Importance of sulfide oxidation in dolomite formation: A field and experimental study.

Abstract

The physical parameters, chemical mechanisms, and geologic settings favorable to dolomite formation remain a conundrum. Based on field evidence from two hypersaline, dolomite-producing lagoons near Rio de Janeiro, Brazil, a link between sulfide oxidation and dolomite formation is herein proposed. Sulfide oxidation is postulated to promote dolomite formation by producing acid that causes undersaturation for competing carbonate phases such as Mg-calcite. Although sulfate reduction and sulfate-reducing bacteria have been thought to promote dolomite formation by removing the proposed inhibitor SO₄, lagoonal sulfate reduction rates are sluggish, SO₄ levels remain at seawater concentrations or higher, and the largest amount of dolomite was observed in the lagoon with the most rapid sulfide oxidation. To further elucidate constraints on rates and mechanisms of dolomite formation, the larger hydrogeochemical and temporal setting of the lagoons is discussed. Dolomite in lagoon sediments appears to be associated with dense, Mg-SO₄-rich brines that migrate from nearby, larger Lagoa Araruama. These brines experience progressive sulfate reduction before being drawn up by capillary forces to the study lagoons' surface, where sulfide is oxidized, dissolution of calcite, aragonite and Mg-calcite is promoted, and supersaturation for dolomite is maintained. Mixing of hypersaline brine, meteoric water and seawater controls saturation states for carbonate minerals in this setting. A series of laboratory experiments is also described. Geochemical parameters from the field study of the two lagoons are mimicked in an effort to precipitate dolomite under low-temperature, atmospheric pressure conditions. The experiments used a calcite chemostat, with calcite dissolution providing the only source of bicarbonate to solution, and carbonate precipitation/dissolution maintaining solutions at approximately calcite equilibrium. Evidence was observed for a very high Mg-calcite overgrowth (∼1% overgrowth of 35 mol% MgCO₃), similar in mol% MgCO₃ to dolomite-associated high Mg-calcites in the Brazilian lagoons and in dolomite-producing localities such as the South Australian Coorong. Sulfide oxidation and associated calcite dissolution may have contributed significantly to carbonate sediment dolomitization during geologic periods of high sealevel stand and widespread shallow carbonate platforms. Changes in seawater composition over time also raise the possibility for significant variation in carbonate saturation state in marine fluids over geologic time.