Systematic study of universal‐stage measurements of planar deformation features in shocked quartz: Implications for statistical significance and representation of results

Abstract

Abstract-The presence of shocked quartz is one of the key lines of evidence for the impact origin of rocks. Crystallographic orientations of planar deformation feature (PDF) sets in shocked quartz have been used to constrain the peak shock pressure that these grains have experienced. So far no systematic and comparative studies of the various orientation measurement methods and their biases are available. Therefore, three shocked-quartz-bearing thin sections from a meta-greywacke clast in breccia, a biotite-gneiss, and a sandstone, respectively, were independently analyzed by three operators (two experienced and one inexperienced) using a four-axis universal-stage (U-stage), in order to evaluate the quality, precision, repeatability, and representativeness of U-stage measurements. Based on the indexing of PDF sets using a new version of the commonly used stereographic projection template, the study of 1751 PDF set orientations in 666 quartz grains in three different shocked rocks shows that differences in abundance and orientation of various PDF sets, as measured by the three separate operators, are rather limited. The precision of U-stage measurements depends mainly on the number of PDF sets investigated, as the ability level of the operator (experienced versus inexperienced) is only responsible for minor deviations in the number of unindexed planes. The frequency percent of dominant PDF planes may vary by up to 20 percentage points (pp) or 81% for a given crystallographic orientation when only 25 sets are measured. When 100 PDF sets are measured, however, this deviation in dominant orientations is reduced to about 7 pp or 28%. We recommend the use of a new stereographic projection template, which plots the pole positions of five additional, commonly occurring PDF orientations, as it can allow indexing of up to 12 pp more PDF planes; these are planes that would previously be considered unindexed and potentially regarded as errors of measurement.Our results suggest that by following a strict measurement procedure, the reproducibility of Ustage measurements is good and the results of different studies can be readily compared. However, it is critical that published PDF orientation histograms clearly define what type of frequency measurement is used, whether or not unindexed PDF sets are included in the frequency calculations, the numbers of grains and sets analyzed, and the relative proportions of each PDF set population that are combined in the histograms. This information appears to be essential for effectively comparing datasets from different studies.

Direct evidence of ancient shock metamorphism at the site of the 1908 Tunguska event

“…To improve the reproducibility of the U-stage measurements we follow the recommendations by Ferrière et al (2009). A histogram of angles between the c-axis and poles to the lamellae in quartz grains from sample SQ(TU08/01), in 5 • bins, was built using the spreadsheet by Huber et al (2011) and it is shown in Fig.…”

Microspherules-Bearing White Sandstone: Implication of Cosmic Impact Event near Jurassic-Cretaceous Boundary in West Central Sinai, Egypt

“…While not representing equivocal shock metamorphic indicators, theses shattered quartzes are similar to those observed in the target rocks of the Libyan Desert glass, which formed via an impact event [38]. Planar fractures occur frequently in shocked quartz, but are not considered a shock diagnostic unless occurring with the associated PDFs [8]. Although the irregular fractures or their inclusions trials in quartz grains are still not supported an impact origin, they occur along with non-planar microdeformation features in impact affected rocks, e.g., at Vredefort [26].…”

Shocked quartz grains in the polymict breccia of the Granby structure, Sweden–‐Verification of an impact

“…Reinhard 1931; Emmons 1943). The orientation of the optic axis and the poles perpendicular to planes of PDF were measured and plotted in a stereographic Wulff net and then indexed with MillerBravais indices (hkil), using a stereographic projection template, displaying the possible pole orientations of common PDF planes within a 5° envelope of measurement error (e.g., Engelhardt and Bertsch 1969;Stöffler and Langenhorst 1994;Langenhorst 2002;Ferrière et al 2009b). The stereographic projection template used in this study is an updated version from Ferrière et al (2009b) including five additional known orientations.…”

“…The orientation of the optic axis and the poles perpendicular to planes of PDF were measured and plotted in a stereographic Wulff net and then indexed with MillerBravais indices (hkil), using a stereographic projection template, displaying the possible pole orientations of common PDF planes within a 5° envelope of measurement error (e.g., Engelhardt and Bertsch 1969;Stöffler and Langenhorst 1994;Langenhorst 2002;Ferrière et al 2009b). The stereographic projection template used in this study is an updated version from Ferrière et al (2009b) including five additional known orientations. Note that, as recommended by Ferrière et al (2009b), all measured orientations that fall into the overlapping zone between {1013} and {1014} have been treated as belonging to {1013} orientations, because the {1014} orientations are not considered to be a major orientation in comparison to {1013}.…”

“…The stereographic projection template used in this study is an updated version from Ferrière et al (2009b) including five additional known orientations. Note that, as recommended by Ferrière et al (2009b), all measured orientations that fall into the overlapping zone between {1013} and {1014} have been treated as belonging to {1013} orientations, because the {1014} orientations are not considered to be a major orientation in comparison to {1013}. in quartz are present to a varying degree in all investigated thin sections.…”