A geochemical hypothesis for dolomitization by ground water

Most modern disordered dolomite has been found in dynamic environments. However, solutions associated with modern dolomite formation do not have a common Mg/Ca ratio; the ratio ranges from about 3 to 100. Ground-water circulation may have a significant role in formation of regional dolomites; one of the primary requirements for regional dolomite formation is a large supply of magnesium ions. An X-ray study of well cuttings from the Tertiary limestone aquifer of central Florida indicates that it is composed primarily of calcite and dolomite with minor amounts of quartz and apatite. The magnesium content of the calcite is slightly lower (0-2 percent MgC03) in the recharge areas than in the deeper confined parts of the aquifer system (2-4 percent MgC03). Our data support other recent work and indicate an equilibrium constant for dolomite of 2 X 10-17. Inasmuch as this value is exactly the square of the calcite equilibrium constant (10~8-35), the Mg/Ca ratio must be unity for the three-phase equilibrium, calcite-dolomite-water. The Mg/Ca ratio in water from the aquifer is as low as 0.05 in the recharge area where the water is also undersaturated with respect to both calcite and dolomite. With time and length of travel path in the system, the water increases systematically in Mg/Ca ratio, which approaches unity; saturation with respect to the two carbonates also increases downgradient until the solution apparently becomes over-saturated with respect to both carbonates. In Tertiary limestones of the Yucatan Peninsula, the Mg/Ca range in water is similar to that for Florida. The small amount of magnesium available from the solution of magnesium calcites and dolomite in the potable zone of active circulation is insufficient to provide the amount required for extensive dolomitization unless enormous quantities of rock are available for dissolution. However, dolomite may be forming in the zones of brackish water that underlie the Florida and Yucatan Peninsulas. The required magnesium may be derived from the readily available ocean water or reflux brines as the hydrologic regimen is changed because of relative fluctuations of sea level.