The microstructural response of quartz and feldspar under shock loading at variable temperatures

Abstract

Shock recovery experiments were carried out on Westerly granite and Hospital Hill quartzite targets in the peak pressure range 8 to 25 GPa, preshock temperatures of 25 ø, 450 ø, and 750øC and pulse durations of 2 to 7 gs using internally heated momentum traps and explosive plane wave generators. Optical and transmission electron microscopy analyses of quartz and feldspar shocked at 25øC revealed the previously documented progression, with increasing pressure: (1) fracturing; (2) planar fractures and shock mosaicism; (3) shock mosaicism and planar deformation features (PDFs); and (4) isotropization. This same sequence is observed for experiments at elevated preshock temperature but with specific microstructures occurring at lower pressures than those in previous experiments at room temperature. At 750øC, strong shock mosaicism, partially thermally recovered, is characteristic of feldspar shocked to 8 GPa, whereas 15 GPa is required for its development in quartz and for the generation of PDFs in both minerals. The results suggest that threshold pressures for formation of the various microstructures and phases are expected to vary systematically as a function of the preshock temperature of the target material. We suggest that PDFs are generated in the shock transition by progressive, heterogeneous, phase transformation of the crystal structure to form dense glass or high pressure polymorphs. The onset pressures for PDFs in specific crystallographic orientations is not influenced strongly by temperature, but the character of the PDFs does change as preshock temperature is increased at the same peak shock stress. The change from multiple sets of thin PDFs at low temperature to thicker single sets of PDFs at moderate temperature, and finally to complete isotropization at high tempera tures, reflects a change in the phase transformation mechanism as a function of temperature. In contrast, development of shock mosaicism in quartz and feldspar occurs throughout the duration of shock loading and is better developed at elevated temperatures where the kinetics are enhanced by the additional thermal energy in the target. crostructures in known volcanic rocks, diatremes, and landslides [Carter et al., 1990; Officer and Carter, 1991] and the apparent differences between these features and classic impactinduced microstructures [Alexopoulos eta/., 1988] indicates that the endogenous microstructures, if of a legitimate shock origin, must be generated in an environment that is quite different from impact. Microstructural data from recent shock recovery experiments on Hospital Hill quartzite and Westerly granite [Reimold and Hiirz, 1986a, b; Huffman et al., 1990] and single crystal quartz [Langenhorst et al., 1992; Gratz et al., ] indicate that preshock temperature may be of fundamental importance to the character of high-strain-rate microstructures at specific peak shock pressures. These observations then raise the question as to what other parameters might influence microstructural development in the dynamic range. Temperature, st...

Scanning electron microscopy, cathodoluminescence, and Raman spectroscopy of experimentally shock‐metamorphosed quartzite

“…This particular rock type was chosen because it occurs, naturally shock deformed, in the central uplift of the Vredefort impact structure, the Vredefort dome. The samples used in this study and previously described by Reimold and Hörz (1986), Huffman et al (1993), and Huffman and Reimold (1996) show virtually no accessory phases (sericite <1 vol%); evidence of tectonic deformation was restricted to occasional weak subgrain development and rare tectonic lamellae. The samples were cut into plates of about 1 mm thickness and embedded in densitymatching epoxy material.…”

“…These include the investigation of the Raman properties of coesite from the Vredefort dome, South Africa (Halvorson and McHone 1992), coesite in suevite from the Chicxulub impact structure, Mexico Ostroumov et al 2002), natural and synthetic coesite (Boyer et al 1985), the coesite-stishovite transition (e.g., Serghiou et al 1995), the densification behavior of silica glasses in shock experiments (Okuno et al 1999), and tektites and impact glasses (Faulques et al 2001) The pre-shock temperature of the target rock also influences the formation and crystallographic orientation of PDFs. Reimold (1988), Huffman et al (1993), and Huffman and Reimold (1996) presented the results of shock experiments with quartzite and granite at room temperature (25°C) and preheated to 450°C and 750°C. They noticed a distinct difference in the relative abundances of the {1013} and {1012} orientations and a large difference in the number of sets of PDFs per grain.…”

Shock experiments with the H6 chondrite Kernouvé: Pressure calibration of microscopic shock effects

“…and rocks are probably due to the major differences in the initial Note that the experiments employed naturally shock-processed propcrtics of Kernouvd and the analogue materials. Shock experiments with hcatcd samples were reported previously for quartz (Langenhorst, 1093;Huffman et al, 1993;Langenhorst and Dcutsch, 1994) and plagioclase (I Iuffman et al., 1993). For the samples preheated to 920 K (high-temperature experiments), the olivine, orthopyroxene, and oligoclase in Kernouve display the same pressure-dependent sequence of shock features as in the low-temperature experiments.…”

The Micromechanics of Westerley Granite at Large Compressive Loads

“…The grain size is uniform with a mean diameter of about 300 lm and a very low porosity ((1%) (BRACE, 1965;HUFFMAN et al, 1993). The Young's modulus is E o ¼ 70 GPa and Poisson's ratio is m = 0.25.…”