ABEE AND RELATED EH CHONDRITE IMPACT-MELT BRECCIAS

Abstract

The Abee EH chondrite is an impact-melt breccia. About 80-90% of the chondrules were melted; many of those that remain have been partly resorbed. Euhedral enstatite phenocrysts are abundant; many have nucleated on the surfaces of relict chondrules and now protrude into kamacite globules. Euhedral laths of graphite, essentially identical to those produced magmatically on Earth, occur throughout the rock. Abee experienced two episodes of impact melting with an intervening period of brecciation. The first impact-melting event produced a relatively homogeneous rock with abundant melt. Upon cooling, the rock was shattered by bombarding meteoroids; igneous oldhamite-rich dark inclusions were introduced at this time. A second impact event produced melt which engulfed the Abee fragments and dark inclusions, primarily digesting the small ones; heat lost to the surrounding clastic debris caused the groundmass melt to quench. Previously described diamonds in Abee containing typical solar-system isotopic compositions of N and Xe were probably produced from graphite by shock.Several other EH chondrites may also be impact-melt breccias. The most extensively melted of these is Adhi Kot, which contains shocked, but largely unmelted, chondrule-rich clasts as well as chondrule-free sulfide- and silica-rich clasts that also appear to be impact products.