GEOLOGY | Volume 43 | Number 1 | www.gsapubs.org 55 High-resolution X-ray computed microtomography: A holistic approach to metamorphic fabric analyses Mohammad Sayab 1 , Jussi-Petteri Suuronen 2 , Pentti Hölttä 1 , Domingo Aerden 3 , Raimo Lahtinen 1 , and Aki Petteri Kallonen 2 1 Geological Survey of Finland, P.O. Box 96, FI-02151 Espoo, Finland 2 Department of Physics, University of Helsinki, PO Box 64, 00014 Helsinki, Finland 3 Departamento de Geodinámica and IACT-CSIC, Universidad de Granada, Granada 18002, Spain ABSTRACT An intrinsic limitation of studying microstructures in thin section is that their spatial (three-dimensional, 3-D) distribution, shape, and orientation have to be inferred by com- bining 2-D data from different sections. This procedure always involves some degree of interpretation that in some cases can be ambiguous. Recent advances in high-resolution X-ray computed microtomography have made possible the direct imaging in 3-D of volumes of rock to centimeter scale. This rapidly evolving technology is nondestructive and provides a holistic approach of microstructural analysis that eliminates interpretative procedures associated with 2-D methods. Spatial images can be generated through any part of the rock sample and used as virtual petrographic sections. Our application of this technique to an oriented drill core sample from the classic Orijärvi metamorphic region of southern Fin- land reveals a number of in situ 3-D aspects, including: (1) the spatial distribution and shape of andalusite porphyroblasts, (2) the geometry of a matrix foliation anastomosing around the porphyroblasts, (3) a millimeter-scale compositional layering that controlled the oscillation of porphyroblasts and sulfide mineralization, and (4) distinct inclusion trail patterns char- acterizing porphyroblast core versus rim zones. The combined data indicate that the steeply dipping bedding-subparallel foliation that characterizes the Orijärvi area formed by bulk north-south crustal shortening and associated vertical stretching. INTRODUCTION Much of our knowledge about crust and mantle dynamics is based on the study of metamorphic minerals and associated micro- structures. Porphyroblastic microstructures in particular represent a unique record of the pres- sure-temperature evolution of a rock linked to its deformation history (e.g., Vernon, 2004). The large majority of this research is based on the study of petrographic thin sections or polished rock surfaces with optical microscope, electron microscope, or microprobe (e.g., Passchier and Trouw, 2005). An important limitation of these tools is their inability to directly visualize micro- structures in three dimensions (3-D). At best, the spatial geometry of rocks can be approximated via the combination of 2-D data from multiple (thin) sections. The lack of full 3-D control commonly introduces ambiguity in microstruc- tural interpretations. For example, sigmoidal inclusion trails have been frequently interpreted in terms of shearing-induced porphyroblast rotation while it was tacitly assumed that the rotation axes must be normal to the stretching lineation (Kriegsman et al., 1989). In a number of cases, however, later work showed that both elements are in fact parallel or oblique, and an alternative origin of the same microstructures via overgrowth of crenulations was concluded (e.g., Sayab, 2005). In complexly deformed rocks, multiple stages of porphyroblast growth are commonly associated with distinctly ori- ented inclusion trail curvature axes. Their dis- tinction and measurement require integrated study of 6–8 differently oriented thin sections of samples (Bell et al., 1995; Aerden, 2003) and even then involve some degree of interpre- tation and extrapolation of 2-D data between sections (Aerden et al., 2010). Recent techni- cal advancements have added a promising new tool to existing microstructural methods: com- puted microtomography (CT) with high-energy X-rays. The main advantage of the technique is that it allows metamorphic microstructures and minerals to be directly visualized in 3-D at high resolution (e.g., Denison et al., 1997; Hud- dlestone-Holmes and Ketcham, 2010), thereby eliminating the interpretative procedures associ- ated with conventional methods. This technique is nondestructive and provides detailed 3-D spa- tial imagery of the internal architecture of a rock by measuring the attenuation of X-rays as they pass through different mineral phases (Carl- son and Denison, 1992; Ketcham and Carlson, 2001; Ketcham, 2005). In addition to 3-D spatial images, an unlimited number of serial cross sec- tions can be generated as a new kind of virtual petrographic section. In this paper the potential of this method is illustrated as applied to a drill core sample from the Orijärvi region, southern Finland, precisely where Eskola (1915) devel- oped the concept of metamorphic facies. Before being extracted, the sample was oriented in the field in order to match 3-D microstructural data to the tectonic framework and mineralization history of the study area (Skyttä et al., 2006). Through virtual scrolling, either horizontally or vertically, along or across the foliation using advanced image processing software, the 3-D shape of metamorphic fabrics can be visualized, and thus provides a new holistic approach for detailed microstructural analysis. The technique allows us to sharply delimit rock volumes with variable compositions in the same sample that then can be separated physically and geochemi- cally analyzed. An alternative nondestructive approach to resolve and segment chemical information in 3-D is combining CT with 2-D micro-X-ray fluorescence imaging (Boone et al., 2011). An additional advantage of CT imag- ing is that it allows us to determine the optimal thin section to cut through a rock. We show how the high-resolution X-ray computed microto- mography (HRXCT) is particularly well suited to resolving the spatial distribution of micro- structural controls on sulfide minerals. Such data extrapolated to regional-scale structures are very relevant to the targeting of ore deposits. SAMPLE DESCRIPTION Sample O1 is a 2.5-cm-diameter, 14-cm-long andalusite-mica schist, vertically drilled using a hand-held drilling machine, from the Orijärvi area, southwest Finland (Finnish National Grid coordinates: 6686250, 3308859). While still in situ, the drill core was marked with a north- pointing oriented groove on the top surface so that it could be easily reoriented in the X-ray scanner (Fig. 1). Cylindrical drill core is ideal for the HRXCT analysis as it images a circu- lar field of view, where the X-ray source and detector remain stationary. The drill core sam- ple is characterized by a steeply (78°) south dipping, east-west–striking pervasive folia- tion (S1 in Figs. 1A and 2A) that is at ~30° to S0. The foliation is associated with regionally developed upright folds formed as a result of a broadly north-south–directed shortening phase of the early Svecofennian orogeny dated as ca. 1875 Ma (Skyttä et al., 2006). In outcrop, the main S1 foliation can be seen to be overprinted by a widely spaced, subvertical S2 foliation striking northeast-southwest, but this fabric is hardly recognizable in the studied drill core (Fig. 1A). The regional distribution of anda- lusite, cordierite, and fibrolitic sillimanite in the Orijärvi area indicate low-pressure, high- temperature amphibolite facies metamorphic conditions, where andalusite to sillimanite progression is reported (Eskola, 1915; Skyttä et al., 2006). GEOLOGY, January 2015; v. 43; no. 1; p. 55–58 | doi:10.1130/G36250.1 | Published online 21 November 2014 © 2014 Geological Society of America. For permission to copy, contact editing@geosociety.org.