Abstract-Shock recovery experiments were performed at 12. 5, 25, 34, 40, and 56 GPa at 25°C, and at 18 and 25 GPa at 400°C, on a high-grade, migmatitic, garnet-cordierite metapelite from the Etive´aureole, Scotland. Objectives for this study were to (1) characterize shock effects in a complex polymineralic rock with a significant proportion of hydrous ferromagnesian minerals, both as a function of variable shock pressure and preshock temperature, and (2) to explore the effects of shock impedance contrast between component minerals on the respective abundances and distribution of these features. At any shock pressure, the order of decreasing intensity of shock metamorphic effects in component phases is: cordierite (Crd) fi biotite (Bt) fi plagioclase (Pl) fi K-feldspar (Kfs) fi quartz (Qtz) fi garnet (Grt) fi orthopyroxene (Opx). Samples shocked to pressures below 40 GPa (25°C) were typically characterized by marked heterogeneous distribution of shock effects on both intragranular and intergranular scales. Shock heterogeneity is mainly attributed to shock impedance contrast between contiguous phases, and manifests as shock amplification locally where shock impedance contrast is greatest, and shock suppression where impedance contrast is least. The heterogeneous distribution of shock metamorphic effects in both experiments and natural rocks is a signature of extreme disequilibrium at the submillimeter scale. The heterogeneous distribution of shock metamorphic effects mitigates against the use of shock effects in minerals exclusively as regional shock pressure barometers, and ought to be augmented by additional constraints on shock pressure from numerical models.
Abstract. Coronas, including symplectites, provide vital clues to the presence of arrested reaction and preservation of partial equilibrium in metamorphic and igneous rocks. Compositional zonation across such coronas is common, indicating the persistence of chemical potential gradients and incomplete equilibration. Major controls on corona mineralogy include prevailing pressure (P ), temperature (T ) and water activity (aH 2 O) during formation, reaction duration (t) single-stage or sequential corona layer growth; reactant bulk compositions (X) and the extent of metasomatic exchange with the surrounding rock; relative diffusion rates for major components; and/or contemporaneous deformation and strain. High-variance local equilibria in a corona and disequilibrium across the corona as a whole preclude the application of conventional thermobarometry when determining P -T conditions of corona formation, and zonation in phase composition across a corona should not be interpreted as a record of discrete P -T conditions during successive layer growth along the P -T path. Rather, the local equilibria between mineral pairs in corona layers more likely reflect compositional partitioning of the corona domain during steadystate growth at constant P and T .Corona formation in pelitic and mafic rocks requires relatively dry, residual bulk rock compositions. Since most melt is lost along the high-T prograde to peak segment of the P -T path, only a small fraction of melt is generally retained in the residual post-peak assemblage. Reduced melt volumes with cooling limit length scales of diffusion to the extent that diffusion-controlled corona growth occurs. On the prograde path, the low melt (or melt-absent) volumes required for diffusion-controlled corona growth are only commonly realized in mafic igneous rocks, owing to their intrinsic anhydrous bulk composition, and in dry, residual pelitic compositions that have lost melt in an earlier metamorphic event.Experimental work characterizing rate-limiting reaction mechanisms and their petrogenetic signatures in increasingly complex, higher-variance systems has facilitated the refinement of chemical fractionation and partial equilibration diffusion models necessary to more fully understand corona development. Through the application of quantitative physical diffusion models of coronas coupled with phase equilibria modelling utilizing calculated chemical potential gradients, it is possible to model the evolution of a corona through P -T -X-t space by continuous, steady-state and/or sequential, episodic reaction mechanisms. Most coronas in granulites form through a combination of these endmember reaction mechanisms, each characterized by distinct textural and chemical potential signatures with very different petrogenetic implications. An understanding of the inherent petrogenetic limitations of a reaction mechanism model is critical if an appropriate interpretation of P -T evolution is to be inferred from a corona. Since corona modelling employing calculated chemical potential gradie...
Abstract. Coronas, including symplectites, are vital clues to the presence of arrested reaction and preservation of partial equilibrium in metamorphic and igneous rocks. Compositional zonation across such coronas is common, indicating the persistence of chemical potential gradients and incomplete equilibration. Major controls on corona mineralogy include P, T and aH2O during formation, continuous or non-continuous corona formation, reactant bulk compositions and extent of metasomatic exchange with the surrounding rock, relative diffusion rates for major components, and/or contemporaneous deformation and strain. High-variance local equilibria in a corona and disequilibrium across the corona as a whole preclude the application of conventional thermobarometry when determining P-T conditions of corona formation, and zonation in phase composition across a corona should not be interpreted as a record of discrete P-T conditions during successive layer growth along the P-T path. Rather, the local equilibria between mineral pairs in corona layers more likely reflect compositional partitioning of the corona domain during steady-state growth at constant P and T. Corona formation in pelitic and mafic bulk rock compositions requires dry, restitic bulk rock compositions. Since most melt is lost at or near peak conditions only a fraction of melt is retained in the restitic post-peak assemblage. Reduced melt volumes with cooling limit length-scales of diffusion to the extent that diffusion-controlled corona growth occurs. On the prograde path, the low melt (or melt-absent) volumes required for kinetically-constrained corona growth are only commonly realised in mafic rocks, owing to their intrinsic anhydrous bulk composition, and in dry, restitic pelitic compositions that have lost melt in an earlier metamorphic event. Mafic and pelitic prograde coronas show similar ranges of thickness and vermicule size; prograde contact aureole coronas display similar thicknesses but slightly longer vermicule lengths compared to regional metamorphic coronas. Retrograde coronas in mafic rocks are significantly thinner than pelitic coronas and have smaller vermicule lengths, whereas retrograde pelitic coronas show similar parameters to their prograde counterparts. Reduced maximum corona thickness and smaller maximum vermicule size in retrograde mafic coronas compared to retrograde pelitic coronas attests to more restricted length-scales of diffusion in melt-poor, anhydrous, mafic bulk rock compositions. Increased maximum layer thickness and vermicule size in prograde mafic coronas compared to retrograde mafic coronas is due to greater length-scales of diffusion in more melt-rich bulk compositions with protracted reaction along the prograde path. Prograde pelitic coronas do not differ significantly from retrograde pelitic coronas with respect to microstructure, owing to the intrinsically more hydrous pelitic bulk compositions and capacity to generate diffusion-enhancing melt during decompression. Through the application of either quantitative physical diffusion modelling of coronas or phase equilibria modelling utilising calculated chemical potential gradients, it is possible to model the evolution of a corona through P-T-X space by continuous or non-continuous processes. Since corona modelling employing calculated chemical potential gradients assumes nothing about the sequence in which the layer forms and is directly constrained by phase compositional variation within a layer, it allows far more nuanced and robust understanding of corona evolution and its implications for the path of a rock in P-T-X space. Key words: corona, chemical potential gradient, diffusion, disequilibrium, metamorphism, mineral compositional zoning, reaction dynamics, reaction texture, symplectite.
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