ABSTRACT: Quantitative Modeling of Water-Rock Interactions in Meteoric Diagenetic Systems

R. Kevin Given, T. M. Quinn, K. C. Lohmann, A. N. Halliday

A quantitative model has been developed that monitors the ^dgr¹⁸O, ^dgr¹³C, and ⁸⁷Sr/⁸⁶Sr and [Sr] of a meteoric fluid as it interacts with surrounding carbonate rocks. The model first establishes the initial chemical signatures of the meteoric water through equilibration with soil-gas CO₂ and concomitant dissolution of carbonate. After the fluid reaches calcite saturation, a small volume of metastable carbonate is dissolved into the fluid and an equal volume of calcite is precipitated. This latter process is repeated, simulating increased dissolution and reprecipitation, and, consequently, lower water/rock ratios. Each system evolves toward closure, i.e., the value in the precipitated calcite equals that in the disso ving rock. This coupling among ^dgr¹³C, ⁸⁷Sr/⁸⁶Sr, and [Sr], and the decoupling of ^dgr¹⁸O is an inescapable conclusion of our modeling efforts.

Covariant trends among ^dgr¹³C, ⁸⁷Sr/⁸⁶Sr, and [Sr] exhibited by sparry calcite cements and the adjacent matrix within Pliocene limestones of Enewetak Atoll are predicted by our model. Bimodal open-system ⁸⁷Sr/⁸⁶Sr data integrated with modeling reveal two discrete episodes of meteoric diagenesis. The first episode represents subaerial exposure shortly after deposition and resulted in calcite with Pliocene ⁸⁷Sr/⁸⁶Sr values. The second episode of meteoric diagenesis occurred during the Pleistocene and resulted in precipitation of calcite with Pleistocene ⁸⁷Sr/⁸⁶Sr values within Pliocene rocks. As the model requires, both calcites have similar ^dgr¹³C and [Sr], indicating that th s diagenetic transfer of younger ⁸⁷Sr/⁸⁶Sr can occur without a recognizable impact on other geochemical tracers.

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