Reaction rim growth in the ternary system CaO-MgO-SiO2 : Diffusion pathways and the effect of water
Abstract
Anhydrous rim growth experiments were performed in the ternary system CaO-MgO- SiO2. A polycrystalline åkermanite (Ca2MgSi2O7) reaction rim develops between monticellite (CaMgSiO4) and wollastonite (CaSiO3) at 1200°C and 0.5 GPa. Platinum particles were initially deposited on the monticellite surface to mark the original interface. These markers are always located in the center of the rim after the experiment, indicating that åkermanite rim growth is solely controlled by MgO-mobility at anhydrous conditions. An effective bulk diffusion coefficient was calculated to 10-15.8+/-0.1m2s-1. During the experiment, grains coarsen in the rim, which is described by a parabolic rate law. Because the grains are coarsening, the volume fraction of grain boundaries and therefore the amount of fast diffusion pathways decreases, as reaction proceeds. The drop of the grain boundary density does, however, not affect rim growth rates, implying that åkermanite rim growth is dominantly controlled by volume diffusion. This is in sharp contrast to results from dry experiments in the binary system MgO-SiO2, where MgO grain boundary diffusion controls rim growth kinetics (Gardés et al. 2010). It is conceivable that the two systems may fundamentally differ in the type of grain boundaries that are present in the reaction rim. Nominally dry experiments using fragments of monticellite crystals embedded in powdered wollastonite as reactant show significantly enhanced reaction rates compared to dry experiments using single crystals. This is interpreted as resulting from traces of water that are adsorbed on the reactant surfaces, which significantly enhances component mobilities. The acceleration of reaction rates by traces of water may either result from enhanced grain boundary diffusion, the formation of OH defects in åkermanite and associated enhancement of volume diffusion or changes in the reaction mechanism by additional mobilization of the CaO and SiO2 components. This issue calls for detailed investigation because the effect of traces of water is of great importance for reaction kinetics in natural rocks. We developed a technique that allows for introduction of specified amounts of water in dry rim growth experiments by using reactants doped with OH-defects. Therefore, single crystals of periclase were brought to 1200 °C at 0.5 GPa for 25 h in the presence of water. We performed rim growth experiments at 1200°C and 0.5 GPa between OH-doped periclase and wollastonite. First results show that minute amounts of water released by periclase decompostion not only enhance reaction rates but also affect relative component mobilities as well as the sequence and organization of reaction rims in a multicomponent system. References: Gardés E, Wunder B, Wirth R, Heinrich W (2010) Growth of multilayered polycrystalline reaction rims in the MgO-SiO2 system, part I: experiments, Contrib Mineral Petrol DOI 10.1007/s00410-010-0517-z.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMMR51B..05J
- Keywords:
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- 3630 MINERALOGY AND PETROLOGY / Experimental mineralogy and petrology;
- 5112 PHYSICAL PROPERTIES OF ROCKS / Microstructure;
- 5139 PHYSICAL PROPERTIES OF ROCKS / Transport properties