Experimental constraints on shock-induced microstructures in naturally deformed silicates

Abstract

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 extinction debate, after mosaicism and microstructures similar to PDFs were reported from the products of explosive volcanism. As a result of this debate, a significant volume of new experimental and observational data on the development of shock-induced microstructures has become available over the last ten years. The results reveal that factors such as pre-shock temperature, pulse duration, and crystallographic orientation of target minerals to the shock wave have a primary influence on how these microstructures develop. Data from diamond anvil cell and high-pressure friction experiments reveal that the same solid-state amorphization process that produces shock-induced PDFs at low temperatures also occurs at much lower strain rates in static experiments.The experimental data indicate that the amorphization process is thermally activated and that the character of the resulting PDFs is a function of the applied strain rate. Shock-induced amorphization occurs along those crystallographic planes that are most readily transformed to the high-pressure phase during very short pulse durations and produces PDFs that are visible at the optical scale. Lower strain rate deformation produces TEM scale amorphization with orientations that are more homogeneously distributed throughout the target mineral and produces no optically visible PDFs. The data confirm the uniqueness of multiple intersecting sets of optically visible PDFs as a diagnostic indicator of hypervelocity impact. The data also support the hypothesis that the amorphization process can occur at a wide range of strain rates, and that the limiting pressure for the process is controlled by the phase stability of the target mineral under the applied loading conditions, not by the HEL. The data also suggest that the onset pressure, the maximum pressure, and the pressure range for producing optically visible PDFs decrease with increasing temperature and decreasing strain rate. As the pressure range for optically visible PDF formation decreases, it is replaced by homogeneous amorphization as observed in the anvil cell experiments. Thus, the uniqueness of multiple intersecting sets of optically visible PDFs to hypervelocity impact is not due to a unique process, but rather to a specific set of loading conditions that produce an optically visible microstructure. Likewise, single sets of microdeformations observed in volcanic rocks are produced by the same process, but at different loading conditions that preclude the development of multiple intersecting sets of PDFs. The data also indicate that shock mosaicism, which occurs above the HEL, represents a plastic response of the target mineral to loading rates that are too large to be accommodated by crystal plastic mechanisms.Observational data for some naturally deformed samples from the K/T Boundary, the Vredefort Dome, and volcanic rocks, along with the experimental observations, are used to constrain the range of conditions under which the natural microstructures form, and to understand the differences between microstructures produced by impact, volcanic, and other natural processes. Finally, some possible mechanisms for producing the microstructures observed in volcanic rocks are proposed.