Electron Microscopy of Clay Minerals in Mudrocks from the San Andreas Fault Observatory at Depth (SAFOD)

The San Andreas Fault Observatory at Depth (SAFOD) drill site situated near Parkfield, California (Hickman et al.,  2004) offers the opportunity to assess the interplay between clay formation, faulting, and fluid migration in an active fault  zone through direct sampling from depth and comparison with exhumed fault strands (Chester et al., 2004; Evans and  Chester, 1995; Solum and van der Pluijm, 2004). Recent  electron microscopy and x-ray diffraction (XRD) studies of  rock fragments and cuttings from this drill site show  abundant clay minerals in the host rock, on fracture surfaces, and within mineralized veins (Schleicher et al., 2006; Solum  et al., 2006). It is, therefore, of importance to determine the  timing of clay formation in relation to pre-, syn-, or postfaulting activities, and to establish their influence on the seismic behavior of the San Andreas Fault (Wu, 1975). Within  this context, the formation of clays at depth is a potential weakening mechanism of crustal-scale faults (Warr and Cox,  2001; Wintsch et al., 1995; Zoback, 2000).


Introduction
The San Andreas Fault Observatory at Depth (SAFOD) drill site situated near Parkfield, California (Hickman et al., site situated near Parkfield, California (Hickman et al., site situated near Parkfield, California (Hickman et al., , 2004) offers the opportunity to assess the interplay between clay formation, faulting, and fluid migration in an active fault , and fluid migration in an active fault and fluid migration in an active fault zone through direct sampling from depth and comparison with exhumed fault strands (Chester et al., 2004;Evans and , 2004;Evans and 2004;Evans and ;Evans and Evans and Chester, 1995;Solum and van der Pluijm, 2004). Recent and van der Pluijm, 2004). Recent van der Pluijm, 2004). Recent , 2004). Recent 2004). Recent electron microscopy and x-ray diffraction (XRD) studies of (XRD) studies of studies of rock fragments and cuttings from this drill site show site show site show abundant clay minerals in the host rock, on fracture surfaces, and within mineralized veins (Schleicher et al., 2006;Solum , 2006;Solum 2006;Solum 2006;Solum Solum et al., 2006). It is, therefore, of importance to determine the , 2006). It is, therefore, of importance to determine the 2006). It is, therefore, of importance to determine the 2006). It is, therefore, of importance to determine the ). It is, therefore, of importance to determine the timing of clay formation in relation to pre-, syn-, or postfaulting activities, and to establish their influence on the seismic behavior of the San Andreas Fault (Wu, 1975). Within , 1975). Within 1975. Within this context, the formation of clays at depth is a potential weakening mechanism of crustal-scale faults (Warr and Cox, and Cox, Cox, , 2001;Wintsch et al., 1995;Zoback, 2000).
In this study of mineral transformations, the scanning electron microscope (SEM) and transmission electron microscope (TEM) are used to provide new insights into the are used to provide new insights into the used to provide new insights into the nano-scale characteristics of clay minerals, and to evaluate the mineralogical characteristics of very fine-grained -grained grained particles in fault rocks from the SAFOD drill hole. Using high-resolution TEM, the following aspects are being investigated: i) the microstructural characteristics of the clay mineral phases down to the nanometer scale, ii) the clay polytype structures by selected-area diffraction analysis (SAED), iii) the microchemistry of clay particles using analytical electron microscopy (AEM) and elemental geochemistry (ICP), and iv) the hydration state of smectite , and iv) the hydration state of smectite and iv) the hydration state of smectite using wet-cell XRD and TEM. Based on these investigations, RD and TEM. Based on these investigations, and TEM. Based on these investigations, different events of clay formation can be assessed in order to characterize diagenetic or hydrothermal fluid-driven growth processes.

Sampling and Methods
Fine-grained, clay-rich rock fragments up to 1 cm in -grained, clay-rich rock fragments up to 1 cm in grained, clay-rich rock fragments up to 1 cm in average length were collected from a spot-core of a clay-rich shear zone at 3067 m measured depth (MD, upper star in Fig. 1). This part of the borehole represents a potentially active section of the San Andreas fault zone, although it is fault zone, although it is ault zone, although it is currently not considered to contain the main fault trace (Zoback et al., 2005). The samples show distinct polishing , 2005). The samples show distinct polishing 2005). The samples show distinct polishing and striations on particle surfaces and on fracture surfaces. Some of these carefully washed rock chips were ultrasonically treated to remove surface particles and to study their surface particles and to study their and to study their to study their study their ir nature.
Other rock fragments with similarly polished surfaces ly polished surfaces polished surfaces were found at 3436 m MD from inside a core catcher after a failed coring attempt (lower star in Fig. 1). In an attempt to preserve the mineralogical structure, natural hydration state, and textural properties, specimens were bottled and , and textural properties, specimens were bottled and and textural properties, specimens were bottled and embedded in resin directly on site. The travel-time of rockchips from the bottom of the hole to the surface is about 7.5 hours. The fractured core chips were cleaned of drilling mud using dry paper towels and placed into small sample bottles containing 50% methanol and 50% LR White resin. In White resin. In hite resin. In In n the laboratory, the solvent was progressively removed by using increasing concentrations of LR White resin following the procedure of Kim et al. (1995) SEM of thin sections prepared from the impregnated rock fragments allowed preselection of locations of interest that were subsequently investigated by TEM. Small Cu-washers were glued on these areas and thinned using an ion mill apparatus. All  In contrast, the matrix shows s small areas with discrete smectite phases; however, most ; however, most however, most , most most smectites occur as mixed-layers of illite-smectite and chloritesmectite. They usually occur together with large packets of illite and chlorite. SEM investigations of these minerals reveal large irregular detrital grains with curved and damaged particle shapes. They are surrounded by a compacted fabric (presumably bedding parallel), perpendicular to the polished fractures and containing notably higher concentrations of K, Mg, and , and and Fe. Illite, chlorite, and inter-, and inter-and interstratifications of smectite with chlorite and illite have also been recognized by Solum et al. (2006) in the whole-rock XRD patterns. Based on first analysis of illite diffraction patterns (SAED), ordered and disordered 1M polytypes can be distinguished, whereas a highertemperature 2M 1 illite polytype has not yet been recognized.
Preliminary observations of samples impregnated on site also indicate the coexistence of authigenic illite-smectite and discrete smectite particles that were not disturbed during embedding and polymerization. However, the characterization of microstructures and comparison with non-impregnated material are not yet fully determined and require more detailed study.

Discussion
Based on the ongoing electron microscopy study of SAFOD drill rocks, illite, chlorite, and smectite in combi-, and smectite in combi-and smectite in combination with diverse mixed-layered minerals occur in the argillaceous lithologies. However, the TEM analysis in in particular shows that the minerals were formed in texturally shows that the minerals were formed in texturally different microstructural sites, suggesting different , suggesting different suggesting different formation processes.
Among the key observations is a natural smectite phase on the fracture surfaces that occurs as oriented platy and fibrous minerals, forming thin film coatings with slickenfibers ( Fig. 2A). This clay phase is interpreted as an authigenic phase formed by dissolution-precipitation reactions dissolution-precipitation reactions electron microscopy work was conducted in the EMAL at the University of Michigan, using the methods described in Warr and Nieto (1998). Microscopy was undertaken using a and Nieto (1998). Microscopy was undertaken using a Nieto (1998). Microscopy was undertaken using a Philips CM12 scanning transmission electron microscope at an operating voltage of 120 kV and a beam current of 20 µA. High-resolution imaging was conducted at magnifications between 50,000�� and 350,000��. Chemical analysis was performed in scanning mode by energy-dispersive spectrometry analysis over areas of 100 nm 2 using a Kevex Quantum solid-state detector.

Results and Ongoing Work
At least three different structural types of smectiteoccurence could be distinguished in the SAFOD mud-rock fragments: i) fracture surfaces with thin films and slicken-: i) fracture surfaces with thin films and slicken-i) fracture surfaces with thin films and slickenfibers ( Fig. 2A, Schleicher et al., 2006), ii) vein mineral-, 2006), ii) vein mineral-2006), ii) vein mineral-2006), ii) vein mineral-), ii) vein mineralization in the rock matrix (Fig. 2B), and iii) the matrix containing chlorite, smectite, chlorite-smectite, and illite-, and illite-and illitesmectite mixed-layerings (Figs. 2C and 2D). The thin films s. 2C and 2D). The thin films . 2C and 2D). The thin films and 2D). The thin films D). The thin films films films that cover the fracture surfaces at 3067 m and 3436 m MD reveal smectitic aluminosilicate minerals with a variable cation content of Na, K, Ca, Mg, and Fe. These compositions differ from the smectite minerals of the fresh drilling mud. A smectite phase with a composition similar to the fracture similar to the fracture the fracture coatings was detected in the mineralized veins at 3436 m MD. Here, chlorite and illite crystals occur as mixed-