falls and Mayo Belwa aubrites

Abstract

Six chondritic clasts in the Cumberland Falls polymict breccia were examined: four texturally resemble ordinary chondrites (OCs) and two are impact melt breccias containing shocked OC clasts adjacent to a melt matrix. The six chondritic clasts are probably remnants of a single OC projectile that was heterogeneously shocked when it collided with the Cumberland Falls host. Mayo Belwa is the first known aubrite impact melt breccia. It contains coarse enstatite grains exhibiting mosaic extinction; the enstatite grains are surrounded by a melt matrix composed of 3-16 μm-size euhedral and subhedral enstatite grains embedded in sodic plagioclase. Numerous vugs, ranging from a few micrometers to a few millimeters in size, constitute ∼5 vol% of the meteorite. They occur nearly exclusively within the Mayo Belwa matrix; literature data show that some vugs are lined with bundles of acicular grains of the amphibole fluor-richterite. This phase has been reported previously in only two other enstatite meteorites (Abee and St. Sauveur), both of which are EH-chondrite impact melt breccias. It seems likely that in Mayo Belwa, volatiles were vaporized during an impact event and formed bubbles in the melt. As the melt solidified, the bubbles became cavities; plagioclase and fluor-richterite crystallized at the margins of these cavities via reaction of the melt with the vapor. © 2010 The Meteoritical Society.