Temperature dependence of the strontium distribution coefficient in calcite: An experimental study from 40 degrees to 200 degrees C and application to natural diagenetic calcites

The temperature dependence of Sr coprecipitation with calcite was determine d experimentally in solutions with Sr/Ca ratios and ionic strengths closely resembling marine pore fluids. Aragonite-to- calcite and dolomite-to-calci te transformations were conducted over a temperature range from 40 degrees to 200 degrees C. Temperature dependence of the distribution coefficient of strontium in calcite (D-Sr(c)) is significant. D-Sr(c) in aragonite-ta cal cite experiments varied from 0.046 at 40 degrees C to 0.068 at 200 degrees C, D-Sr(c) in dolomite-to-calcite experiments varied from 0.034 at 40 degre es C to 0.062 at 200 degrees C, Experimental values of D-Sr(c) 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 Jua n de Fuca Ridge spreading center (Ocean Drilling program Holes 857C and 858 D), Low-Mg calcites from Hole 858D have Sr contents appropriate for their p resent-day in situ temperatures and pore-water Sr values. In contrast, high -Mg calcites from Hole 858D that precipitated from supersaturated pore wate rs at inferred high growth rates have elevated Sr contents relative to calc ulated equilibrium values. The Sr contents of calcites from Hole 857C are c lose to predicted equilibrium values in some instances but are substantiall y different in others. Deviations from predicted equilibrium Sr contents, e specially in nodules, are attributed to the formation of mixed generations of carbonate precipitating with increasing burial depths and temperatures t hrough pore waters with variable Sr composition.