The catalysis of mineral reactions by water and restrictions on the presence of aqueous fluid during metamorphism

Abstract

ABSTRACT, The problems of characterizing intergranular regions and of estimating rates of intergranular diffusion in metamorphic rocks are discussed. Intergranular regions can be anhydrous, hydrated but undersaturated with H20, or saturated with H20, but only in the latter case can a free aqueous fluid phase be present. Estimates of intergranular diffusion coefficients (D mR) at 550 ~ derived from a variety of published experimental work, vary from ~ 10 -8 m 2 s -1 for diffusion of species through an intergranular fluid film to ~< 4 • 10-24 m 2 s 1 for diffusion of SiO2 or O in anhydrous grain boundaries in quartzite. Estimates of D mR for hydrated grain boundaries vary from ~ 10 -13 m 2 s -1 to ~ 10 -zl m 2 s 1; the concentration of H20 in the grain boundaries and the identity of the diffusing species (generally unknown) may be important controlling factors, and there exists the possibility of a spectrum of values between these two extremes.Using available kinetic data it is shown that a free aqueous fluid could never have been present in parts of the basement terrane of the Sesia Zone (Western Alps) during uplift from the eclogite facies, except possibly late in the cooling history. The breakdown of sodic pyroxene+ quartz occurred in response to the localized infiltration of catalytic aqueous fluid, possibly over a time interval as short as 6-6000 a, and possibly under conditions remote from equilibrium. H20-present conditions during a dehydration reaction in metapelites of the Adula nappe (central Alps) could also have been of short duration. These examples are consistent with a model in which basement rocks at deep crustal levels are dry for long periods of time and in which the development of equilibrium mineral assemblages and microstructures generally occurs over relatively short periods of time under transitory fluid-present conditions (caused by devolatilization and/or infiltration).

The effect of water on intergranular mass transport: new insights from diffusion-controlled reaction rims in the MgO–SiO2 system

“…For example, the grain boundary diffusivity of aluminium in 'hydrous fluid-saturated' rocks is enhanced by seven orders of magnitude compared to 'nearly anhydrous' rocks (Carlson 2010), and Gasc et al (2011) reported the same range of variations for the electrical conductivity in polycrystalline brucite from 'dry' to 'wet' conditions. Based on a consensus from literature, Farver and Yund (1995) proposed that intergranular diffusion can be classified into three categories, depending on water activity in the intergranular medium: 'water-absent', 'water-unsaturated' and 'water-saturated' regimes (see also Rubie 1986;Carlson 2010). However, the physico-chemical nature of the intergranular medium corresponding to these regimes remains unclear, and the transitional steps from one to another are not well specified so far.…”

Syn-kinematic hydration reactions, grain size reduction, and dissolution–precipitation creep in experimentally deformed plagioclase–pyroxene mixtures

“…As the pressurization and heating procedure required to attain the experimental Pc-T conditions takes place within 5-8 h, the sample material is brought rapidly to a metastable state. Although this rapid change in P -T conditions is unique to experiments, there is widespread evidence from observations of natural rocks that, similarly, metastable mineral assemblages can be sustained even at high-grade conditions and to large overstepping of reaction boundaries when rocks are dry (e.g., Rubie, 1986;Austrheim, 1987;Wayte et al, 1989;Krabbendam et al, 2000;Austrheim, 2013;Jamtveit et al, 2016). Only where fluid infiltrates are mineral reactions enabled and equilibrium can be attained.…”

The reaction antigorite → olivine + talc + H2O in the Bergell aureole, N. Italy

“…Conventional wisdom states that metamorphic fluids are held in the semi-amorphous, grain-boundary region. This fluid phase could facilitate the rapid diffusive interaction between the reacting antigorite and the olivine because diffusion along wet grain boundaries is much faster than along dry ones (Rubie, 1986). …”