Holocene Dolomitization of Calcium Carbonate Sediments from the Coastal Sabkhas of Abu Dhabi, U.A.E.: A Stable Isotope Study

Abstract

Secondary dolomitization of Holocene calcium carbonate sediments occurs beneath the coastal sabkhas of Abu Dhabi, U.A.E. A stable isotope study of the sabhka sediments and brines indicates that the calcium-rich dolomite precipitates in isotopic equilibrium with brines at temperatures between 34° and 49°C. These calculated temperatures correspond to measured temperatures in the sediments where the dolomite is forming. Aragonite and perhaps high Mg-calcite serve as intermediates in the formation of the dolomite, which proceeds via a dissolution-reprecipitation process. With time, the dolomite appears to "age"; there is increased ordering in the crystal lattice, an increase in the crystal size, and continued isotopic equilibration with brines at the lower temperatures found deeper under the sabkha surface. A process of diagenesis and isotopic reequilibration also occurs in the coexisting calcium-carbonate phases. The final product of the "aging" is a calcite and dolomite mixture in which the carbonate phases are in isotopic equilibrium with each other and their environment at approximately 35°C (36 and 34°C calculated for calcite and dolomite, respectively, and 32.4°C measured in late summer). The oxygen-18 fractionation between the two end-products () falls within the predicted range suggested experimentally for secondary replacement dolomite in isotopic equilibrium with coexisting calcite (Fritz and Smith 1970; Matthews and Katz 1977).