[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 )

キーワード：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