Experimental Study of Grain-Scale Processes during Chemical Transport in Rocks
Abstract
Transport of water into the mantle, metamorphism in the crust, hydrothermal alteration, and weathering near the surface are all mediated by reaction between a grain-boundary fluid and mineral. Such fluid-mineral reactions commonly result in a change in the solid volume, which can provide either positive or negative feedback because of the change in permeability. To determine this effect, we measured extents of reaction in three systems: dolomite to periclase + calcite with a large -Δ Vsolid, alkali feldspar cation exchange with a small ± Δ Vsolid, and olivine to serpentine with a large + Δ Vsolid. Our experiments were conducted in cold-seal hydrothermal vessels, at values of temperature and pressure appropriate for the middle to upper part of the crust, and for periods up to two months in duration. We used both single grains and small samples of rock as starting materials to compare results from systems with large and small fluid-solid ratios. The experimental results are characterized by extensive porosity development, by rates that are limited by diffusion through a reacted layer coating the grains, by pseudomorphic replacement, and, in the case of serpentinization, the formation of grain-crossing fractures. We find also that isotope-exchange takes advantage of other reactions that result in the formation of new minerals or in recrystallization or solution-reprecipitation of replacement minerals. The experimental textures are similar to those in natural systems, even in some metasomatic systems, although the experiments are far from equilibrium initially. The rate of reaction in the dolomite-calcite-periclase system is the same in both grain and rock experiments. The rate of cation exchange in the feldspar system depended more on the porosity of the rock. The rate of serpentinization is substantially slower in the rock samples than in individual grains. The image shows a 4 × 6 mm dunite with serpentine in black.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.V23D2852L
- Keywords:
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- 1012 GEOCHEMISTRY / Reactions and phase equilibria