High‐resolution X‐ray computed tomography of impactites

Abstract

High‐resolution X‐ray computed tomography (HRXCT) is a nondestructive method used to study the interiors of opaque solid objects. Here we present the results of a first application of the HRXCT method to imaging the interior of impactites, in particular, suevites (glass‐bearing impact breccias) from the Bosumtwi (Ghana) and Ries (Germany) craters and a Muong Nong–type tektite from Thailand. The aim of these studies was to determine the three‐dimensional (3‐D) distribution of clasts within the matrix of the suevites, to test this technique with respect to its suitability for the recognition of different clast types of different densities, and to determine textural characteristics of the tektite. The main part of the study concentrated on a large suevite sample (about 10 × 7 × 5 cm) from the Bosumtwi impact structure. Target rock fragments in the Bosumtwi sample consist of greywacke and sandstone/quartzitic rocks, shale and phyllite, and granites. Another large clast component is composed of impact melt and glass fragments. Macroscopic petrography and thin section petrography were used to identify the clast types in the specimen for correlation with its HRXCT signatures. The results show that HRXCT allows the easy discrimination of the relatively frothy inclusions of glassy melt in the suevites, as they are darker than the matrix in the raw X‐ray scans and can be traced through the whole sample. Color or gray scale applications allow the distinction of at least four different clast types based on density differences. The size of the smallest discernable clasts (about 0.5 mm) is determined by the resolution of the measurements, which, in turn, is a function of scan slice thickness and field of view. In the case of the tektite sample, we were also able to image the 3‐D distribution of vesicles, possibly indicating glass flow. The HRXCT method allows us to determine the three‐dimensional distribution of clast populations in impact breccias by image processing techniques and to quantify their abundances in volume percent.