EXPERIMENTAL CONSTRAINTS ON SHOCK-INDUCED MICROSTRUCTURES IN NATURALLY DEFORMED SILICATES

Planar deformation features (PDFs) in various minerals have long been accepted as evidence of impact-induced deformation. The uniqueness of this association was challenged in the context of the K/T Boundary ext inction debate, after mosaicism and microstructures similar to PDFs we re reported from the products of explosive volcanism. As a result of t his debate, a significant volume of new experimental and observational data on the development of shock-induced microstructures has become a vailable over the last ten years. The results reveal that factors such as pre-shock temperature, pulse duration, and crystallographic orient ation of target minerals to the shock wave have a primary influence on how these microstructures develop. Data from diamond anvil cell and h igh-pressure friction experiments reveal that the same solid-state amo rphization process that produces shock-induced PDFs at low temperature s also occurs at much lower strain rates in static experiments. The ex perimental data indicate that the amorphization process is thermally a ctivated and that the character of the resulting PDFs is a function of the applied strain rate, Shock-induced amorphization occurs along tho se crystallographic planes that are most readily transformed to the hi gh-pressure phase during very short pulse durations and produces PDFs that are visible at the optical scale, Lower strain rate deformation p roduces TEM scale amorphization with orientations that are more homoge neously distributed throughout the target mineral and produces no opti cally visible PDFs. The data confirm the uniqueness of multiple inters ecting sets of optically visible PDFs as a diagnostic indicator of hyp ervelocity impact. The data also support the hypothesis that the amorp hization process can occur at a wide range of strain rates, and that t he limiting pressure for the process is controlled by the phase stabil ity of the target mineral under the applied loading conditions, not by the HEL. The data also suggest that the onset pressure, the maximum p ressure, and the pressure range for producing optically visible PDFs d ecrease with increasing temperature and decreasing strain rate. As the pressure range for optically visible PDF formation decreases, it is r eplaced by homogeneous amorphization as observed in the anvil cell exp eriments, Thus, the uniqueness of multiple intersecting sets of optica lly visible PDFs to hypervelocity impact is not due to a unique proces s, but rather to a specific set of loading conditions that produce an optically visible microstructure, Likewise, single sets of microdeform ations observed in volcanic rocks are produced by the same process, bu t at different loading conditions that preclude the development of mul tiple intersecting sets of PDFs. The data also indicate that shock mos aicism, which occurs above the HEL, represents a plastic response of t he target mineral to loading rates that are too large to be accommodat ed by crystal plastic mechanisms. Observational data for some naturall y deformed samples from the K/T Boundary, the Vredefort Dome, and volc anic rocks, along with the experimental observations. are used to cons train the range of conditions under which the natural microstructures form, and to understand the differences between microstructures produc ed by impact, volcanic, and other natural processes. Finally, some pos sible mechanisms for producing the microstructures observed in volcani c rocks are proposed.