Diffusion‐controlled growth of wollastonite rims between quartz and calcite: comparison between nature and experiment

Abstract

Growth rates of wollastonite reaction rims between quartz and calcite were experimentally determined at 0.1 and 1 GPa and temperatures from 850 to 1200 °C. Rim growth follows a parabolic rate law indicating that this reaction is diffusion‐controlled. From the rate constants, the D′δ‐values of the rate‐limiting species were derived, i.e. the product of grain boundary diffusion coefficient D′ and the effective grain boundary width, δ. In dry runs at 0.1 GPa, wollastonite grew exclusively on quartz surfaces. From volume considerations it is inferred that (D′CaOδ)/(D′SiO2δ)≥1.33, and that SiO2 diffusion controls rim growth. D′SiO2δ increases from about 10−25 to 10−23 m3 s−1 as temperature increases from 850 to 1000 °C, yielding an apparent activation energy of 330±36 kJ mol−1. In runs at 1 GPa, performed in a piston‐cylinder apparatus, there were always small amounts of water present. Here, wollastonite rims always overgrew calcite. Rims around calcite grains in quartz matrix are porous and their growth rates are controlled by a complex diffusion‐advection mechanism. Rim growth on matrix calcite around quartz grains is controlled by grain boundary diffusion, but it is not clear whether CaO or SiO2 diffusion is rate‐limiting. D′δ increases from about 10−21 to 10−20 m3 s−1 as temperature increases from 1100 to 1200 °C. D′SiO2δ or D′CaOδ in rims on calcite is c. 10 times larger than D′SiO2δ in dry rims at the same temperature. Growth structures of the experimentally produced rims are very similar to contact‐metamorphic wollastonite rims between metachert bands and limestone in the Bufa del Diente aureole, Mexico, whereby noninfiltrated metacherts correspond to dry and brine‐infiltrated metacherts to water‐bearing experiments. However, the observed diffusivities were 4 to 5 orders of magnitude larger during contact‐metamorphism as compared to our experimental results.

Rheology of calcite–quartz aggregates deformed to large strain in torsion

“…Wollastonite grew on calcite grains (Figures 14 and 15) suggesting the presence of water [ Tanner et al , 1985]. Wollastonite formation and growth under wet conditions may occur by dissolution of silica and transport through a fluid‐filled intergranular network [ Tanner et al , 1985] or by grain boundary diffusion rate limited by CaO or SiO 2 [ Milke and Heinrich , 2002]. In SEM backscattered images of some samples deformed at 1000 K we observed thin white streaks in the calcite matrix, possibly representing wollastonite particles decorating grain boundaries (Figure 14b).…”

Arrested development – a comparative analysis of multilayer corona textures in high-grade metamorphic rocks

“…These models have been substantiated by a vast body of rigorous experimental work constraining the kinetics of reaction rim growth (e.g. Farver and Yund, 1996;Yund, 1997;Fisler et al, 1997;Milke et al, 2001;Watson and Price, 2002;Milke and Heinrich, 2002;Milke and Wirth, 2003;Abart and Schmidt, 2004;Schmid et al, 2009;Götze et al, 2010;Keller et al, 2008;Niedermeier et al, 2009;Dohmen and Milke, 2010;Keller et al, 2010;Joachim et al, 2011a, b;Mueller et al, 2012;Helpa et al, 2014Helpa et al, , 2015Jonas et al, 2015;Abart et al, 2016). Phenomenological models of disequilibrium elemental and isotopic compositions produced experimentally with incomplete diffusive element exchange (e.g.…”

Porphyroblast crystallization: linking processes, kinetics, and microstructures

“…An approach sometimes taken in such circumstances is to conduct experiments at higher temperatures, then extrapolate back down to natural temperatures of interest (e.g. Tanner et al 1985;Fisler et al 1997;Liu et al 1997;Yund 1997;Milke et al 2001;Milke and Heinrich 2002;Keller et al 2008), but this is commonly not feasible for studies relevant to the crystallization of most porphyroblasts because the appropriate assemblages are not stable at the high temperatures required for appreciable reaction. There is much promise in the experimental approach taken by Farver and Yund (1995a,b, 1996, 2000a, who successfully quantified rates of intergranular diffusion of K, Ca, Si, and O by measuring the bulk penetration of isotopically labelled tracers into very fine-grained polycrystalline aggregates of feldspar, quartz, calcite, and forsterite.…”