[P-CG23] Shock responses of planetary materials elucidated from meteorites and laboratory experiments

[PCG23-01] Shock heating origin of anomalous achondrite NWA 6704 from infrared spectroscopy and modal mineralogy

*Matthew Izawa¹, Sean Yokoyama², Shigeru Yamashita¹, Takuo Okuchi¹, Takuya Moriguti¹, Andrew P Jephcoat¹, Vishnu Reddy³, Allison M McGraw³, Lucille Le Corre⁴, Edward A Cloutis⁵ (1.Institute for Planetary Materials, Okayama University, 827 Yamada, Misasa, Tottori 682-0193, Japan, 2.University of Toronto, 27 King's College Cir, Toronto, ON M5S, Canada , 3.Lunar and Planetary Laboratory, University of Arizona, 1629 E University Blvd Tucson AZ 85721-0092 USA, 4.Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ, USA, 5.Center for Terrestrial and Planetary Exploration, University of Winnipeg, 515 Portage Avenue, Winnipeg, Manitoba, R3B 2E9 Canada )

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 [1] and impact-melt [2] 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 [3] 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.



[1] Warren et al., Geochimica et Cosmochimica Acta 107:135–154 2013 [2] Hibiya et al., Geochimica et Cosmochimica Acta 245(15) 597-627, 2018 [3] McGraw et al., in press, Astrophysical Journal 2020