Shock metamorphism of the Coconino Sandstone at Meteor Crater, Arizona

Abstract

A study of the shocked Coconino sandstone from Meteor Crater, Arizona, was undertaken to examine the role of porosity in the compression of rocks and in the formation of highpressure phases. A suite of shocked Coconino specimens collected at the crater is divided into five classes, arranged in order of decreasing quartz content. The amounts of coesite, stishovite (measured by quantitative X‐ray diffraction), and glass vary systematically with decreasing quartz content. Coesite may comprise 1/3 by weight of some rocks, whereas the stishovite content does not exceed 1%. The five classes of rocks have distinct petrographic properties, correlated with the presence of regions containing coesite, stishovite, or fused silica. Very few occurrences of diaplectic glass are observed. In the lowest stages of shock metamorphism (class 1), the quartz grains are fractured, and the voids in the rock are filled with myriads of small chips derived from neighboring grains. The fracture patterns in the individual quartz grains are controlled by the details of the initial morphology of the colliding grains. In one weakly shocked rock, it was possible to map the general direction of shock passage by recording the apparent direction of collision of individual grains. The principal mechanism of energy deposition by a shock wave in a porous material is the reverberation of shock and rarefaction waves through grains due to collisions with other grains. A one‐dimensional model of the impact process can predict the average pressure, volume, and temperature of the rock if no phase changes occur but cannot predict the observed nonuniformity of energy deposition. In all rocks shocked to higher pressure than was necessary to close the voids, high‐pressure and/or high‐temperature phases are present. Locally high pressures enduring for microseconds and high temperatures enduring for milliseconds controlled the phases of SiO_2 that formed in the rock. Collapsing pore walls became local hot spots into which initial deposition of energy was focused. Microcrystalline coesite in class 2 rocks occurs in symplektic regions on quartz grain boundaries that were regions of initial stress and energy concentration, or in sheared zones within the grains. The occurrence and morphology of the coesite‐rich regions can be explained only if the transformation from quartz to coesite proceeds slowly in the shock wave. In class 3 rocks, microcrystalline coesite occurs in opaque regions that surround nearly isotropic cores of cryptocrystalline coesite. The cores are interpreted to be the products of the inversion of stishovite (or a glass with Si in sixfold coordination) that initially formed in the shock front in regions of grains shocked to pressures near 300 kb. Stishovite is preserved only in the opaque regions, which are believed to have been cooler than the cores. In class 4 rocks vesicular glass occurs in core regions surrounded by opaque regions containing coesite. The relation of the glass to the coesite and quartz suggests that the glass was formed by inversion of stishovite formed above 350 kb on release to lower pressure. Class 5 rocks are composed almost entirely of glass, with vesicles uniformly distributed in the glass. These vesicles were probably formed by exsolution of water that had been dissolved in melted SiO_2 during passage of the shock.