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Vilen Fel'dman, Moscow State University (Russian Federation)
Lyudmila Sazonova, Moscow State University (Russian Federation)
Eugen Kozlov, Institute of Thechnical Physic (Russian Federation)
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It is now known that the material of impact crater can contain high-pressure polymorphmodifications of four minerals: coesite and stishovite (SiO2), diamond and lonsdaleite (C), majorite (MgSiO3) and ringwoodite (Mg2SiO4). All of these minerals are known in meteorites and have been synthesized in laboratory experiments on the shock wave loading of rocks.
The analysis of conditions under wich high-density phases occur in astroblemes (Fel'dman, 1990) indicates that the mechanisms that produced these phases can likely be grouped within the scope of following three major variants: (i) crystallizacion from an impact melt, (ii) martensite phase transition, and (iii) migration phase generation (recrystallization couplend with the migrations of chemical components during the solid-state stage of shock metamorphism). The crystallization from impact melt was described in application to all four of the aforementionedhigh-pressure minerals both in nature and laboratory experiments; martensite phase transitions are known to occur in stishovite and lonsdaleite (also in nature and experiments) and in diamond (only in experiments); and migrations were identified in soesite (in nature and experiments), diamond (togorite, in nature), and ringwoodite and the high-pressure phase of pyroxen cjmposition (in experiments).
High-pressure polymorph of MgSiO3 and Mg2SiO4 were first obtained in our laboratory experiments on the loading of rocks by spherical convergent shock waves under pressures of 30-70 GPa (Kozlov et al., 2002, 2003, 2004; Sazonova et al., 2006; Fel'dman et al., 2006, 2007). In these experiments both aforementioned high-pressure minerals were generated by shock-wave decomposition of biotite, garnet and hornblend. This transformation is associated with the active migrations of kations both from and into the crystal. Such migrations is clearly controlled by the characteristics of crystalline lattice of minerals: amorphization is typical for minerals with ring and framework structures, and shock-thermal decomposition affects with layer, band and other structures (Fel'dman et al., 2003). The minimum values of the shock pressure at wich the shock-wave decomposition of a mineral starts with the origin of high-density phases depend on the type of the crystal structure of precursor mineral. It was experimentally confirmed that the high-density phases are formed by the shock-thermal decomposition of rock-forming minerals depending on the contents of the precursor minerals in the rock, because the shock wave energy is distributed between minerals composing a rock proportionally to the contents of these minerals.
Our results obtained on minerals discussed above let us to conclude that these minerals can occur in nature only in rocks that cooled in the regime of quenching. Yigh-density phases occurring in a naturely impactites can hardly be utilized as geobarometers of shock metamorphism.
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