Talk:Shocked Quartz

Shocked Quartz

Shocked quartz is a form of quartz that has a microscopic structure that is different from normal quartz. Under intense pressure (but limited temperature), the crystalline structure of quartz will be deformed along planes inside the crystal. These planes show up as lines under a microscope, which are called shock lamellae.

Discovery Shocked quartz was discovered after underground nuclear bomb testing, which caused the intense pressures required to form shocked quartz. Eugene Shoemaker showed that shocked quartz also is found inside of craters, such as the Barringer Crater. The presence of shocked quartz proved that these craters were formed by an impact: a volcano would not generate the pressure required.

Prevalence Shocked quartz is also found worldwide, in a thin layer at the boundary between Cretaceous and Tertiary rocks. This is further evidence (in addition to iridium enrichment) that the transition between the two geological eras was caused by a large impact.

Structure Shocked quartz is associated with two pressure polymorphs of silicon dioxide: coesite and stishovite. These polymorphs have a different molecular structure than standard quartz. Again, this structure can only be formed by intense pressure, but moderate temperatures. High temperatures would anneal the quartz back to its standard form. Coesite and stishovite are thus also indicative of impact (or nuclear explosion).

Shocked Minerals Recent discussion on the characteristics and origin of shock features can be found in French [1990]; Grieve et al. [1990]; and Stöffler and Langenhorst [1994]. Shock deformation features were first observed in quartz grains from the Clearwater Lake impact structure in Quebec [ McIntyre, 1962]. Since then these structural defects have been abundantly documented in a variety of minerals from many impact structures around the world. Often found in association with typical features such as shatter cones, extensive grain fracturing, shock mosaics, high pressure polymorphs of SiO (coesite and stishovite) and diaplectic glass lamellae, these features are considered to be characteristic of shock metamorphism caused by meteoritic impact [ Grieve et al., 1990]. Similar features occur in minerals at nuclear explosion sites or when material is experimentally shocked to pressures above  5 Gigapascals. The types of shock features and their associations allow estimates of the pressure that was applied to the grain.

Shocked minerals are now commonly found in association with the KT boundary event. Shocked quartz are abundant within the KT boundary clay layer in the Western Interior from Alberta to New Mexico [ Bohor, 1990; Izett, 1990], in a number of deep-sea cores in the Pacific Ocean [ Kyte et al., 1994] and in the polymict breccia from the Chicxulub structure in Yucatan [ Hildebrand et al., 1991; Schuraytz et al., 1994]. At other KT sites shocked quartz appears much less common. Shock features in zircons also provide interesting clues to the KT impact debate. Bohor et al. [1993] proposed that shocked features in zircons may be related to increasing degrees of shock metamorphism and correlate with proportionate resetting of the U-Pb isotopic system. Krogh et al. [1993a, 1993b] dated single zircons from the KT boundary sections in the US Western Interior, Haiti and from the impact breccia at Chicxulub. They determined the original U-Pb age of the parent material, to be around 550 Ma and that U-Pb isotopic resetting consistent with partial Pb loss took place around 65 Myr, which is consistent with the age of the KT boundary impact. The Pan-African (_ 550 Ma) age for the target rock agrees well with the basement age at Chicxulub in Yucatan but is incompatible with the mid-Proterozoic age (_ 2200 Ma) of the target beneath the Manson structure in Iowa. Shocked zircons are therefore one of the key arguments used to rule out the Manson crater as the source of the KT boundary ejecta.