Simulation of carbonate diagenetic processes: formation of dolomite, huntite and monohydrocalcite by the reactions between nesquehonite and brine

Abstract

A tank system containing sediments and brine chosen to simulate an evaporative shallow-water organic-rich sedimentary basin was allowed to evolve for ten months. Early diagenetic reactions between nesquehonite and brine resulted in a marked increase in alkalinity. The first major change was the precipitation of monohydrocalcite spherules which formed a crust between supernatant liquid and sediment. Cores of the nesquehonite layer showed dissolution of the nesquehonite and the formation of huntite and protohydromagnesite, followed by the formation of Mg-rich dolomite, and then dolomite of stoichiometric proportions. Calcite precipitated with dolomite within the nesquehonite layer. A second crust, precipitated within a layer of decaying filamentous algae, between the nesquehonite and calcite layers was composed of calcite, dolomite and halite. Precipitation within the nesquehonite layer was controlled by reactions between brine and nesquehonite, leading to dissolution of nesquehonite and hydrolysis of carbonate. The formation of huntite and dolomite appears to have been aided by a high CO 2− 3 concentration in solution, which was effected by the high salinity, and maintained by the continued dissociation of nesquehonite.