Effect of differential stress on strontium partitioning in calcite

Strontium content of calcite has been used to aid in the interpretation of diagenesis of carbonate rocks. However, the distri bution coefficient for s trontium partitioning between calcite and solution (K-D(Sr-Ca)) is subject to both kinetic and thermodynamic factors that are incompletely understood. The effect of stress (which may involve both of these factors) on the part itioning of trace-element cations in the carbonate lattice has not previous ly been investigated. Aragonite-to-calcite transformation may occur under a xial-stress burial conditions if aragonite-rich sediments escape near-surfa ce diagenetic alteration. The effect of stress on K-D(Sr-Ca) during aragoni te ta calcite conversion was investigated through experimental compaction s tudies. Experiments investigated partitioning behavior under conditions of purely h ydrostatic fluid pressure surrounding growing calcite crystals. Changes in hydrostatic pressure (up to 35 MPa, isothermal) had little or no effect on the partitioning of strontium into calcite. Varying solution Sr/Ca ratios a t constant temperature likewise had no effect on the distribution coefficie nt. Hydrostatic experiments also investigated the temperature dependence of partitioning. Experimental data indicate that K-D(Sr-Ca) at STP is about 0 .028. An experimental design was also constructed to test whether differential st ress affects the K-D(Sr-Ca) value, A compaction apparatus was used to gener ate an effective stress of over 5 MPa, simulating lithostatic burial load a t approximately 240 m. Temperature was held constant during experimental co mpaction runs. Data from compaction experiments show an order-of-magnitude depression of K-D(Sr-Ca) with increasing effective stress in the range from 0 to 5.35 MPa, These empirical results may aid in the interpretation of lo w-Sr limestones in the geologic record.