Diaplectic quartz glass and SiO2 melt experimentally generated at only 5 GPa shock pressure in porous sandstone: Laboratory observations and meso-scale numerical modeling

We have investigated the carbonates in the impact melts and in a monolithic clast of highly shocked Coconino sandstone of Meteor Crater, AZ to evaluate whether melting or devolatilization is the dominant response of carbonates during high-speed meteorite impact. Both melt-and clast-carbonates are calcites that have identical crystal habits and that contain anomalously high SiO 2 and Al 2 O 3 . Also, both calcite occurrences lack any meteoritic contamination, such as Fe or Ni, which is otherwise abundantly observed in all other impact melts and their crystallization products at Meteor Crater. The carbon and oxygen isotope systematics for both calcite deposits suggest a low temperature environment (<100°C) for their precipitation from an aqueous solution, consistent with caliche. We furthermore subjected bulk melt beads to thermogravimetric analysis and monitored the evolving volatiles with a quadrupole mass spectrometer. CO 2 yields were <5 wt%, with typical values in the 2 wt% range; also total CO 2 loss is positively correlated with H 2 O loss, an indication that most of these volatiles derive from the secondary calcite. Also, transparent glasses, considered the most pristine impact melts, yield 100 wt% element totals by EMPA, suggesting complete loss of CO 2 . The target dolomite decomposed into MgO, CaO, and CO 2 ; the CO 2 escaped and the CaO and MgO combined with SiO 2 from coexisting quartz and FeO from the impactor to produce the dominant impact melt at Meteor Crater. Although confined to Meteor Crater, these findings are in stark contrast to Osinski et al. (2008) who proposed that melting of carbonates, rather than devolatilization, is the dominant process during hypervelocity impact into carbonate-bearing targets, including Meteor Crater.

Section: Discussionmentioning

confidence: 99%

Devolatilization or melting of carbonates at Meteor Crater, AZ?

Hörz

Archer

Niles

et al. 2015

“…; Kowitz et al. ); however, the term PDF then would be misleading. We believe that PDF can appear bent but then the host crystal lattice has been disturbed subsequently to the impact event (e.g., Trepmann ; Reimold et al.…”

Section: Samples and Methodologymentioning

confidence: 99%

“…Kowitz et al. (, ) reported that porosity in sandstone is a critical parameter that determines at what shock pressures the onset of formation of diagnostic shock features (e.g., diaplectic quartz glass, SiO 2 melt, and PDF) occurs. They noted first PDF development at 17.5 GPa for a Seeberger sandstone (L5); a arenitic quartz sandstone with an average porosity of 12.2–19.2 vol%.…”

Section: Samples and Methodologymentioning

confidence: 99%

Microcomputed tomography and shock microdeformation studies on shatter cones

Zaag

Reimold

Hipsley

2016

One of the aspects of impact cratering that are still not fully understood is the formation of shatter cones and related fracturing phenomena. Yet, shatter cones have been applied as an impact‐diagnostic criterion for decades without the role of shock waves and target rock defects in their formation having been elucidated ever. We have tested the application of the nondestructive microcomputed tomography (μCT) method to visualize the interior of shatter cones in order to possibly resolve links between fracture patterns and shatter cone surface features (striations and intervening “valleys”). Shatter‐coned samples from different impact sites and in different lithologies were investigated for their μCT suitability, with a shatter cone in sandstone from the Serra da Cangalha impact structure (Brazil) remaining as the most promising candidate because of the fracture resolution achieved. To validate the obtained CT data, the scanned specimen was cut into three orthogonal sets of thin sections. Scans with 13 μm resolution were obtained. μCT scans and microscopic analysis unraveled an orientation of subplanar fractures and related fluid inclusion trails, and planar fracture (PF) orientations in the interior of shatter cones. Planar deformation features (PDF) were observed predominantly near the shatter cone surface. Previously undescribed varieties of feather features (FF), in the form of lamellae emanating from curviplanar and curved fractures, as well as an “arrowhead”‐like FF development with microlamellae originating from both sides of a PF, were observed. The timing of shatter cone formation was investigated by establishing temporal relations to the generation of various shock microscopic effects. Shatter cones are, thus, generated post‐ or syn‐formation of PF, FF, subplanar fractures, and PDF. The earliest possible time for shatter cone formation is during the late stage of the compressional phase, that is, shock wave passage, of an impact event.

“…Flyer plate tests, which are performed in addition to the impact experiments (Kowitz et al. , ), can probably contribute much better to compare shock effects in detail with modeled shock attenuation. Nevertheless, in addition to the described macroscopic features, the shatter cone fragments microscopically show also a distinctive fracture behavior due to the passage of the shock wave.…”

Section: Discussionmentioning

confidence: 99%

Formation of shatter cones in MEMIN impact experiments

Wilk

Kenkmann

2016

Shatter cones are the only macroscopic feature considered as evidence for shock metamorphism. Their presence is diagnostic for the discovery and verification of impact structures. The occurrence of shatter cones is heterogeneous throughout the crater record and their geometry can diverge from the typical cone shape. The precise formation mechanism of shatter cones is still not resolved. In this study, we aim at better constraining the boundary conditions of shatter cone formation in impact experiments and test a novel approach to qualitatively and quantitatively describe shatter cone geometries by white light interferometry. We recovered several ejected fragments from MEMIN cratering experiments that show slightly curved, striated surfaces and conical geometries with apices of 36°–52°. These fragments fulfilling the morphological criteria of shatter cones were found in experiments with 20–80 cm sized target cubes of sandstone, quartzite and limestone, but not in highly porous tuff. Targets were impacted by aluminum, steel, and iron meteorite projectiles at velocities of 4.6–7.8 km s−1. The projectile sizes ranged from 2.5–12 mm in diameter and produced experimental peak pressures of up to 86 GPa. In experiments with lower impact velocities shatter cones could not be found. A thorough morphometric analysis of the experimentally generated shatter cones was made with 3D white light interferometry scans at micrometer accuracy. SEM analysis of the surfaces of recovered fragments showed vesicular melt films alternating with smoothly polished surfaces. We hypothesize that the vesicular melt films predominantly form at strain releasing steps and suggest that shatter cones are probably mixed mode fractures.