HP-HT EXCURSIONS AND FALSE BULK SHOCK READINGS: CONSTRAINTS ON ATTENUATION RATE

Many meteorites contain shock veins and melt pockets that exhibit evidence for extreme excursions in pressure and temperature. They are also referred to as hot spots. They are characterized by the development of HP-HT polymorphs, which are otherwise absent from the bulk of the rock (i.e., beyond shock veins and melt pockets). Shock veins are typically <2 mm wide, while melt pockets can be several mm in diameter. Shock veins have also been described from a few terrestrial impact craters, where their spatial context provides valuable insight into formation mechanisms. Two notable examples are Vredefort in South Africa, and Manicouagan in Canada. The presence of HP-HT polymorphs can be incorrectly interpreted to indicate bulk shock conditions, whereupon the shock pressure is overestimated. Given this insight, it is important to evaluate the shock state of minerals beyond the localized excursion effects. In the case of Manicouagan, the development of stishovite and maskelynite in the anorthositic lithologies of the central uplift indicate pressures of 12-22 GPa, but the bulk shock is <10 GPa and probably 5 GPa, based on the occurrence of shatter cones and some PDFs. A bulk pressure of 5-10 GPa is more compatible with shock loading at a depth of approximately 8 km, from which the uplift was elevated during the late excavation and modification stages of the impact process. Excursion conditions are realized upon the shock front encountering discontinuities, such as grain boundaries, different mineral species, pore space, fractures, microfaults and other heterogeneities. Solid-state phase transformations to denser states can cause secondary shock effects via implosion, which can further enhance excursion conditions on the grain scale. This presentation will discuss methods for hot spot recognition in planetary materials using terrestrial, lunar, martian and asteroid examples, with suggestions for obtaining realistic bulk shock determinations and accompanying shock attenuation rates.