Mechanically‐ v. diffusion‐controlled metamorphic microstructure: a symplectite example from Rhodope Metamorphic Complex (Greece)

Abstract

Gradients in chemical potential are the driving force for chemical diffusion and their spatial distribution is thus essential to investigate equilibrium in metamorphic rocks. At high temperatures (>650°C), where chemical diffusion is relatively fast, the development and preservation of compositional zoning in minerals can be controlled by mechanically maintained pressure variations. Therefore, the dependence of chemical potentials on pressure plays an important role in correct interpretations of rock microstructures. Distinguishing between pressure-controlled chemical zoning and zoning reflecting chemical diffusion is a challenge. To tackle this challenge, we investigate a symplectitic microstructure around kyanite in an amphibolitized eclogite from the Rhodope Metamorphic Complex (Greece). The sample recrystallized at high temperatures (~720°C) and low pressures (<1 GPa) during which kyanite was replaced by fine-grained symplectites of sapphirine, spinel, plagioclase and corundum. The plagioclase rim around kyanite shows a cusp-shaped chemical zoning of calcium along the grain boundaries. The results from combined phase equilibria and diffusion modelling show that the cusp-like shapes were developed by fast grain boundary diffusion at constant pressure after the relaxation of grain-scale pressure variations across the plagioclase rim. The results illustrate an example where petrographic observations help to distinguish between mechanically-and diffusion-controlled chemical zoning.

Garnet zoning in kyanite‐bearing eclogite from Golfo Aranci: New data on the early prograde P–T evolution in NE Sardinia, Italy

“…Similar textures of sapphirine–plagioclase, spinel–plagioclase, and corundum–plagioclase symplectites replacing kyanite have been described in Baldwin, Powell, White, and Štípská () in quartz–plagioclase–garnet–kyanite granulite‐facies gneisses from the southern domain of the Athabasca granulite terrane, Canada. Such textures have been also reported from the Rhodope Massif, Greece (Liati & Seidel, ; Moulas, Kostopoulos, Connolly, & Burg, ; Moulas, Tajčmanová, Vrijmoed, & Podladchikov, ), the Armorican Massif, France (Godard & Mabit, ), in Central Swiss Alps (Brouwer & Engi, ), the Bohemian Massif (Štípská, Powell, White, & Baldwin, ), and Luotian dome of North Dabie Zone (Groppo, Rolfo, Liu, Deng, & Wang, ).…”

Crystal-Chemical and Structural Characterization of Omphacite in High-Pressure Eclogites From the Arquía Complex on Southwestern Pijao, Central Cordillera (Colombian Andes)

“…We confirm that the chemical homogeneity of omphacite is probably due to reaching equilibrium at the peak PT conditions. Microtextural analysis reveals that the colonies of symplectites of the analyzed sample are morphologically similar to those described in previous studies (e.g., Mysen and Griffin, 1973;Boland and van Roermund, 1983;Anderson and Moecher, 2007;Su et al, 2011;Moulas et al, 2015). The symplectite formation is promoted in quartz-bearing eclogites unlike other contexts (quartz-free eclogites) in which this process may require intergranular transport of silica in a fluid phase or oxidation of omphacite (Mysen and Griffin, 1973).…”

Corundum-bearing and spinel-bearing symplectites in ultrahigh-pressure eclogites record high-temperature overprint and partial melting during slab exhumation

“…4c), suggesting that the kyanite is a reactant rather than a product of reaction. The interpretation is supported by many previous studies (e.g., Nakamura and Hirajima, 2000;Janák et al, 2012;Moulas et al, 2015), which reveal that kyanite could be replaced by the symplectites of Crn + Pl during HT metamorphism (≥ 700 • C). Thus, we conclude that the reaction responsible for the CPB symplectite formation is as follows: Ph + Ky → Crn + Pl + Bt + melt.…”