Supplementary material relevant to the manuscript: “Fast and spatially heterogeneous cooling rates at amphibolite-facies conditions reveal the significance of local heat sources: a case study from the Lepontine Alps (Switzerland)” by Tagliaferri, A., Moulas, E., Schmalholz, S. M., & Schenker, F. L. Submitted to American Journal of Science.
A kyanite-eclogite that occurs as lenses in orthogneisses from Thermes village was used to unravel the pressuretemperature-time path of the (U)HP rocks from the Rhodope. The peak-pressure mineral assemblage is omphacite, garnet, kyanite, phengite, rutile, apatite and zircon. Quartz is absent from the matrix and it can be found either as inclusions in garnet or as post-peak veins. This late quartz contains primary and secondary fluid inclusions implying the presence of a fluid phase during post high-pressure metamorphism. Kyanite is never observed in direct contact with quartz being armoured by an intervening stripe of plagioclase which itself shows zoning, becoming increasingly albitic towards quartz. Plagioclase formation at the Ky-Qtz interface requires certain Na and Ca influx that was provided by matrix omphacite. Two types of symplectites were formed by reaction between omphacite and kyanite. Firstly, corundum+plagioclase symplectites were formed at the expense of the jadeitic component of omphacite during decompression. Subsequently, the residual diopsidic component of omphacite reacted with the already formed corundum to give rise to spinel+plagioclase symplectites. The previous mechanisms demonstrate metasomatism in the micro-scale by diffusion controlled processes. During decompression matrix omphacite was decomposed to amphibole+plagioclase symplectites which reacted with garnet to form coronas consisting of two amphiboles (orthoand clino-), plagioclase, ilmenite and magnetite. Biotite and plagioclase are also found as symplectites replacing phengite during decompression. Thermodynamic modelling of the symplectitic domains that replace kyanite shows that the stability of these domains is sensitive to the effective local chemical composition; in addition, analysis of phase relationships demonstrated the existence of the observed assemblages at pressures lower than 1.3GPa.
Abstract. Residual pressure can be preserved in mineral inclusions, e.g. quartz-in-garnet, after exhumation due to differential expansion between inclusion and host crystals. Raman spectroscopy has been applied to infer the residual pressure and provides information on the entrapment temperature and pressure conditions. However, the amount of residual pressure relaxation cannot be directly measured. An underestimation or overestimation of residual pressure may lead to significant errors between calculated and actual entrapment pressure. This study focuses on three mechanisms responsible for the residual pressure modification: (1) viscous creep; (2) plastic yield; (3) proximity of inclusion to the thin-section surface. Criteria are provided to quantify how much of the expected residual pressure is modified due to these three mechanisms. An analytical solution is introduced to demonstrate the effect of inclusion depth on the residual pressure field when the inclusion is close to the thin-section surface. It is shown that for a quartz-in-garnet system, the distance between the thin-section surface and inclusion centre needs to be at least 3 times the inclusion radius to avoid pressure release. In terms of viscous creep, representative case studies on a quartz-in-garnet system show that viscous relaxation may occur from temperatures as low as 600–700 ∘C depending on the particular pressure–temperature (P–T) path and various garnet compositions. For quartz entrapped along the prograde P–T path and subject to viscous relaxation at peak T above 600–700 ∘C, its residual pressure after exhumation may be higher than predicted from its true entrapment conditions. Moreover, such a viscous resetting effect may introduce apparent overstepping of garnet nucleation that is not related to reaction affinity.
Strength contrasts and spatial variations in rheology are likely to produce significant stress differences in the Earth’s crust. The buildup and the relaxation of stresses have important consequences for the state of stress of the brittle crust, its deformational behaviour and seismicity. We performed scaled analogue experiments of a classic wedge-type geometry wherein we introduced a weak, fluid-filled body representing a low-stress heterogeneity. The experiments were coupled to direct pressure measurements that revealed significant pressure differences from their surrounding stressed matrix. The magnitude of the pressure variations is similar to the magnitude of the differential stress of the strongest lithology in the system. When rocks with negligible differential stresses are considered, their pressure can be more than twice larger than the surrounding lithostatic stress. The values of the pressure variations are consistent with the stresses that are estimated in analytical studies. This behaviour is not restricted to a particular scale or rheology, but it requires materials that are able to support different levels of stress upon deformation. For non-creeping rheological behaviours, the stress and pressure variations are maintained even after deformation ceases, implying that these stress variations can be preserved in nature over geological timescales.
The Rhodope Metamorphic Complex (RMC) in northern Greece and southern Bulgaria is a synmetamorphic nappe pile that developed during the Alpine-Himalayan orogen. The nappe system is deformed and forms dome-andbasin structures that indicate syn- to post-convergent exhumation. High-pressure rocks showing variable degrees of retrogression occur in the intermediate high-pressure imbricate units. We document the deformation style and present new thermobarometric and geochronological constraints for the Kesebir-Kardamos dome in the eastern RMC in an attempt to comprehend the major mechanisms involved in the exhumation of high-pressure (HP) and high-temperature (HT) rocks. Thermodynamic modeling and thermobarometry applied to the high-grade rocks from the intermediate thrust sheets of the core of the Kardamos dome suggest peak conditions at 1.2GPa and ca. 750 C. U-Pb SHRIMP dating of zircons from rocks from the same unit reveals Early Cretaceous (144 Ma) as the time of the major metamorphic overprint whereas some zircon rims yield reset ages at Eocene times (53 and 44 Ma). Kinematic indicators from the imbricate units suggest a continuum from ductile to brittle conditions during exhumation. The exhumed high-grade rocks were covered by marine sediments soon after their exhumation (Lutetian-Priambonian?). Slumps in sediments suggest that sedimentation took place in a tectonically active environment. Our new structural, petrological and geochronological data suggest that the major shear zone in the core of the Kesebir-Kardamos dome is equivalent to the Nestos-Chepelare suture zone.
Abstract Quantifying natural processes that shape our planet is a key to understanding the geological observations. Many phenomena in the Earth are not in thermodynamic equilibrium. Cooling of the Earth, mantle convection, mountain building are examples of dynamic processes that evolve in time and space and are driven by gradients. During those irreversible processes, entropy is produced. In petrology, several thermodynamic approaches have been suggested to quantify systems under chemical and mechanical gradients. Yet, their thermodynamic admissibility has not been investigated in detail. Here, we focus on a fundamental, though not yet unequivocally answered, question: which thermodynamic formulation for petrological systems under gradients is appropriate—mass or molar? We provide a comparison of both thermodynamic formulations for chemical diffusion flux, applying the positive entropy production principle as a necessary admissibility condition. Furthermore, we show that the inappropriate solution has dramatic consequences for understanding the key processes in petrology, such as chemical diffusion in the presence of pressure gradients.
Retrogression and hydration commonly affect large portions of the crust, causing variable degrees of chloritization, sericitization and/or serpentinization depending on the protolith and the conditions of fluid ingression. Retrograde overprint involving hydration is a strongly exothermic process, and leads to a thermal perturbation around the pressure–temperature conditions of hydration, which in the case of chloritization of felsic rocks typically occurs at <500°C. These conditions of retrogression overlap with the closure temperatures of some isotopic systems commonly used for geochronology, for example 40Ar/39Ar in micas and feldspars. The exothermicity of hydration therefore disturbs the recorded apparent ages and cooling histories of reworked terranes. Using an average metapelite composition as case study, we estimate that hydration and retrogression of a high-grade amphibolite facies assemblage to a low-grade greenschist paragenesis involves approximately a twofold increase of the mineral-bound water content and releases about 50 kJ.kg-1 latent heat. Using a simple 1-dimensional numerical model, we solve the heat equation for a steady-state continental geotherm that is advected towards the surface and track the cooling rates for markers that exhume from different depths. Assuming enthalpy production at 380°C to simulate exothermic hydration, the cooling rate is significantly reduced until the markers are exhumed to the temperature/depth of hydration and reaction. The calculated cooling paths feed into KADMOS (Moulas & Brandon, 2022), a set of MATLAB routines designed to calculate apparent 40Ar/39Ar ages as function of customized thermal histories. KADMOS solves the equation of 40Ar production from 40K decay and thermally-activated diffusive loss of 40Ar for time (Fig. 1). Our results reveal that for intermediate exhumation rates, spherical muscovite grains with <100 µm in diameter are affected by a ~10% age error when latent heat is considered (Fig. 1b). Such muscovites in rocks exhuming with a velocity of, for example, 4 mm/year would record an apparent 40Ar/39Ar age of c. 10 Ma (Fig. 1a) and be affected by an absolute age error of ~1 Ma from thermal buffering by hydration, yielding an apparent age of 10 ± 1 Ma (Fig. 1b). Our calculations indicate that latent heat released from exothermic hydration may significantly disturb low-temperature isotopic systems, thereby complicating the cooling histories and obscuring the temporal constraints deduced from state-of-the-art geochronological systems.Figure 1 – Exhumation velocity vs. grainsize contoured for apparent 40Ar/39Ar age in muscovite (a) and relative error when latent heat is considered (b)REFERENCESEvangelos Moulas, & Mark T Brandon. (2022). KADMOS: a Finite Element code for the calculation of apparent K-Ar ages in minerals (Version 1). Zenodo. https://doi.org/10.5281/zenodo.7358138
Abstract Strength contrasts and spatial variations in rheology are likely to produce significant stress differences in the Εarth’s crust. The buildup and the relaxation of stresses have important consequences for the state of stress of the brittle crust, its deformational behaviour and seismicity. We performed scaled analogue experiments of a classic wedge-type geometry wherein we introduced a weak, fluid-filled body representing a low-stress heterogeneity. The experiments were coupled to direct pressure measurements that revealed significant pressure differences from their surrounding stressed matrix. The magnitude of the pressure variations is similar to the magnitude of the differential stress of the strongest lithology in the system. When rocks with negligible differential stresses are considered, their pressure can be more than twice larger than the surrounding lithostatic stress. The values of the pressure variations are consistent with the stresses that are estimated in analytical studies. This behaviour is not restricted to a particular scale or rheology, but it requires materials that are able to support different levels of stress upon deformation. For non-creeping rheological behaviours, the stress and pressure variations are maintained even after deformation ceases, implying that these stress variations can be preserved in nature over geological timescales.
<div> <p>A recent focus of studies in geodynamic modeling and magmatic petrology is to understand the coupled behavior between deformation and magmatic processes. Here, we present a 2D numerical model of an upper crustal magma (or mush) chamber in a visco-elastic host rock, with coupled thermal, mechanical and chemical processes, accounting for thermodynamically consistent material parameters. The magma chamber is isolated from deeper sources of magma (at least periodically) and it is cooling, and thus shrinking. We quantify the changes of pressure and stress around a cooling magma chamber and a warming host rock, using a compressible visco-elastic formulation, considering both simplified idealized and more complex and realistic geometries of the magma chamber.</p> </div><div> <p>We present solutions based on a self-consistent system of the conservation equations for coupled thermo-mechanical-chemical processes, under the assumptions of slow (negligible inertial forces), visco-elastic deformation and constant chemical bulk composition. The thermodynamic melting/crystallization model is based on a pelitic melting model calculated with Perple_X, assuming a granitic composition and is incorporated as a look-up table. We will discuss the numerical implementation, show the results of systematic numerical simulations, and illustrate the effect of volume changes due to temperature changes (including the possibility melting and crystallization) on stress and pressure evolution in magmatic systems.</p> </div>
Studies of host rock deformation around magmatic intrusions usually focus on the development of stresses directly related to the intrusion process. This is done either by considering an inflating region that represents the intruding body, or by considering multiphase deformation. Thermal processes, especially volume changes caused by thermal expansion are typically ignored. We show that thermal stresses around upper crustal magma bodies are likely to be significant and sufficient to create an extensive fracture network around the magma body by brittle yielding. At the same time, cooling induces decompression within the intrusion, which can promote the appearance of a volatile phase. Volatile phases and the development of a fracture network around the inclusion may thus be the processes that control magmatic-hydrothermal alteration around intrusions. This suggests that thermal stresses likely play an important role in the development of magmatic systems. To quantify the magnitude of thermal stresses around cooling intrusions, we present a fully compressible 2D visco-elasto-plastic thermo-mechanical numerical model. We utilize a finite difference staggered grid discretization and a graphics processing unit based pseudo-transient solver. First, we present purely thermo-elastic solutions, then we include the effects of viscous relaxation and plastic yielding. The dominant deformation mechanism in our models is determined in a self-consistent manner, by taking into account stress, pressure, and temperature conditions. Using experimentally determined flow laws, the resulting thermal stresses can be comparable to or even exceed the confining pressure. This suggests that thermal stresses alone could result in the development of a fracture network around magmatic bodies.
