Shock experiments on quartz targets pre-cooled to 77 K

Abstract

The influence of temperature on the development of shock deformation effects in quartz single-crystal targets has been investigated in this parameter study. Quartz targets were pre-cooled to 77 K and shocked parallel to their $\left(101{;}^{-}0\right)$ orientation in shock reverberation experiments of microsecond shock duration. Shock deformation effects in quartz were characterized by optical and spectroscopic methods. The degree of shock metamorphism in pre-cooled quartz targets has then been compared with literature data from μs-shock reverberation experiments on quartz targets shocked parallel to the same crystallographic orientation but at initial temperatures of 293–903 K.

The degree of shock metamorphic overprint in quartz was found to be rather insensitive to temperature variation in the low-temperature region (<548 K), but sensitive to temperature variation in the high-temperature region up to 903 K. This temperature dependence allows to distinguish between different mechanisms that had been proposed to explain the formation of shock deformation effects in quartz. The identified temperature dependence points toward a simple mechanism in which the crystal lattice collapses due to overpressure at low to moderate temperatures. Very high temperatures result in weakening of the silicon-oxygen bonds that support the crystal lattice, leading to a decrease of the onset pressure for structural collapse.

Introduction

Hypervelocity impact of dust- to kilometer-sized projectiles are part of the background processes in space, having left their marks on every solid object in space – planetary bodies and human-made probes, satellites, and the space station. In addition, impacts on asteroids [1], [2] and on planetary bodies such as Mars [3], [4] and the Moon [5], [6] have been capable to eject rock fragments into space; some of these ejecta may eventually arrive on Earth and could be recovered as meteorites. These meteorites represent unique samples from different types of planetary bodies that allow the study of the history of the solar system [7], [8] and – to some extent – of pre-solar stars, which produced the elements of which our solar system is made [9]. In theory, meteorites impact-ejected from Mars-like planets could also serve as agents for the exchange of viable organisms between several planets in a given solar system [10].

Almost all meteorites accessible for scientific analysis show that they were subjected to shock events caused by hypervelocity collisions. Therefore, it is important to understand the physical and chemical modifications in meteorites subjected to weak and strong shock waves [11], [12]. Shock effects in rock-forming minerals of meteorites have been studied extensively [12], [13] and can be categorized into destructive (decreasing crystal order) and constructive (increasing crystal order) shock metamorphism.

Constructive shock metamorphism includes the formation of high-pressure phases, e.g., quartz can be transformed into stishovite and coesite [14], [15], graphite into diamond [16], and olivine into wadsleyite and ringwoodite [17], [18]. By “overshooting”, i.e., subjecting a specimen to shock pressures exceeding the stability field of high-pressure phases, stishovite [14], diamond [16], or wadsleyite [17] have been experimentally produced in microsecond (μs) shock experiments. In addition, a great variety of high-pressure phases of the major rock-forming minerals have been discovered in highly shocked L6 type ordinary chondrites [18], [19] and martian meteorites [20], [21]; however, the formation and preservation of most high-pressure polymorphs is restricted to locally occurring shock veins and melt pockets reflecting local shock pressure and shock-temperature excursions [18], [19], [20], [21], [22], whereby these high-pressure phases form under decreasing shock pressure and temperature.

The destructive shock effects – such as mosaicism, shear fractures, high dislocation densities, planar deformation features (PDFs), diaplectic glass or fused glass [13], [15] – in silicate minerals, which comprise the main mass of terrestrial and extraterrestrial rocks, are used to determine the maximum bulk shock pressure at a given rock experienced (quantitative shock pressure barometry) (e.g., [12], [13]). For a variety of important rock-forming minerals, such as olivine, pyroxene, plagioclase, and quartz, the destructive shock deformation effects have been generally pressure-calibrated by shock recovery experiments conducted at room temperature. Continuous cooperation for nearly 40 years between the working groups of D. Stöffler (MfN, Berlin) and U. Hornemann (EMI) has resulted in the standard classification scheme for shock metamorphism in rocks and minerals [13]. It is important to identify those shock deformation effects which are thought to be rather insensitive to temperature, because natural rocks can have a broad range of initial temperatures depending on the internal heat flux and ambient surface temperatures of the parent body at the time of the impact event. In addition to the initial temperature of the rock, porosity affects the amount of heat that is deposited by the shock wave. Pore-space collapse, which occurs at relatively low shock pressures, involves a larger amount of plastic (pressure-volume) work than the compression of a nonporous material, for the same level of shock pressure to be achieved. The additional P-V work in shocked porous rocks is turned into heat as a result of the compaction of pores and may have a similar effect as a higher ambient temperature at production of shock effects in minerals [23].

In order to investigate the influence of temperature on the destructive shock effects in silicate, we have conducted a series of shock recovery experiments on single-crystal quartz targets that had been pre-cooled to 77 K with liquid nitrogen. Single-crystal quartz represents the most intensively studied model system for shock metamorphism of silicates (e.g., [14], [15], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38]). Quartz single crystals are chemically homogeneous, and crystallographically, and texturally identical to other quartz single crystal specimens. Thus, our results can be directly compared to those of a large number of shock experiments performed at room temperature and of experiments where the sample was pre-heated by several hundred degrees [15], [29], [30], [31], [32], [33], [34], [35], [36] before it was subjected to shock pressures. The most important shock deformation effects in quartz include (i) planar deformation features (PDF) and mosaicism in quartz, which develop at pressures of 8–36 GPa; (ii) in addition, quartz with reduced refractive indices, density and birefringence that contains abundant PDF develops at pressures of 24–36 GPa; (iii) between 36 and glass; (iv) above 50 quartz transforms into a quasi-amorphous diaplectic glass (from greek diaplêsso = destroy by striking or beating [25]) with density and refractive indices intermediate between quartz and thermal quartz glass, and (iv) above 50 GPa quartz is melted (fused; formation of lechatelierite of n = 1.459) (e.g., [15], [33], [37]).

This study provides the first results of shock experiments with pre-cooled quartz targets, investigating the transformation of quartz to quartz with reduced refractive indices and density, and at higher shock pressure, to diaplectic quartz glass. Combining our experimental results with literature data for shock experiments at room temperature and with pre-heated quartz targets allows to constrain the influence of temperature on the shock effects in quartz single crystals.