High‐pressure phase transitions of α‐quartz under nonhydrostatic dynamic conditions: A reconnaissance study at PETRA III

Abstract-Coesite and stishovite are developed in shock veins within metaquartzites beyond a radius of~30 km from the center of the 2.02 Ga Vredefort impact structure. This work focuses on deploying analytical field emission scanning electron microscopy, electron backscattered diffraction, and Raman spectrometry to better understand the temporal and spatial relations of these silica polymorphs. a-Quartz in the host metaquartzites, away from shock veins, exhibits planar features, Brazil twins, and decorated planar deformation features, indicating a primary (bulk) shock loading of >5 < 35 GPa. Within the shock veins, coesite forms anhedral grains, ranging in size from 0.5 to 4 lm, with an average of 1.25 lm. It occurs in clasts, where it displays a distinct jigsaw texture, indicative of partial reversion to a less dense SiO 2 phase, now represented by microcrystalline quartz. It is also developed in the matrix of the shock veins, where it is typically of smaller size (<1 lm). Stishovite occurs as euhedral acicular crystals, typically <0.5 lm wide and up to 15 lm in length, associated with clast-matrix or shock vein margin-matrix interfaces. In this context, the needles occur as radiating or subparallel clusters, which grow into/over both coesite and what is now microcrystalline quartz. Stishovite also occurs as more blebby, subhedral to anhedral grains in the vein matrix (typically <1 lm). We propose a model for the evolution of the veins (1) precursory frictional melting in a microfault (~1 mm wide) generates a molten matrix containing quartz clasts. This is followed by (2) arrival of the main shock front, which shocks to 35 GPa. This generates coesite in the clasts and in the matrix. (3) On initial shock release, the coesite partly reverts to a less dense SiO 2 phase, which is now represented by microcrystalline quartz. (4) With continued release, stishovite forms euhedral needle clusters at solid-liquid interfaces and as anhedral crystals in the matrix. (5) With decreasing pressure-temperature, the matrix completes crystallization to yield a microcrystalline quasi-igneous texture comprising quartz-coesite-stishovite-kyanite-biotitealkali feldspar and accessory phases. It is possible that the shock vein represents the locus of a thermal spike within the bulk shock, in which case there is no requirement for additional pressure (i.e., the bulk shock was ≃35 GPa). However, if that pressure was not realized from the main shock, then supplementary pressure excursions within the vein would have been required. These could have taken the form of localized reverberations from wave trapping, or implosion processes, including pore collapse, phase change-initiated volume reduction, and melt cavitation.

Section: Discussionsupporting

confidence: 69%

Quartz–coesite–stishovite relations in shocked metaquartzites from the Vredefort impact structure, South Africa

Spray

Boonsue

2017

“…Within MEMIN, the formation of high‐pressure phases of α‐quartz was studied under dynamic and nonhydrostatic conditions in a novel approach utilizing membrane‐driven diamond anvil cells at pressures up to 66 GPa, at ambient temperature, and at different loading rates (Carl et al. ), and up to 70 GPa at high temperatures (Carl et al. Forthcoming).…”

Section: Experimental Studies On Shock Metamorphismmentioning

confidence: 99%

Experimental impact cratering: A summary of the major results of the MEMIN research unit

Kenkmann

Deutsch

Thoma

et al. 2018

Self Cite

This paper reviews major findings of the Multidisciplinary Experimental and Modeling Impact Crater Research Network (MEMIN). MEMIN is a consortium, funded from 2009 till 2017 by the German Research Foundation, and is aimed at investigating impact cratering processes by experimental and modeling approaches. The vision of this network has been to comprehensively quantify impact processes by conducting a strictly controlled experimental campaign at the laboratory scale, together with a multidisciplinary analytical approach. Central to MEMIN has been the use of powerful two‐stage light‐gas accelerators capable of producing impact craters in the decimeter size range in solid rocks that allowed detailed spatial analyses of petrophysical, structural, and geochemical changes in target rocks and ejecta. In addition, explosive setups, membrane‐driven diamond anvil cells, as well as laser irradiation and split Hopkinson pressure bar technologies have been used to study the response of minerals and rocks to shock and dynamic loading as well as high‐temperature conditions. We used Seeberger sandstone, Taunus quartzite, Carrara marble, and Weibern tuff as major target rock types. In concert with the experiments we conducted mesoscale numerical simulations of shock wave propagation in heterogeneous rocks resolving the complex response of grains and pores to compressive, shear, and tensile loading and macroscale modeling of crater formation and fracturing. Major results comprise (1) projectile–target interaction, (2) various aspects of shock metamorphism with special focus on low shock pressures and effects of target porosity and water saturation, (3) crater morphologies and cratering efficiencies in various nonporous and porous lithologies, (4) in situ target damage, (5) ejecta dynamics, and (6) geophysical survey of experimental craters.

“…The pressure and temperature required for the formation and preservation of high‐pressure phases of quartz seem to be different when comparing impact environments with quasi‐static conditions (Carl et al. ). At room temperature and static conditions, α‐quartz transforms to coesite with fourfold coordinated Si at 2 GPa, and to stishovite with sixfold coordinated Si (Stishov and Popova ) at 8 GPa (Kirfel et al.…”

Section: Introductionmentioning

confidence: 99%

“…However, in a reconnaissance study conducted by Carl et al. (), SiO 2 phase transformations were monitored at compression rates of up to ~2 GPa s −1 using synchrotron diffraction techniques. The work by Carl et al.…”

Section: Introductionmentioning

confidence: 99%

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Phase transitions of α‐quartz at elevated temperatures under dynamic compression using a membrane‐driven diamond anvil cell: Clues to impact cratering?

Carl

Liermann²,

Ehm

et al. 2018

Self Cite

Coesite and stishovite are high‐pressure silica polymorphs known to have been formed at several terrestrial impact structures. They have been used to assess pressure and temperature conditions that deviate from equilibrium formation conditions. Here we investigate the effects of nonhydrostatic, dynamic stresses on the formation of high‐pressure polymorphs and the amorphization of α‐quartz at elevated temperatures. The obtained disequilibrium states are compared with those predicted by phase diagrams derived from static experiments under equilibrium conditions. We analyzed phase transformations starting with α‐quartz in situ under dynamic loading utilizing a membrane‐driven diamond anvil cell. Using synchrotron powder X‐ray diffraction, the phase transitions of SiO2 are identified up to 77.2 GPa and temperatures of 1160 K at compression rates ranging between 0.10 and 0.37 GPa s−1. Coesite starts forming above 760 K in the pressure range between 2 and 11 GPa. At 1000 K, coesite starts to transform to stishovite. This phase transition is not completed at 1160 K in the same pressure range. Therefore, the temperature initiates the phase transition from α‐quartz to coesite, and the transition from coesite to stishovite. Below 1000 K and during compression, α‐quartz becomes amorphous and partially converts to stishovite. This phase transition occurs between 25 and 35 GPa. Above 1000 K, no amorphization of α‐quartz is observed. High temperature experiments reveal the strong thermal dependence of the formation of coesite and stishovite under nonhydrostatic and disequilibrium conditions.