ABSTRACT

The temperature dependence of Sr coprecipitation with calcite was determined experimentally in solutions with Sr/Ca ratios and ionic strengths closely resembling marine pore fluids. Aragoniteto-calcite and dolomite-to-calcite transformations were conducted over a temperature range from 40° to 200°C. Temperature dependence of the distribution coefficient of strontium in calcite (D^c_Sr) is significant. D^c_Sr in aragonite-to-calcite experiments varied from 0.046 at 40°C to 0.068 at 200°C. D^c_Sr in dolomite-to-calcite experiments varied from 0.034 at 40°C to 0.062 at 200°C. Experimental values of D^c_Sr are somewhat dependent on the precursor phase.

The experimental results are applied to natural diagenetic and hydrothermal calcites recovered from the sediment-covered Middle Valley part of the Juan de Fuca Ridge spreading center (Ocean Drilling Program Holes 857C and 858D). Low-Mg calcites from Hole 858D have Sr contents appropriate for their present-day in situ temperatures and pore-water Sr values. In contrast, high-Mg calcites from Hole 858D that precipitated from supersaturated pore waters at inferred high growth rates have elevated Sr contents relative to calculated equilibrium values. The Sr contents of calcites from Hole 857C are close to predicted equilibrium values in some instances but are substantially different in others. Deviations from predicted equilibrium Sr contents, especially in nodules, are attributed to the formation of mixed generations of carbonate precipitating with increasing burial depths and temperatures through pore waters with variable Sr composition.