Keywords:Shock melting, Anomalous achondrites, Infrared spectroscopy
The anomalous achondrite Northwest Africa (NWA) 6704 has isotopic affinities with carbonaceous chondrites and shows geochemical evidence for a highly oxidized precursor and no sign of equilibration with a metallic melt. NWA 6704 and its paired samples are dominated by pyroxene with sodic plagioclase, Ni-bearing olivine, and rare chromite and awaruite (Ni3Fe). The isotopic and geochemical link between the NWA 6704 pairing group and carbonaceous chondrites is consistent with significant hydration of the precursor materials. Both endogenic  and impact-melt  origins for NWA 6704 meteorites have been proposed. Here, we use infrared spectroscopy of pyroxene grains from NWA 6704 to show a distinct lack of hydration in the parent melt of NWA 6704, consistent with an origin as a superheated impact melt. The recent discovery of plausible parent asteroids for NWA 6704  demonstrates that some apparently dry, “igneous” asteroids (including in the outer asteroid belt) may have originated as impact-melted material, possibly during catastrophic collisional disruptions.
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