Grain‐scale pressure variations and chemical equilibrium in high‐grade metamorphic rocks

Abstract

In the classical view of metamorphic microstructures, fast viscous relaxation (and so constant pressure) is assumed, with diffusion being the limiting factor in equilibration. This contribution is focused on the only other possible scenario – fast diffusion and slow viscous relaxation – and brings an alternative interpretation of microstructures typical of high‐grade metamorphic rocks. In contrast to the pressure vessel mechanical model applied to pressure variation associated with coesite inclusions in various host minerals, a multi‐anvil mechanical model is proposed in which strong single crystals and weak grain boundaries can maintain pressure variation at geological time‐scales in a polycrystalline material. In such a mechanical context, exsolution lamellae in feldspar are used to show that feldspar can sustain large differential stresses (>10 kbar) at geological time‐scales. Furthermore, it is argued that the existence of grain‐scale pressure gradients combined with diffusional equilibrium may explain chemical zoning preserved in reaction rims. Assuming zero net flux across the microstructure, an equilibrium thermodynamic method is introduced for inferring pressure variation corresponding to the chemical zoning. This new barometric method is applied to plagioclase rims around kyanite in felsic granulite (Bohemian Massif, Czech Republic), yielding a grain‐scale pressure variation of 8 kbar. In this approach, kinetic factors are not invoked to account for mineral composition zoning preserved in rocks metamorphosed at high grade.

Thermodynamic equilibrium at heterogeneous pressure

“…For such applications, the chemical potential may be modified to account for the externally imposed force. Another chemical potential-based equilibrium approach was used by Tajčmanová et al (2014) in a geologically relevant application to obtain pressure gradients from chemically zoned minerals in a binary system.…”

“…Later, mechanical models adopted viscoelastic solutions in which pressure gradually decreased from the inclusion into the host (Zhang 1998a). Recently, Tajčmanová et al (2014) used an elastic-plastic solution to expand the mechanical feasibility of maintaining pressure variations to polycrystalline materials or radially cracked inclusions.…”

“…If the pressure variations occur within a solid solution, such as plagioclase, chemical equilibrium will result in chemical zoning. Tajčmanová et al (2014) developed an alternative geobarometer to infer pressure variations from chemical zoning by use of homogeneity of chemical potentials in which pressure was spatially varied.…”

Significance of “stretched” mineral inclusions for reconstructing P–T exhumation history

“…6). At P-T conditions well removed from this triple point, kyanite is expected to retain positive inclusion pressures (e.g., Tajčmanová et al 2014, where encapsulation occurred at 1.6-1.8 GPa and 850-900 °C). These residual stresses would be sufficiently large to result in the transformation of kyanite to andalusite when projected into the tensile regime.…”

On the significance of short-duration regional metamorphism

“…Additionally, new concepts in metamorphic petrology offer an alternative explanation relating to maintenance of sharp compositional gradients as a result of significant, long-lived, grain-scale pressure variations and their influence on chemical activity and diffusion (e.g. Tajcmanová et al 2014;Wheeler 2014).…”