Petrographic investigation of shatter cone melt films recovered from MEMIN impact experiments in sandstone and iSALE modeling of their formation boundary conditions

Abstract

Shatter cones are diagnostic for the recognition of meteorite impact craters. They are unambiguously identifiable in the field and the only macroscopic shock deformation feature. However, the physical boundary conditions and exact formation mechanism(s) are still a subject of debate. Melt films found on shatter cone surfaces may allow the constraint of pressure–temperature conditions during or immediately after their formation. Within the framework of the MEMIN research group, we recovered 24 shatter cone fragments from the ejecta of hypervelocity impact experiments. Here, we focus on silicate melt films (now quenched to glass) found on shatter cone surfaces formed in experiments with 20–80 cm sized sandstone targets, impacted by aluminum and iron meteorite projectiles of 5 and 12 mm diameter at velocities of 7.0 and 4.6 km s−1, respectively. The recovered shatter cone fragments vary in size from 1.2 to 9.3 mm. They show slightly curved, striated surfaces, and conical geometries with apical angles of 36°–52°. The fragments were recovered from experiments with peak pressures ranging from 46 to 86 GPa, and emanated from a zone within 0.38 crater radii. Based on iSale modeling and petrographic investigations, the shatter coned material experienced low bulk shock pressures of 0.5–5 GPa, whereas deformation shows a steep increase toward the shatter cone surface leading to localized melting of the rock, resulting in both vesicular as well as polished melt textures visible under the SEM. Subjacent to the melt films are zones of fragmentation and brittle shear, indicating movement away from the shatter cone apex of the rock that surrounds the cone. Smearing and extension of the melt film indicates subsequent movement in opposite direction to the comminuted and brecciated shear zone. We believe the documented shear textures and the adjacent smooth melt films can be related to frictional melting, whereas the overlying highly vesiculated melt layer could indicate rapid pressure release. From the observation of melting and mixing of quartz, phyllosilicates, and rutile in this overlying texture, we infer high, but very localized postshock temperatures exceeding 2000 °C. The melted upper part of the shatter cone surface cross‐cuts the fragmented lower section, and is accompanied by PDFs developed in quartz parallel to the {11true2false¯2} plane. Based on the overprinting textures and documented shock effects, we hypothesize shatter cones start to form during shock loading and remain an active fracture surface until pressure release during unloading and infer that shatter cone surfaces are mixed mode I/II fracture surfaces.

Microstructures in shocked quartz: linking nuclear airbursts and meteorite impacts

“…In quartz grains experimentally shocked at 5 to 17.5 GPa, Fazio et al [5] observed glass veins composed of amorphous silica generally thicker than 50 nm, extending several microns in length. Wilk et al [6] found amorphous silica in experimentally shocked rocks called shatter cones that formed at low shock pressures of 0.5-5 GPa. Laboratory shock experiments by Martinelli et al [79] used quartz crystals with a minimum diameter of 3400 µm, larger than we tested.…”

“…For example, in quartz grains experimentally shocked at 5 to 17.5 GPa, Fazio et al [5] observed glass veins composed of amorphous silica extending across several microns in length and generally thicker than 50 nm. Wilk et al [6] found amorphous silica in experimentally shocked rocks called shatter cones that formed at low shock pressures of 0.5-5 GPa. Shatter cones are considered to be a classic impact indicator.…”

Microstructures in Shocked Quartz: Linking Nuclear Airbursts and Meteorite Impacts

“…In quartz grains experimentally shocked at 5 to 17.5 GPa, Fazio et al [5] observed glass veins composed of amorphous silica that extend across several microns in length and that are generally thicker than 50 nm. Wilk et al [6] found amorphous silica in experimentally shocked rocks called shatter cones that formed at low shock pressures of 0.5-5 GPa. In addition, Carl et al [7] conducted experiments demonstrating that extensive amorphization of quartz begins at ~10 GPa.…”

Abu Hureyra, Syria, Part 1: Shock-fractured quartz grains support 12,800-year-old cosmic airburst at the Younger Dryas onset

“…In quartz grains experimentally shocked at 5 to 17.5 GPa, Fazio et al [88] observed glass veins composed of amorphous silica extending several microns in length and generally thicker than 50 nm. • Wilk et al [89] found amorphous silica in experimentally shocked rocks called shatter cones that formed at low shock pressures of 0.5-5 GPa.…”