Asymmetrically zoned reaction rims: assessment of grain boundary diffusivities and growth rates related to natural diffusion-controlled mineral reactions

This study explores garnet coronas around hedenbergite, which were formed by the reaction plagioclase + hedenbergite→garnet + quartz, to derive information about diffusion paths that allowed for material redistribution during reaction progress. Whereas quartz forms disconnected single grains along the garnet/hedenbergite boundaries, garnet forms ～20‐μm‐wide continuous polycrystalline rims along former plagioclase/hedenbergite phase boundaries. Individual garnet crystals are separated by low‐angle grain boundaries, which commonly form a direct link between the reaction interfaces of the plagioclase|garnet|hedenbergite succession. Compositional variations in garnet involve: (i) an overall asymmetric compositional zoning in Ca, Fe²⁺, Fe³⁺ and Al across the garnet layer; and (ii) micron‐scale compositional variations in the near‐grain boundary regions and along plagioclase/garnet phase boundaries. These compositional variations formed during garnet rim growth. Thereby, transfer of the chemical components occurred by a combination of fast‐path diffusion along grain boundaries within the garnet rim, slow diffusion through the interior of the garnet grains, and by fast diffusion along the garnet/plagioclase and the garnet/hedenbergite phase boundaries. Numerical simulation indicates that diffusion of Ca, Al and Fe²⁺ occurred about three to four, four and six to seven orders of magnitude faster along the grain boundaries than through the interior of the garnet grains. Fast‐path diffusion along grain boundaries contributed substantially to the bulk material transfer across the growing garnet rim. Despite the contribution of fast‐path diffusion, bulk diffusion through the garnet rim was too slow to allow for chemical equilibration of the phases involved in garnet rim formation even on a micrometre scale. Based on published garnet volume diffusion data the growth interval of a 20‐μm‐wide garnet rim is estimated at ～10³–10⁴ years at the inferred reaction conditions of 760 ± 50 °C at 7.6 kbar. Using the same parameterization of the growth law, 100‐μm‐ and 1‐mm‐thick garnet rims would grow within 10⁵–10⁶ and 10⁶–10⁷ years respectively.     

Pressure solution at grain-to-grain contacts

Whereas much petrographic evidence for pressure solution, in sedimentary rocks has been accumulated since Sorby's work, its thermodynamic justification has never been clearly established, and has been challenged by some authors. Difficulties disappear when it is recognized that in the most general case migration of chemical components proceeds down chemical-potential gradients rather than down concentration gradients. Along a grain contact a chemical-potential gradient is produced by variations in contact pressure and by local variations in Helmholtz energy of the solid. For example, in a sand made up of even-sized spherical grains buried 500 m, the ratio of the diameter (D) of the grains to that of a spherical elastic contact circle (a) is D/a ～- 26. The chemical potential at the center of such an elastic contact is ～- 14kcal mol^?1 higher than in surrounding pore water saturated with respect to quartz. For comparison, at a temperature of 30°C, saturation with respect to amorphous silica rather than quartz raises the chemical potential by only 1.6 kcal mol^?1. If the diameter of the contact circle has enlarged to e.g. a = D/5, the chemical potential at its center is still 0.5 kcal mol ^?1 greater than that of free quartz under hydrostatic pressure. The corresponding potential gradients are the driving force of pressure solution. The concept of pressure solution thus does not contradict any thermodynamic principle; in particular, it does not require that the chemical component of the solid have a smaller partial volume in solution than in the solid state. Petrographic and experimental evidence can therefore be accepted without reservation.     

Domainal fabrics of hematite in schistose,shear zone-hosted high-grade Fe ores: The product of the interplay between deformation and mineralization

Schistose high-grade hematite orebodies (>64 wt % Fe) in the Iron Quadrangle, Minas Gerais, were formed in shear zones by hydrothermal alteration of the Paleoproterozoic Cauê BIF during the Transamazonian orogenesis. The ore is comprised of platy hematite (specularite) grains that define the foliation and overprint a relict banded martite-hematite fabric resembling, at first sight, a mylonite. The EBSD analyses of a m-scale schistose orebody from the Pau Branco mine show that specularite grew as elongated plates with the (00.1) plane parallel to the foliation. The population of the measured grain aspect ratio (GAR) is homogenous in different scales, and the longest axes of the crystals align with the stretching lineation (L//X) building continuous domains, or anastomose around stretched iron oxide aggregates and rootless fold hinges. The pole figure of the (00.1) plane shows usually a maximum centered on the pole of the foliation Z often elongated on a girdle perpendicular to the lineation L. The {10.4} pole figure has the configuration of a symmetric cleft girdle and the corresponding {11.0} and {10.0} pole figures present well developed girdles parallel to the foliation with an elongated maximum centered on X. Microstructures associated with crystal-plastic behavior and dynamic recrystallization are missing and the fabric of the orebody probably results from precipitation of strain-controlled oriented hematite plates and anisotropic syntaxial growth of favorably oriented grains with the intervention of hydrothermal fluids during Fe enrichment. The shear zone provided pathways for the percolation of mineralizing fluids under temperatures that varied from 140 to 350 °C or higher, under ductile or ductile–brittle conditions. The orthorhombic fabric and CPO (crystallographic preferred orientation) of the ore nevertheless contrast with the asymmetry of simple shear as observed in the torsion experiments by Siemes et al., 2010, Siemes et al., 2011, probably due to volume loss and possibly a flattening component of deformation in the ore zone.     

Pressure-solution deformation of the purgatory conglomerate,rhode island (u.s.a.): quantification of volume change,real strains and sedimentary shape factor

Pressure solution has caused substantial volume redistribution within the Purgatory Conglomerate from Rhode Island. Material has been removed from quartzite cobble surfaces parallel to the fold axes and mostly redeposited as fibrous pressure shadows at the long axis terminations of the cobbles. In the hinges of folds, 23% of the mean cobble volume has been removed, and in highly deformed and overturned fold limbs up to 55% volume reduction has occurred. The initial cobble shape and orientation can be measured at an undeformed locality and the deformation path can be easily deduced; thus these are real cobble volume reductions. Apparent volume losses (initial shapes not removed) range from 70% to 89%. Real strains for cobbles (axial ratios ranging from 1:0.65:0.38 to 1:0.47:0.15) have values of e_x, e_y, e_z which range from 0%, −20%, −11% to 0%, −37%, −42%, respectively, depending on structural position. The conglomerate itself has been extended parallel to the fold axes (e_r), but the extension is not recorded by the cobble shapes.     

Strain analysis of deformed granitic rocks (Helikian), Muskoka District,Ontario

Three-dimensional analysis of irrotational, longitudinal, finite strain was carried out on samples from a crescentic sheet which intruded and was deformed with a host gneiss unit of probable Helikian age. Analytical methods were compared using deformed feldspar grains representing four ideal degrees of strain intensity observed in the porphyritic sheet. The polar plot and R_f/φ, R_s methods proved most reliable and sensitive.Data derived from fabric and strain analysis at 38 sites in the units suggest a two-stage deformational sequence. The first stage produced recumbent, isoclinal, similar (class 2) folds with northwest-trending hinge surface traces. This fold form was modified during the second stage to produce an overall type 2 fold interference pattern. The second stage produced upright, open buckle folds as well as the resultant strain fabric currently observed. Strain analysis confirms the general fold geometries of the model, and also documents competency contrasts between the matrix and feldspar grains with increased strain intensity and magnitude. Deformation of feldspar grains in the sheet involved modification of a fabric of low strain magnitude (?_s = 0.3) and a k value near unity to magnitudes of ?_s = 2.6 and k = 0.6. Matrix strain intensities and magnitudes are consistently higher than those of the feldspar markers in the sheet. This variation is related to competency differences between the matrix and the feldspar grains. Fabric anisotropy accounts for the strain gradient observed between the sheet and gneiss.     

Oxidation potential and the composition of metamorphic fluid as a solution to the inverse problem of convex programming

Using the Selektor-C software program package, oxidation potential and the composition of metamorphogenic fluid were determined for mineral assemblages from nine samples of granulite-grade metamorphic rocks by solving the inverse problems of convex programming. The calculated and real mineral assemblages are in good agreement with respect to the composition and association of minerals, which is compelling evidence for the attainment of chemical equilibrium (minimum of Gibbs free energy) under given P-T conditions. Based on the dual solution of the inverse problem, a new approach was proposed for the estimation of the oxidation potential of fluid and mineral assemblages, which can be used to determine oxygen potential for almost any mineral association, independent of the presence of magnetite, ilmenite, or graphite. It was found that magnetite-free mineral associations are characterized by highly reducing conditions corresponding to oxygen potentials close to the CCO buffer. The external metamorphic fluid that was present during granulite-facies metamorphism was probably formed in the graphite stability field. The results of calculations for the model aqueous electrolyte solution-mineral assemblage suggest high SiO₂ solubility in the metamorphogenic fluid. Therefore, the process of granulite metamorphism may be a potent geochemical factor of the redistribution and transportation of silica from lower to upper crustal levels.     

Effects of grain size distribution on mineralogical and chemical compositions: a case study from size‐fractional sediments of the Huanghe (Yellow River) and Changjiang (Yangtze River)

The influence of hydrodynamics on the chemical composition of sediments is based on the uneven distribution of element abundances in different size fractions. In this study, 72 size‐fractional sediments from the Huanghe (Yellow River) and Changjiang (Yangtze River) riverbeds were measured with XRD, SEM, ICP‐AES and ICP‐MS. The analysis results show that the mineral and chemical characteristics change with grain size in the Huanghe and Changjiang sediments. According to the principal components analysis, three independent geochemical factors were found. The first factor elements, Zr, Hf, Th, U, Y, La and TiO₂ are influenced by the existence of heavy minerals. The second factor elements, Al₂O₃, alkalis, alkaline earth (excluding Ca and Sr) and most of the transitional metals are dominated by clay minerals. The third factor group includes Ca and Sr, which were controlled by calcium‐bearing mineral contents and chemical weathering intensities. The various grain size distributions greatly affect the mineralogical and chemical compositions of bulk sediments. Compared to other size fractions, the 5–6PHI size fractions of the Huanghe and Changjiang sediments have special mineralogical and chemical compositions, and intermediate volume percentages. Weight or volume percentage of each size fraction may be more suitable than mean grain‐size of the bulk sediment to elucidate the grain size effects. Chemical Index of Alteration (CIA) values increase steeply with decreasing grain size, while Weathering Index of Parker (WIP) values are relatively stable. Because of the big influence of the abundance of clay minerals on CIA values, it is questionable to use CIA as a proxy of weathering intensity. Considering the clay mineral effects, stability in values and heterogeneous material properties, WIP has the potential to indicate the chemical weathering intensity of sediments. Copyright © 2014 John Wiley & Sons, Ltd.     

The effects of stress on reactions in the Earth: Sometimes rather mean,usually normal,always important

Stress affects chemical processes on all scales in the Earth but the magnitude of its effect is debated. Here, I give a new synthesis of the theory that describes the effects of stress on chemistry, elaborating upon work in Materials Science which is built from fundamental thermodynamic laws, and show its significance in Earth Science. There are separate but compatible relationships describing what happens (1) at interfaces and (2) within grains. (1) The main chemical effects of stress in the Earth are due to variations in normal stress along grain interfaces and between interfaces with different orientations. For reactions involving diffusion these variations give effects on mineral stability broadly equivalent to pressure changes of (molar volume)/(molar volume change during reaction) × (stress variation). The volume ratio is generally large and so the effects of normal stress variations are always important since all stressed rocks have interfaces supporting different normal stresses. There is no global chemical equilibrium in a stressed system, so reaction kinetics contribute to ongoing evolution until stresses relax: this evolution can include deformation by diffusion creep and pressure solution, possibly with new mineral growth. These effects are relevant for predicting the conditions for reactions involving fluids, such as serpentinite formation and breakdown (relevant for the Earth's volatile cycles) and for other reactions such as ringwoodite breakdown (relevant for understanding the 660 km mantle discontinuity). (2) Within stressed solid solution grains it is not possible to define chemical potentials of all chemical components since one has to be specified as “immobile.” The chemical potential of a “mobile” component such as an exchange vector can be defined. It depends on the “partial molar strain,” a second rank tensor defining the variation in unit cell geometry with composition. In cubic crystals the partial molar strain is isotropic and the chemical potential of a mobile component depends on mean stress. In other crystal systems the partial molar strain is anisotropic and the chemical potential depends on a “weighted” mean stress; orientation as well as magnitude of stress has an influence. I propose “chemical palaeopiezometry”—the possibility of measuring past stress levels via chemistry. Examples show that stress variations in hundreds of MPa to GPa are required to produce 2% variations in composition but high stresses and/or precise chemical analyses will allow this proposal to be tested. High stresses around inclusions and dislocations could be targeted. So, the weighted mean stress inside grains has an effect which is relatively minor although potentially valuable in explaining chemical variations; the normal stress at interfaces plays the main role in chemical processes and its effects are of significant magnitude.     

Quartz c-axis fabric development and large-scale post-nappe folding (Wandfluhhorn Fold,Penninic nappes)

Quartz c-axis fabrics have been investigated within a suite of quartz veins and monomineralic layers around a major post-nappe fold hinge (the Wandfluhhorn Fold) in the Bosco area (Swiss-Italian border) within the lower Penninic nappes.Two kinematic domains which are separated by the axial plane trace of the Wandfluhhorn Fold are recognized; on the lower limb the measured quartz c-axis fabric asymmetry indicates a sense of shear in which the overlying layers move to the southwest (i.e. top-to-SW) whereas on the upper limb the shear sense is reversed with the top moving to northeast. The shear direction (N60°E–N80°E), however, is constant in both areas and oblique to an older stretching lineation as well as to the D₃ fold hinge. Such a distribution of asymmetric quartz c-axis fabrics and the constant orientation of their interpreted shear direction, which is apparent only from the fabric data and not from field evidence, indicates fabric development pre- or early syn-Wandfluhhorn folding, with subsequent folding and modification of the existing textures and possibly rotation of the initial fold axis.An overall westward-directed shear has been suggested for the whole of the Lepontine Alps. However, this study demonstrates that this simple general pattern has been modified locally by later folding. It also demonstrates that the dominant lineation may be a finite stretching lineation due to more than one phase of deformation and is not necessarily related to any particular transport direction.     

Foliations developed during slump deformation of Miocene marine sediments,Cyprus

Foliations within a Miocene slumped bioclastic sandstone unit of the Pakhna Formation, southern Cyprus, were investigated in order to assess the importance of slump strain, liquifaction and compaction in their generation. There are two approximately orthogonal sets of folds, F₁ and F₂. F₁ folds are upright to inclined slump folds formed during slope-failure translation of the sediment. The cores of upright F₁ folds have a steeply dipping macroscopic fabric defined by the axial surfaces of small tight folds in compositional layering. F₂ folds occur on steeply-dipping limbs of F₁ folds. F₂ folds are small and asymmetric with flat-lying axial surfaces, and are interpreted as compaction generated. A pervasive flat-lying microscopic fabric defined by grain and pore long axis orientation is found in both fold sets, and is probably a liquifaction fabric enhanced by compaction. A pervasive steeply-dipping microfabric parallel to the axial planes of slump folds is not present in any of the slumps investigated.     

Quantitative analysis of mass transfer during polymetamorphism in pelites of the Transangarian Yenisei Ridge

The study provides geological, structural, mineralogical, petrological, and geochronological evidence for polymetamorphic evolution of gneisses from the Garevka complex of the Yenisei Ridge. The results of the study provide significant insight into the geochemical behavior of major and trace elements in zoned garnet crystals and mineral inclusions formed during prograde and retrograde metamorphism of pelitic rocks. It was shown that the concentrations of Y and HREE in garnet decrease with increasing P and T and increase with decreasing pressure and temperature. The combined study of multicomponent chemical zoning patterns of coexisting minerals and metamorphic mineral reactions in metapelites was conducted. The results show that the main reason for a drastic increase in CaO content in garnets during collisional metamorphism is a mass exchange between garnet and plagioclase. The deviation from this trend, as indicated by the concurrent increase inthe grossular content of garnet and anorthite content of plagioclase, arises from the breakdown of epidote. The calculated metamorphic reactions, mass balance analysis, and changes in mineral chemistry during metamorphism reinforce the evidence for the isochemical character of processes with respect to most components of the system. The minimum volume of the system in which chemical exchange between reacting phases is balanced for all major and trace elements did not exceed ~ 1 mm³. The total HREE balance requires a greater reaction volume (up to ~ 8 mm ) involved in the redistribution of these elements, which provide evidence for their relatively higher mobility during metamorphism relative to other rare earth elements. The specific distribution and quite substantial mass transport of HREE are controlled by heterovalent isomorphic substitution between these elements and CaO in garnet.     

Pencil structure and strain in weakly deformed mudstone and siltstone

Weakly deformed mudstone and siltstone (Middle Ordovician Knobs Formation) of the Appalachian Valley and Ridge Province, south-western Virginia, U.S.A. show strain-dependent transitions between bedding fissility, pencil structure and cleavage. Pencil structures are associated with a bulk inhomogeneous shortening deformation where minimum principal strain (Z) ranges between 9 and 26% shortening (assuming a plane strain and constant volume). Where strains are less, bedding fissility dominates. Pencil fragments are defined by intersecting fracture sets subparallel to the pre-existing bedding fissility and cleavage. Their long axes are both parallel to the bedding-cleavage intersection and to the inferred Y axis of the tectonic strain ellipsoid. Pencil development is considered to result from fracturing along both fabric anisotropies during weathering and post-tectonic stress relaxation. Pencils show variations in size and shape depending on lithology (grain size and composition), degree of initial clay preferred orientation, degree of cleavage development, type of cleavage, total bulk strain and degree of strain homogeneity. Their shape (l/w) however is a direct measure of total Y/Z strain since strain determinations from chlorite pressure-fringes on framboidal pyrite within the pencil fragments give

$\text{(}\text{Y}\text{Z}\text{) = 0.913 + 0.019 (l/w)}$

. Pencil structure is therefore a potential strain marker in weakly deformed rocks.     

Spinifex biogeochemistry across arid Australia: Mineral exploration potential and chromium accumulation

Spinifex (Triodia spp.) grasslands cover vast areas of arid Australia, across a variety of soils and landscapes. These grasses are deep rooted and long lived, hence have great potential as a biogeochemical sampling medium for mineral exploration. This study discusses the results of analyses of Triodia pungens and Triodia scariosa from field sites over buried Au mineralisation (Coyote, Oberon and Tunkillia Prospects). At each site there is a multi-element anomaly in the vegetation over the projected mineralisation, the haloes are of different scales depending on the local landscape setting and dispersion potential of each element associated with mineralisation. The magnitude of the anomalies is similar for each site independent of underlying substrate. Overall, spinifex chemical composition has the potential to act as a point indicator of substrate geochemistry with very minimal dispersion (hundreds of metres only) that can delineate the extent of a potential ore deposit.This study also discusses the Cr accumulation potential of T. pungens and T. scariosa, discovered during the mineral exploration studies, from several field sites (Coyote, Oberon, Tunkillia and North Miitel Prospects). Triodia species are shown to be able to accumulate Cr up to potentially toxic levels independent of substrate concentration. This could be due to accumulation (active transport) or the lack of a barrier mechanism (passive uptake) within the plant.     

Complex chemical zoning in eclogite facies garnet reaction rims: the role of grain boundary diffusion

In metapelites of the Saualpe complex (Eastern Alps) continuous 10 µm to 20 µm wide garnet reaction rims formed along biotite-plagioclase and biotite-perthite interfaces. The pre-existing mineral assemblages are remnants of low pressure high temperature metamorphism of Permian age. The garnet reaction rims grew during the Cretaceous eclogite facies overprint. Reaction rim growth involved transfer of Fe and Mg components from the garnet-biotite interface to the garnet-feldspar interface and transfer of the Ca component in the opposite direction. The garnets show complex, asymmetrical chemical zoning, which reflects the relative contributions of short circuit diffusion along grain boundaries within the polycrystalline garnet reaction rims and volume diffusion through the grain interiors on bulk mass transfer. It is demonstrated by numerical modelling that the spacing of the grain boundaries, i.e. the grain size of the garnet in the reaction rim is a first order control on its internal chemical zoning.     

Microstructure and fabric development in ice: Lessons learned from in situ experiments and implications for understanding rock evolution

In this contribution we present a review of the evolution of microstructures and fabric in ice. Based on the review we show the potential use of ice as an analogue for rocks by considering selected examples that can be related to quartz-rich rocks. Advances in our understanding of the plasticity of ice have come from experimental investigations that clearly show that plastic deformation of polycrystalline ice is initially produced by basal slip. Interaction of dislocations play an essential role for dynamic recrystallization processes involving grain nucleation and grain-boundary migration during the steady-state flow of ice. To support this review we describe deformation in polycrystalline ‘standard’ water-ice and natural-ice samples, summarize other experiments involving bulk samples and use in situ plane-strain deformation experiments to illustrate the link between microstructure and fabric evolution, rheological response and dominant processes. Most terrestrial ice masses deform at low shear stresses by grain-size-insensitive creep with a stress exponent (n ≤ 3). However, from experimental observations it is shown that the distribution of plastic activity producing the microstructure and fabric is initially dominated by grain-boundary migration during hardening (primary creep), followed by dynamic recrystallization during transient creep (secondary creep) involving new grain nucleation, with further cycles of grain growth and nucleation resulting in near steady-state creep (tertiary creep). The microstructural transitions and inferred mechanism changes are a function of local and bulk variations in strain energy (i.e. dislocation densities) with surface grain-boundary energy being secondary, except in the case of static annealing. As there is a clear correspondence between the rheology of ice and the high-temperature deformation dislocation creep regime of polycrystalline quartz, we suggest that lessons learnt from ice deformation can be used to interpret polycrystalline quartz deformation. Different to quartz, ice allows experimental investigations at close to natural strain rate, and through in-situ experiments offers the opportunity to study the dynamic link between microstructural development, rheology and the identification of the dominant processes.     

Magnetic fabrics, rock magnetism, cathodoluminescence and petrography of apparently undeformed Bambui carbonates from São Francisco Basin (Minas Gerais State, SE Brazil): An integrated study

Magnetic measurements were performed on apparently undeformed limestones and carbonate shales from 44 sites in nearly horizontal stratigraphic layers mainly from the basal units of the Neoproterozoic Bambui Group in the southern part of the São Francisco Basin. Rock magnetism, cathodoluminescence, transmitted and reflected light microscopy analyses reveal that there is a mix of ferromagnetic minerals, mainly magnetite and pyrrhotite, in most sites. In some sites, however, the ferromagnetic minerals are magnetite and hematite. Fine-grained pyrrhotite and pyrite accompany rare fine-grained graphite and probably amorphous carbon in some of stylolites, while pyrrhotite is also present as larger interstitial masses in coarse-grained domains outside, but close to the stylolites. Magnetic fabrics were determined applying both anisotropy of low-field magnetic susceptibility (AMS) and anisotropy of anhysteretic remanence magnetization (AAR). The AAR tensor was less well defined than the AMS fabric due to the low ferromagnetic mineral content. The analysis at the individual-site scale defines three AMS fabric types. The first type (two sites) shows K_min perpendicular to the bedding plane, while K_max and K_int are scattered within bedding plane itself. This fabric is usually interpreted as primary (sedimentary-compactional), typical of totally undeformed sediments. The second type shows the three well-clustered AMS axes with K_min still perpendicular to the bedding plane. This fabric is the most important since it was found in the majority of the sites. The third type (two sites) is characterized by well-clustered K_max in the bedding plane, while K_min and K_int are distributed along a girdle. The second and third fabric types are interpreted as combinations of sedimentary-compactional and tectonic contributions at the earliest, and at a slightly later stage of deformation, respectively. AMS represents the contribution of all the rock-forming minerals, while AAR isolates the contribution of remanence-bearing minerals from the matrix minerals. However, rock magnetism shown that anhysteretic remanence only reaches grains with coercivity < 100 mT because the maximum AF in the majority of the available instruments is 100 mT. Therefore, hematite and pyrrhotite probably do not contribute to AAR, which is due to the shape-preferred orientation of magnetite grains. For some sites, the AMS and AAR fabric orientations are different, mainly with respect to the lineation orientations (K_max and A_max, respectively). In general, K_max is well developed and follows the trend of the main regional thrusts, fold axes and faults generated in the first deformational phase, while A_max follows both this trend and that of structural lineaments formed during the second deformational phase. These deformation phases arose from the compression, which occurred during the evolution of the Brasília fold belt during the last stages of the Brasiliano event. The magnetic fabrics of the apparently undeformed Bambui limestones are typical of very weakly deformed sediments, in which the depositional-compaction fabric has been partly overprinted by a tectonic one, with minimum susceptibility direction remaining perpendicular to bedding. This result is in agreement with the textures given by the petrographic observations.     

Grain coarsening in polymineralic contact metamorphic carbonate rocks: The role of different physical interactions during coarsening

The microstructural evolution of polymineralic contact metamorphic calcite marbles (Adamello contact aureole) with variable volume fractions of second-phase minerals were quantitatively analyzed in terms of changes in grain size and nearest neighbor relations, as well as the volume fractions, dispersion and occurrences of the second phases as a function of changing metamorphic conditions. In all samples, the calcite grain size is controlled by pinning of grain boundaries by second phases, which can be expressed by the Zener parameter (Z), i.e., the ratio between size and volume fraction of the second phases. With increasing peak metamorphic temperature, both the sizes of matrix grains and second phases increase in dependence on the second-phase volume fraction. Two distinct coarsening trends are revealed: trend I with coupled grain coarsening limited by the growth of the second phases is either characterized by large-sized or a large number of closely spaced-second phase particles, and results finally in a dramatic increase in the calcite grain size with Z. Trend II is manifest by matrix controlled grain growth, which is retarded by the presence of single second-phase particles that are located on calcite grain boundaries. It is supported by grain boundary pinning induced by triple junctions, and the calcite grain size increases moderately with Z. The two different grain coarsening trends manifest the transition between relatively pure polymineralic aggregates (trend II) and microstructures with considerable second-phase volume fractions of up to 0.5. The variations might be of general validity for any polymineralic rock, which undergoes grain coarsening during metamorphism. The new findings are important for a better understanding of the initiation of strain localization based on the activation of grain size dependent deformation mechanisms.     

Advective heat transfer and fabric development in a shallow crustal intrusive granite — the case of Proterozoic Vellaturu granite,south India

Syntectonic plutons emplaced in shallow crust often contain intermediate-to low-temperature deformation microstructures but lack a high-temperature, subsolidus deformation fabric, although the relict magmatic fabric is preserved. The Proterozoic Vellaturu granite emplaced at the eastern margin of the northern Nallamalai fold belt, south India during the late phase of regional deformation has a common occurrence of intermediate-to low-temperature deformation fabric, superimposed over magmatic fabric with an internally complex pattern. But high-T subsolidus deformation microstructure and fabric are absent in this pluton. The main crystal plastic deformation and fluid enhanced reaction softening was concentrated along the margin of the granite body. Resulting granite mylonites show Y-maximum c-axis fabric in completely recrystallized quartz ribbonds, dynamic recrystallization of perthites, and myrmekite indicative of fabric development under intermediate temperature (∼ 500–400°C). The weakly-deformed interior shows myrmekite, feldspar microfracturing and limited bulging recrystallization of quartz. The abundance of prism subgrain boundaries is indicative of continuing deformation through low-temperature (∼ 300°C). The relative rates of cooling influenced by advective heat transfer and deformation of the pluton seem to control the overall subsolidus fabric development. The rapid advective heat transfer from the interior in the early stages of subsolidus cooling was followed by slow cooling through intermediate temperature window as a well-developed phyllosilicate rich mylonitic skin around the granite body slowed down conductive heat loss. Low-T crystal plastic deformation of quartz was effected at a late stage of cooling and deformation of the shallow crustal granite body emplaced within the greenschist facies Nallamlai rocks.