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Calcite and aragonite precipitation from seawater solutions of various salinities: Precipitation rates and overgrowth compositions |
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| Authors: | Shaojun Zhong Alfonso Mucci |
| Institution: | Department of Geological Sciences, McGill University, Montréal, Qué. H3A 2A7, Canada |
| Abstract: | The reaction rate and composition of calcite and aragonite overgrowths precipitated from seawater solutions of various salinities (i.e. S=5, 15, 25, 35, 44) were determined at 25°C and 10?2.5-atm. CO2 partial pressure using a constant disequilibrium seeded technique. The rate data were fitted to an empirical rate law of the form: logR=n(ωc(or a)-1)+logk where n is the empirical reaction order; and k is the rate constant. Calcite precipitation rates in seawater solutions do not vary appreciably as a result of salinity variations over the range investigated, while those for aragonite show an increase in going from the higher (i.e. S=35, 44) to the lower (i.e. S=5, 15, 25) salinity range. This study also confirms previously published findings that above a given saturation state (i.e. Ωc>/2.6) aragonite precipitates more rapidly than calcite at 25°C. The incorporation of Sr2+ in aragonite and Mg2+ in calcite overgrowths are independent of the precipitation rate. The partition coefficient of Sr2+ in aragonite is approximately equal to unity and is unaffected by salinity variations between 5 and 44. However, the Mg2+ partition coefficient in calcite, increases with decreasing salinity of the parent seawater solutions, possibly as a result of variations in the sulfate content of the solutions and solids. The experimental results were discussed in the context of a number of geological environments. |
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