Dolomite

Introduction Dolomite mineral –CaMgCO3 Dolomite used for mineral & rock –use dolomite (mineral) dolostone (rock)

Dolomite Precipitation Precipitation requires elevated Mg/Ca ratio – Problem: How get elevated ratios Precipitation: Ca2+ (aq) + Mg2+ (aq) +2CO32- (aq) = CaMg(CO3)2 (solid) Dolomitization: 2CaCO3 (solid) + Mg2+ (aq) = CaMg(CO3)2 (solid) + Ca2+ (aq)

Most dolomite diagenetic Early/late diagenetic replacement May cut across depositional units May be associated with unconformities May destroy textural features May occur as randomly distributed rhombs Some dolomite precipitated from water 500-million-year-old dolomite nonconformably overlies 1.5-billion-year-old rhyolite.dolomiterhyolite

Paleozoic Dolomite, UK

Typical dolomite outcrop

Stromatolites in dolomite Precambrian stromatolites South Africa Modern stromatolite mats, Brazil vsites.unb.br/ig/sigep/sitio041/fig2.jpg

Classification A. According to carbonate content –limestone = 0-10% dolomite –dolomitic limestone = 10-50% –calcitic limestone = 50-90% –dolomite = % B. According to grain size –dolorudite, dolarenite, dolosparite, dolomicrite C. Can use Folk or Dunham modifiers: –e.g. dolomitic biosparite or dolobiosparite

Dolomite increases back through time Possibilities: Change in seawater chemistry More dolomite forming environments in past Diagenetic replacement of older limestones

Dolomite forming environments Bahamas, Arabian Gulf, Bonaire Island, and Florida –hard crust on surface--supratidal to 1m depth –dolomitized pellets, gastropods = early replacement –In Arabian Gulf & Bonaire evaporites associated with dolomitic seds –All above cases Mg/Ca ratio 3-4x seawater

Dolomite forming environments B. San Andreas Island, Colombia –Pleistocene l.s. dolomitized by downward moving brines –brines formed through evaporation- supratidal zone

Dolomite forming environments C. Coorong, S. Australia –dolomite forms in coastal lagoons & lakes –no evaporation associated –groundwater modified through evaporation, enters lakes

Controversial Origin of Dolomite Seepage-reflux model –Covers San Andreas Island –Flooding and capillary movement of seawater –Evaporation = pore waters w/high Mg/Ca ratio –Pore waters move downward causing dolomitization –Problem--this type of dolomitization not demonstrated on geologic scale

Controversial Origin of Dolomite Evaporative pumping –Covers Arabian coast –Seawater moves laterally from lagoon to supratidal seds –Process driven by evaporation in sabkhas –Evaporation results in high Mg/Ca ratios; get dolomitization –Widespread dolomitization possible during regressions

Seepage-reflux & Evaporative pumping models –imply dolomitization = supratidal process –some dolomites not supratidal

Alternative Models groundwater/seawater mixing--Andros IS –Seawater supersaturated w.r.t. to dolomite –Easier to precipitate aragonite and calcite –Hghly ordered dolomite structure difficult to produce –Mix meteoric water with seawater produce undersaturation w.r.t. calcite but increased saturation of dolomite –Then contact with freshwater –Freshwater mixing with seawater in buried seds also possible –Problem: slow rate of dolomite precip=no large scale dolomite formation

Alternative Models Precipitation of scattered dolomite –Mg leaching from high Mg calcite or from clays

Dedolomitization Calcite replaced by dolomite – Rhomb occupied by calcite –Or calcite replaces edges The centers of dolomite rhombs have been leached and later refilled with single crystals of calcite. Some of the rhombs in this photograph are still hollow.

Paleozoic Dolomite, UK