The first proposal linking asteroid impact to
a terrestrial crater was made in 1906, when D.M. Barringer argued that the simple crater in Arizona now named for him was produced by the high-speed impact of a large meteorite.10 Fragments of meteoritic iron—a nickel-iron alloy containing rare metals in concentrations unlike those in any terrestrial rock—had been discov- ered on the rim of the crater. Barringer was con- vinced that a large meteorite was buried below the crater floor, and he started a mining company to drill holes in search of the iron mass. His claim was a contentious one, in part because he was never able to uncover the meteorite, which had vaporized upon contact. His opponents argued into the 1950s that the crater was caused by vol- canism or collapse. The discovery of pulverized deposits and ultrahigh-pressure mineral phases helped convince geologists and astronomers that the Barringer crater and many others are scars of asteroid impact. Because in most cases the asteroid is
destroyed upon impact, remnants that prove impact origin are hard to find. Erosion and burial further complicate the situation. Therefore, sci- entists have developed diagnostic criteria for identifying and confirming impact structures on Earth. In the absence of the extraterrestrial projectile or geochemical evidence thereof, the following characteristics are deemed most impor- tant for confirming asteroid impact: evidence of shock metamorphism, crater morphology and geophysical anomalies. Of these three, only diag- nostic shock-metamorphic effects can provide unambiguous evidence of impact origin. The impact shock wave causes compression of
target rocks at pressures far exceeding the Hugoniot elastic limit (HEL)—the maximum stress a material will reach without permanent distortion.11
3,000
Vaporization
2,000
Zircon decomposes Quartz melts
Sphene melts 1,000 0
Zeolite Hornfels Sanidinite
0.1
Granulite Amphibolite Greenschist Glaucophane Schist
0.5 1 Eclogite
Shatter cones
5 Pressure, GPa
> Pressures and temperatures of impact-related mineral changes. Conditions that cause ordinary subsurface metamorphism are shaded in blue. High-velocity impact, typically generating pressures greater than 5 GPa, causes phase transformations—such as the change from quartz polymorph coesite to stishovite—and shock metamorphism, which is characterized by planar deformation features, diaplectic glass, shock melting and vaporization.
deformation features
10 50 100 Planar
Melting
Diaplectic glass
except in impact structures. Another phase change that can occur at impact pressures is from graphite to diamond. High-pressure phase changes typically involve closer packing of the mineral’s constituent molecules, resulting in a high-density version of the mineral. These changes can be detected by optical and scanning electron microscopy, X-ray diffraction and nuclear magnetic resonance measurements.
Structural changes can occur on macroscopic
and microscopic scales. A macroscopic indicator of impact shock is the occurrence of shatter cones. These structures, which are cones with regular thin grooves that radiate from the apex, develop better in some lithologies than in others. They form at pressures from 2 to 30 GPa [290,000 to 4,400,000 psi] and range in size from milli- meters to meters (below).
The HEL for most minerals and rocks
is 5 to 10 GPa [725,000 to 1,450,000 psi]. The only natural process on Earth known to produce shock pressures exceeding these levels is asteroid impact; static pressures involved in deep meta- morphism may approach 5 GPa, and volcanic activity does not exceed pressures of 1 GPa [145,000 psi]. At impact pressures, two types of shock meta-
morphism can occur: phase changes and struc- tural changes (above right). In a phase change, a mineral transforms from one phase to another. For example, quartz undergoes solid-state phase transitions, forming coesite and then stishovite at even higher pressures. The stishovite poly- morph of quartz has never been found in nature
4 cm
> Shatter cone in limestone. This shatter cone was retrieved from the Steinheim crater, in Germany. The crater was formed about 15 million years ago and is still visible on the Earth’s surface. (Photograph courtesy of Christian Koeberl.)
Oilfield Review Autumn 09 Impact Fig. 5
ORAUT09-Impact Fig. 5
Winter 2009/2010
19
Temperature, °C
Coesite Stishovite
Quartz Coesite
Graphite Diamond
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