SHOCK METAMORPHISM OF QUARTZ IN NATURE AND EXPERIMENT .2. SIGNIFICANCE IN GEOSCIENCE

The occurrence of shock metamorphosed quartz is the most common petrog raphic criterion for the identification of terrestrial impact structur es and lithologies. Its utility is due to its almost ubiquitous occurr ence in terrestrial rocks, its overall stability and the fact that a v ariety of shock metamorphic effects, occurring over a range of shock p ressures, have been well documented. These shock effects have been gen erally duplicated in shock recovery experiments and, thus, serve as sh ock pressure barometers. After reviewing the general character of shoc k effects in quartz, the differences between experimental and natural shock events and their potential effects on the shock metamorphism of quartz are explored. The short pulse lengths in experiments may accoun t for the difficulty in synthesizing the high-pressure polymorphs, coe site and stishovite, compared to natural occurrences. In addition, pos t-shock thermal effects are possible in natural events, which can affe ct shock altered physical properties, such as refractive index, and ca use annealing of shock damage and recrystallization. The orientations of planar microstructures, however, are unaffected by post-impact ther mal events, except if quartz is recrystallized, and provide the best n atural shock barometer in terms of utility and occurrence. The nature of planar microstructures, particularly planar deformation features (P DFs), is discussed in some detail and a scheme of variations in orient ations with shock pressure is provided. The effect of post-impact even ts on PDFs is generally limited to annealing of the original glass lam ellae to produce decorated PDFs, resulting from the exsolution of diss olved water during recrystallization. Basal (0001) PDFs differ from ot her PDF orientations in that they are multiple, mechanical Brazil twin s, which are difficult to detect if not partially annealed and decorat ed. The occurrence and significance of shock metamorphosed quartz and its other phases (namely, coesite, stishovite, diaplectic glass and le chatelierite) are discussed for terrestrial impact structures in both crystalline (non-porous) and sedimentary (porous) targets. The bulk of past studies have dealt with crystalline targets, where variations in recorded shock pressure in quartz have been used to constrain aspects of the cratering process and to estimate crater dimensions at eroded structures. In sedimentary targets, the effect of pore space results i n an inhomogeneous distribution in recorded shock pressure and tempera ture, which requires a different classification scheme for the variati on of recorded shock compared to that in crystalline targets. This is discussed, along with examples of variations in the relative abundance s of planar microstructures and their orientations, which are attribut ed to textural variations in sedimentary target rocks. Examples of the shock metamorphism of quartz in distal ejecta, such as at the K/T bou ndary, and from nuclear explosions are illustrated and are equivalent to that of known impact structures, except with respect to characteris tics that are due to long-term, post-shock thermal effects. Finally, t he differences between the deformation and phase transformation of qua rtz by shock and by endogenic, tectonic and volcanic processes are dis cussed. We confirm previous conclusions that they are completely dissi milar in character, due to the vastly different physical conditions an d time scales typical for shock events, compared to tectonic and volca nic events. Well-characterized and documented shock effects in quartz are unequivocal indicators of impact in the natural environment.