Dolomitization Models
Dolomitization is the process by which existing calcium carbonate minerals (calcite or aragonite) are replaced by dolomite [CaMg(CO3)2], expressed by the replacement reaction: 2CaCO3 (solid) + Mg2+ (aq) = CaMg(CO3)2 (solid) + Ca2+ (aq). [1] Direct precipitation from aqueous solution — the other chemical pathway — requires temperatures far in excess of normal surface conditions, so most geologists believe that dolomitization by replacement was the dominant mechanism for forming ancient dolomite bodies.
Kinetic Requirements
Theoretical considerations suggest that dolomite formation is favored kinetically by high Mg2+/Ca2+ ratios, low Ca2+/CO32– ratios, and low salinity. [1] Higher temperatures are also favorable, and at temperatures exceeding about 100°C most kinetic inhibitors — including Mg2+ hydration — become ineffective. [1] These requirements explain why researchers have focused heavily on finding natural settings where elevated Mg/Ca ratios and fluid movement can drive the replacement reaction at near-surface conditions.
The Three Principal Models
Three principal models have been proposed to explain how dolomitization occurs in natural environments near the Earth’s surface. [1] Models 1 and 3 invoke seawater — concentrated by evaporation in the case of Model 1 — as the dolomitizing fluid, while Model 2 requires mixing of seawater and fresh (meteoric) water. [1]
1. Hypersaline (Sabkha / Evaporation-Reflux) Model
In hypersaline environments such as the sabkhas of the Persian Gulf — coastal plains characterised by the presence of evaporites — intense evaporation concentrates Mg in brines, raising the Mg2+/Ca2+ ratio to levels that favor dolomitization, commonly in association with gypsum and anhydrite formation. [1] The mechanism involves evaporative concentration of seawater in a supratidal or lagoonal setting; as CaSO4 minerals (gypsum, anhydrite) precipitate and remove Ca from solution, the Mg/Ca ratio rises. Dense hypersaline brine then sinks and refluxes downward and seaward through underlying carbonate sediments, driving dolomitization in the subsurface. This explains the frequent spatial association of ancient dolomites with evaporite sequences in the stratigraphic record.
2. Mixed-Water (Mixing-Zone) Model
The mixing-zone model proposes that where meteoric (fresh) groundwater and marine porewaters meet in the subsurface, the resulting mixed fluid is undersaturated with respect to calcium carbonate but supersaturated with respect to dolomite. [1] This zone of mixing — typically beneath coastal carbonate platforms — provides the chemical conditions (lower Ca2+/CO32– ratios, lower effective salinity) that reduce kinetic inhibitors and allow dolomite to nucleate and replace precursor carbonates. The position of the mixing zone shifts with sea level, so dolomitization can sweep through large volumes of rock as the zone migrates over time.
3. Seawater (Shallow-Subtidal) Model
The seawater model proposes that normal or slightly modified seawater, pumped or flushed through carbonate sediments by tidal or thermal circulation, can serve as the dolomitizing fluid. [1] Although modern seawater has a Mg/Ca ratio too high for calcite to precipitate freely but not high enough to overcome all dolomite kinetic barriers, the enormous volumes of seawater that can flow through a permeable carbonate platform over geologic time provide a vast reservoir of Mg ions. Given sufficient time and fluid flux, even normal-salinity seawater may drive dolomitization in shallow-subtidal settings where temperatures are relatively elevated.
Scale of the Problem
The three models each address some of the kinetic requirements for dolomitization, but none of them has been universally accepted as a complete explanation, because none fully accounts for the immense volumes of ancient dolomite preserved in the stratigraphic record. [1] The mismatch between the small volumes of modern dolomite being formed today and the thick ancient sequences raises the question of whether early-formed near-surface processes, or burial diagenetic replacement, or some combination of both, were responsible for the bulk of the world’s dolomite.
References & Citations
- 1.Principles of Sedimentology and Stratigraphy Boggs
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