SHOCK METAMORPHISM

by: Charles O’Dale

1. Introduction;
2. Impactites;
3. Pressure/temperature conditions;
4. High pressure minerals;
5. Shock pressures;
6. Shock compression;
7. Impact melt;
8. Pseudotachylite;
9. Planar deformation features (pdf).

1. INTRODUCTION

Research has established that near the hypervelocity impact point, initial shock pressures can exceed 100 GPa, resulting in the total melting and vaporization of a large volume of target rock together with virtually all of the impactor. Passing outward, the lower shock pressures produce a series of distinctive effects in the target rocks (Koeberl, French, 2009):

The distinctive mark of an impact crater is the presence of rock that has undergone shock-metamorphic effects, such as shatter cones, melted rocks, and crystal deformations. The problem is that these materials tend to be deeply buried, at least for simple craters. They tend to be revealed in the uplifted center of a complex crater, however.

High-temperature rock types, including laminated and welded blocks of sand, spherulites and tektites, or glassy spatters of molten rock. The impact origin of tektites has been questioned by some researchers; they have observed some volcanic features in tektites not found in impactites. Tektites are also drier (contain less water) than typical impactites. While rocks melted by the impact resemble volcanic rocks, they incorporate unmelted fragments of bedrock, form unusually large and unbroken fields, and have a much more mixed chemical composition than volcanic materials spewed up from within the Earth. They also may have relatively large amounts of trace elements that are associated with meteorites, such as nickel, platinum, iridium, and cobalt. Note: it is reported in the scientific literature that some “shock” features, such as small shatter cones, which are often reported as being associated only with impact events, have been found in terrestrial volcanic ejecta.

(AstroNotes October 2010, March 2011

The burden of proof for an impact origin generally lies with the documentation of the occurrence of shock-metamorphic effects.

Impacts produce distinctive “shock-metamorphic” effects that are found in situ within the crater and allow impact sites to be distinctively identified. Such shock-metamorphic effects, in addition to the shatter cones and slickenslides, include brecciated rocks, suevites, impact melts and pseudotachylites. They attest to the destructive power of the impact event.

Schematic radial cross-section through one-half of a simple impact structure, showing locations of different impact-produced lithologies. Curved lines show isobars of shock pressures (in GPa) produced in the basement rocks by the impact.The rocks at an impact target site are melted, shattered, and mixed during the impact explosion. When the site finally settles and cools, a new composite rock, impact breccia in bodies tens to hundreds of meters in size, is the result.

Lithologies showing these unique diagnostic shock effects, formed at pressures ≥10 GPa, tend to be restricted to two locations:

• crater-fill materials (suevites, melt breccias, and fragmental impact breccias) deposited in the crater; and
• brecciated basement rocks, often containing shatter cones, near the center of the structure.

The magnitudes of the impact shock relative to the point of impact that form the shock metamorphic effects are quantified for reference:

^a Hypervelocity – 11.2 km/sec to 70 km/sec.

^b Crater: impactor size ratio ranges from 20:1 to 50:1 (Shoemaker 1963, Baldwin 1963).

^c The standard unit of pressure is the Pascal, abbreviated Pa, which is equivalent to 1 kilogram per square meter. A GPa is a gigapascal (giga means billion), a measurement of pressure, and is equal to 10,000 times the atmospheric pressure at the Earth’s surface.

2. IMPACTITES

Impactite is the term used for all rocks produced or affected by a hypervelocity impact event (a.k.a. instant rocks). Impactites range from completely reconstituted lithologies, such as impact melt rocks, to fractured target rocks. They generally, but not always, contain evidence of shock metamorphism.

3. PRESSURE-TEMPERATURE CONDITIONS

On earth the lowest impact velocity with an object from space is the gravitational escape velocity of 11 km / sec. The highest velocity impact is more than 70 km/s:
-earth escape velocity [11 km/s] + sun escape at the earth’s orbit [30 km/s] +
-earth’s orbit velocity [30 km/s] = ~70 + km/sec.
The average earth impact velocity is 17 km/s.
At these speeds, impacts produce shock waves in solid materials. Both meteorite and target rocks are compressed to high density, then rapidly depressurized and exploding violently. Since craters are caused by explosion they are always circular with only very low angle impacts causing elliptical craters.

Unique deformation effects occurred as changes in minerals such as mineral deformations and melting under the extreme high pressure and temperature (e.g., the shock pressure may be than 60 GPa and post-shock temperature may be 2000◦C). Quartz begins to convert to coesite (another polymorph or atomic-structural form of silica) at about 20 kilobars (that pressure is reached in the solid Earth at about a depth of 70 km (43 miles). Solids begin to convert to glass at 400 kb, to melt at ~500 kb, and to vapor (gas) at a megabar. Thus, the range of shock metamorphism is from 0.02 to 1 megabar – these pressures are known to occur in the Earth only in its mantle and core but the rock types affected by shock metamorphism are dominantly those of the crust where the natural pressure gradient achieves values less than those affecting shocked rocks.

• rock melting (≥60 GPa);
• selective mineral melting (40–60 GPa);
• diaplectic glass phases (30–45 GPa);
• high-pressure minerals – coesite and stishovite (12–30 GPa),
• planar deformation features (PDFs) in quartz (10–25 GPa);
• multiple fracturing (cleavage) and basal Brazil twinning in quartz (5–10 GPa);
• rock fracturing (2–5 GPa);
• shatter cones (≥2-30 GPa);
• slickensides (frictional faulting).

Metastable preservation of coesite and stishovite requires rapid cooling prior to amorphization. Stishovite is unstable above about 300-600°C, whereas coesite is stable up to about 1100°C, suggesting that the quartz grains studied here were quenched at relatively high postshock temperatures exceeding the stability range of stishovite, but within the stability range facilitating preservation of coesite.

Shock Metamorphism of Terrestrial Impact Structures and its Application in the Earth and Planetary Sciences[A. Gucsik (ed.), Cathodoluminescence and its Application in the Planetary Sciences, 23 c Springer-Verlag Berlin Heidelberg 2009]

Unique deformation effects occurred as changes in minerals such as mineral deformations and melting under the extreme high pressure and temperature (e.g., the shock pressure may be than 60 GPa and post-shock temperature may be 2000◦C). Quartz begins to convert to coesite (another polymorph or atomic-structural form of silica) at about 20 kilobars (that pressure is reached in the solid Earth at about a depth of 70 km (43 miles). Solids begin to convert to glass at 400 kb, to melt at ~500 kb, and to vapor (gas) at a megabar. Thus, the range of shock metamorphism is from 0.02 to 1 megabar – these pressures are known to occur in the Earth only in its mantle and core but the rock types affected by shock metamorphism are dominantly those of the crust where the natural pressure gradient achieves values less than those affecting shocked rocks.

Metastable preservation of coesite and stishovite requires rapid cooling prior to amorphization. Stishovite is unstable above about 300-600°C, whereas coesite is stable up to about 1100°C, suggesting that the quartz grains studied here were quenched at relatively high postshock temperatures exceeding the stability range of stishovite, but within the stability range facilitating preservation of coesite.

Shock Metamorphism of Terrestrial Impact Structures and its Application in the Earth and Planetary Sciences[A. Gucsik (ed.), Cathodoluminescence and its Application in the Planetary Sciences, 23 c Springer-Verlag Berlin Heidelberg 2009]

4. HIGH PRESSURE MINERALS

• Coesite^a,  SiO₂  – is a high-pressure polymorph of quartz (SiO₂)  that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (700 °C, 1,300 °F), are applied to quartz.

High pressure destructs the crystal lattice characteristic of quartz and compresses the silicon and oxygen atoms into an amorphous system. The result is high-density glass. Once the pressure has surpassed a certain threshold, the amorphization process becomes irreversible and the material can no longer return to a crystalline configuration.

Coesite has two morphologies: fine grade needle-like crystals or as greenish aggregates (a.k.a. “granular coesite”).

In 1960, coesite was found by Edward C. T. Chao, in collaboration with Eugene Shoemaker, to naturally occur in the Barringer Crater. This was evidence that the crater must have been formed by an impact.

Coesite from the Wanapitei Impact structure, Dence 1974.

• Magnesioferrite, MgFe₂O₄ A rare spinel mineral crystallizes as black metallic octahedral crystals and has been documented in the recrystallized impact breccias of the Steen River Impact Structure (SRIS).  It is named after its chemical composition of magnesium and ferric iron. The density is 4.6 – 4.7 (average = 4.65), and the diaphaniety is opaque.
• Majorite, Mg₃(MgSi)(SiO₄)₃ A type of garnet mineral found in the upper mantle of the Earth. It is distinguished from other garnets in having Si in octahedral as well as tetrahedral coordination. Majorite was first described in 1970 from the Coorara Meteorite of Western Australia and has been reported from various other meteorites in which majorite is thought to result from an extraterrestrial high pressure shock event. Mantle derived xenoliths containing majorite have been reported from potassic ultramafic magmas on Malaita Island on the Ontong Java Plateau Southwest Pacific.
• Maskelynite^a A clear, glassy pseudomorph of plagioclase produced by a relatively low pressure (250-300 kilobars) and low temperature (350°C) shock wave. It is found in the rocks of the central peaks of Clearwater West and Manicouagan craters, Quebec, Canada. Heated maskelynite reverts to crystalline plagioclase, indicating only slight structural disordering unlike fused plagioclase glass.
• Reidite, ZrSiO⁴ A high-pressure polymorph of zirconium orthosilicate that forms >30 GPa and is an important accessory mineral in studies of shock metamorphism as its formation conditions have been experimentally constrained. Naturally occurring reidite is rare. Laboratory and theoretical results show how zircon transforms into reidite and then back into zircon.^b
• Ringwoodite, Mg₂SiO₄  Formed at high temperatures and pressures of the Earth’s mantle between 525 and 660 km (326 and 410 mi) depth. It is polymorphous with the olivine phase forsterite (a magnesium iron silicate).
• Selenite, CaSO⁴.H²O  (hydrothermal) The colorless and transparent variety of gypsum (calcium sulfate: ) that shows a pearl like luster and has been described as having a moon-like glow. The word selenite comes from the greek word for Moon and means moon rock. Gypsum is one of the more common minerals in formed sedimentary environments, such as tropical seas.

  

  The heat source at Haughton were the pale gray impact melt breccias which were originally at temperatures of >1000°C. As groundwater and rainwater came into contact with these hot rocks, these fluids were heated and circulated through the crater. Some of the target rocks at Haughton contained sedimentary gypsum, which was dissolved by these hot hydrothermal fluids. These fluids then migrated through the crater and re-deposited gypsum or selenite within cavities in the impact melt breccias as they cooled.

• Stishovite^c, SiO₂ An extremely hard, dense tetragonal form (polymorph) of silicon dioxide found as microscopic grains in impact craters and in ultra-high pressure rocks.. It is very rare on the Earth’s surface, however, it may be a predominant form of silicon dioxide in the Earth, especially in the lower mantle.  
• Wadsleyite, β-Mg₂SiO₄ A high-pressure phase of polymorphous Mg₂SiO₄. An orthorhombic mineral. It was first found in nature in the Peace River meteorite from Alberta, Canada.

^a   NATURAL TERRESTRIAL MASKELYNITE, Dence, et al THE AMERICAN MINERALOGIST 1967
^b  Structure and stability of ZrSiO⁴ under hydrostatic pressure, Marqués, et al APS PHYSICS  2006
^c   Metastable preservation of coesite and stishovite requires rapid cooling prior to amorphization. Stishovite is unstable above about 300-600°C, whereas coesite is stable up to about 1100°C, suggesting that the quartz grains studied at the Chesapeake Bay impact crater were quenched at relatively high postshock temperatures exceeding the stability range of stishovite, but within the stability range facilitating preservation of coesite.

5. SHOCK PRESSURES

6 . SHOCK COMPRESSION

Most of the structural and phase changes in minerals and rocks are uniquely characteristic of the high pressures (diagnostic shock effects are known for the range from 8 to >50 GPa) and extreme strain rates (up to 108 /s) (for comparison: a bat hitting a baseball generates a strain rate of ~102/s) associated with impact. The products of static compression, as well as those of volcanic or tectonic processes, differ from those of shock metamorphism, because of peak pressures and strain rates that are lower by many orders of magnitude.

Is an informal term describing a rock created or modified by the impact of a meteorite. The term encompasses shock-metamorphosed target rocks, melts, breccias, suevites and mixtures, as well as sedimentary rocks with significant impact-derived components (shocked mineral grains, tektites, anomalous geochemical signatures, etc).

 

COMMINUTION – the reduction of solid materials from one average particle size to a smaller average particle size, by crushing, grinding, cutting, vibrating, or other processes. In geology, it occurs naturally during faulting in the upper part of the Earth’s crust.

Shock metamorphism of the Coconino Sandstone at Meteor Crater, Arizona
Susan Werner Kieffer
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 SiO₂ 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 SiO₂ during passage of the shock.

Journal of Geophysical Research 10 August 1971

SHOCKED GNEISS

Gneiss (pronounced “nice”) is normally a dark dense rock, but at Haughton, the gneiss resembles pumice stone – it is ash-white, porous and very lightweight. In fact, some of these fragments float in water! The reason why this gneiss is so light is due to the air spaces or bubbles, which formed as the gneiss was compressed by the shock wave, and then released. Certain minerals in the rock were also vaporized, leaving behind a porous ghost of the gneiss it originally was.

 

7. IMPACT MELT

Rock that has been made temporarily molten as a result of the energy released by the impact of a large colliding body. Impact melts include small particles, known as impact melt spherules, that are splashed out of the impact crater, and larger pools and sheets of melt that coalesce in low areas within the crater. They are composed predominantly of the target rocks, but can contain a small but measurable amount of the impactor.

Shock pressures in excess of roughly 60 GPa (600 kbar) are required for total rock melts. Impact melts are extremely uniform in their composition but highly variable in texture. They are composed predominantly of the target rocks but may contain a small but measurable amount of the impactor.

Impact melt rocks are basically volcanic rocks, such as basalt lava, and they attest to the extreme conditions generated by the impact event. Pressures and temperatures in the target rocks surrounding the point where the asteroid or comet hits are so high that large volumes or rock can be instantaneously melted. Pieces of this melt can cool rapidly to form glass and be incorporated in suevites. However, sometimes, so much melt is produced that it forms a pool in the central parts of an impact crater to form crater-fill deposits. This pool of melt then cools slowly over time and solidifies to form a new rock, which we term impact melt rock. An impact melt rock contains only a few fragments of target rock, maybe up to ~25% fragments in extreme cases. If the melt contains a lot of fragments of target rock, then we term this an “impact melt breccia”. Impact melt rocks can be found in crater-fill deposits  and in ejecta deposits.

Impact melt found in the Charlevoix impact structure. Note the country rock fragment in the inclusion.

Melt rocks in impact structures may also result from frictional melting in strong dynamic metamorphism a pseudotachylite.

8. PSEUDOTACHYLITE

Pseudotachylite is a fault rock that has the appearance of the basaltic glass, tachylyte. It is dark in color and has a glassy appearance. However, the glass has normally been completely devitrified into very fine-grained material with radial and concentric clusters of crystals. It may contain clasts of the country rock and occasionally crystals with quench textures that began to crystallize from the melt. It is formed when a high pressure from an impact is applied to country rocks and then abruptly released. This causes the rock along and within fracture lines or faults to partly melt. The fractures or faults containing the pseudotachylite are welded shut as soon as the motion created by the impact stops.

The entire period of activity of a fracture or fault filled with pseudotachylite may be measured in minutes.

A tachylite is a black volcanic glass formed by the chilling of basaltic magmas.

9. PLANAR DEFORMATION FEATURES (pdf)

The passage of the shock wave through the rock changes the structure of some of the enclosed minerals. IE: change is possible in the feldspar mineral plagioclase. The shock wave can break down the structure of the mineral, changing parts of it into a diapletic glass (glass formed at high-pressure in the solid-state) which is isotropic, or uniform in all directions.

Upon bolide impact, the passage of the resultant shock wave through the rock changes the structure of some of the enclosed minerals.

Planar deformation features (PDFs) are not cracks in quartz, but are “… multiple sets of closed, extremely narrow, parallel planar regions …” that are typically less than 2-3 μm wide and spaced around 2-10 μm apart (French, 1998: 42).

Planar deformation features, or PDFs, are optically recognizable microscopic features in grains of silicate minerals (usually quartz or feldspar), consisting of very narrow planes of glassy material arranged in parallel sets that have distinct orientations with respect to the grain’s crystal structure.

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