14 Dolomite and dolomitization

Dolomite formula

CaMg(CO3)2

How to lower SO4-2

(1) Microbial reduction in anoxic marine pelagic settings or as in algal mat inter- and supra-tidal flats. (2) Precipitation of gypsum and anhydrite. (3) Dilution w/meteoric waters.

Dolomite precipitation

- Although normal seawater is supersaturated with respect to dolomite, no one has been able to precipitate (abiogenic)dolomite at low temperatures from natural waters...aka. the dolomite 'problem'

Likely reasons for dolomite problem being prevented by kinetic barriers

- Dolomite is highly ordered; aragonite and calcite precipitate preferentially because of simpler structure. - High ionic strength of seawater (common ion problem). - Mg +2 is more strongly hydrated than Ca+2 and hydration/ion pairing of Mg +2 makes a barrier for CO3-2.

Barriers may be overcome by

- Evaporating seawater - Diluting sea water - Raising temperature - Lowering SO-2 4

Dolomite named after

- French engineer who first described its properties - Déodat Guy Sylvain Tancrède Gratet de Dolomieu

Dolomite/dolostones

- Much more common than limestones - Much more stable at depth; some of the oldest sedimentary rocks on earth - Common oil & gas reservoirs - Hosts for metallic mineral deposits, Mississippi-valley type Pb-Zn deposits, for example

Overcoming barriers: LOWERING SO4‐2

- even small amounts of SO4-2 inhibit dolomitization of calcite and aragonite -very abundant in seawater but hardly present in freshwater

Overcoming barriers: evaporating seawater

- gypsum, anhydrite and aragonite are precipitated raising the Mg/Ca ratio of pore fluids. - ALSO: Mg+2 is less strongly hydrated

Models of dolomization: Meteoric‐marine mixing

- in coastal settings with active groundwater movement - fave model in 20s

Overcoming barriers: Diluting sea water

- reduces ionic strength and the rapid rates of aragonite + calcite precipitation. - Minimal Mg/Ca ratio (~ 5) must be maintained for dolomite stability field - stream freshwater into seawater

Models of dolomization

1. Evaporitic settings 2. Meteoric‐marine mixing 3. Burial settings or hydrothermal 4. Seawater dolomitization 5. Microbial precipitation

A and B- lab culture, clean, pristine, no heterogeneity C and D - from actual sediments; lagoons and upper triassic microbiolite - microbes make dolomite and generate local chemical environment to facilitate dolomite - role in weathering

Dolomitization models

Models of dolomization: Microbial precipitation

as anoxic, organic-rich sediments or within microbial mats in shallow hyper-saline lagoons.

Models of dolomization: Evaporitic settings

high intertidal to supratidal sabkhas or coastal evaporitic lakes subject to seawater flooding

Dolomite problem; precipitation prevented by

kinetic barriers

Overcoming barriers: Raising temperature

overcomes hydration of Mg+2

Models of dolomization: Burial settings or hydrothermal

raised temperatures and Mg+2 sourced from compactional dewatering of basin mudrocks migrating into adjacent shelf-edge and platform carbonates

Models of dolomization: seawater dolomitization

salinity-driven circulation of deep, cold seawater into isolated carbonate platforms.