Polycrystalline calcite to aragonite transformation kinetics: experiments in synthetic systems

The kinetics of the calcite to aragonite transformation have been investigated using synthetic polycrystalline calcite aggregates, with and without additional minerals present. The reaction progresses as a function of time were measured at four temperature/pressure conditions: (1) 550 °C/1.86 GPa; (2) 600 °C/2.11 GPa; (3) 650 °C/2.11 GPa, and (4) 700 °C/2.29 GPa. Experiments reveal that Mg-calcite and Fe-calcite transforms to aragonite at considerably slower rates than pure calcite, and that Sr-bearing calcite and calcite + quartz aggregates transform at significantly higher rates than pure calcite. The reaction progresses vs. time data for pure calcite were fitted to Cahns grain-boundary nucleation and interface-controlled growth model. Evidence for interface-controlled growth is provided by petrographic observations of grain boundaries. The activation energy for aragonite growth from the synthetic polycrystalline calcite determined in this study is significantly lower than that previously determined from a natural marble. The discrepancy in rates and activation energy may be attributed to the nature of grain boundaries, to deformational strain or the presence of impurities in the studied samples, and likely to uncertainties in experimental conditions. The results of this study imply that the variation of local petrologic conditions, in addition to temperature, pressure and grain size, may play an important role in determining the rates for the calcite to aragonite transformation in nature.Editorial responsibility: W. Schreyer     

Strain localisation in the subcontinental mantle — a ductile alternative to the brittle mantle

It is now admitted that the high strength of the subcontinental uppermost mantle controls the first order strength of the lithosphere. An incipient narrow continental rift therefore requires an important weakening in the subcontinental mantle to promote lithosphere-scale strain localisation and subsequent continental break-up. Based on the classical rheological layering of the continental lithosphere, the origin of a lithospheric mantle shear/fault zone has been attributed to the existence of a brittle uppermost mantle. However, the lack of mantle earthquakes and the absence of field occurrences in the mantle fault zone led to the idea of a ductile-related weakening mechanism, instead of brittle-related, for the incipient mantle strain localisation. In order to provide evidence for this mechanism, we investigated the microstructures and lattice preferred orientations of mantle rocks in a kilometre-scale ductile strain gradient in the Ronda Peridotites (Betics cordillera, Spain). Two main features were shown: 1) grain size reduction by dynamic recrystallisation is found to be the only relevant weakening mechanism responsible for strain localisation and 2), with increasing strain, grain size reduction is coeval with both the scattering of orthopyroxene neoblasts and the decrease of the olivine fabric strength (LPO). These features allow us to propose that grain boundary sliding (GBS) partly accommodates dynamic recrystallisation and subsequent grain size reduction.A new GBS-related experimental deformation mechanism, called dry-GBS creep, has been shown to accommodate grain size reduction during dynamic recrystallisation and to induce significant weakening at low temperatures (T < 800 °C). The present microstructural study demonstrates the occurrence of the grain size sensitive dry-GBS creep in natural continental peridotites and allows us to propose a new rheological model for the subcontinental mantle. During dynamic recrystallisation, the accommodation of grain size reduction by three competing deformation mechanisms, i.e., dislocation, diffusion and dry-GBS creeps, involves a grain size reduction controlled by the sole dislocation creep at high temperatures (> 800 °C), whereas dislocation creep and dry-GBS creep, are the accommodating mechanisms at low temperatures (< 800 °C). Consequently, weakening is very limited if the grain size reduction occurs at temperatures higher than 800 °C, whereas a large weakening is expected in lower temperatures. This large weakening related to GBS creep would occur at depths lower than 60 km and therefore provides an explanation for ductile strain localisation in the uppermost continental mantle, thus providing an alternative to the brittle mantle.     

Microfabric of folded quartz veins in metagreywackes: dislocation creep and subgrain rotation at high stress

The microfabrics of folded quartz veins in fine‐grained high pressure–low temperature metamorphic greywackes of the Franciscan Subduction Complex at Pacheco Pass, California, were investigated by optical microscopy, scanning electron microscopy including electron backscatter diffraction, and transmission electron microscopy. The foliated host metagreywacke is deformed by dissolution–precipitation creep, as indicated by the shape preferred orientation of mica and clastic quartz without any signs of crystal‐plastic deformation. The absence of crystal‐plastic deformation of clastic quartz suggests that the flow stress in the host metagreywacke remained below a few tens of MPa at temperatures of 250–300 °C. In contrast, the microfabric of the folded quartz veins indicates deformation by dislocation creep accompanied by subgrain rotation recrystallization. For the small recrystallized grain size of ～8 ± 6 μm, paleopiezometers indicate differential stresses of a few hundred MPa. The stress concentration in the single phase quartz vein is interpreted to be due to its higher effective viscosity compared to the fine‐grained host metagreywacke deforming by dissolution–precipitation creep. The fold shape suggests a viscosity contrast of one to two orders of magnitude. Deformation by dissolution–precipitation creep is expected to be a continuous process. The same must hold for folding of the vein and deformation of the vein quartz by dislocation creep. The microfabric suggests dynamic recrystallization predominantly by subgrain rotation and only minor strain‐induced grain boundary migration, which requires low contrasts in dislocation density across high‐angle grain boundaries to be maintained during climb‐controlled creep at high differential stress. The record of quartz in these continuously deformed veins is characteristic and different from the record in metamorphic rocks exhumed in seismically active regions, where high‐stress deformation at similar temperatures is episodic and related to the seismic cycle.     

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.     

Post-deformational annealing of calcite rocks

The evolution of microstructure and crystallographic preferred orientation (CPO) during post-deformational annealing was studied on three calcite rock types differing in purity and grain size: Carrara marble (98% calcite, mean grain size of 115 μm), Solnhofen limestone (96%, 5 μm) and synthetic calcite aggregates (99%, 7 μm). Samples were first deformed in torsion at 727 °C at a shear strain rate of 3 × 10^{− 4} s^{− 1} to a shear strain of 5 and subsequently heat-treated at 727 °C for various durations between 0 and 24 h. Microstructures and CPOs were analysed by optical microscopy, image analysis and electron backscatter diffraction (EBSD).All rock types deformed in the dislocation creep field at the same applied conditions, but their microstructures and CPOs after deformation and after annealing differed depending on starting grain size and material composition. In Carrara marble and in the synthetic calcite aggregate, a strong CPO developed during deformation accompanied by dynamic recrystallisation with significant changes in grain size. During annealing, widespread grain growth and subtle changes of CPO occurred, and equilibrated foam microstructures were approached after long annealing times. The CPO is the only feature in annealed samples indicating an earlier deformation phase, although it is not always identical to the CPO formed during deformation. In the more impure Solnhofen limestone, secondary phases on grain boundaries suppressed grain boundary mobility and prevented both the formation of a recrystallisation CPO during deformation and grain size modification during deformation and annealing.     

The transition between deformation regimes dominated by intercrystalline diffusion and intracrystalline creep evaluated by oxygen isotope thermometry

The dominant flow mechanism in tectonic processes depends on the rheological properties of geological materials and the physical conditions prevailing during deformation. We have evaluated the relative importance of intercrystalline diffusion and intracrystalline creep in crustal deformation in terms of temperature and grain size.Oxygen isotope thermometry has been used to elucidate the thermal environment obtaining during deformation and contemporaneous metamorphism of Dalradian rocks from Southwest Scotland. The temperature and grain size data, applied in conjunction with microstructural criteria for evaluating independent mechanisms of steady-state flow, allow recognition of a low-temperature deformation regime dominated by intercrystalline diffusion, and a high-temperature regime dominated by dislocation processes.The transition between the fields of intercrystalline diffusion and dislocation creep for quartz and calcite of 100 Mm grain size occurs at about 450° C and about 300° C, respectively. These empirically derived results are consistent with the temperature intervals over which intercrystalline diffusion and dislocation creep, respectively, are predicted to be dominant at geologically reasonable strain rates, as derived from theoretically formulated deformation mechanism maps for quartz and calcite.Grain growth may play an important role in delimiting the higher-temperature boundary of the intercrystalline diffusion field. Intercrystalline diffusion is the only deformation mechanism that involves mass transfer over distances that are large in relation to the grain size. This result has important consequences for geochemical transport phenomena.     

Transformation kinetics of polycrystalline aragonite to calcite: new experimental data,modelling, and implications

Experimental data are used to model the transformation rate of polycrystalline aragonite (grain diameter 80 m) to calcite. Optimized values for nucleation and growth rates were obtained by numerically fitting the overall transformation rates from 280° to 380°C and 0.10 MPa to an expression for a grain-boundary-nucleated and interface-controlled transformation. The nucleation rate is 4–5 orders of magnitude faster than for calcite nucleated within aragonite grains, and the growing in rate is slower below 300°C than for calcite growing in aragonite single crystals. The activation enthalpy for growth in polycrystalline aggregate is 247kJ/mol compared to 163 kJ/mol for growth in single crystals. Permanent deformation of the phases limits the elastic strain energy due to the 7% volume change and reduces the coherency of the calcite/aragonite interace. Theoretical expressions are used to extrapolate the data for nucleation and growth to other temperatures, and data from 0.10 to 400 MPa are used to evaluate the effect of pressure on the grain-boundary nucleation rate. Because of permanent deformation of the phases, the effective strain energy for nucleation is 0.55 kJ/mol, which is less than a quarter of the value for purely elastic deformation. These data are used to predict the percent transformation for various P-T-t paths; without heating during uplift partial preservation of aragonite in dry blueschist facies rocks can occur if the calcite stability field is entered at 235° C, and the kinetic data are also consistent with published P-T-t paths which include heating during uplift. The predicted percent transformation is relatively insensitive to variations in the initial grain size of the aragonite, but strongly dependent on the effective strain energy.     

Low-Temperature Plasticity of Naturally Deformed Calcite Rocks

Optical, cathodoluminescence and transmission electron microscope (TEM) analyses were conducted on four groups of calcite fault rocks, a cataclastic limestone, cataclastic coarse-grained marbles from two fault zones, and a fractured mylonite. These fault rocks show similar microstructural characteristics and give clues to similar processes of rock deformation. They are characterized by the structural contrast between macroscopic cataclastic (brittle) and microscopic mylonitic (ductile) microstructures. Intragranular deformation microstructures (i.e. deformation twins, kink bands and microfractures) are well preserved in the deformed grains in clasts or in primary rocks. The matrix materials are of extremely fine grains with diffusive features. Dislocation microstructures for co-existing brittle deformation and crystalline plasticity were revealed using TEM. Tangled dislocations are often preserved at the cores of highly deformed clasts, while dislocation walls form in the transitions to the fine-grained     

Partial high-pressure aragonitization of micritic limestones in an accretionary complex, Tavşanlı Zone, NW Turkey

Pelagic micritic limestones within an upper Cretaceous accretionary complex in the Tavşanlı Zone, NW Turkey, preserve textures indicating incomplete prograde transformation of micritic calcite to aragonite, representing the only known example of this type. Aragonitization starts at the central parts of the micritic limestone beds and advances towards the lower and upper parts of the layers at the expense of micrite. Micrite is very fine grained (<0.003 mm) and contains radiolaria, foraminifera and thin shell fragments. Aragonite forms large crystals, up to 3 cm across, with straight grain boundaries and c-axis mostly subparallel to the carbonate beds. Relict micritic portions are devoid of any aragonite grain. Stylolites characterized by the accumulation of clay minerals, Fe-Mn-hydroxides and quartz are concentrated in the upper and lower parts of the beds. Stylolite formation precedes aragonitization. Conditions of aragonitization are estimated as 200 ± 50 °C and 0.45–0.65 GPa, based on metamorphic mineral assemblages observed in associated basalts. Several features such as (i) constant composition of micritic calcite (98–99 mol.% CaCO₃) throughout individual beds, (ii) enormous grain size difference between micritic calcite and aragonite (up to 1200 times), and (iii) absence of any aragonite grains within the relict micritic portions suggest that kinetic rather than thermodynamic factors controlled selective aragonite formation in the central portions of carbonate layers.     

岩石扩散蠕变及其地质意义

总结了近十年来岩石扩散蠕变显微构造鉴别特征，研究方法的最新进展，矿物颗粒大小和流体（包括熔体和水）等是影响扩散蠕变的重要因素。扩散蠕变与位错蠕变和超塑性变形有密切关系，它的研究对大陆裂谷化过程，大陆碰撞带中岩石圈地幔强度弱化，中下地壳韧性剪切应变局限化以及与相变有关的矿物粒度细小化作用的分布有重要应用意义。     

超高压变质岩的塑性流变：显微构造和变形机制

超高压变质岩是大陆深俯冲作用的产物。超高压变质岩在深俯冲和快速折返过程中,经历了长距离地构造搬运和构造力的作用。其构造变形主要集中在韧性剪切带中,并发生强烈地塑性流变。研究超高压变质构造岩的显微构造及其变形机制对于深入了解大陆壳岩石在深俯冲过程中的流变学行为有十分重要的意义,山东仰口的超高压韧性剪切带中榴辉岩质和花岗质糜棱岩记录了超高压变形的历史。在超高压条件下的稳定矿物绿辉石、多硅白云母、兰晶石和钾长石具有不规则波状消光、亚晶界、核幔构造和动态重结晶等显微构造特征,TEM 研究揭示了大量的位错构造,表明位错蠕变是其主要的变形机制。在花岗质糜棱岩中,金红石在刚性矿物的压力影中沉积,细粒的石榴石条带平行片理延伸,都说明超高压变形过程中有流体存在,流体助力的物质扩散迁移是又一个重要的变形机制。依据现有的流变学定律估算的流变应力应该在几十兆帕以上。     

Macro‐and Microstructural,Textural Fabrics and Deformation Mechanism of Calcite Mylonites from Xar Moron‐Changchun Dextral Shear Zone,Northeast China

The calcite mylonites in the Xar Moron-Changchun shear zone show a significance dextral shearing characteristics. The asymmetric(σ-structure) calcite/quartz grains or aggregates, asymmetry of calcite c-axes fabric diagrams and the oblique foliation of recrystallized calcite grains correspond to a top-to-E shearing. Mineral deformation behaviors, twin morphology, C-axis EBSD fabrics, and quartz grain size-frequency diagrams demonstrate that the ductile shear zone was developed under conditions of greenschist facies, with the range of deformation temperatures from 200 to 300°C. These subgrains of host grains and surrounding recrystallized grains, strong undulose extinction, and slightly curved grain boundaries are probably results of intracrystalline deformation and dynamic recrystallization implying that the deformation took place within the dislocation-creep regime at shallow crustal levels. The calculated paleo-strain rates are between 10~(–7.87)s~(–1) and 10~(–11.49)s~(–1) with differential stresses of 32.63–63.94 MPa lying at the higher bound of typical strain rates in shear zones at crustal levels, and may indicate a relatively rapid deformation. The S-L-calcite tectonites have undergone a component of uplift which led to subhorizontal lifting in an already non-coaxial compressional deformation regime with a bulk pure shear-dominated general shear. This E-W large-scale dextral strike-slip movement is a consequence of the eastward extrusion of the Xing'an-Mongolian Orogenic Belt, and results from far-field forces associated with Late Triassic convergence domains after the final closure of the Paleo-Asian Ocean.     

The effect of dissolved magnesium on creep of calcite II: transition from diffusion creep to dislocation creep

We extended a previous study on the influence of Mg solute impurity on diffusion creep in calcite to include deformation under a broader range of stress conditions and over a wider range of Mg contents. Synthetic marbles were produced by hot isostatic pressing (HIP) mixtures of calcite and dolomite powders for different intervals (2–30 h) at 850°C and 300 MPa confining pressure. The HIP treatment resulted in high-magnesian calcite aggregates with Mg content ranging from 0.5 to 17 mol%. Both back-scattered electron images and chemical analysis suggested that the dolomite phase was completely dissolved, and that Mg distribution was homogeneous throughout the samples at the scale of about two micrometers. The grain size after HIP varied from 8 to 31 μm, increased with time at temperature, and decreased with increasing Mg content (>3.0 mol%). Grain size and time were consistent with a normal grain growth equation, with exponents from 2.4 to 4.7, for samples containing 0.5–17.0 mol% Mg, respectively. We deformed samples after HIP at the same confining pressure with differential stresses between 20 and 200 MPa using either constant strain rate or stepping intervals of loading at constant stresses in a Paterson gas-medium deformation apparatus. The deformation tests took place at between 700 and 800°C and at strain rates between 10⁻⁶ and 10⁻³ s⁻¹. After deformation to strains of about 25%, a bimodal distribution of large protoblasts and small recrystallized neoblasts coexisted in some samples loaded at higher stresses. The deformation data indicated a transition in mechanism from diffusion creep to dislocation creep. At stresses below 40 MPa, the strength was directly proportional to grain size and decreased with increasing Mg content due to the reductions in grain size. At about 40 MPa, the sensitivity of log strain rate to log stress, (n), became greater than 1 and eventually exceeded 3 for stresses above 80 MPa. At a given strain rate and temperature, the stress at which that transition occurred was larger for samples with higher Mg content and smaller grain size. At given strain rates, constant temperature, and fixed grain size, the strength of calcite in the dislocation creep regime increased with solute content, while the strength in the diffusion creep regime was independent of Mg content. The results suggest that chemical composition will be an important element to consider when solid substitution can occur during natural deformation.     

Can metamorphic reactions proceed faster than bulk strain?

Available constraints on metamorphic reaction rates derived from the study of natural systems are similar to, or slightly lower than, the bulk strain rates measured in the same rocks. Here, we explore whether this apparent relationship is merely coincidence or due to a more fundamental mechanistic link between reaction and strain. Grain boundary migration accommodated dislocation creep (GBMDC) or grain boundary diffusion creep (GBDC) (i.e. pressure solution), both of which involve dissolution-precipitation as we define it, will occur simultaneously with mineral reactions involving dissolution-precipitation in the presence of a non-zero deviatoric stress. The exact relationships between reaction and strain are different depending on whether GBMDC or GBDC is controlling strain, but the mechanistic link exists in both cases. We present theoretical arguments which show that bulk strain by GBMDC or GBDC, which may additionally be accommodated by processes not involving dissolution-precipitation, such as dislocation glide and climb or grain boundary sliding, should in most cases be somewhat faster than the bulk reaction rates as observed. With few exceptions, for natural metamorphic systems undergoing plastic deformation, strain rates provide an upper limit for bulk reaction rates occurring simultaneously in the same rocks. The data suggest that mineral reaction rates may typically be within one order of magnitude of the strain rate.Editorial responsibility: T.L. Grove     

多相糜棱岩内第二相对应变局部化的影响——以秦岭群花岗质糜棱岩为例

糜棱岩韧性变形发生的应变局部化过程,尤其是多相糜棱岩第二相对基质相变形的影响一直是显微构造研究难点.研究表明糜棱岩借助颗粒边界滑移实现多相混合,形成多矿物相集合体.在多相糜棱岩内,第二相在基质相颗粒边界施加齐纳阻力,牵制基质相颗粒边界的迁移速率,破坏基质相颗粒的动态平衡过程,使基质相颗粒位于古应力计对应的颗粒粒度以下,...     

Microstructural evolution and trace element mobility in Witwatersrand pyrite

Microstructural analysis of pyrite from a single sample of Witwatersrand conglomerate indicates a complex deformation history involving components of both plastic and brittle deformation. Internal deformation associated with dislocation creep is heterogeneously developed within grains, shows no systematic relationship to bulk rock strain or the location of grain boundaries and is interpreted to represent an episode of pyrite deformation that predates the incorporation of detrital pyrite grains into the Central Rand conglomerates. In contrast, brittle deformation, manifest by grain fragmentation that transects dislocation-related microstructures, is spatially related to grain contacts and is interpreted to represent post-depositional deformation of the Central Rand conglomerates. Analysis of the low-angle boundaries associated with the early dislocation creep phase of deformation indicates the operation of <010>{100} slip systems. However, some orientation boundaries have geometrical characteristics that are not consistent with simple <010>{100} deformation. These boundaries may represent the combination of multiple slip systems or the operation of the previously unrecognized <001>{120} slip system. These boundaries are associated with order of magnitude enrichments in As, Ni and Co that indicate a deformation control on the remobilization of trace elements within pyrite and a potential slip system control on the effectiveness of fast-diffusion pathways. The results confirm the importance of grain-scale elemental remobilization within pyrite prior to their incorporation into the Witwatersrand gold-bearing conglomerates. Since the relationship between gold and pyrite is intimately related to the trace element geochemistry of pyrite, the results have implications for the application of minor element geochemistry to ore deposit formation, suggest a reason for heterogeneous conductivity and localized gold precipitation in natural pyrite and provide a framework for improving mineral processing.     

Crystal plasticity and superplasticity in quartzite; A natural example

Plastically deformed quartzites from the Betic Movement Zone (Betic Cordilleras, Spain) exhibit microstructures indicative of crystal plasticity on a mineral grain scale. Quartzites with dynamically recrystallized grain sizes larger than 10 μm have strong crystallographic preferred orientations, narrow grain boundaries, little creep damage, and an inverse proportionality of dislocation density and grain size. Mylonites with grain sizes smaller than 10 μm have low crystallographic preferred orientations, wide grain boundaries (up to 1000 Å), abundant creep damage, and decreasing dislocation density with diminishing grain size. This is thought to reflect a clear-cut shift in deformational regimes from dislocation creep to superplastic flow at 10 μm grain size. Superplasticity can be acquired by quartzites which suffer dynamic recrystallization to grain sizes smaller than 10 μm during an initial dislocation creep stage. Dislocation motion is the major accomodating mechanism for strain incompatibilities that arise during grain-boundary sliding in the mylonites.It seems reasonable to estimate flow stresses from unbound dislocation densities and dynamically recrystallized grain sizes in the tectonite specimens. In the mylonites, dynamically recrystallized grain size probably reflects the stress magnitude before the shift in deformational mechanisms, and an estimate for late stage stresses is provided by unbound dislocation densities. In both deformational regimes the flow strength appears to depend on the extent of dynamic recrystallization.     

Fluid-assisted fracturing,cataclasis, and resulting plastic flow in mylonites from the Moresby Seamount detachment,Woodlark Basin

The Moresby Seamount detachment (MSD) in the Woodlark Basin (offshore Papua New Guinea) is a large active low-angle detachment excellently exposed at the seafloor, and cutting through mafic metamorphic rocks. Hydrothermal infiltration of quartz followed by that of calcite occurred during cataclastic deformation. Subsequent deformation of these a priori softer minerals leads to mylonite formation in the MSD. This study aims at a better understanding of the deformation mechanism switch from cataclastic to plastic flow. Deformation fabrics of the fault rocks were analyzed by light-optical microscopy. Rheologically critical phases were mapped to determine distributions and area proportions, and EBSD was used to measure crystallographic preferred orientation (CPO). Strong calcite CPOs indicate dominant dislocation creep. Quartz CPOs, however, are weak and more difficult to interpret, suggesting at least some strain accommodation by diffusion creep mechanisms. When quartz aggregates are intermixed with the polymineralic mylonite matrix diffusion creep grain boundary sliding may be dominant. The syntectonic conversion from mafic cataclasites to more siliceous and carbonaceous mylonites induced by hydrothermal processes is a critical weakening mechanism enabling the MSD to at least intermittently plastic flow at low shear stresses. This is probably a crucial process for the operation of low-angle detachments in hydrated and dominantly mafic crust.     

Dolomite microstructures between 390° and 700 °C: Indications for deformation mechanisms and grain size evolution

Dolomitic marble on the island of Naxos was deformed at variable temperatures ranging from 390 °C to >700 °C. Microstructural investigations indicate two end-member of deformation mechanisms: (1) Diffusion creep processes associated with small grain sizes and weak or no CPO (crystallographic preferred orientation), whereas (2) dislocation creep processes are related with larger grain sizes and strong CPO. The change between these mechanisms depends on grain size and temperature. Therefore, sample with dislocation and diffusion creep microstructures and CPO occur at intermediate temperatures in relative pure dolomite samples. The measured dolomite grain size ranges from 3 to 940 μm. Grain sizes at T_max >450 °C show an Arrhenius type evolution reflecting the stabilized grain size in deformed and relative pure dolomite. The stabilized grain size is five times smaller than that of calcite at the same temperature and shows the same Arrhenius-type evolution. In addition, the effect of second phase particle influences the grain size evolution, comparable with calcite. Calcite/dolomite mixtures are also characterized by the same difference in grain size, but recrystallization mechanism including chemical recrystallization induced by deformation may contribute to apparent non-temperature equilibrated Mg-content in calcite.     

Ca-Mg inter-diffusion in synthetic polycrystalline dolomite-calcite aggregate at elevated temperatures and pressure

This study measures the reaction rate of dolomite and aragonite (calcite) into Mg-calcite at 800, 850, and 900°C and 1.6 GPa. The dry synthetic dolomite-aragonite aggregate transformed very rapidly into dolomite-calcite polycrystalline aggregate while Mg-calcites formed at a relatively slow rate, becoming progressively richer in Mg with run time. We modeled the reaction progress semi-empirically by the first-order rate law. The temperature dependence of the overall transport rate of MgCO₃ into calcite can be described by the kinetic parameters (E?=?231.7 kJ/mol and A _o ?=?22.69 h^?1). Extrapolation using the Arrhenius equation to the conditions during exhumation of UHPM rocks indicates that the reaction of dolomite with aragonite into Mg-saturated calcite can be completed as the P-T path enters the Mg-calcite stability field in a geologically short time period (<1 Ky). On the other hand, the extrapolation of the rate to prograde metamorphic conditions reveals that the Mg-calcite formed from dolomitic marble in the absence of metamorphic fluid may not reach Mg-saturation until temperatures corresponding to high-grade metamorphism (e.g., >340°C and >10 My). SEM-EDS analysis of individual calcite grains shows compositional gradients of Mg in the calcite grains. The Mg-Ca inter-diffusion coefficient at 850°C is around 1.68?×?10^?14 m²/sec if diffusion is the major control of the reaction. The calculated closure temperatures for Ca-Mg inter-diffusion as a function of cooling rate and grain size reveal that Ca/Mg resetting in calcite in a dry polycrystalline carbonate aggregate (with grain size around 1 mm) may not occur at temperatures below 480°C at a geological cooling rate around 10°C/My, unless other processes, such as short-circuit interdiffusion along grain boundaries and dislocations, are involved.