Reaction rates and δO 18 variation for the carbonate-phosphoric acid preparation method

Abstract

Reliable analyses of coexisting calcite and dolomite for δO 18 pose a major problem in sample preparation when the phases cannot be physically separated and complicate attempts to interpret the geologic significance of the two carbonates. When mixtures of calcite and dolomite are reacted with phosphoric acid, CO 2 is evolved from both phases simultaneously. An investigation of the reaction rates of pure calcite and pure dolomite with 100% phosphoric acid, indicates that for mixtures the particle size range should be limited to minimize the cross-contamination of evolved CO 2 which is later analyzed for δO 18. For crushed samples of most carbonate rocks 5–44 μ is a satisfactory size range. Prolonged fine-grinding of calcite and dolomite mixtures results in more cross-contamination than with ordinary crushing. Tests with pure calcite and pure dolomite samples reacted at 25°C with phosphoric acid show that the O 18 O 16 ratio in the CO 2 evolved increases during the course of a reaction. We found that CO 2 from the first-reacting carbonate had δO 18 as much as 3 per mil lighter than CO 2 from the last reacting carbonate, for either calcite or dolomite. This change in δO 18 during reaction is related in part to particle size of the reacting carbonate ( Fritz and Fontes, 1966) and perhaps in part also to particle surface strain caused by crushing and grinding. However, there is also an additional kinetic isotope fractionation which may be associated with diffusion of the reaction products CO 2 (gas), CO 2 (dissolved) and H 2O (?) away from the reacting particle surfaces. The variation of reaction rates for calcite and dolomite as a function of particle size and the change in δO 18 as a function of reaction completion are especially important for carbonates having very large or very small ratios of calcite to dolomite; for these the δO 18 determination of the minor component is very uncertain using traditional methods.