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 Porosity waves are a mechanism by which fluid generated by devolatilization and melting, or trapped during sedimentation, may be expelled from ductile rocks. The waves correspond to a steady‐state solution to the coupled hydraulic and rheologic equations that govern flow of the fluid through the matrix and matrix deformation. This work presents an intuitive analytical formulation of this solution in one dimension that is general with respect to the constitutive relations used to define the viscous matrix rheology and permeability. This generality allows for the effects of nonlinear viscous matrix rheology and disaggregation. The solution combines the porosity dependence of the rheology and permeability in a single hydromechanical potential as a function of material properties and wave velocity. With the ansatz that there is a local balance between fluid production and transport, the solution permits prediction of the dynamic variations in permeability and pressure necessary to accommodate fluid production. The solution is used to construct a phase diagram that defines the conditions for smooth pervasive flow, wave‐propagated flow, and matrix fluidization (disaggregation). The viscous porosity wave mechanism requires negative effective pressure to open the porosity in the leading half of a wave. In nature, negative effective pressure may induce hydrofracture, resulting in a viscoplastic compaction rheology. The tubelike porosity waves that form in such a rheology channelize fluid expulsion and are predicted by geometric argumentation from the one‐dimensional viscous solitary wave solution.
Abstract Recent crystallographic data indicate that in biotite Ti orders preferentially onto the M2 octahedral site rather than onto the M1 site as assumed in previous solution models for K 2 O–FeO–MgO–Al 2 O 3 –SiO 2 –H 2 O–TiO 2 –O 2 (KFMASHTO) biotite. In view of these data, we reformulate and reparameterize former biotite solution models. Our reparameterization takes into account Fe–Mg order–disorder and ferric iron contents of natural biotite as well as both natural and experimental observations on biotite Ti‐content over a wide range of physicochemical conditions. In comparison with previous biotite models, the new model reproduces the Ti‐content and stability field of biotite as constrained by experiments with significantly better accuracy. The predictive power of the model is tested by comparison with petrologically well‐characterized natural samples of SiO 2 ‐saturated and SiO 2 ‐undersaturated rocks that were not used in the parameterization. In all these tests, the reformulated model performs well.
