Phyllosilicate Grains Research Articles

The fate of primary iron sulfides in the CM1 carbonaceous chondrites: Effects of advanced aqueous alteration on primary components

AbstractWe have carried out a SEM‐EPMA‐TEM study to determine the textures and compositions of relict primary iron sulfides and their alteration products in a suite of moderately to heavily altered CM1 carbonaceous chondrites. We observed four textural groups of altered primary iron sulfides: (1) pentlandite+phyllosilicate (2P) grains, characterized by pentlandite with submicron lenses of phyllosilicates; (2) pyrrhotite+pentlandite+magnetite (PPM) grains, characterized by pyrrhotite–pentlandite exsolution textures with magnetite veining and secondary pentlandite; (3) pentlandite+serpentine (PS) grains, characterized by relict pentlandite exsolution, serpentine, and secondary pentlandite; and (4) pyrrhotite+pentlandite+magnetite+serpentine (PPMS) grains, characterized by features of both the PPM and PS grains. We have determined that all four groups were initially primary iron sulfides, which formed from crystallization of immiscible sulfide melts within silicate chondrules in the solar nebula. The fact that such different alteration products could result from the same precursor sulfides within even the same meteorite sample further underscores the complexity of the aqueous alteration environment for the CM chondrites. The different alteration reactions for each textural group place constraints on the mechanisms and conditions of alteration with evidence for acidic environments, oxidizing environments, and changing fluid compositions (Ni‐bearing and Si‐Mg‐bearing).

Micromechanisms leading to shear failure of Opalinus Clay in a triaxial test: a high-resolution BIB–SEM study

Abstract. A microphysics-based understanding of mechanical and hydraulic processes in clay shales is required for developing advanced constitutive models, which can be extrapolated to long-term deformation. Although many geomechanical tests have been performed to characterise the bulk mechanical, hydro-mechanical, and failure behaviour of Opalinus Clay, important questions remain about micromechanisms: how do microstructural evolution and deformation mechanisms control the complex rheology? What is the in situ microstructural shear evolution, and can it be mimicked in the laboratory? In this contribution, scanning electron microscopy (SEM) was used to image microstructures in an Opalinus Clay sample deformed in an unconsolidated–undrained triaxial compression test at 4 MPa confining stress followed by argon broad ion beam (BIB) polishing. Axial load was applied (sub-)perpendicular to bedding until the sample failed. The test was terminated at an axial strain of 1.35 %. Volumetric strain measurements showed bulk compaction throughout the compression test. Observations on the centimetre to micrometre scale showed that the samples exhibited shear failure and that deformation localised by forming a network of micrometre-wide fractures, which are oriented with angles of 50∘ with respect to horizontal. In BIB–SEM at the grain scale, macroscale fractures are shown to be incipient shear bands, which show dilatant intergranular and intragranular microfracturing, granular flow, bending of phyllosilicate grains, and pore collapse in fossils. Outside these zones, no deformation microstructures were observed, indicating only localised permanent deformation. Thus, micromechanisms of deformation appear to be controlled by both brittle and ductile processes along preferred deformation bands. Anastomosing networks of fractures develop into the main deformation bands with widths up to tens of micrometres along which the sample fails. Microstructural observations and the stress–strain behaviour were integrated into a deformation model with three different stages of damage accumulation representative for the deformation of the compressed Opalinus Clay sample. Results on the microscale explain how the sample locally dilates, while bulk measurement shows compaction, with an inferred major effect on permeability by an increase in hydraulic conductivity within the deformation band. Comparison with the microstructure of highly strained Opalinus Clay in fault zones shows partial similarity and suggests that during long-term deformation additional solution–precipitation processes operate.

The evolution of slate microfabrics during progressive accretion of foreland basin sediments

Here, we study slate microfabrics from the exhumed accretionary wedge of the central European Alps and focus on the development of foliation. High-resolution micrographs from novel BIB-SEM imaging and Synchrotron X-ray Fluorescence Microscopy are analysed with 2D auto-correlation functions to quantify the geometry and spacing of slate microfabrics along a metamorphic gradient covering the outer and inner wedge (200–330 °C). The sedimentary layering primarily controls the morphology of the slate microfabrics. However, from outer to inner wedge, a fabric evolution is observed where diagenetic foliations gradually transform to secondary continuous and spaced foliations. With increasing metamorphic grade, the amount of recrystallized phyllosilicate grains and their interconnectivity increase, as does clast/microlithon elongation (aspect ratios up to 11), while foliation spacing decreases to <20 μm. This foliation evolution under non-coaxial deformation involves a combination of mechanical rotation of phyllosilicates, fracturing, and fluid-assisted pressure-dissolution-precipitation creep. The latter is the dominant deformation mechanism at T > 230 °C and accommodates background strain in the inner wedge. The evolving microstructural anisotropy is interpreted to lead to strain weakening by structural softening and may provide preferential fluid pathways parallel to the foliation, enabling the dehydration of large rock volumes in accretionary sediment wedges undergoing prograde metamorphism.

The interplay between phyllosilicates fabric and mechanical response of deep-seated landslides. The case of El Forn de Canillo landslide (Andorra)

Deep-seated landslides are among the most devastating natural hazards on earth, usually involving a rigid rock mass sliding over a weak, clayey shear band rich in phyllosilicates. The mechanical response of this shear band to the loading of the overburden is, therefore, critical for the stability and the evolution of a landslide. We hereby show that this mechanical response is strongly associated with mineralogy and microstructure of clay minerals forming the shear band, and vice versa. By presenting a detailed mineralogical, textural, and mechanical characterization of a shear band of an active deep-seated landslide, we attempt to shed light on processes determining the failure mechanism of a large deep-seated landslide. The case study chosen is the El Forn landslide located in Andorra Principality (Eastern Pyrenees). Its shear band is formed of black shales of the Silurian period. Using core samples of this landslide, we have performed mineralogical and microstructural analyses (XRPD, SEM-EDS, and MicroCT), combined with mechanical tests (liquid limit, plastic limit) to study the interplay between the internal texture of the material of the shear band and its mechanical response. Our results show that the highest mechanical alteration of the material occurs at the center of the shear band, where the phyllosilicates are perfectly aligned parallel to the shearing direction. The alignment of the crystals and their face-to-face contact increases the plasticity index of the material and reduces its porosity. Hence, the shearing movement rearranges the contacts of the phyllosilicate grains inside the shear band, reducing the resistance of the material and promoting the slip of the overburden.

Comparative multi-scale analysis of filamentous microfossils from the c. 850 Ma Bitter Springs Group and filaments from the c. 3460 Ma Apex chert

Filamentous microfossils belonging to Cephalophytarion from the 850 Ma Bitter Springs Group have previously been used as key analogues in support of a biological interpretation for filamentous objects from the 3460 Ma Apex chert. Here we provide a new perspective on this interpretation by combining Raman data with correlative electron microscopy data from both Cephalophytarion and Apex specimens. We show that, when analysed at high spatial resolution, the Apex filaments bear no morphological resemblance to the younger Bitter Springs microfossils. Cephalophytarion filaments are shown to be cylindrical, comprising chains of box-like cells of approximately constant dimensions with lateral kerogenous walls and transverse kerogenous septa. They exhibit taphonomic shrinkage and folding, possess fine cylindrical sheaths and are permineralized by sub-micrometric quartz grains. They fulfil all established biogenicity criteria for trichomic microfossils. In contrast, Apex filaments do not possess lateral cell walls, are not cylindrical in nature, and vary considerably in diameter along their length. Their kerogenous carbon does not have a cell-like distribution and their chemistry is consistent with an origin as exfoliated phyllosilicate grains. This work demonstrates the importance of high-resolution data when interpreting the microstructure, and origins, of putative Precambrian microfossils.

Fluid‐Assisted Strain Localization in Quartz at the Brittle/Ductile Transition

AbstractA mylonitic quartzite with conjugate and synthetic shear bands was investigated by Electron Backscatter Diffraction and optical microscopy to obtain insights on recrystallization mechanisms and strain localization in quartz at plastic to semibrittle conditions close to the brittle‐ductile transition. The mylonitic quartzite deformed during late Miocene thrusting coeval with contact metamorphism in the high‐strain domains of the Calamita Schists (Elba Island, Italy). Mylonitic deformation occurred from amphibolite to lower greenschist facies conditions during cooling of the aureole. Dynamic recrystallization, dominated by the activity of dislocation creep by prism &lt;a&gt; slip, produced recrystallized quartz layers mantling relic large quartz porphyroclasts. Under decreasing temperature and fluid‐rich conditions, quartz porphyroclasts acted as relatively rigid bodies and fractured along synthetic and conjugate C′ shear bands. Shear bands developed along kinematically favored orientations, just locally assisted by weak crystallographic planes in quartz. Fracturing along shear bands was assisted by cataclasis and fluid infiltration enhancing fracture propagation and healing by recrystallization and authigenesis of new quartz and phyllosilicate grains. The process enhanced the propagation of and strain localization in shear bands, with the development of bands of weak phyllosilicates. Furthermore, we observed the development of a c axis preferred orientation (CPO) related to dissolution and precipitation of new grains with their c axis oriented parallel to shear bands. This study highlights the importance of the interplay between brittle and crystal‐plastic processes and fluid ingress in the semibrittle regime to understand deformation partitioning and strain localization.

Zn-Al-rich chlorite in interleaved phyllosilicate grains from the low-temperature metamorphic Ordovician terrane of Iglesiente, south-west Sardinia, Italy

Zn-Al chlorite with a ZnO content of 2–3 wt% has been found in the Monte Argentu Formation that belongs to the external zone of the Variscan Sardinian Belt in SW Sardinia. This chlorite occurs in metasiltite rocks characterized by the alternation of dark-coloured layers rich in Fe- and Ti-oxides and light-coloured layers rich in quartz and phyllosilicates. The Zn-Al chlorite is associated with K-white mica, pyrophyllite, and quartz. The most common interleavings consist of K-white mica + Zn-Al chlorite, K-white mica + pyrophyllite, K-white mica + Zn-Al chlorite + pyrophyllite, and Zn-Al chlorite + pyrophyllite. The X-ray patterns of the <2 μm fraction of the investigated samples are characterized by peaks for illite, pyrophyllite, chlorite and quartz. Metamorphic temperatures of 300°–400°C have been estimated on the basis of the pyrophyllite occurrence. Pressure estimated on the basis of K-white mica composition is in the 0.3–0.6 GPa range.

Contrasting alteration processes in hydrothermally altered dolerites from the Betic Cordillera, Spain

AbstractDolerites from the Sierra de San Pedro (Betic Cordillera, southern Spain) develop three main types of microsystems during hydrothermal alteration: (1) centimetre-sized veins cross-cutting the dolerites; (2) microfractures in feldspar and diopside grains; and (3) alterations involving primary-igneous mafic phyllosilicate grains. The vein and microfracture sites developed alteration assemblages of randomly oriented smectites (saponite and beidellite) and halloysite. At these sites, the alteration mechanism was governed by complete dissolution of the parent material with subsequent crystallization of the products, with no parent-mineral crystallographic control. Pseudomorphed mafic phyllosilicate sites are characterized by oriented complex mineral assemblages made of chlorite, chlorite/smectite mixed layers, corrensite, saponite and relicts of biotite. These assemblages formed during the initial high-temperature stage of the hydrothermal process. In these microsites, the alteration sequence was controlled by the parent-mineral structure and chemistry, with products determined by structural relations with the parent phyllosilicate. Alteration of one phyllosilicate to the next most stable proceeds via interstratification of the parent and product phyllosilicates.

Seismic anisotropy as an indicator of reservoir quality in siliciclastic rocks

Abstract Improving the accuracy of subsurface imaging is commonly the main incentive for including the effects of anisotropy in seismic processing. However, the anisotropy itself holds valuable information about rock properties and, as such, can be viewed as a seismic attribute. Here we summarize results from an integrated project that explored the potential to use observations of seismic anisotropy to interpret lithological and fluid properties (the SAIL project). Our approach links detailed petrofabric analyses of reservoir rocks, laboratory based measurements of ultrasonic velocities in core samples, and reservoir-scale seismic observations. We present results for the Clair field, a Carboniferous–Devonian reservoir offshore Scotland, west of the Shetland Islands. The reservoir rocks are sandstones that are variable in composition and exhibit anisotropy on three length-scales: the crystal, grain and fracture scale. We have developed a methodology for assessing crystal-preferred-orientation (CPO) using a combination of electron back-scattered diffraction (EBSD), X-ray texture goniometry (XRTG) and image analysis. Modal proportions of individual minerals are measured using quantitative X-ray diffraction (QXRD). These measurements are used to calculate the intrinsic anisotropy due to CPO via Voigt-Reuss-Hill averaging of individual crystal elasticities and their orientations. The intrinsic anisotropy of the rock is controlled by the phyllosilicate content and to a lesser degree the orientation of quartz and feldspar; the latter can serve as a palaeoflow indicator. Our results show remarkable consistency in CPO throughout the reservoir and allow us to construct a mathematical model of reservoir anisotropy. A comparison of CPO-predicted velocities and those derived from laboratory measurements of ultrasonic signals allows the estimation of additional elastic compliance terms due to grain-boundary interactions. The results show that the CPO estimates are good proxies for the intrinsic anisotropy of the clean sandstones. The more micaceous rocks exhibit enhanced anisotropy due to interactions between the phyllosilicate grains. We then compare the lab-scale predictions with reservoir-scale measurements of seismic anisotropy, based on amplitude variation with offset and azimuth (AVOA) analysis and non-hyperbolic moveout. Our mathematical model provides a foundation for interpreting the reservoir-scale seismic data and improving the geological modelling of complex reservoirs. The observed increases in AVOA signal with depth can only be explained with an increase in fracturing beneath the major unit boundaries, rather than a change in intrinsic CPO properties. In general, the style and magnitude of anisotropy in the Clair field appears to be indicative of reservoir quality.

Effect of plastic deformation in laboratory conditions on magnetic anisotropy of sedimentary rocks

In order to correlate the degree of plastic deformation and low-field magnetic anisotropy, a series of laboratory pressure experiments were carried out on a batch of grey marls. Samples were gradually deformed using triaxial high-pressure device. The confining pressure of 300 MPa was used, yielding maximum relative deformation up to 20% depending upon the uniaxial differential stress. In the range of initial deformation, irregular changes of the anisotropy parameters were typically observed. This effect is related to variable pre-deformation orientation of the anisotropy ellipsoid in the samples. At higher deformation, samples are characterized by increasing degree of magnetic anisotropy and by increasing foliation. Reorientation of paramagnetic phyllosilicate grains due to plastic deformation seems to be the most probable mechanism of the magnetic anisotropy changes.

Origin of mixed-layered (R1) muscovite-chlorite in an anchizonal slate from Puncoviscana Formation (Salta Province, Argentina)

AbstractMica-chlorite mixed-layering was identified by X-ray diffraction (XRD) as a major or subordinate constituent in several slates of the Puncoviscana Formation from Sierra de Mojotoro (Eastern Cordillera, NW Argentina). In order to determine the crystallochemical characteristics of these mixed-layered sequences and interpret their petrological meaning, anchizonal slate P90 was chosen for TEM observations. In this slate, dioctahedral mica and chlorite form interleaved phyllosilicate grains (IPG) or stacks, up to 110 um long, preferentially oriented with (001) planes at a high angle to the slaty cleavage but also oblique to S0.In agreement with XRD results, the main phyllosilicates identified by transmission electron microscopy (TEM) were dioctahedral mica and random mixed-layer muscovite-chlorite, with chlorite in subordinate amounts and scarce smectite. In the lattice-fringe images of mixed-layer packets, a sequence of irregular stacking that produced apparent 24 Å (10 + 14) layers was observed, but it was frequently possible to distinguish the 10 Å layers from adjacent 14 Å layers. In nearly all packets, 14 Å layers prevail, exhibiting 14 Å:10 Å ratios between 1:1 and 3:1. Some elongated lenticular fissures which are probably a consequence of layer collapse caused by the TEM vacuum were identified in these packets. The straight, continuous appearance of lattice fringes plus the scarce evidence of collapsed layers identified suggest that these packets correspond principally to mixed-layer muscovite-chlorite, which is confirmed by analytical electron microscopy analyses. However, smectite-like layers are probably the third component of some of these mixed-layer sequences, which may account for their high Si and low (Fe + Mg) contents, their low interlayer charge in relation to theoretical interlayer muscovite-chlorite, and for the presence of Ca in the interlayer site.Textural relationships between chlorite and muscovite packets in IPG along with the observed transformations from 14 Å to 10 Å along the layer, is compatible with a prograde metamorphic replacement of chlorite in stacks by dioctahedral mica layers, probably in the presence of an aqueous fluid.

Static recrystallization and preferred orientation of phyllosilicates: Michigamme Formation, northern Michigan, USA

The Michigamme Formation of the Marquette District in Michigan's Upper Peninsula comprises a sequence of cleaved rocks of increasing metamorphic grade. Because metamorphism in the area occurred after cleavage formation, the rocks provide an opportunity to study preferred orientation development of phyllosilicates under conditions of static recrystallization. X-ray texture goniometry on samples from the greenschist-facies zone that were collected at varying distances from the bounding biotite-in and garnet-in isograds, shows that: (1) the preferred orientation of phyllosilicates is always parallel to the mesoscopic cleavage, and (2) the degree of preferred orientation of phyllosilicates improves as a function of increasing metamorphic grade (from <4 to >9 m.r.d.). Scanning electron microscopy on these samples shows that: (1) the length/width ratio increases with increasing grade, and (2) grain shapes are better defined with increasing grade. Previous work on slates showed mechanical processes dominate at very low-grade metamorphism, whereas chemical processes are favored at higher grades. The Michigamme samples show that improvement of preferred orientation occurrred by grain dissolution and crystallization. Noncleavage-parallel phyllosilicate grains were preferentially dissolved, probably facilitated by internal strain energy from mineral defects, aided by chemical energy, whereas cleavage-parallel phyllosilicates were hosts for new growth along their basal planes. These results show that significant fabric strengthening can be achieved by grain dissolution and crystallization in the absence of tectonic stress.

Chemical Weathering in a Tropical Watershed, Luquillo Mountains, Puerto Rico: II. Rate and Mechanism of Biotite Weathering

Samples of soil, saprolite, bedrock, and porewater from a lower montane wet forest, the Luquillo Experimental Forest (LEF) in Puerto Rico, were studied to investigate the rates and mechanisms of biotite weathering. The soil profile, at the top of a ridge in the Rio Icacos watershed, consists of a 50–100-cm thick layer of unstructured soil above a 600–800 cm thick saprolite developed on quartz diorite. The only minerals present in significant concentration within the soil and saprolite are biotite, quartz, kaolinite, and iron oxides. Biotite is the only primary silicate releasing significant K and Mg to porewaters. Although biotite in samples of the quartz diorite bedrock is extensively chloritized, chlorite is almost entirely absent in the saprolite phyllosilicates. Phyllosilicate grains are present as 200–1000 μm wide books below about 50 cm depth. X-ray diffraction (XRD) and electron microprobe analyses indicate that the phyllosilicate grains contain a core of biotite surrounded by variable amounts of kaolinite. Lattice fringe images under transmission electron microscope (TEM) show single layers of biotite altering to two layers of kaolinite, suggesting dissolution of biotite and precipitation of kaolinite at discrete boundaries. Some single 14-Å layers are also observed in the biotite under TEM. The degree of kaolinitization of individual phyllosilicate grains as observed by TEM decreases with depth in the saprolite. This TEM work is the first such microstructural evidence of epitaxial growth of kaolinite onto biotite during alteration in low-temperature environments. The rate of release of Mg in the profile, calculated as a flux through the soil normalized per watershed land area, is approximately 500 mol hectare −1 yr −1 (1.6 × 10 −9 mol Mg m soil −2 s −1). This rate is similar to the flux estimated from Mg discharge out the Rio Icacos (1000 mol hectare −1 yr −1, or 3.5 × 10 −9 mol Mg m soil −2 s −1), indicating that scaling up from the soil to the watershed is possible for Mg release. The rate of Mg release from biotite, normalized to Brunauer-Emmett-Teller (BET) surface area, is calculated using a mass balance equation which includes the density and volume of phyllosilicate grains, porewater chemistry and flux, and soil porosity. The mean rates of biotite weathering calculated from K and Mg release rates are approximately 6 and 11 × 10 −16 mol biotite m biotite −2 s −1 respectively, significantly slower than laboratory rates (10 −12 to 10 −11 mol biotite m biotite −2 s −1). The discrepancy in scaling down from the soil to the laboratory is probably explained by (1) differences in weathering mechanism between the two environments, (2) higher solute concentrations in soil porewaters, (3) loss of reactive surface area of biotite in the saprolite due to kaolinite and iron oxide coatings, and/or (4) unaccounted-for heterogeneities in flow path through the soil.

Microstructures of intergrown phyllosilicate grains from Verrucano metasediments (northern Apennines, Italy)

Interleaved phyllosilicate grains (IPG) of various compositions are widespread in low-grade Verrucano metasediments of the northern Apennines (Italy). They are ellipsoidal or barrel shaped, up to 300–400 μm long and they are often kinked and folded; phyllosilicate packets occur as continuous lamellae or as wedge-shaped layers terminating inside the grain. Using electron microscopy techniques (SEM, TEM) six types of IPG have been distinguished on the basis of their mineralogical composition: (1) Chl+Ms ± Kln; (2) Chl+ Ms+Pg ± Kln; (3) Ms+Prl ± Pg; (4) Ms+Prl+Su; (5) Ms+Prl+Chl+Su; (6) Su+Ms. Types (1) and (2) are mainly composed of chlorite, with Ms and Pg as minor phases; Kln grows on Ms in highly weathered samples. Types (3), (4), (5), and (6) are composed of muscovite, with intergrown Prl, Chl, Su and new-formed muscovite. The IPG show all kinds of contacts: from coherent grain boundaries with parallel basal planes and along-layer transitions to low- and high-angle grain boundaries. The IPG formed on pristine minerals such as chlorite and muscovite. The transformations took place during the prograde and retrograde metamorphic path of the rocks: they were facilitated by deformation and they occurred in equilibrium with a fluid phase, which allowed cation diffusion. Prograde reactions [Chl = Ms (or Pg); Ms = Prl; Ms = Chl] involve dehydration and sometimes a decrease in volume, whereas retrograde reactions (Ms = Kln; Ms = Su) involve hydration and an increase in volume. These transformations do not simply occur through an interchange of cations, but often involve deep structural changes: transitions from one phyllosilicate to another generally proceed through dissolution-recrystallization reactions. In conclusion, Verrucano IPG represent microstructural sites which have not completely equilibrated with the whole rock and whose mineral assemblage depends on the original composition of the microstructural sites.

Fluid‐rock reaction weakening of fault zones

The presence of weak phyllosilicates may explain the low shear strengths of fault zones if they define well‐developed fabrics. The growth of phyllosilicates is favored in meteoric water‐dominated granitic fault systems, where mineral‐aqueous fluid equilibria predict that modal phyllosilicate will increase via feldspar replacement reactions. In deeper, more alkaline, rock‐dominated regimes, the reactions reverse, and feldspars tend to replace phyllosilicates. In Mg‐rich mafic rocks, however, phyllosilicates (chlorite, biotite) can replace stronger framework and chain silicates in both shallower (&lt;∼10 km) meteoric H2O‐dominated and in deeper, alkaline, rock‐dominated regimes. Where these phyllosilicates precipitate in active fault zones, they contribute directly to reaction softening. Because low‐temperature deformation of phyllosilicates is not governed by factional processes alone but can occur by pressure‐independent dislocation glide, the strength of phyllosilicate‐rich fault rocks can be low at all depths. Low strain rate creep during interseismic periods can align phyllosilicate grains in foliated gouge and phyllonites. Where preferred orientations are strong and contiguity of phyllosilicates is large, strengths of rocks within fault zones may approach minimum strengths defined by single phyllosilicate crystals. Fault zones containing localized high concentrations of phyllosilicates with strong preferred orientations in well‐defined folia can exhibit aseismic slip, especially where mafic Mg‐rich rocks occur along the fault (like parts of the San Andreas Fault).

Primary and tectonic fabric intensities in mudrocks

In this paper, we present a summary of quantitative measurements of the preferred orientation of phyllosilicate grains in a variety of tectonically weakly deformed to undeformed, fine-grained, pelitic rocks. We also briefly examine a small amount of data on clay fabrics in deep sea cores, and compare them to fabrics in deformed equivalents from the Mariposa Formation and Calaveras Complex, Sierra Nevada, California. The measured fabric ellipsoids support the following conclusions: (1) a bedding-parallel foliation forms in pelitic rocks during compaction that in our samples is associated with shortening between 3% and 74%; (2) the amount of compaction is in part controlled by the quartz-feldspar content: interbedded sandy layers undergo little to no compaction and do not develop bedding parallel foliations; and (3) it is impossible to determine precisely the amounts of compaction, horizontal shortening, and later tectonic strain in deformed samples because of the variability of initial compactions. Our data, and results presented by others, indicate that strain paths in pelitic rocks always include early compaction, followed by an episode of simultaneous compaction and plane strain in accretionary wedges, and by a variety of other types of strain in other settings.

Preservation of Clay Minerals in the Precambrian (1.1 GA) Nonesuch Formation in the Vicinity of the White Pine Copper Mine, Michigan

AbstractThe Middle Proterozoic (1.1 Ga) Nonesuch Formation, host of the stratiform copper deposit at White Pine, Michigan, consists of 200 m of principally dark grey clastic sediments which contain detritus obtained dominantly from underlying mafic to intermediate volcanic rocks. Clay minerals from samples collected from the mine area and drill holes up to 100 km away have been studied using SEM, EMPA, TEM and AEM. Two morphologies of phyllosilicates, both including white mica and chlorite, occur in the ‘lower’ Nonesuch Formation: (1) detrital-shaped and (2) matrix. Detrital-shaped phyllosilicate grains are up to 450 microns long with long axes parallel to bedding. Matrix phyllosilicates occur as packets typically &lt;200 Å thick and as pore-filling cement.TEM images of detrital-shaped chlorite generally display 14-Å periodicity, although 24-Å corrensite- like units occur locally. Most detrital-shaped chlorite from the mine area samples have a relatively restricted range of Fe/(Mg+Fe) ratios from 0.52 to 0.58, but the Fe/(Fe+Mg) ratios of detrital-shaped chlorite outside the mine area range from 0.27 to 0.64. TiO2 crystals occur within and surrounding the detrital-shaped chlorite. Matrix chlorite has Fe/(Fe+Mg) ratios of 0.47 to 0.63, indicating that it is relatively homogeneous and enriched in Fe compared to detrital-shaped chlorite.Detrital-shaped white mica occurs as a 2Mi polytype and generally has a phengitic composition. Matrix illite-rich I/S occurs as a 1Md polytype, is K and Al deficient relative to end-member muscovite and contains significant Fe and Mg.The data are consistent with homogenization of detrital-shaped chlorite in the White Pine mine area by hydrothermal fluids during copper mineralization. The TiO2 crystals and corrensite-like units in detrital- shaped chlorite imply that it is at least in part derived from alteration of biotite. The presence of immature 1Md illite-rich I/S and a one layer chlorite polytype with stacking disorder suggests that the matrix clays are in their original, post-smectite state of formation as consistent with an authigenic origin during early burial diagenesis; i.e., they have not undergone subsequent transformation even though sedimentation and ore deposition occurred prior to 1 Ga.

Incipient metamorphism and deformation in the Variscides of SW Dyfed, Wales: first steps towards isotopic equilibrium

ABSTRACTIncipient metamorphism accompanying thrusting, folding and cleavage development has been investigated in a varied sequence of Palaeozoic sediments near the Variscan front in SW Dyfed, Wales. The aim was to evaluate a critical stage in the progression from heterogeneous sediment, whose detrital phases are neither in equilibrium with one another, nor with pore fluids, through indurated sedimentary rock to metamorphic rock comprising newly formed crystals that equilibrated with one another as they grew.Quartz veins are widely developed in the area, especially in the more psammitic lithologies, while finer grained rocks became cleaved during tectonic deformation. Mineralogical constraints and fluid inclusion measurements suggest maximum temperatures around 200‐310d̀ C (slightly higher in the Marloes‐Musselwick Thrust Sheet than in other parts of the structural succession) at depths of the order of 6‐13 km.Quartz veins yield distinctly heavier oxygen isotopic compositions than detrital quartz grains in the adjacent wall rocks, although care must be taken in interpreting the data because slivers of detrital grains may become incorporated into veins, while matrix detrital grains may incorporate veinlets or rims of newly formed quartz. It is concluded that vein quartz grew in isotopic equilibrium with a fluid phase whose isotopic composition was primarily controlled by exchange with phyllosilicates, not detrital quartz grains. Vein and matrix quartzes from the Marloes‐Musselwick Thrust Sheet are distinctly lighter (δ18Oveins=+14 to +18% and δ18Omatrix=+11 to +14%) than those from other thrust sheets (δ18O =+17 to +20% and +14 to +17%, respectively).We conclude that vein quartz and phyllosilicate grains in cleavage domains probably attained equilibrium with a locally buffered pore fluid at the peak of metamorphism, but many relict grains of different chemical and isotopic composition remained elsewhere in the rock. Local fluid migration along veins and through cleavage lamellae facilitated the attainment of equilibrium, but there is little evidence for large‐scale infiltration of externally derived fluids. With further metamorphism the quartz in these rocks would attain an isotopic composition intermediate between that of the heavy vein material and light detritus which coexist here.

Pre-lithification structures, deformation mechanisms, and fabric ellipsoids in slumped turbidites from the Pigeon Point Formation, California

Paterson, S.R. and Tobisch, O.T. 1993. Pre-lithification structures, deformation mechanisms, and fabric ellipsoids in slumped turbidites from the Pigeon Point Formation, California. Tectonophysics, 222: 135–149. Quantitative fabric, structural, and microstructural analyses of pre-lithification folds, foliations, and lineations formed by slumping of turbidite sequences in the Cretaceous Pigeon Point Formation, California, provide a useful comparison with strain and microstructures developed in lithified and tectonically deformed turbidites. Our results indicate the following: 1. (1) multiple generations of folds, cleavages, and lineations can develop prior to any post-lithification tectonic deformation 2. (2) individual grains in sandstones have variable axial ratios, but the ratios and orientations of large populations of grains define fabric ellipsoids with small axial ratios ( ave. = 1.25:1.13:1) 3. (3) phyllosilicate grains define moderate flattening fabrics (reflecting 20–40% shortening or volume loss), with the intensity of alignment partly controlled by the percent of quartz and feldspar grains 4. (4) the fabric ellipsoids in sand-rich layers largely reflect deposition and slumping: pre- and post-slump compactions did not occur, in sand-rich units but did align clay particles in mud-siltstone units, and 5. (5) intra-grain microstructures in quartz and feldspar (e.g., undulose extinction, subgrains) are inherited or recycled features rather than representing effects of post-lithification strains. Our data also suggest that prelithification slumping occurred by pervasive grain rotation and grain boundary sliding in saturated sands with some local movement of material along bedding horizons. A likely model for the folding and associated fabrics is that buckling and fold-hinge flattening drove fluid expulsion, which in turn caused local grain-scale realignment, transposition of bedding, and the development of an axial planar cleavage in the hinge zones. Continued fluid flow was channeled by the clay-silt layers in fold limbs and axial plane cleavege in fold hinges.

Magnetic Fabric in Two Sedimentary Rock-Types from the Southern Pyrenees.

The anisotropy of magnetic susceptibility (AMS) of some red sandstones, grey marly limestones and fine sandstones in various structural settings from the Southern Pyrenees has been analyzed. Red beds have been studied in a slightly deformed Eocene foreland sequence and in a meter-scale fold involved in a thrust sheet. Marly limestones of the same age as the redbeds were sampled in both thrust sheets included in the Upper Thrust sheets with no cleavage developed and units which belong to the cleaved Lower Thrust sheets (Gavarnie Thrust sheet). In every case the K1 (maximum) susceptibility axis lies in the axis of intersection of the plane of bedding and the plane of tectonic flattening or cleavage. The K3 (minimum) susceptibility axis tends to be normal to the bedding plane if cleavage is weak and normal to the cleavage plane if cleavage is well developed. In these cases the AMS ellipsoid is oblate. Where both bedding and cleavage compete to determine the AMS ellipsoid, the K3 axis lies in a girdle between the normals to the two planes, the K2 (intermediate) axis is automatically forced to lie on the same great circle, and the AMS ellipsoid may be prolate. The measurement of AMS of both marly limestones and sandstones with the same degree of deformation, reveals that the AMS of the first group is sensitive to the imprint of a tectonic fabric, whereas that of the second group reflects the sedimentary fabric. Circumstantial evidence suggests that phyllosilicate grains are more important than iron oxides in determining the AMS of the studied rocks.