53-8 Is cubic zirconia heritage a diagnostic indicator for lunar impact melts ?

Abstract:

Apollo sample 14073, collected from within a trench at Station G, has been classified as a KREEP basalt with 50% plagioclase, 25% clinopyroxene, and 20% orthopyroxene, minor ilmenite (2%), troilite, Fe-metal, phosphates, Fe-Ti Zr silicates and mesostasis (Gancarz et al., 1971).

In a thin section of 14073, 11, we investigated the crystallographic properties of zirconia grains with electron backscatter diffraction (EBSD)-based crystallographic and phase mapping. The EBSD analyses were performed with a JEOL 7900F field emission SEM equipped with an Oxford Instruments Symmetry detector at NASA JSC. One baddeleyite grain shows evidence for phase heritage of a high symmetry, consistent with a cubic zirconia polymorph. Crystallographic orientation maps of the grain revealed the presence of several twin planes with 90°, 115°, and 180° disorientations about the <100> axis. Additionally, the cubic zirconia heritage was identified through the characteristic crystallographic orientation relationships observed in the <100>, <010> and <001> pole figures, and disorientation axes plots of the grain.

In terrestrial impactites, the presence of these unique orientation relationships has been attributed to the heating of zircon to over 2370° C at Earth surface pressure, producing melt, which causes zircon to decompose to the high symmetry cubic zirconia phase (Chinchalkar et al., 2024), and subsequent martensitic reversion which subsequently reverts to the more stable monoclinic zirconia (baddeleyite) during cooling. No other shock metamorphic or triple junction features have been identified in other phases in the sample supports that it did not experience shock metamorphism or thermal annealing after crystallization. Therefore, it is unlikely that the cubic zirconia grain is related to impact-induced superheating or high-pressure shock metamorphism. In addition, recently identified cubic phase heritage in zirconia grains in Chang'e 5 mare basalt (Huang et al., 2024 ) that is not related to superheating may indicate that cubic zirconia crystallization on the Moon may form under different thermal and redox regimes than those expected in terrestrial impactites and may not require superheating of impact melts.

References:

Chinchalkar, et al., 2024. Earth Planet. Sci. Lett. 636, 118714. https://doi.org/https://doi.org/10.1016/j.epsl.2024.118714

Gancarz et al. (1971) Earth Planet. Sci. Lett. V. 12, 1-18.

Huang, et al., 2024. J. Geophys. Res. Planets 129. https://doi.org/10.1029/2023JE007955

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