The equilibrium H- and O-isotope fractionations can be approximated by the following equations which are based on experimental, empirical and/or theoretical data: Hydrogen: 1000 In ~kaolinite-water ~" -2.2 x 106 X T -2 - 7.7 Oxygen: 1000 In ~kaolinite-water = 2.76 x 106  _ 6.75 1000 In (~smectite-water = 2.55 x 106x T -2 - 4.05 1000 In ~illite-water = 2.39 X 106  T -2 - 3.76 The equilibrium H-isotope fractionation factors vs. 106  T -2 for kaolinite and probably smectite and illite are monotonic curves between 350-0~ More complex curves, with a minimum fractionation near 200~ are probably influenced by surface effects and/or disequilibrium fractionations among the different hydrogen sites. The H-isotope fractionations between smectite-water increase by 70% from Fe-poor montmorillonite to nontronite at low temperatures. The pore-interlayer water in smectite H-isotope fractionation at low temperatures is ~20__+ 10%. The presence of organic matter can modify both the ~SD value of the clay analysis and its 'water' content. Clays -- kaolinite, illite, smectite and probably halloysite -- tend to retain their D/H and 180/160 ratios unless subjected to more extreme diagenetic or metamorphic conditions or special local processes. Kinetic information is still only qualitative: for comparable grain sizes, hydrogen exchanges more rapidly than oxygen in the absence of recrystallization. Low-temperature diffusion coefficients cannot be calculated with sufficient precision from the higher temperature exchange data. The H- and O-isotope studies of clays can provide useful information about their conditions of formation.

Stable isotope geochemistry of clay minerals; The story of sloppy, sticky, lumpy and tough, Cairns-Smith (1971)

Colloid chemistry of clay minerals: the coagulation of montmorillonite dispersions

The section of the San Andreas fault located between Cholame Valley and San Juan Bautista in central California creeps at a rate as high as 28 mm yr(-1) (ref. 1), and it is also the segment that yields the best evidence for being a weak fault embedded in a strong crust. Serpentinized ultramafic rocks have been associated with creeping faults in central and northern California, and serpentinite is commonly invoked as the cause of the creep and the low strength of this section of the San Andreas fault. However, the frictional strengths of serpentine minerals are too high to satisfy the limitations on fault strength, and these minerals also have the potential for unstable slip under some conditions. Here we report the discovery of talc in cuttings of serpentinite collected from the probable active trace of the San Andreas fault that was intersected during drilling of the San Andreas Fault Observatory at Depth (SAFOD) main hole in 2005. We infer that the talc is forming as a result of the reaction of serpentine minerals with silica-saturated hydrothermal fluids that migrate up the fault zone, and the talc commonly occurs in sheared serpentinite. This discovery is significant, as the frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on the shear strength of the fault, and its inherently stable sliding behaviour is consistent with fault creep. Talc may therefore provide the connection between serpentinite and creep in the San Andreas fault, if shear at depth can become localized along a talc-rich principal-slip surface within serpentinite entrained in the fault zone.

Talc-bearing serpentinite and the creeping section of the San Andreas fault

Laboratory measurements of the strength of core samples from a drill hole located northwest of Parkfield, California, near the southern end of a creeping zone of the San Andreas fault, demonstrate that the fault is profoundly weak at this location and depth. This is because of the presence of the smectite clay mineral saponite — one of the weakest phyllosilicates known. The finding suggests that deformation of the mechanically unusual creeping portions of the San Andreas fault system is controlled by the presence of weak minerals, rather than by high fluid pressure or other proposed mechanisms. This study reports on laboratory-strength measurements of fault core materials from a drill hole located northwest of Parkfield, California, near the southern end of a creeping zone of the San Andreas fault. It is found that the fault is profoundly weak at this location and depth, owing to the presence of the smectite clay mineral saponite—one of the weakest phyllosilicates known. These findings provide strong evidence that deformation of the mechanically unusual creeping portions of the San Andreas fault system is controlled by the presence of weak minerals rather than by high fluid pressure or other proposed mechanisms. The San Andreas fault accommodates 28–34 mm yr−1 of right lateral motion of the Pacific crustal plate northwestward past the North American plate. In California, the fault is composed of two distinct locked segments that have produced great earthquakes in historical times, separated by a 150-km-long creeping zone. The San Andreas Fault Observatory at Depth (SAFOD) is a scientific borehole located northwest of Parkfield, California, near the southern end of the creeping zone. Core was recovered from across the actively deforming San Andreas fault at a vertical depth of 2.7 km (ref. 1). Here we report laboratory strength measurements of these fault core materials at in situ conditions, demonstrating that at this locality and this depth the San Andreas fault is profoundly weak (coefficient of friction, 0.15) owing to the presence of the smectite clay mineral saponite, which is one of the weakest phyllosilicates known. This Mg-rich clay is the low-temperature product of metasomatic reactions between the quartzofeldspathic wall rocks and serpentinite blocks in the fault2,3. These findings provide strong evidence that deformation of the mechanically unusual creeping portions of the San Andreas fault system is controlled by the presence of weak minerals rather than by high fluid pressure or other proposed mechanisms1. The combination of these measurements of fault core strength with borehole observations1,4,5 yields a self-consistent picture of the stress state of the San Andreas fault at the SAFOD site, in which the fault is intrinsically weak in an otherwise strong crust.

Low strength of deep San Andreas fault gouge from SAFOD core

Chapter 1.INTRODUCTION TO OXYGEN FUGACITY AND ITS PETROLOGIC IMPORTANCE

The continuous conversion of smectite to illite in samples from the Shinzan hydrothermal alteration area of Japan has been examined by X-ray powder diffraction (XRD) and transmission (TEM) and analytical transmission electron microscopy (AEM). TEM shows that randomly interstratified illite/ smectite (I/S) containing 100-50% expandable layers exhibits a flakey shape, whereas regularly and partially ordered interstratified I/S having 50-0% expandable layers exhibits a lath-like habit. An early- formed lath of regularly interstratified I/S is typically <35 A in thickness and 300–500 A in width; these dimensions gradually increase with decreasing percentage of expandable layers. XRD shows that the lathshaped I/S has a 1M polytype mica structure. AEM shows that the interlayer K content of flakey I/S increases monotonously with decreasing percentage of expandable layers in the range 100-50% expandable layers, whereas the interlayer K content of lath-shaped I/S increases along a different trend from that for the flakey I/S in the range 50-0% expandable layers. These observations suggest that randomly interstratified I/S is fundamentally smectite that is undergoing K-fixation and dissolution and that regularly and partially ordered interstratified I/S are immature illite which is still growing. Consequently, they suggest a mechanism for the hydrothermal smectite-to-illite conversion that is based on the K-fixation in and dissolution of smectite and the precipitation and growth of thin illite particles. Furthermore, these data suggest that the kinetics of smectite dissolution and illite growth are the most important factors controlling the smectite-to-illite conversion.

Chemical and morphological evidence for the conversion of smectite to illite

Mechanism of illite formation during smectite-to-illite conversion in a hydrothermal system

Smectite-to-chlorite transformation in thermally metamorphosed volcanoclastic rocks in the Kamikita area, northern Honshu, Japan

Low-temperature chlorites formed in diagenetic to low-grade metamorphic environments generally have greater Si contents and larger numbers of octahedral vacancies, and smaller Fe+Mg contents than higher-grade metamorphic chlorites. The compositional variations are characterized approximately by four end-member components: Al-free trioctahedral chlorite, chamosite, corundophilite, and sudoite. The solid solution is considered to be a random mix of cations and vacancies in the octahedral sites. Using the compositions of chlorites from Niger, Rouez, and Saint Martin diagenetic-hydrothermal series, a new, more convenient geothermometer, applicable to low-T chlorites is proposed and comparison made with geothermometers proposed previously. The chlorites studied contain appreciable amounts of Fe(III) (>14% of the total Fe), determined by Mossbauer spectroscopy. The calculations under which all Fe was regarded as ferrous gave considerable overestimates for the formation temperature, irrespective of the geothermometer used. This problem was reduced by taking into account the presence of Fe(III) in the octahedral sites. The geothermometer from this study gave more reasonable estimates than the geothermometers proposed by Walshe (1986) and Vidal et al. (2001), particularly in the case of the Niger chlorites which crystallized in the lowest-temperature conditions. The ordered-site substitution model of solid solution developed by Vidal et al. (2001) predicted satisfactorily the formation temperature of the Rouez chlorites and of some of the Saint Martin chlorites, suggesting that the chlorite compositions are controlled by the ${\rm{Si}}\square R_{ - 2}^{2 + }$
 exchange at low-T conditions while they are controlled by Tschermak exchange at higher temperatures. The decreasing number of vacancies with temperature are poorer in Fe-rich than in Fe-poor chlorites. Furthermore, the ordered-site occupation of cations and vacancies in trioctahedral chlorite occurs concomitantly with the compositional changes ruled by increasing temperature conditions.

Application of chemical geothermometry to low-temperature trioctahedral chlorites

This present study provides an overview of the clay-mineral reactions involved in the chloritization process in a mixed-layer mineral series, and focuses on the properties of the resulting low-temperature chlorites (formed at <220°C) in diagenetic and hydrothermal systems. According to the literature, most chlorite species occurring in low-temperature geological systems are derived from specific clay precursors except for direct precipitates from solution in veins. In addition, three main types of clay-mineral series have been associated with these chloritization processes: saponite-to-chlorite, berthierine-to-chlorite and kaolinite-to-sudoite reactions. The conversion of saponite to chlorite results in the most common sequence of trioctahedral clay minerals related to the occurrence of Mg-Fe trioctahedral chlorite in a wide variety of hydrothermal and diagenetic to very low-grade metamorphism environments. Two models were proposed in the literature to describe the saponite-to-chlorite conversion through corrensite. The first model is a continuous transition model based on solid-state transformation (SST) mechanisms and is valid in rock-dominated systems (closed micro-systems with very low fluid-rock ratios). The second model is a stepwise transition model based on dissolution-crystallization mechanisms (DC) and is efficient in fluid-dominated systems (open systems with high fluid-rock ratio). The berthierine to Fe-chlorite transition results in a sequence of trioctahedral phases which are related to chloritization processes in iron-rich and reducing environments. This transformation is a cell-preserved phase transition dominated by a SST mechanism that operates simultaneously in different domains of the parental mineral and may be considered as a polymorphic mineral reaction. Finally, the conversion of kaolinite to sudoite (Al-Mg di-trioctahedral chlorite) has not been documented like the other two aforementioned conversion series. Despite the scarcity of detailed investigations, the conversion of kaolinite to sudoite through tosudite is considered a stepwise mineral reaction that is dominated by a DC mechanism. From a compilation of literature data, it appears that several parameters of hydrothermal and diagenetic chlorites differ, including the minimal temperature, the textural and structural characteristics and the extents of compositional fields. In hydrothermal systems, discrete chlorite occurs at a minimal temperature near 200°C, regardless of its chemical composition. In diagenetic systems, discrete chlorite occurs at minimal temperatures that vary according to its crystal chemistry (100–120°C for Mg-chlorite as opposed to 40–120°C for Fe chlorite). The strong discrepancy between the lowest temperature at which Mg- and Fe-chlorite form in buried sediments and in geothermal systems should result from drastically different heating rates, heat-flow conditions and tectonism between basins at passive margins and geothermal systems at active margins. The morphology, structure and compositional fields of the diagenetic Fe-rich chlorite may have been inherited from those of the berthierine precursor. All of the chlorite species formed through the DC mechanism have good geothermometry potential. However, the SST mechanism in which berthierine is transformed into chlorite could have unexplored consequences regarding the use of the chemistry (including stable isotope composition) of diagenetic Fe-chlorite for reconstructing the burial history of sediments. Further investigations regarding the formation mechanisms of mixed-layer minerals are required to provide us with insight to understand the chloritization process in low-temperature geological systems.

Atsuyuki Inoue

Papers

Chemical and morphological evidence for the conversion of smectite to illite

Mechanism of illite formation during smectite-to-illite conversion in a hydrothermal system

Smectite-to-chlorite transformation in thermally metamorphosed volcanoclastic rocks in the Kamikita area, northern Honshu, Japan

Application of chemical geothermometry to low-temperature trioctahedral chlorites

Chlorite and chloritization processes through mixed-layer mineral series in low-temperature geological systems – a review