Discussion of the unique “eclogitic” clast reported from Northwest Africa 801 (CR2) and the possibility of ejection of foundered crust from deep in a molten Moon‐sized projectile

Abstract

AbstractA unique 2 mm‐wide clast of fine‐grained garnet–omphacite peridotite with chondritic chemistry was reported from the CR2 chondrite Northwest Africa 801 by Hiyagon et al. (2016, Geochimica et Cosmochimica Acta, 186, 32–48). Those authors described the clast as eclogitic and inferred from its mineral compositions that the rock formed quickly, during perhaps 100–1000 years of burial, deep within a Moon‐sized body at about 3 GPa (30 kbar) pressure and 1000°C. Such conditions are unprecedented among meteorites and invite scrutiny. Here, we discuss the clast and its origin. The inferred conditions appear justified, but the published idea of burial during a protoplanetary merger and, soon after, exhumation in a violent collision seems improbable and contrary to the clast's low shock levels. We find that exhumation is better explained by pull‐apart of a projectile in a low velocity hit‐and‐run collision. We try inconclusively to explain near‐simultaneous burial and exhumation in a hit‐and‐run return scenario. Taking a different approach, and to conclude, we speculate that an approximately lunar‐mass body was molten beneath a thin dense chondritic crust, of which fragments foundered and sank deep into the magma ocean as an ongoing process. Fragments that were changing to garnet–omphacite peridotite were exhumed when the molten body was pulled apart in a hit‐and‐run collision with a larger body.