11th Planetary Crater Consortium 2020 (LPI Contrib. No. 2251) 2018.pdf

Shock metamorphism of opal-A by laser-induced microprojectile impact experiments S. Lee1,2,3, J. Cai4, S. Jin3, D. Zhang5,6, R. Thevamaran4, H. Xu3. 1Lunar and Planetary Institute, USRA, Houston, TX 77058, ([email protected]), 2ARES, NASA Johnson Space Center, Houston, TX 77058, 3Department of Geoscience, University of Wisconsin– Madison, Madison, WI 53706, 4Department of Engineering Physics, University of Wisconsin-Madison, Madison, WI 53706, 5Hawaii Institute of Geophysics and Planetology, University of Hawaii at Manoa, Honolulu, HI 96822, 6GeoSoilEnviroCARS, University of Chicago, Lemont, IL60439, USA

Introduction: Shock metamorphism, also called Sample and Methods: The opal-A (hyalite opal) impact metamorphism, is the progressive breakdown sample is from the Chalice mine, North Carolina and deformation of underlying rock layers and their showing the globular and botryoidal shape of constituent minerals during an impact event [1,2]. amorphous silica with glass-like appearance. The Natural shock metamorphism is the physical polished thin section of opal-A (~50 μm) is prepared for consequence of meteorite impacts on planetary bodies, the impact experiment. The shock metamorphism of leading to the circular or crater-like geological opal-A induced by supersonic microprojectile impacts structures, deformed bedrock and sediment, large in an advanced laser-induced projectile impact test igneous provinces, economic deposits, and biological (Figure 1) was investigated using synchrotron XRD extinction events [1,2]. The shock metamorphic (beamline 13-BM-C, at the Advanced Photon Source) signatures are widely observed in impact craters on and TEM (Philips CM200-UT). The 3D topographical Earth, lunar rocks, meteorites, and many other types of features of impact areas were studied by surface asteroids [2,3]. mapping interferometer (Zygo NewView 6300) Here, we report an experimental shock scanning white-light interferometer. metamorphism of opal-A induced by supersonic Results: The topographical features of craters can microprojectile impacts using an advanced laser- provide valuable information on impact processes induced projectile impact test (LIPIT). Opal-A is a including physical and geological processes. The 3D hydrated amorphous silica species and a widespread topography of impact areas in our opal-A samples is mineraloid of Earth’s crust that occurs in vesicles, veins, similar to craters formed by meteorite impact, showing and fissures of many rocks (e.g., commonly found with a circular outline with the uplifted rim as well as a depth sandstone, chalk deposits, limonite, and rhyolite) [3]. that is shallow relative to the diameter (Figure 2). The On Mars, the Compact Reconnaissance Imaging impact velocity of the silica projectile and its angle of Spectrometer for Mars (CRISM) instrument suggests impact with respect to the surface normal direction was the presence of opaline silica deposits on the Martian measured using the multiple-exposure images. The surface [4]. We report the first impact experiments on silica projectile was accelerated to velocities in the opal-A The phase transformation and deformation range of 478 m/s (Figure 1). features of shocked opal-A were investigated by X-ray Synchrotron XRD patterns from the same area diffraction (XRD) and transmission electron before and after the impact were obtained (Figure 3). microscopy (TEM) analyses. The results of this study The diffraction pattern indicates the phase provide important information on the shock transformation of coesite, cristobalite, and tridymite metamorphism of opal-A from macro to nanoscales. from opal-A. No quartz was observed in the impacted opal. TEM analysis was also used to identify the

Figure 1. An illustration of the laser-induced projectile Figure 2. Optical microscopic images of initial and impact (LIPIT) experiment and a multi-exposure shocked opal-A and 3-D topography of impact areas image of a projectile captured by 103 ns intervals. characterized the topography of single impact. 11th Planetary Crater Consortium 2020 (LPI Contrib. No. 2251) 2018.pdf

minerals and textures from shocked opal (Figure 3). We weakening effect and diffusion of water-related point identified opal-CT, coesite, tridymite, and cristobalite defect could lower the transition boundary of opal. from the shocked opal-A. We also tested the shock Similarly, porous sandstone can form a coesite phase at metamorphism of quartz and novaculite (micro- relatively low shock pressure. crystalline quartz) under the same impact condition but Conclusions: The investigation of the minerals via the phase transformation was not observed in the high-velocity micro-projectile impacts using LIPIT experiments. technique provides a new way to explore shock Discussions: Combined method of synchrotron metamorphism. Synchrotron XRD and TEM techniques XRD and TEM shows the phase transformation of opal- reveal nano- and micro-scaled phases in the shocked A to opal-CT, tridymite, cristobalite, and coesite that opal-A. The results show the phase transformation of was induced by the microprojectile impacts (Figure 3). opal-A to coesite, opal-CT, tridymite, and cristobalite at Based on the impact simulation, the normal shock the shocked area where experienced pressures up to estimations—an upper bound for the pressure generated ~0.6 – 0.7 GPa. by spherical projectiles—suggest that the shocked areas The main advantage of LIPIT technique is that the experienced pressures up to ~0.6 – 0.7 GPa, which is size of projectile and the impact speed can be controlled lower than the pressure of the phase transition boundary in desired ways and provides dynamic experiments at (> 2GPa) between α-quartz and coesite. length scales that enable holistic analysis of impact A plausible hypothesis for this opal-A to coesite phenomena using electron microscopy and synchrotron transition at low pressures could be associated with opal X-ray diffraction techniques. The method can be structure [5]. According to pair distribution function applied to other rock-forming mineral systems in analysis, the opal structure mainly consisted of six- various experimental conditions to mimic collision membered and four-membered rings of SiO4 tetrahedra. environments in the laboratory through adjusting The four-membered ring of SiO4 (i.e., local structure of density, speed, and temperature of the projectiles. coesite) could be the precursor to the formation of Furthermore, the shock metamorphic experiments can coesite phase, whereas the six-membered rings can be be used to study the specific orientations of minerals the precursor to the formation of tridymite (two-layer that respond to impact along with different incident stacking of the SiO4 tetrahedra ring) and cristobalite directions. In summary, the integrated LIPIT micro- (three-layer stacking). In nature, some opal coexisted projectile experiments and post-impact electron with low-tridymite and -cristobalite without quartz in microscopy and synchrotron characterization methods hydrothermal environments [6,7]. along with potential computational modeling will The presence of water in opal could also have benefit the study of shock metamorphism in minerals, contributed to lower the phase transition pressures. The and lead to expand our knowledge of shock opal is a hydrated amorphous phase of silica metamorphism on Earth, Moon, Mars, and other (SiO2·nH2O) and this sample contains ~6.7 wt.% water. planetary bodies. Previous experiments reported wet quartz, containing Acknowledgements: We acknowledge NASA ~400 ppm of water, displays plastic behavior and Astrobiology Institute (NNA13AA94A) for supporting deformations around 0.5 to 0.6 GPa [8]. The hydrolytic this study. Part of this work was performed at GeoSoilEnviroCARS (Sector 13), Partnership for Extreme Crystallography program (PX^2), Advanced Photon Source (APS), and Argonne National Laboratory. GeoSoilEnviroCARS is supported by the National Science Foundation—Earth Sciences (EAR- 1634415) and Department of Energy—Geosciences (DE-FG02-94ER14466). References: [1] Stöffler, D. and Langenhorst, F. (1994) Meteoritics, 29, 155-181. [2] French, B. M. (1966) Science, 903-906. [3] Langenhorst, F. and Deutsch, A (2012) Elements, 8, 31-36. [4] Milliken R. E. et al. (2008) Geology, 36, 847-850. [5] Lee, S. et al. (2020) American Mineralogist, (in review) [6] Lee, S. Figure 3. A slice of 2-D X-ray diffraction of initial and and Xu, H. (2019) Acta Cryst. B, 75, 160-167. [7] Lee, shocked opal. TEM images of initial and shocked opal S and Xu, H. (2020) Minerals, 10, 124. [8] Doukhan, J. showing products of tridymite, coesite, and cristobalite C. and Trépied, L. (1985) Bull. Mineral.108, 97-123. together with opal-CT.