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 combining 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 Finland 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 characterizing 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 microstructures. Porphyroblastic microstructures in particular represent a unique record of the pressure-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 microstructures 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 microstructural 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 oriented inclusion trail curvature axes. Their distinction 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 interpretation and extrapolation of 2-D data between sections (Aerden et al., 2010). Recent technical advancements have added a promising new tool to existing microstructural methods: computed 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; Huddlestone-Holmes and Ketcham, 2010), thereby eliminating the interpretative procedures associated with conventional methods. This technique is nondestructive and provides detailed 3-D spatial imagery of the internal architecture of a rock by measuring the attenuation of X-rays as they pass through different mineral phases (Carlson and Denison, 1992; Ketcham and Carlson, 2001; Ketcham, 2005). In addition to 3-D spatial images, an unlimited number of serial cross sections 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) developed 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 geochemically 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 imaging 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 microtomography (HRXCT) is particularly well suited to resolving the spatial distribution of microstructural 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 circular field of view, where the X-ray source and detector remain stationary. The drill core sample is characterized by a steeply (78°) south dipping, east-west–striking pervasive foliation (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 andalusite, 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).

Two phases of andalusite porphyroblasts have been recognized in the horizontally and vertically oriented thin sections (And 1, And 2; Fig. 2B). Porphyroblast rims include well-aligned trails of needle-shaped inclusions that are generally continuous with the intensely developed matrix foliation. Porphyroblast cores are more densely populated with mineral inclusions, but these are mainly equidimensional and do not exhibit a preferred orientation. Tectonic implications of these textures are discussed herein.

HIGH-RESOLUTION X-RAY COMPUTED MICROTOMOGRAPHY

HRXCT can precisely image the interior of solid materials such as rocks. In contrast to medical X-ray computed tomography, the small X-ray source size and/or smaller detector pixels used in microtomography permit higher resolution, and longer exposure times are possible because the irradiated material is inanimate. The scanner generates a series of grayscale radiographs of a given rock sample that are then reconstructed into a 3-D volumetric image of the internal structure of the sample. The gray value of each cubic volume element, or voxel (cf. pixel: picture element), reflects the relative linear X-ray attenuation coefficient, which is dependent on the density and average atomic number of the mineral, and X-ray energy. The 3-D volume can be viewed as individual cross-sectional images (slices) or a 3-D rendering, and quantitatively analyzed with 3-D image processing techniques. A more detailed and technical account of CT and its applications to geological materials can be found in Ketcham and Carlson (2001) or Cnudde and Boone (2013).

The HRXCT used in this study is the Nanotom 180 (Phoenix|X-ray Systems and Services, Germany; now part of GE Measurement Systems and Solutions) hosted in the University of Helsinki Department of Physics (Fig. 1B). X-rays from a tungsten target were used with the X-ray tube voltage set to 160 kV, the beam current set to 120 µA, and 0.5 mm of copper used to filter the X-ray beam. We acquired 1440 views per 360°, with 8 s total exposure time per view. Two segments of the drill core sample (O1-A and O1-B) measuring 2.5 × 5 cm were scanned separately (Fig. 1B). The 3-D images are composed of 2000 horizontal slices from each drill core segment with a voxel size of 14 × 14 × 14 μm. Minerals of interest were manually delineated in approximately every 30th slice, based on the gray values and texture in the 3-D image, and segmented by interpolating the selections. The resulting images were double-checked using reflected and polarized light microscopes in order to precisely determine minerals (Figs. 2A and 3A). Horizontal elongated holes generated in the matrix are due to anastomosing effects of the main matrix foliation around uneven surfaces of andalusite porphyroblasts (Fig. 4A).

3-D VISUALIZATION AND ANALYSES

The CT data were processed using Avizo Fire software (www.fei.com/software/avizo-fire-ms-brochure.pdf) with built-in algorithms for 3-D visualization and rendering. Contrasting grayscale values allowed us to segment and separate andalusite porphyroblasts and their inclusions, sulfides, quartz, and mica in the matrix. The brightest grains are sulfides, followed by mica, and the darkest are andalusite and quartz (Fig. 3). North-south and east-west vertical sections (Figs. 3B and 3C, respectively) cutting andalusite porphyroblast rims reveal steeply pitching S1 inclusion trails subparallel to matrix S1. Vertical and horizontal slices oriented perpendicular to the matrix foliation exhibit tight F1 folding of relict sedimentary bedding (S0) with steeply south dipping axial planes parallel to S1 (Fig. 3B) and moderately east plunging fold axes (Fig. 3F).

In all horizontal slices of both the O1-A and O1-B segments of the drill core, andalusite porphyroblasts are preferentially aligned west-northwest–east-southeast (Figs. 3A and 3D), whereas the anastomosing matrix foliation (S1) is deviated around them (Figs. 3E, 4A, and 4B). The horizontal thin section of the sample exhibits similar textural relationships (Fig. 2B). The 3-D imagery evidences two separate west-northwest–east-southeast–striking layers within the drill core. The southern layer lacks porphyroblasts, but contains numerous small sulfide grains (∼100–1000 µm) that are preferentially distributed along the matrix foliation (Figs. 4B and 4C). The northern layer hosts andalusite porphyroblasts, but only scarce sulfide grains (Figs. 3B, 3D, 4B, and 4C). The two layers that probably represent sedimentary bedding were physically separated for detailed chemical analysis by X-ray fluorescence (Table 1); results show the marked differences in Al2O3, CaO, Na2O, and S contents in the northern and southern layers.

Andalusite porphyroblasts are mostly tabular but show strong elongation in vertical direction (Figs. 4E and 4F). The largest porphyroblast is 18 × 5 × 4 mm and has a core containing unoriented inclusions surrounded by a rim with well-aligned inclusions (Fig. 4E). This microstructure indicates that prior to development of the steeply dipping foliation during F1 folding, no metamorphic fabric, or a very weak one, existed in the rock. Thus, the andalusite porphyroblasts grew mostly prior to development of the regional S1 cleavage. This fabric progressively intensified against porphyroblast margins and eventually included late-stage andalusite rims (Aerden et al., 2010).

DISCUSSION AND CONCLUSIONS

Until 1990, the majority of workers studied thin sections that were not precisely oriented relative to geographic coordinates and were cut either perpendicular and or parallel to dominant matrix fabrics. This approach changed when Hayward (1990) introduced a technique for determining the orientation of crenulation axes preserved within porphyroblasts and matrix from radial sets of vertical thin sections of single samples. The method was further refined by Bell et al. (1995), and its application since then has allowed us to resolve the tectonometamorphic histories of numerous mountain belts in unprecedented detail. A closely related computer technique, FitPitch (developed by Aerden, 2003), allowed calculation of preferred orientations of internal foliations (inclusion trails) from pitch and strike measurements collected in sets of differently oriented thin sections. HRXCT has already been widely used for 3-D visualization of igneous (e.g., Jerram et al., 2009), metamorphic (e.g., Huddlestone-Holmes and Ketcham, 2010), and ore minerals (Barnes et al., 2008; Liu et al., 2014). Our HRXCT data illuminate the complete internal architecture of metamorphic fabrics and minerals in a drill core sample. Chemically and texturally distinct metamorphic layers enriched in either sulfide minerals or andalusite porphyroblasts have been recognized. The sulfide grains are preferentially oriented along the main matrix foliation, demonstrating sulfide remobilization during formation of the S1 foliation in the Svecofennian orogeny, dated as ca. 1875 Ma (Skyttä et al., 2006). Thus, extensive syntectonic remobilization of volcanogenic massive sulfide–type sulfide deposits in the Orijärvi area probably took place at the time this fabric formed. Because inclusion trails in andalusite cores are unaligned, no foliation, or only a very weak foliation, existed at the time of nucleation. Porphyroblast growth was subsequently coeval with the progressive development of a steeply dipping, east-west–striking anastomosing foliation (S1) reflecting broadly north-south shortening accompanying the metamorphism early (ca. 1875 Ma) during the Svecofennian orogeny (Fig. 5). Porphyroblast rim growth, including S1, probably occurred early during development of the younger S2 northeast-southwest–striking spaced foliation. The Orijärvi domain of the Uusimaa Belt (southern Finland) is characterized by a steep bedding-subparallel tectonic foliation that formed during a broadly north-south crustal shortening phase of the ca. 1875 Ma Svecofennian event (Skyttä et al., 2006), associated with subvertical stretching (Cagnard et al., 2007) and oroclinal bending (Lahtinen et al., 2014). Thus, the elongated tabular shape of andalusite crystals (Figs. 4 and 5) was probably controlled by vertical stretching during this event.

Discussions with Mikko Nironen and Matti Pajunen of the Geological Survey of Finland (GTK) are acknowledged. Aerden acknowledges research projects P09-RNM-5388 and CGL2010-21048. We thank Ritva Serimaa, Department of Physics, Helsinki University, for extending the computed microtomography facilities to the GTK, and Bob Holdsworth and three anonymous reviewers for constructive reviews.