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Oscillatory compositional zoning in minerals has been observed in hydrothermal, magmatic, and metamorphic environments and is commonly attributed to chemical or physical cyclical changes during crystal growth. Chemical zoning is a common feature of solid solutions, which has been rarely reported in phyllosilicates. In this study, oscillatory zoning in chlorite is described in samples from the Pic-de-Port- Vieux thrust, a minor thrust fault associated to the major Gavarnie thrust fault zone (Central Pyrenees, Spain). The Pic-de-Port-Vieux thrust sheet comprises a 1-20 m thick layer of Triassic red pelite and sandstone thrust over mylonitized Cretaceous dolomitic limestone. The thrust fault zone deformation comprises secondary faults and cleavage affecting the Triassic pelite and sandstone. An important feature responsible to this deformation is a set of veins filled by quartz and chlorite. Chlorite is present in crack-seal extension veins and in shear veins; both structures opened under the same stress conditions. In some shear veins, chlorite occurs as pseudo-uniaxial plates arranged in rosette-shaped aggregates. These aggregates appear to have developed as a result of radial growth of the chlorite platelets. Oscillatory zoning has been imaged by backscattered scanning electron microscopy and by X‑ray quantitative micro-mapping. These oscillations correspond to chemical zoning with alternating iron-rich and magnesium-rich bands. The chlorite composition ranges from a Fe-rich pole to a Mg-rich pole. Fe3+/ΣFe values were measured in chlorite using μ-XANES spot analyses and vary from 0.23 to 0.44. The highest values are in the Ferich area. Temperature maps, built from standardized microprobe X‑ray images and redox state using the program XMapTools, indicate oscillatory variations from about 310 to 400 ± 50 °C during chlorite crystallization. These temperature variations are correlated with a Fe3+/ΣFe variation by Al3+Fe3+-1and ditrioctahedral substitutions highlighted by Mg and FeTot contents (Fe-Mg zoning). Chemical variations could be then explained by alternation of cooling times and cyclical pulses of a fluid hotter than the host rock. It is however not excluded that kinetic effects influence the incorporation of Mg or Fe during chlorite crystallization.

Abstract The syn-sedimentary Lower Eocene deformation of the Ainsa basin (Central Pyrenees, Spain) resulted in the emplacement of the Cotiella nappe, with thrust faults affecting Cuisian turbidites. Three thrustfault zones have been studied in order to investigate the role of deformation on mineral transformations. Two of the thrust faults contain a thin shear band comprised of intensely cleaved black marls with calcite veins which are distinct from the surrounding grey marls. The mineral assemblages in the grey marls are quartz, calcite, illite, chlorite with minor dolomite and feldspars, the clay fraction is composed of illite, chlorite and minor I/S mixed layers. No major mineralogical changes related to deformation are observed except in the most strained sediments in which kaolin has been detected. Kaolin is the major component of the black marls, with a minor amount of detrital quartz, illite and chlorite. The kaolin poly type is dickite according to powder XRD and IR spectroscopy. Texture analysis shows that the black marls are composed of millimetric lenses of undeformed silt embedded in strongly deformed illite-dickite-rich sediment. The low-magnification transmission electron microscope images reveal that clay minerals are oriented with their (001) plane parallel to the foliation and that dickite occurs as large (3 µm) deformed crystals, or as small defect-free crystals generally embedded in illite. The 2M poly type nature of dickite has been characterised by HRTEM and selected-area electron diffraction. Longsequence poly types and complex disordered structures are also locally observed in large crystals. Microtextural relationships suggest that dickite is a syn-kinematic mineral which precipitated after partial illite dissolution. The potassium released during the reaction may have left the system through fluid circulation in the shear zone. The occurrence of syn-kinematic dickite in highly strained shear zones of thrust faults suggests that deformation is an important factor promoting fluid-sediment interactions and clay mineral reactions.

We describe the structure, microstructure, and petrophysical properties of fault rocks from two normal fault zones formed in low-porosity turbiditic arkosic sandstones, in deep diagenesis conditions similar to those of deeply buried reservoirs. These fault rocks are characterized by a foliated fabric and quartz-calcite sealed veins, which formation resulted from the combination of the (1) pressure solution of quartz, (2) intense fracturing sealed by quartz and calcite cements, and (3) neoformation of synkinematic white micas derived from the alteration of feldspars and chlorite. Fluid inclusion microthermometry in quartz an calcite cements demonstrates fault activity at temperatures of 195°C to 268°C. Permeability measurements on plugs oriented parallel with the principal axes of the finite strain ellipsoid show that the Y axis (parallel with the foliation and veins is the direction of highest permeability in the foliated sand stone (10-2 md for Y against 10-3 md for X, Z, and the protolith, measured at a confining pressure of 20 bars). Microstructural observations document the localization of the preferential fluid path between the phyllosilicate particle forming the foliation. Hence, the direction of highest permeability in these fault rocks would be parallel with the fault an subhorizontal, that is, perpendicular to the slickenlines representing the local slip direction on the fault surface. We suggest that a similar relationship between kinematic markers and fault rock permeability anisotropy may be found in other fault zone types (reverse or strike-slip) affecting feldspar-rich lithologies in deep diagenesis conditions. [ABSTRACT FROM AUTHOR]

Abstract Nacrite, a two-layer polytype of kaolin (space group Cc) generally formed at high temperature, was found in the Lodeve Basin, France. It occurs in dolomite cavities in the Cambrian basement of the Permian basin, as authigenic, euhedral, up to millimetre-sized crystals, associated with barite deposits of hydrothermal origin. Formation of the mineral deposits is attributed to subbasinal fluid discharge and trapping during a Late Permian extensional regime. Textural relations suggest that nacrite and barite formed contemporaneously. The combination of transmission electron microscopy (TEM) images and electron diffraction patterns (SAED) confirm the two-layer periodicity of nacrite. In low-magnification images, nacrite displays a lamellar structure defined by a high density of planar defects parallel to (00 I). Each lamella is from 1.4 to 600 nm thick. SAED and high-resolution images imply that these planar defects are "twin" boundaries. Fluid inclusions occurring in barite are mostly one-phase, but coexist with rare two-phase inclusions. Ice-melting temperatures indicate the presence of a high-salinity brine (up to 25 wt% NaCI equiv.), whereas heating runs indicate formation at low temperature, approximately 80 to 100°C. Oxygen isotope data for nacrite are consistent with that low temperature range. These data imply that the occurrence of nacrite rather than other kaolin-family poly types can not generally be used as a qualitative geothermometer that implies formation at relatively high temperatures

The nature, composition, and relative abundance of clay minerals in the sandstones of the Brent Group reservoir were studied between 3200–3300 m in a well of the Ellon Field (Alwyn area, North Sea). The sandstones have a heterogeneous calcite cement which occurred during early-diagenesis. Clay diagenesis of the cemented and uncemented sandstones was investigated using optical microscopy, scanning electron microscopy (SEM), X-ray diffraction analyses (XRD), and infrared spectroscopy (IR). The influence of cementation on clay neoformation is demonstrated in this study. Detrital illite and authigenic kaolinite are present in both the calcite-cemented and uncemented sandstones suggesting that kaolinite precipitated before calcite cementation. In the uncemented sandstones, blocky dickite replaces vermiform kaolinite with increasing depth. At 3205 m, authigenic illite begins to replace kaolinite and shows progressive morphological changes (fibrous to lath-shape transition). At 3260 m, all sandstones are not cemented by calcite. Illite is the only clay mineral and shows a platelet morphology. In the cemented samples, vermiform kaolinite is preserved at all depths, suggesting that dickite transformation was inhibited by the presence of the calcite cement. This observation suggests that calcite cement would prevent fluid circulation and dissolution-precipitation reactions.

Abstract Authigenic euhedral kaolins of nano-to-millimetre size were found in dolomite cavities in the Cambrian basement of the Lodève Basin, France. X-ray diffraction, DTA and infrared-spectroscopy data demonstrate that nacrite is the only kaolin phase present in the cavities. Nacrite is associated with authigenic quartz, barite, dolomite and calcite. Petrographic data indicate a contemporaneous growth of nacrite and quartz after dolomite. In some cavities, barite and nacrite crystallized together. The crystallization of quartz, nacrite and barite could be the result of fluid influx and trapping in a karstified basement during the Late Permian extensional tectonic regime that affected the Lodève Basin.

Sediments from depths to 670 m in the Barbados accretionary complex and transecting the décollement zone have been studied by transmission and analytical electron microscopy (TEM/AEM). The sediments consist of claystone and mudstone intercalated with layers of volcanic ash. Smectite comprises the bulk of the noncalcareous sediments and forms a continuous matrix enveloping sparse, irregular, large grains of illite, chlorite, kaolinite and mixed-layer illite/chlorite of detrital origin at all depths. The detrital origin of illite is implied by illite-smectite textural relations, well-ordered 2Mpolytypism, and a muscovite-like composition. K is the dominant interlayer cation in smectite at all depths, in contrast to the Na and Ca that are normally present in similar rocks. Deeper samples associated with the décollement zone contain small (up to 100 Å thick) illite packets included within still-dominant subparallel layers of contiguous smectite. AEM analyses of these packets imply illite-like compositions. Selected area electron diffraction (SAED) patterns show that this illite is the 1Md polytype. Packets display step-like terminations like those seen in illite of hydrothermal origin. The data collectively demonstrate that smectite transforms progressively to illite via a dissolution-re-crystallization process within a depleting matrix of smectite, and not by a mechanism of layer replacement. This illite seems to form at depths as shallow as 500 m and temperatures of 20°-30°C, which is in marked contrast to the much higher temperature conditions normally assumed for this transformation. This implies that the high water/rock ratios associated with the décollement zone are significant in promoting reaction.

Sediments and basalts were drilled along an E-W transect in the eastern flank of the Juan de Fuca ridge during the ODP Leg 168. In situ temperature measurements and chemical analyses of the sediment pore waters were also realized. The preliminary results of the Leg show that the basement relief controls the fluid circulation and that a continuous sediment layer provides a hydrologically resistive barrier to fluid exchange. Fluid discharge is restricted to basement outcrops. Lateral fluid flow over long distances (100 km) is also demonstrated.

X-ray powder diffraction (XRD) and energy-dispersive X-ray analyses (EDX) of individual clay particles from hydrothermal mounds in the Galapagos spreading center (GSC) (Deep Sea Drilling Project, hole 509B) and high-resolution transmission electron microscopy (HRTEM) of the <2-µm size fraction of these sediments were carried out to document the mineralogy, geochemistry, and evolution of their clay horizons. The hydrothermal clay minerals of the GSC mounds were found to be intercalated with pelagic sediments and occurred as irregular interstratified illite/smectite according to X-ray powder diffraction analyses. On the basis of TEM, HRTEM, and EDX data, two types of clays appeared to coexist; these types differed in morphology, potassium content, and mode of stacking sequence. Lath-shaped particles having regular 10-Å, spacings were identified as glauconite, and filmy or veil-like particles, having curly edges and variable 10-13-Å spacings were identified as Fe-smectite (nontronite and Fe-montmorillonite). The absence of lattice fringes between Fe-smectite and glauconite crystallites was observed by HRTEM in clay aggregates. This structural discontinuity between Fe-smectite and glauconite layers suggests that a dissolution-recrystallization mechanism was responsible for the textural and chemical transition from the filmy Fe-smectite to the lath-like glauconite.