DISCOVERY OF SHOCK-TWINNED ZIRCON AT KAMIL CRATER, EGYPT

With an age of less than ~5000 yr and diameter of 45 m, Kamil crater in Egypt [1] is one of the youngest and smallest terrestrial impact craters. Abundant shock evidence has been reported from Kamil, including impact melt, coesite, stishovite, and PDF and PF in quartz, giving rise to estimates of shock pressures from 20-60 GPa [2,3]. Here we studied zircon in sample L23, a cm-sized clast comprised of shocked and partially melted sandstone ejecta, with lechatelierite, stishovite, coesite, and quartz with PDF and PF [2,3]. Four of twenty zircon grains analyzed by electron backscatter diffraction contain {112} deformation twin lamellae. Lamella lengths range from 1-2 µm, and thus are shorter than those reported from other impact structures, such as Vredefort [4], Sudbury [5], Rock Elm [6], and Santa Fe [7], where individual lamellae are 10s of µm in length. Empirical studies indicate formation of {112} twins at ca. 20 GPa [8], a finding supported by their co-existence (in some rocks) with high-pressure phases [6,9; this study]. The only available laboratory constraint is a diamond anvil cell (DAC) experiment that found {112} twins in zircon powder quenched at 20 GPa [10]. While the DAC experiment did not constrain the pressure at which twins formed, the authors cited plastic deformation of the twins as evidence that that may have formed <11 GPa. Given the porous nature of Kamil target rocks, magnification of a 5-10 GPa shock wave can readily produce >20 GPa conditions locally [11], along with high-temperature. The presence of coesite, stishovite, lechatelierite, and shocked quartz with PDF in sample L23 are consistent with empirically-derived pressure estimates for {112} twin formation in shocked zircon at Kamil crater. Kamil represents the smallest impact structure where shock-twinned zircon has been reported; given the apparent efficiency of {112} twin formation (20% of grains in sample L23), shock-twinned zircon is here shown to provide a robust record of diagnostic shock deformation at even the smallest known impact craters.

[1] Folco et al. 2010 Science. [2] Fazio et al. 2014 MAPS. [3] Folco et al. 2018 Geology. [4] Erickson et al. 2013 GCA. [5] Thomson et al. 2014 GSAB. [6] Cavosie et al. 2015 Geology. [7] Montalvo et al. 2018 GSAB. [8] Moser et al. 2011 CJES. [9] Erickson et al. 2017 CMP. [10] Morozova et al. 2018 AGU. [11] Güldemeister et al. 2013 MAPS.