Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufmmr53d..08d&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #MR53D-08
Other
3654 Ultra-High Pressure Metamorphism, 3919 Equations Of State, 3924 High-Pressure Behavior, 3944 Shock Wave Experiments, 6240 Meteorites And Tektites (1028, 3662)
Scientific paper
About 50 years ago, shock wave researchers were confounded by the apparent rapidity of shock-induced phase transformations and invoked special mechanisms as an explanation. Some workers speculatively interpret shock wave data as indicative of very rapid reconstructive phase transformations of minerals at modest temperatures. The belief that kinetic effects are negligible has justified the use of microsecond duration shock recovery experiments to interpret much longer duration shock effects in terrestrial impact craters and in meteorites. Here we summarize the extensive evidence that phase transformations under shock compression are governed by the same kinetics as phase transitions under static compression. Hugoniot measurements on quartz and feldspars have been interpreted as indicating phase transitions to dense phases at pressures near 35 GPa. Release adiabat measurements imply that these phases persist on release of pressure down to about 7 GPa, below which there is expansion to a final density appropriate to a glass. Microsecond-duration shock recovery experiments support this interpretation; a low density amorphous phase (diaplectic glass) is recovered. A similar effect is observed in static compression experiments conducted at room temperature at a lower pressure, 12-15 GPa. The pressure difference is a kinetic effect; the transition requires higher shock pressures (implying higher shock temperatures) to run to completion on a microsecond time scale. Direct evidence for kinetic effects on the tectosilicate transformation is found in studies of naturally shocked meteorites that contain melt veins. When the melt veins solidify under pressure, the resultant mineralogy together with vein cooling calculations constrains the shock pressure-time history. In some samples, in which the shock pressure is constrained by vein mineralogy to a range of 17-25 Gpa, one may observe the presence of diaplectic glass in only near-vein regions heated by conduction from the vein. In other samples, constrained to the same pressure range, long-duration (> 0.1 s) shock pressures are inferred from constant high-pressure mineralogy across wide (>1mm) veins and diaplectic glass is found throughout the meteorite. Evidence for kinetic effects may also be inferred from studies of samples from large impact craters. Pressure estimates based on shock recovery experiments imply post-shock temperatures that are too high for the survival of metastable high-pressure phases, such as stishovite, present in the samples. Release adiabat measurements on quartz also account for the presence of coesite in association with stishovite and diaplectic glass. Release wave velocitie slow below about 7 GPa, providing a relatively long period for the growth of coesite in its stability field from shock heated silica. Studies of high-pressure phases of olivine and pyroxene in meteorites are also concordant with both static high pressure data and inferences from Hugoniot and release measurements. These phases are not found in microsecond duration shock recovery experiments. Finally experiments on the shock synthesis of diamond are in complete accord with evidence for shock synthesized diamond in large impact craters and with static high pressure data on the direct uncatalyzed transition of graphite to diamond.
de Carli Paul S.
El Goresy Ahmed
Sharp Thomas G.
Xie Zhaohua
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