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abstract

The textural, physical and chemical properties of limestones are often controlled by diagenetic reactions involving dissolution and precipitation of carbonate minerals. To constrain the rates and paths of these reactions, chemically controlled dissolution and precipitation experiments were performed using various carbonate solids in a number of solution types at 25°C and a PCO2 of 10−2.5 atm. The solution chemical variables included Mg/Ca ratio, salinity and dissolved SO4−2 and PO4−3 Dissolution rates of calcite, aragonite and Mg-calcite as a function of saturation state were similar regardless of changes in Mg/Ca ratio (over the range 1–5), salinity (0–35%c) and SO4 −2 (0–0.028 m). The addition of dissolved orthophosphate inhibited dissolution of all three carbonate minerals to similar degrees.

In contrast, calcite precipitation rates in seeded growth experiments responded strongly to changes in Mg/Ca ratio and dissolved SO4 −2 concentration. Changes in salinity had no influence on calcite growth rates. The relative precipitation rates of calcite versus aragonite were strongly affected by the presence of SO4 −2 and PO4−3 Calcite precipitation was selectively inhibited by SO4 −2, whereas aragonite precipitation was selectively inhibited by PO4−3 Thus, not only absolute but also relative growth rates of carbonate cements may be influenced by the nature of the geochemical environment. The results suggest that sedimentary microevironments, where PO4−3 is released from organic matter degradation and SO4 −2 is depleted via bacterial SO4 −2 reduction, may be favorable sites for calcite or Mg-calcite precipitation.

The data also suggest that carbonate precipitation reactions are much more readily inhibited by various chemical species than are dissolution reactions. This gives rise to a large difference in relative rates of dissolution and precipitation in certain geochemical environments, which generally favors the dissolution side of the reaction.

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