Carbonatisation of Weathered Peridotites in Laboratory Experiments
Abstract
Enhanced in-situ carbonatisation of ultramafic rocks has been proposed as a strategy for a permanent and safe storage of CO2 in order to reduce anthropogenic greenhouse gas emissions (e.g., Kelemen and Matter 2008). This idea emerged from studies of natural examples demonstrating that ultramafic rocks react extensively with CO2 to form ophicarbonates. However, despite their Mg-rich nature, ultramafic rocks are often associated with calcite (CaCO3) rather than magnesite (MgCO3) and dolomite (CaMg(CO3)2). Whether these so-called ophicalcites represent sedimentary or tectonic breccias or are produced during hydrothermal alteration of ultramafic rocks, has been discussed for many years (e.g., Folk and McBride 1976). The view that reactions between hydrothermal fluids and ultramafic rocks can result in the formation of ophicalcite was recently supported by Beinlich et al. (2010), who documented Ca- and CO2-metasomatism and extreme Mg depletion in serpentinised and weathered peridotite clasts from the conglomerates of the Solund basin (SW Norway). This study also suggests that weathering is an important factor for the carbonatisation of ultramafic rocks. We have performed hydrothermal experiments on weathered peridotites in order to better constrain the mechanisms and conditions that trigger Mg-loss from ultramafic rocks and subsequent calcite precipitation. Un-crushed, partly serpentinised and weathered peridotite samples were allowed to react in a Ca-bearing saline solution under CO2 pressure (PCO2: 130-160 bar) at 200°C. We were able to illustrate the textural and chemical evolution during the reaction through a detailed comparison of the solid and fluid samples before and after the experiments. The initial samples showed a typical mesh texture with veins of serpentine surrounding meshes filled either with fresh or weathered olivine. The experimentally treated samples reveal a strongly reacted rim, predominantly composed of calcite, but still showing ghosts of the former mesh texture. Meshes that were initially filled with weathered olivine, were preferred sites of reaction relative to fresh olivine and serpentine. Dissolution of the mesh fillings and subsequent replacement by calcite resulted in Mg- and Si-enrichment in the fluid. The results confirm that hydrothermal alteration of ultramafic rocks may lead to Mg-depletion and ophicalcite formation and, in particular, highlight the role of weathering in enhancing the carbonatisation of peridotites. Our study has potential implications for industrial mineral sequestration of CO2 since weathering is commonly extensive in peridotites. The removal of Mg from the site of carbonatisation would be an undesired effect during CO2 injection into ultramafic rocks, but with a Ca-source available carbonatisation may still be effective. References: Beinlich, A., Austrheim, H., Glody, J., Erambert, M. and Andersen, T.B. (2010), Geochim. Cosmochim. Acta, in press. Folk, R.L. and McBride, E.F. (1976), Geology, 4(6): 327-332. Kelemen, P.B. and Matter, J. (2008), PNAS, 105(45): 17295-17300.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2010
- Bibcode:
- 2010AGUFMGC31B0862H
- Keywords:
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- 3612 MINERALOGY AND PETROLOGY / Reactions and phase equilibria;
- 3617 MINERALOGY AND PETROLOGY / Alteration and weathering processes;
- 3625 MINERALOGY AND PETROLOGY / Petrography;
- microstructures;
- and textures;
- 3630 MINERALOGY AND PETROLOGY / Experimental mineralogy and petrology