Abstract

Strontium content of calcite has been used to aid in the interpretation of diagenesis of carbonate rocks. However, the distribution coefficient for strontium partitioning between calcite and solution (K D (super 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 partitioning of trace-element cations in the carbonate lattice has not previously been investigated. Aragonite-to-calcite transformation may occur under axial-stress burial conditions if aragonite-rich sediments escape near-surface diagenetic alteration. The effect of stress on K D (super Sr-Ca) during aragonite-to-calcite conversion was investigated through experimental compaction studies. Experiments investigated partitioning behavior under conditions of purely hydrostatic 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 at constant temperature likewise had no effect on the distribution coefficient. Hydrostatic experiments also investigated the temperature dependence of partitioning. Experimental data indicate that K D (super Sr-Ca) at STP is about 0.028. An experimental design was also constructed to test whether differential stress affects the K D (super Sr-Ca) value. A compaction apparatus was used to generate an effective stress of over 5 MPa, simulating lithostatic burial load at approximately 240 m. Temperature was held constant during experimental compaction runs. Data from compaction experiments show an order-of-magnitude depression of K D (super Sr-Ca) with increasing effective stress in the range from 0 to 5.35 MPa. These empirical results may aid in the interpretation of low-Sr limestones in the geologic record.

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