Progress Of Metamorphic Reactions Research Articles

3D reconstruction of fabric and metamorphic domains in a slice of continental crust involved in the Alpine subduction system: the example of Mt. Mucrone (Sesia–Lanzo Zone, Western Alps)

Geological mapping, multiscale structural analysis, and estimations of the degree of fabric evolution and of reaction progress allow the construction of a 3D quantitative model of structural and metamorphic gradients in a portion of continental crust deeply involved in the Alpine subduction system and mainly structured under eclogite-facies conditions before the continental collision. The investigated and modelled rocks outcrop in the surroundings of Mt. Mucrone, in the central Sesia–Lanzo Zone. The Geomodeller® software allowed a quantitative 3D estimation of domains characterized by homogeneous fabric evolution and metamorphic reaction progress (DFE and DRP, respectively) for two of the seven structural and metamorphic imprints detected in this area: the D2-eclogitic and D5-greenschist stages, which are the most pervasive at km scale. Such a 3D modelling clarifies mutual relationships between fabric and metamorphic gradients and indicates that: (i) DFE and DRP are closely related regardless of the rock type; (ii) the syn-deformational thermal regime can influence the degree of metamorphic transformation, if DFE remains below the 60% threshold; (iii) the phase transitions can not be properly implemented in quantitative geodynamic modelling without considering the heterogeneity of reaction progress and fabric evolution.

Contrasting fracture patterns induced by volume-increasing and -decreasing reactions: Implications for the progress of metamorphic reactions

Hydration and dehydration reactions during metamorphism cause drastic changes in porosity and fluid pressure in a rock, and some of these reactions proceed with fracturing. For this paper we have developed a model for coupled hydraulic–chemical–mechanical processes, using the distinct element method, in order to understand the relationships among chemical reactions, fluid flow, and fracturing during metamorphism. The model considers fluid advection along fractures and the dependence of reaction rates on fluid pressure. With the help of the model, volume-decreasing dehydration reactions and volume-increasing hydration reactions are investigated in detail as analogs of prograde and retrograde reactions, respectively. Both types of reaction proceed inwards from drained boundaries or fractures, but they show contrasting fracture patterns. A volume-decreasing dehydration reaction produces tree-type fractures, with the new fractures generated as branches of a pre-existing fracture. A volume-increasing hydration reaction produces a polygonal network of fractures, where new fractures nucleate at sites far from the pre-existing fracture, and extend to form T-junctions. These contrasting fracture patterns are essentially controlled by solid volume changes during reaction rather than by the fluid pressure gradient. In the case of the tree-type fractures, the fractures are continuously generated at the reaction front, and the reaction proceeds smoothly by positive feedbacks between reaction, fracturing, and fluid flow. In contrast, in a volume-increasing hydration reaction, fracturing initially occurs in response to the irregular boundary shape, but as the reaction progresses, a compressive stress field is generated, which inhibits further fracture generation. The compressive stress field also prevents fluid flow by closing the pre-existing fractures, which slows down the reaction. These contrasting feedback systems between volume-decreasing dehydration and volume-increasing hydration reactions help to explain why prograde metamorphic reactions proceed pervasively, and why the progress of a retrograde hydration reaction tends to be localized along fractures so that relics of peak metamorphism are commonly preserved.

Metamorphic reaction rate controlled by fluid pressure not confining pressure: implications of dehydration experiments with gypsum

Pressure is a key control on the progress of metamorphic reactions. When fluids are present in rocks, the fluid pressure is commonly different to the load supported by the solid framework. Here, we show experimentally that, when the two pressures are varied independently, fluid pressure exerts the dominant control on reaction rate, even when the rock is compacting. We present 35 experiments on gypsum dehydration with independently controlled confining pressure, pore fluid pressure and temperature. Results show that a pore fluid pressure decrease at constant confining pressure has a strong effect on the average rate of the reaction. A decrease in confining pressure at constant pore fluid pressure has relatively little effect. Our results have implications for reaction kinetics: even though the product phase is supporting more and more load as reaction proceeds, that load does not appear to exert a chemical effect. On the large scale, our results imply that changes in fluid pressure will drive or stop the progress of metamorphic reactions. When estimating depth at which a metamorphic devolatilization reaction occurs, knowledge of the pore fluid pressure may be necessary rather than commonly used lithostatic pressure. This is relevant for basin diagenesis, mineralization in hydrothermal systems and chemical evolution after pore fluid pressure is perturbed by earthquakes.

Porphyroblast crystallization: linking processes, kinetics, and microstructures

Analysis of the processes, kinetics, and microstructures that characterize porphyroblast crystallization identifies the primary factors that govern the progress of metamorphic reactions and highlights the importance of feedbacks among those factors. Commonly, the kinetics of nucleation and the kinetics of intergranular diffusion are rate-limiting in porphyroblast crystallization. That finding should inspire petrologic vigilance, as it implies strong potential for significant thermal overstepping of reactions, crystallization at high levels of chemical affinity, reactions that span protracted intervals of time and temperature, and limited length scales for chemical equilibration.

Permeability of the continental crust: dynamic variations inferred from seismicity and metamorphism

Geofluids (2010) 10, 193–205AbstractThe variation of permeability with depth can be probed indirectly by various means, including hydrologic models that use geothermal data as constraints and the progress of metamorphic reactions driven by fluid flow. Geothermal and metamorphic data combine to indicate that mean permeability (k) of tectonically active continental crust decreases with depth (z) according to log k ≈ −14–3.2 log z, where k is in m2 and z in km. Other independently derived, crustal‐scale k–z relations are generally similar to this power‐law curve. Yet there is also substantial evidence for local‐to‐regional‐scale, transient, permeability‐generation events that entail permeabilities much higher than these mean k–z relations would suggest. Compilation of such data yields a fit to these elevated, transient values of log k ≈ −11.5–3.2 log z, suggesting a functional form similar to that of tectonically active crust, but shifted to higher permeability at a given depth. In addition, it seems possible that, in the absence of active prograde metamorphism, permeability in the deeper crust will decay toward values below the mean k–z curves. Several lines of evidence suggest geologically rapid (years to 103 years) decay of high‐permeability transients toward background values. Crustal‐scale k–z curves may reflect a dynamic competition between permeability creation by processes such as fluid sourcing and rock failure, and permeability destruction by processes such as compaction, hydrothermal alteration, and retrograde metamorphism.

Three-dimensional evaluation of fabric evolution and metamorphic reaction progress in polycyclic and polymetamorphic terrains: a case from the Central Italian Alps

Abstract The 3D reconstruction of geological bodies is an excellent tool for the representation of crustal structures and is applied here to understand related heterogeneities in the grain-scale fabrics; the western portion of the Languard–Tonale Alpine tectono-metamorphic unit (Austroalpine domain, Central Alps) allows evaluation of the per cent volume of textural reworking during polyphase pre-Alpine and Alpine deformations. The structural and metamorphic overprinting during the last deformation imprint involved less than 50% of rock volume; this estimate is obtained by discriminating domains that homogeneously recorded structural and metamorphic re-equilibration during crenulation–decrenulation cycles. These domains are reconstructed using a geograhpical information system (GIS) to manipulate field data and interpretative cross-sections as a means to constrain their 3D volumes. The degree of fabric evolution is integrated at the microscale with the estimate of the reactants/products ratio to infer the progress of metamorphic transformation related to advancing degree of mechanical reactivation. The correlation between degree of fabric evolution and progress of synkinematic metamorphic reactions shows that differences between pristine mineral assemblages v. pre-existing fabrics influence the rate of reaction accomplishment. Fabric evolution and degree of metamorphic transformation increase proportionally once above the threshold value of 60% of volume affected by fabric rejuvenation; metamorphic degree also influences the progress of metamorphic reactions.

Effect of metamorphic reactions on thermal evolution in collisional orogens

AbstractThe effects of metamorphic reactions on the thermal structure of a collisional overthrust setting are examined via forward numerical modelling. The 2D model is used to explore feedbacks between the thermal structure and exhumation history of a collisional terrane and the metamorphic reaction progress. The results for average values of crustal and mantle heat production in a model with metapelitic crust composition predict a 25–40 °C decrease in metamorphic peak temperatures due to dehydration reactions; the maximum difference between the P–T–t paths of reacting and non‐reacting rocks is 35–45 °C. The timing of the thermal peak is delayed by 2–4 Myr, whereas pressure at peak temperature conditions is decreased by more than 0.2 GPa. The changes in temperature and pressure caused by reaction may lead to considerable differences in prograde reaction pathways; the consumption of heat during dehydration may produce greenschist facies mineral assemblages in rocks that would have otherwise attained amphibolite facies conditions in the absence of reaction enthalpy. The above effects, although significant, are produced by relatively limited metamorphic reaction which liberates only half of the water available for dehydration over the lifetime of the prograde metamorphism. The limited reaction is due to the lack of heat in a model with the average thermal structure and relatively fast erosion, a common outcome in the numerical modelling of Barrovian metamorphism. This problem is typically resolved by invoking additional heat sources, such as high radiogenic heat production, elevated mantle heating or magmatism. Several models are tested that incorporate additional radiogenic heat sources; the elevated heating rates lead to stronger reaction and correspondingly larger thermal effects of metamorphism. The drop in peak temperatures may exceed 45 °C, the maximum temperature differences between the reacting and non‐reacting P–T–t paths may reach 60 °C, and pressure at peak temperature conditions is decreased by more than 0.2 GPa. Field observations suggest that devolatilization of metacarbonate rocks can also exert controls on metamorphic temperatures. Enthalpies were calculated for the reaction progress recorded by metacarbonate rocks in Vermont, and were used in models that include a layer of mixed metapelite–metacarbonate composition. A model with the average thermal structure and erosion rate of 1 mm year−1 can provide only half of the heat required to drive decarbonation reactions in a 10 km thick mid‐crustal layer containing 50 wt% of metacarbonate rock. Models with elevated heating rates, on the other hand, facilitated intensive devolatilization of the metacarbonate‐bearing layer. The reactions resulted in considerable changes in the model P–T–t paths and ∼60 °C drop in metamorphic peak temperatures. Our results suggest that metamorphic reactions can play an important role in the thermal evolution of collisional settings and are likely to noticeably affect metamorphic P–T–t paths, peak metamorphic conditions and crustal geotherms. Decarbonation reactions in metacarbonate rocks may lead to even larger effects than those observed for metapelitic rocks. Endothermic effects of prograde reactions may be especially important in collisional settings containing additional heat sources and thus may pose further challenges for the ‘missing heat’ problem of Barrovian metamorphism.

Numerical simulation of kinetically-controlled calc-silicate reactions and fluid flow with transient permeability around crystallizing plutons

Numerical simulations were performed to examine the relationships between variable contact-aureole permeability, the kinetics of calc-silicate reactions, and the fluxes of mixed CO 2 -H 2 O fluids around a crystallizing granite laccolith. The role of magmatic water was explicitly considered. The Notch Peak contact-metamor- phic aureole in Utah was used to define the stratigraphy of the model domain and rock compositions. Reactions that were considered include the major isograd-defining reactions that occurred in the Notch Peak aureole. The half-space model domain had the laccolith, 2 km thick at the middle apex. Only 1-phase fluid flow was considered. Results show that the evolution of the fluid flow-field is highly dependent on the pressure (P) boundary condition at the top of the model domain. When P at the top boundary is allowed to increase, P in most of the top half of the domain eventually exceeds the lithostatic pressure plus the assumed tensile strength of rocks of 15 MPa. This boundary condition simulates an unvented flow-system. A more realistic boundary condition, one that simulates a system that is able to vent to the surface, is when Pat the top boundary is held at a hydrostatic pressure. In this case, the flow-field is determined largely by pressure gradients between the overpressured magmatic fluid exsolving from the pluton and the lower pressures at the domain boundaries. Fracturing is predicted to occur early after pluton emplacement as the pore fluid is heated and metamorphic reactions produce CO 2 . Although fracturing and reaction-enhanced porosity and permeability influence the local flow-field, the domain-scale flow-field is controlled by long-distance pressure gradients. The domain-scale flow-field and tem- perature distribution impose the largest control on the distribution of major mineral assemblages in the metamorphic aureole. Transient changes in permeability due to fracturing and volume changes in the solid matrix that accompany reactions have a smaller control on the distribution of minerals. The simulations predict significant overstepping and coeval progress of metamorphic reactions. Reaction rates range from 5x 10 -10 to 10 -14 kmol/m 2 /sec, depending on the actual P-T-XCO 2 f conditions and the abundance of the rate-controlling mineral. Throughout the metamorphic aureole, XCO 2 f approaches 1 as CO 2 evolved by reactions displaces H 2 0. High pore pressures prevent magmatic H 2 O from infiltrating the aureole until pressures drop when reactions are approaching completion. Consequently, only in the inner aureole, which is eventually infiltrated by magmatic H 2 O, can minerals such as wollastonite and vesuvianite be produced. An integrated H 2 O flux of 3×10 4 kmol/m 2 is required to produce the width of the model wollastonite zone by 20 ky. Because of pressure gradients, CO 2 that is produced in the inner aureole flows outward into colder rocks even before these rocks are heated. The T-XCO 2 f paths in outer aureole rocks cut across the H 2 O-CO 2 solvus, which predicts that in nature H 2 O and CO 2 should unmix and behave as two separate fluid phases. The average yearly CO 2 flux at the top of the model domain, 2.3 × 10 3 mol/m 2 /y, is comparable to fluxes of metamorphic carbonic fluids in active geothermal fields.

Effects of fluid circulation in subducting crust on Nankai margin seismogenic zone temperatures

Vigorous fluid circulation maintained in newly subducted ocean crust significantly affects subduction zone temperatures on the Nankai margin, Japan. The shallow part of the igneous ocean crust is pervasively fractured and thus highly permeable, allowing vigorous hydro thermal circulation. This circulation has been recognized as an important control on the thermal budget and evolution of ocean crust worldwide. However, existing subduction zone thermal models either do not include hydrothermal circulation in ocean crust or assume that it abruptly stops upon subduction. Here we use a conductive proxy to incorporate the thermal effects of high Nusselt number fluid circulation in subducting crust into a subduction zone thermal model. Hydrothermal circulation reduces temperatures in the seismogenic zone of the Nankai margin plate boundary fault by ~20 °C at the updip limit of seismicity and ~100 °C at the downdip limit. With improved thermal models for subduction zones that include the effects of hydrothermal circulation in subducting crust, estimates of metamorphic reaction progress and interpretations of fault zone processes on various margins may need to be revisited.

Fluid Immiscibility in Metamorphic Rocks

Evidence of fluid immiscibility in metamorphic rocks comes, in the best of all worlds, from fluid inclusion observations. In fact, fluid immiscibility should be anticipated over the full range of metamorphic conditions provided that appropriate bulk fluid compositions are present. In a review paper entitled “ Metamorphic Fluids: the Evidence from Fluid Inclusions ,” Crawford and Hollister (1986) summarized: “The role of fluid immiscibility in metamorphism was only recognized after serious study of fluid inclusions in metamorphic rocks was underway in the late 1970s and has yet to attract the attention of more than a handful of petrologists studying metamorphic rocks.” Since then, tremendous progress has been made. A huge amount of fluid inclusion studies have convincingly shown that immiscible fluids can be present in rocks of all bulk compositions and at all P-T conditions. Thermodynamic databases of minerals, solid solutions and fluid mixtures along with the development of Gibbs free energy minimization programs now allow for detailed prediction of the evolution of phase assemblages and mineral compositions in P-T space. Significant progress has also been made in understanding reaction kinetics, diffusional and convective mass transport and the interpretation of fluid-rock interaction during metamorphism. Given the vast fluid inclusion evidence of immiscibility it would appear, however, that phase petrology and mass transport in any specific rock where fluids are involved are not correctly interpreted if possible or proven immiscibility is not considered. Relatively few studies directly link mineral reactions with immiscible fluids. Therefore, this review aims at working out the effects of fluid immiscibility on the progress of metamorphic reactions and fluid transport over a wide range of conditions. Questions arise directly from the fluid inclusion record in metamorphic rocks. This chapter, however, is organized the other way round. At first it summarizes phase relations of fluid systems pertinent to …

Relationships between very low‐grade metamorphism and tectonic deformation: examples from the southern Central Iberian Zone (Iberian Massif, Variscan Belt)

We have studied the syn‐kinematic very low‐grade metamorphism in a polyphase Variscan deformed region using X‐ray diffraction techniques. Two phases of regional metamorphism are related to their respective episodes of penetrative deformation in the southern Central Iberian Zone. The data obtained suggest that the rocks did not reach metamorphic equilibrium, but strain favoured the progress of mineral reactions in the more deformed parts. The first deformation is Devonian in age and consists in a heterogeneous ductile shearing coeval with large‐scale recumbent folding that were produced under high‐anchizone to epizone metamorphic conditions. The heterogeneity of the shearing originated strain gradients that can be said to enhance the growth of new minerals and the illite polytype transformation in the highly strained overturned limb of the preserved pile of recumbent folds, but illite crystallinity remained constant throughout the structure. The second deformation is Mid‐Carboniferous in age and consists in an upright folding that took place under late diagenesis to low‐anchizone metamorphic conditions. The distribution of mineral parageneses and illite crystallinity across one of the upright folds suggests that strain gradients favoured the metamorphic reaction progress from the hinge (low strain) towards the limbs (high strain). Other characteristics of the region such as a metamorphic gap associated with an unconformity at the base of the Lower Carboniferous rocks, or cryptic contacts aureoles surrounding volcanic intercalations and a large granitic batholith, are also studied.

Near infrared spectra of muscovite, Tschermak substitution, and metamorphic reaction progress: Implications for remote sensing

Near infrared (NIR) spectra of Precambrian metagraywacke in the Black Hills, South Dakota, demonstrate that reflectance spectroscopy can be used to monitor progressive changes in mineral chemistry as a function of metamorphic grade. The wavelength of a combination Al-O-H absorption band in muscovite, measured using both laboratory and field-portable NIR spectrometers, shifts from 2217 nm in the biotite zone to 2199 nm in the sillimanite + K-feldspar zone. The band shift corresponds to an increase in the Al vi content of muscovite, determined by electron microprobe, and is thus a monitor of Al 2 Si -1 (Fe,Mg) -1 (Tschermak) exchange. Spectroscopic measurements such as these are useful in the case of aluminum-deficient rocks, which lack metamorphic index minerals or appropriate assemblages for thermobarometric studies, and in low-grade rocks (subgarnet zone), which lack quantitative indicators of metamorphic grade and are too fine grained for petrographic or microprobe studies. More important, spectroscopic detection of mineral-chemical variations in metamorphic rocks provides petrologists with a tool to recover information on metamorphic reaction histories from high-spectral-resolution aircraft or satellite remote sensing data.

Contact metamorphism of layered carbonate-shale sequences in the Oslo Rift. II: Migration of isotopic and reaction fronts around cooling plutons

Intrusion of felsic plutons of Permian age into Lower Paleozoic shale-carbonate sequences caused extensive contact metamorphism in the Oslo Rift. Oxygen and carbon isotope data from both carbonates and shales are presented to demonstrate that the progress of metamorphic reactions in the contact aureole around the Drammen granite was, to a large extent, controlled by the infiltration of water-rich fluids of a magmatic oxygen and with a carbon isotope signature. Highly integrated fluid fluxes were confined to shale-rich lithologies; massive limestones were relatively impermeable to fluid flow. The spatial distribution of isotopic and reaction fronts in the aureole and coupled 18O 13C isotope variations of carbonates from interlayered shale-carbonate units fits well to a 1-D transport model, with fluid flow away from the intrusive contact; whereas the same isotope variations in limestones containing thin shale intercalations do not. The high fluid fluxes estimated from 1-D models (> 2000 m 3/m 2) require either focused flow of magmatic fluids in the study area or large-scale convective flow, possibly involving fluids of meteoric origin. Limited kinetic broadening of the oxygen isotopic front indicates fluid fluxes of < 10 −8 m/s. Fluid flow must have persisted for > 10 4 yr to have moved the oxygen front 1000 m from the intrusive contact. Simple 2-D transport models, involving dominant fluid flow along aquifers or fractures and involving diffusion as well as advection, explain most of the coupled 18O 13C trends reported for carbonates from contact aureoles.