ABSTRACT: Interpreting Water-Rock Interaction in Ancient Carbonates Using Multi-Method Microanalysis

Scott J. Carpenter, K. C. Lohmann, A. N. Halliday, L. M. Walter, P. Holden

Routine geochemical measurements of Sr, O, and C isotope ratios and minor element concentrations of carbonate rocks have been conducted on relatively large samples (larger than 25 mg total sample) for several decades. However, several different cement generations occur in most ancient rocks, and even smaller scale chemical heterogeneities can exist within single crystals. Such heterogeneity demands smaller samples for chemical analysis. Modern mass spectrometers reliably measure very small samples, (i.e., 1 µmol of CO₂ and < 100 nanograms of Sr). It is now possible to analyze Sr, O, and C isotope ratios and Ca, Mg, Fe, Mn, and Sr concentrations using a total sample of 1.0 mg. Such analyses can provide fine-scale geochemical trends that would be obscured by bulkrock analyses.

As a case study, diagenetic phases from a Late Devonian pinnacle reef from the Alberta basin have been microanalyzed to quantify water-rock interaction (marine cements, caliche, pendant, vadose cements, and sparry calcite cements). Unaltered marine cements have the lowest ⁸⁷Sr/⁸⁶Sr ratios and the highest ^dgr¹⁸O values of all carbonate phases (0.70805 ± 2, -4.8^pmil 0.5) as well as the highest concentrations of [Mg] and [Sr] (approximately 8500 and 350 ppm, respectively). These are assumed to represent average, initial rock compositions. In ⁸⁷Sr/⁸⁶Sr-^dgr¹⁸O crossplots, altered marine cement data diverge from this value toward higher ⁸⁷Sr/⁸⁶Sr ratios and lower ^dgr¹⁸O values (to 0.7 842 and -8.0 ^pmil). Vadose cements, caliche, and meteoric phreatic spars have ⁸⁷Sr/⁸⁶Sr ratios and ^dgr¹⁸O values identical to altered marine cement data, with some spars having the same compositions as unaltered marine cements. These trends can be explained by two-component mixing models that use low W/R ratios, reasonable Sr concentrations, and distribution coefficients (0.05). However, a source of radiogenic strontium for diagenetic fluids is required to change initial ⁸⁷Sr/⁸⁶Sr ratios to observed values.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990