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_Journal of Structural Geology 32 (2010) 478e489_ Contents lists available at ScienceDirect Journal of Structural Geology journal homepage: www.elsevier.com locate jsg Preferred orientation of phyllosilicates: Comparison of fault gouge, shale and schist Hans-Rudolf Wenk*, Waruntorn Kanitpanyacharoen, Marco Voltolini Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA Article info Article history: Received 22 March 2009 Received in revised form 6 January 2010 Accepted 17 February 2010 Available online 1 March 2010 Keywords: Phyllosilicate fabrics Fault gouge Shale Schist Abstract Samples of fault gouge from the San Andreas Fault drill hole (SAFOD), a shale from the North Sea sedimentary basin and schists from metamorphic rocks in the Alps have been analyzed with high energy synchrotron X-rays to determine preferred orientation of mica and clay minerals. The method relies on obtaining 2D diffraction images which are then processed with the crystallographic Rietveld method, implemented in the software MAUD, allowing for deconvolution of phases and extraction of their orientation distributions. It is possible to distinguish between detrital illite muscovite and authigenic illite smectite, kaolinite and chlorite, and muscovite and biotite, with strongly overlapping peaks in the diffraction pattern. The results demonstrate that phyllosilicates show large texture variations in various environments, where different mechanisms produce the rock microfabrics: fault gouge fabrics are quite weak and asymmetric with maxima for (001) in the range of 1.5e2.5 multiples of random distribution (m.r.d.). This is attributed to heterogeneous deformation with randomization, as well as dissolution-precipitation reactions. Shale fabrics have maxima ranging from 3 to 9 m.r.d. and this is due to sedimentation and compaction. The strongest fabrics were observed in metamorphic schists (10e14 m.r.d.) and developed by deformation as well as recrystallization in a stress field. In the analyzed samples, fabrics of co-existing quartz are weak. All phyllosilicate textures can be explained by orientation of (001) platelets, with no additional constraints on a-axes. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction Phyllosilicates in many rocks are distinctly oriented. In fact, their alignment often defines the macroscopic schistosity, bedding plane, and cleavage. Phyllosilicates differ from many other rock-forming minerals by their dominant platy morphology with (001) as both sheet and cleavage plane. Also, anisotropy of physical properties is extreme, with elastic stiffness more than three times higher parallel to the sheet plane than perpendicular to it. Grain shape and physical properties play a major role in the alignment of these minerals, e.g., during compaction of a sediment, ductile deformation or crystallization under stress. Much early work has been dedicated to quantify the alignment of mica platelets which can be easily measured with the universal stage and petrographic microscope (e.g., Sander, 1930). Interestingly, already Sander (1934) used X-ray diffraction to characterize preferred orientation in shale (e.g., his Figure 14). With advances in X-ray diffraction techniques, a pole figure goniometer method in transmission geometry was applied to fine-grained slates (e.g., Oertel, 1983) and this method was refined. * Corresponding author. E-mail address: wenk@berkeley.edu (H.-R. Wenk). 0191-8141 $ e see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsg.2010.02.003 The method relies on positioning the detector at the Bragg angle for a basal reflection diffraction peak and recording intensity changes with sample orientation. Thus, a fair amount of information is available about (001) phyllosilicate pole figures in a variety of rocks, including gneisses, schists, slates and, more recently, shales. The investigations document considerable diversity that depends on formation conditions and compositions. With the universal stage and with X-ray pole figure goniometry only (001) pole figures can be measured, and thus, there is currently very little information about the orientation of a-axes. Furthermore, gouge, shales and schists are composed of many phases with strongly overlapping diffraction peaks. This applies particularly to the 10° peak that is used for most pole figure goniometry measurements. In shales, the detrital illite muscovite peak is overlapped with the authigenic illite smectite peak; in schists, the muscovite peak is superposed on the biotite peak, and these distinct phases cannot be separated. Here a diffraction method that relies on full diffraction spectra, rather than individual diffraction peaks has advantages (e.g., Lonardelli et al., 2005) and we apply it in this study to quantify preferred orientation of major mineral components of three samples from the drill core of the H.-R. Wenk et al. Journal of Structural Geology 32 (2010) 478e489 479 Table 1 Details about sample locations. SAFOD _1 arkose Box 20, Run 4 (18e20 cm) 3064.5 m, 5025_1_C_4 SAFOD _2 gouge Box 23, Run 5 (14e18 cm) 3067.0 5025_1_C_5 SAFOD _3 sheared sandstone-gouge Hole G, Run 2, Section 4, 3149 m Kimmeridge shale, North Sea drill hole, 3750 m Brg 929 Folded graphite schist, Septimer Pass, Cureglia, 2500 m Brg 1118 Phyllite, folded, Vor dem Berg, Val Bergalga (Avers), 2680 m Brg 1295 Schist, Plan V?st above Soglio, Bergell Alps, 1850m SAFOD project in the vicinity of the San Andreas fault, a shale, and three metamorphic schists. 2. Samples, experimental techniques and data analysis Location information of the samples is summarized in Table 1. The San Andreas Fault Observatory at Depth (SAFOD) is a project to investigate mechanical, seismic and chemical processes in situ at depth, as well as providing samples for detailed laboratory analysis (Hickman et al., 2004). We analyzed three samples of clay-rich arkosic composition from shear zones that are, however, not the main San Andreas fault zone. In the following discussion we refer to the samples as SAFOD _1 (Box 20, Run 4 (18e20 cm) 3064.5 m, 5025_1_C_4), SAFOD _2 (Box 23, Run 5 (14e18 cm) 3067.0 m, 5025_1_C_5) and SAFOD _3 (Hole G, Run 2, section 4, 3149 m) (see also: http://www.icdp-online.org/content/icdp/front_content.php?idart=1037). Similar samples were analyzed for mineralogical composition and microstructure and we will not go into those details (Schleicher et al., 2006; Solum et al., 2006). There is considerable heterogeneity, particularly in SAFOD _2 and _3 with local shear bands. SEM micrographs reveal a complex microstructure with detrital quartz, feldspars, mica and authigenic illite smectite clay (Fig. 1aed). SAFOD _3 displays a very fine-grained matrix with folded veins of calcite (Fig. 1c). The shale of Kimmeridge age is from a borehole in the North Sea at 3750 m depth (Hornby, 1998). It has a porosity of 2.5% and, based on infrared spectrometry, a composition of 35% illite smectite mica, 22% kaolinite, 30% quartz, 5% albite and 4% pyrite was suggested (new results, discussed below, suggest a somewhat different composition). The shale has a high elastic anisotropy as determined from ultrasound velocity measurements (C11 = 49.8 GPa, C33 = 29.5 GPa, where C33 is perpendicular to the bedding plane). The microstructure reveals clearly detrital micaceous particles, as well as authigenic clay (Fig. 1d). Three schists are from greenschist facies rocks of the upper Pennine Suretta nappe in the central Alps in Grisons, Switzerland. Brg 929 is a phyllite from Cureglia (2500 m), near Septimer Pass. The sample contains muscovite with subordinate chlorite. There is a fine-grained population and coarser crystals. The larger crystals are mechanically bent, and occasionally kinked, in fold-like structures (Fig. 2a). Brg 1118 is a fine-grained muscovite-graphite schist with albite porphyroclasts from Vor dem Berg (2680 m) in Val Bergalga (Avers). Fine-grained muscovite is concentrated in layers and highly aligned. Some clusters of larger muscovite crystals (up to 0.5 mm) are more randomly oriented (Fig. 2b). Brg 1295 is a fine-grained biotite-muscovite schist from the base of the Suretta nappe near Plan V?st, 1850 m and is of higher metamorphic grade. Biotite is strongly aligned, muscovite and chlorite are generally fine-grained and associated with alteration of feldspars (Fig. 2c). Fig. 1. Backscattered SEM images of samples from SAFOD and Kimmeridge shale (Hornby, 1998). (a) and (b) SAFOD _2, (c) SAFOD _3, and (d) shale. Bright particles are rich in heavy elements such as pyrite spherulites in shale (d). Dark zones indicate porosity. 480 H.-R. Wenk et al. Journal of Structural Geology 32 (2010) 478e489 Fig. 2. Optical micrographs of greenschist facies schists. Plane polars. (a) Brg 929 muscovite-chlorite schist with local crenulation folds. (b) Brg 1118 graphite-bearing muscovite-chlorite phyllite with albite porphyroclast. (c) Brg 1295 biotite-muscovite chlorite schist. Note the two generations of muscovite, one interlayered with biotite and the second as alteration of feldspar. Samples were first embedded in epoxy. Then 2 mm-thick slices were prepared with a microsaw, using kerosene as a cooling agent. The surface of the slice was about 1 cm². These slices were then used for synchrotron diffraction experiments in transmission. The method has been described by Wenk et al. (2008a) in some detail. The samples were measured at the high-energy beamline BESSRC 11-ID-C of APS (Advanced Photon Source) of Argonne National Laboratory, with a monochromatic wavelength of 0.107863 x17A. At these short wavelengths X-rays can penetrate mm of material without significant absorption and thus can be used to analyze large volumes, comparable to neutrons. Contrary to neutrons, X-ray data collection can be done much faster (100 s versus hours). Beam-size was 0.5 × 0.5 mm and sample to detector distance was about 2 m. The sample's' Ключевые слова: smectite, authigenic, pole, van der, foliation, journal, case, der pluijm, biotite, safod box, anisotropy, re?nement, rietveld renement, plane, maximum, oriented, background function, texture development, chlorite occurs, pole ?gure, unique axis, ?gures, sheet silicate, pole ?gures, schist, rietveld, elastic, map plot, random orientation, von, bedding, analysis, hammersley, stress, table, layer, platelet, peak, clay, texture strength, mineral, texture analysis, oertel, method relies, voltolini, structural, applied, slice, metamorphic schist, metamorphic, calculation, journal structural, preferred orientation, sample, illite smectite, brg brg, tilt, lonardelli, van der pluijm, journal structural geology, illite muscovite, san, muscovite, debye ring, clay minerals, fault gouge, doi, random, weak texture, phyllosilicate, safod sample, quartz, bedding plane, springer, method, authigenic phyllosilicates, phase, preferred, pole gure, standard deviation, brg, tectonophysics, geophysical, schistosity plane, scale, texture, maud, strongest, experiment, kronenberg, bell, andreas, considerable heterogeneity, strongest texture, fabric, image, graphite, ha, stochastic odf, perpendicular, structure, property, geophysics, rock, kimmeridge shale, physical property, universal stage, macroscopic foliation, ?gure, overlapping peak, american mineralogist, strength, study, crystal structure, clay mineral, weak, van, wenk journal, intensity variation, authigenic illitesmectite, microstructures, texture maximum, phyllosilicates, textures, von dreele, synchrotron, detrital illitemuscovite, march, orientation distribution, pattern, shale, diffraction image, matthies, distribution, wa, a-axes, structural geology, diffraction peak, orientation, minerals, role, -r, fault, detrital illite, random distribution, geology, phyllosilicate texture, lineation direction, crystal, phyllosilicate fabric, sample safod, gouge, alignment, high, horizontal axis, renement, sander, detrital, microstructure, deformation, kaolinite, journal geophysical, der, safod, illite, pluijm, chlorite, authigenic illite, mica, considerably weaker, x-ray, authigenic clay, spectrum, elastic anisotropy, wenk, bish, schist brg, issue, illitesmectite, pole gures, relies, hornby, basal plane, kimmeridge, rotational freedom, scale parameter, montmorillonite, range, mineralogist, compaction, diffraction, odf, axes, peacor, metamorphic rock, sample slab, american, diffraction pattern