Dolomitization

Abstract

Dolomite, CaCO3.MgCO3, is a mineral commonly found with limestone and carbonate sediments of coral reefs. Dolomite does not precipitate from normal seawater or solutions at low temperature and atmospheric pressure, yet it is a very common carbonate mineral in sedimentary rocks throughout the geological record. This paradox is referred to as the Dolomite problem. Dolomite is not a product of a CaCO3-MgCO3 solid solution series but it is a mixed lattice crystal whose nucleation cannot be explained in terms of classical (equilibrium) thermodynamics where Ostwald’s step rule governs the course of irreversible geochemical reactions. It was as recently as 1999 that Deelman suggested that Ostwald’s Rule can be broken when fluctuations of sufficient amplitude, frequency, and duration occur and they involve changes in pH conditions related to variable pCO2. Under such conditions a metastable precursor to dolomite (such as magnesium calcite) forms and through repeated alternations of dissolution and precipitation, change to more stable phases (such as magnesite or dolomite). He proposes that such fluctuations allow for both the metastable and stable phases to nucleate simultaneously and that during such fluctuations, conditions opposing the subsequent growth of the metastable phase will favor continued growth of the stable phase. It has recently been suggested by other researchers that the presence of microbial mediation could overcome the thermodynamic and kinetic barriers to dolomite nucleation in the natural environment. The majority of hydrological settings in which dolomite is known to occur involve special conditions which are characteristically highly variable and fluctuating and are dominated by enhancement of fluid flow, usually of modified seawater through preexisting carbonate sediments and sedimentary rocks, especially coral reefs. The occurrences of dolomite in Modern coral reefs and atolls has been known for over 100 years, yet the process of dolomite nucleation has evaded adequate chemical explanation until 1999 when Deelman proposed a way that it could happen. The deep borings of the Pacific Ocean Atolls and Guyots have provided a window to view the occurrence of dolomite and the processes of its formation. Of special significance have been the results of scientific investigation at Funafuti Atoll, Enewetak Atoll, Midway Atoll, Niue, Kita-daito-jima Atoll, Mururoa Atoll, Fangataufa Atoll, Christmas Island and Resolution Guyot, Wodejebato Guyot, and MIT Guyot. Equally important have been those deep borings where dolomite was not reported. They include the Great Barrier Reef, Bikini Atoll, Makatea Island, Guam Island, to name just a few. No discernable pattern of dolomite occurrence could be established. The overarching dolomitization process in all the hydrological models involves seawater, somehow modified, and dissolution – precipitation associated with repeated fluctuations such as tidal pumping, wave action, deep ocean currents, sea-level oscillations, pulses of thermally elevated water flow, etc. The process of dolomite formation may be penecontemporaneous with sedimentation but it is more frequently postdepositional, even diagenetic. Sometimes, there can be a considerable time delay of the order of 100 m.a. between sedimentation and dolomite formation. The more recent realization that anaerobic bacteria can play a role in the primary precipitation of dolomite is an avenue for fruitful research in addressing the topic of dolomite formation and occurrence.