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Dolomitization 2/25/2009 Dolomitization A Short Course VU March, 2009 Peter Swart University of Miami www.aoqz76.dsl.pipex.com/ Landscapes.htm Dieudonné Sylvain Guy Tancrede de Gratet de Dolomieu usually known as Déodat de Dolomieu (Dolomieu June 23, 1750 - Chateuneuf November 28, 1801) a French geologist; the rock Dolomite was named after him. During one of his field trips to the Alps of South Tyrol (today part of northeastern Italy) he discovered a calcareous rock which, unlike limestone, did not effervesce in weak acid. He published these observations in 1791 in the Journal de Physique. The following year, in the same journal, the rock was named dolomie (or dolomite, in English) by Nicolas-Théodore de Saussure. Today both the rock and its major mineral constituent bear the name of Dolomieu, as do the Dolomites, the mountain range in northwestern Italy, where he first identified the rock. http://en.wikipedia.org/wiki/Dolomieu 1 2/25/2009 1980 1994 "Dolomite is a complicated mineral. It exhibits a wide range in the concentration of major elements and is as complex as Feldspar" (Land, 1980) 2 2/25/2009 CaMg(CO3)2 Dolomites appear to be OXYGEN between 3 to 4 per mille heavier (more O-18) than co-occuring carbonates. +3 Dolomite CARBON Calcite +1 Dolomites are about 1 per mille heavier in carbon Stable Isotopes Natural Variations UNDA 18 Oo/oo - - - - 6 2 4 0 4 8 2 12345 6 200 Bulk Dolomites 400 o 3 /oo 600 (fbmp) Depth 800 1000 1200 1400 3 2/25/2009 Experimental CaMgCO3+H3PO4=CO2+H2O+CaMgHPO4 1/2 1/4 = [CO2] [H2O][CaMgHPO4] /[CaMgCO3][H3PO4] 4 2/25/2009 45 40 35 30 25 20 mineral-water a 15 Northrup and Clayton (1966) Delt 10 O.Neil and Epstein (1966) Sheppard and Schwarcz, (1970) 5 O'Neil et al, (1969) Vasconcelos et al (2005) 0 0 50 100 150 200 Temperature oC Water or Temperature? 45 Northrup and Clayton (1966) O.Neil and Epstein (1966) 40 Sheppard and Schwarcz, (1970) r O'Neil et al, (1969) Vasconcelos et al (2005) 35 mineral-wate 30 a Delt 25 20 0 1020304050 Temperature oC 5 2/25/2009 200 180 -8 160 -4 0 140 4 120 8 100 80 Temperature 60 40 20 0 -12 -10 -8 -6 -4 -2 0 2 Oxygen After Land (1980) 45 0 40 0.5 35 1 30 1.5 25 2 20 2.5 15 3 Delta calcite-dolomite Delta mineral-water Delta 10 3.5 5 4 0 4.5 0 50 100 150 200 250 Temperature oC Northrup and Clayton (1966) O.Neil and Epstein (1966) Sheppard and Schwarcz, (1970) O'Neil et al, (1969) Delta calcite-dolomite ARABIAN SAUDI GULF ARABIA Swart, Cantrell, Handford, Kendall & Westphall 2005 Ghawar Field QATAR 13 o 18 o Mineralogy % Permeability mD Porosity % C /oo O /oo Sr (ppm) R value dolomite-calcite 050 0.1 10 1000 0.0 10.0 20.0 0.50 1.50 2.50 -7.00 -5.00 -3.00 50 100 150 200 -1 1 15 6550 6555 6560 6565 6570 6575 Depth (ft) 6580 6585 6590 6595 6600 6 2/25/2009 Isotopes and Dolomites • Dolomites appear to be between 3 to 4 per mille heavier (more O-18) than co-occuring carbonates. • Synthesis: Uncertainties arises because dolomites are difficult ((p)if not impossible) to synthesize at room temperatures. Hence results are often interpolated from high temperatures. • Co-occuring: Dolomites co-occuring with calcites tend to be 3 per mille heavier, but are they coprecipitates? • Dolomites are about 1 per mille heavier in carbon What is up with Carbon? CaCO3 + Mg 2+ = CaMg(CO3)2 + Ca2+ 1 Ca2+ + Mg2+ + 2CO32- = CaMg(CO3)2 2 Dolomite can form from a range of different equations, the two end members of which are shown above. Equation one needs input of Mg2+ but is energentically less favourable than equation 2. Hence dolomite is favoured in environments with high alkalinity. What is up with Carbon? CaCO3 + Mg 2+ = CaMg(CO3)2 + Ca2+ Ca2+ + Mg2+ + 2CO32- = CaMg(CO3)2 In one 1l of seawater there are over 20 M of oxygen but only 2 mM of carbon. Hence during recrystallization it is more difficult to change the carbon isotopic values and these remain more positive despite the fact the the oxygen values can be reset to lower values. 7 2/25/2009 Dolomitization 2CH2O + SO42- = 2CO2 + H2O + H2S H2S = H+ + HS- CaCO3 = Ca2+ + CO32- Ca2+ + Mg2+ + 2CO32- = CaMg(CO3)2 Decomposition of organic material in either oxic or anoxic environments favours dissolution of carbonates and promotes dolomitization by increasing the concentration of carbonate ions Stoichiometry of Dolomitization 2+ 2+ 2- •Ca + Mg + 2CO3 = CaMg(CO3)2 2+ 2+ • 2CaCO3 + Mg = CaMg(CO3) + Ca 2+ 2+ 2- + •Ca +Mg +2HCO3 = CaMg(CO3)2 + 2H 2+ 2+ •2CaCO3 + Mg = CaMg(CO3) + Ca 200 186 2.71 2.83 186/2.83 / 200/2.71 = 0.89 11% increase in porosity Beumont 1837 8 2/25/2009 What is Needed for Dolomitization? • Supply of Magnesium – Seawater • Seawater is about 1000x oversaturated with respect to dolomite – High-Mg Calcite • Mechanism of supplying Magnesium Dolomitization Models • Mixing Zone – Chemical model – Supply model • Reflux of Hypersaline fluids • Thermal Convection – Low Temperature – High temperature • Bacterial • Special Geochemical Environments 9 2/25/2009 Mixing-zone driven flow Mixing Zone Figure from Moore 2001 10 2/25/2009 Figure from Moore 2001 Thermal Convection Cold Cold Hot Wilson et al. (1992) 11 2/25/2009 Wilson et al. (1992) Wilson et al. (1992) 12 2/25/2009 200 180 -8 160 -4 0 140 4 120 8 100 80 Temperature 60 40 20 0 -12 -10 -8 -6 -4 -2 0 2 Oxygen After Land (1980) Reflux Evaporation Figure from Moore 2001 13 2/25/2009 Figure from Moore 2001 Figure from Moore 2001 Dolomitization Models • Mixing Zone – Chemical model – Supply model • Reflux of Hypersaline fluids • Thermal Convection – Low Temperature – High temperature • Bacterial • Special Geochemical Environments 14 2/25/2009 1-Hardgrounds with increased dolomite content below 2- Background dolomite 1. Burrowing, formation of firmground, pause in sedimentation Seawater Open burrows Firmground surface Mudstone-Wackestone Mineralogy along the Bahamas Transect 10 km 15 2/25/2009 127°E 128°E 129°E 33°S 100 Eastern Transect 200 W estern Transect 1129 1132 1131 1127 1130 Eyre 500 1126 1134 1133 Terrace 1000 34°S 2000 3000 1128 4000 4500 0 50 km 1127 1131 1129 Eyre Terrace Precam- brian 200 km 16 2/25/2009 Other Reactions • Oxidation of Organic Material 2- - –2CH2O + SO4 = 2HCO3 + H2S • Dissolution 2- 2+ 2+ 2+ – Ca(1-x-y)SrxMgyCO3 = CO3 + Ca + xSr yMg • Precipitation 2- 2+ 2+ 2+ = –CO3 + Ca + xSr yMg Ca(1-x-y)SrxMgyCO3 • Alkalinity 2- - – Alkalinity= CO3 + HCO3 17 2/25/2009 Mineralogy along the Bahamas Transect 10 km N 1127 1131 1129 0 Eyre Terrace 1 Ce noz oic Sequences 2 3 km Mesozoic 4 5 Precambrian 200 km 6 Mineralogy in Unda and Clino within Sequence Stratigraphic Framework 18 2/25/2009 Dolomite from 628 127°E 128°E 129°E 33°S 100 Eastern Transect 200 Western Transect 1129 1132 1131 1127 1130 Ey r e 500 1126 1134 1133 Terr ace 1000 34°S 2000 3000 1128 4000 4500 0 50 km 19 2/25/2009 N 1127 1131 1129 0 Eyre Terrace 1 Ce noz oic Sequences 2 3 km Mesozoic 4 5 Precambrian 200 km 6 Site 1127 Site 1131 Site 1129 0123 km water bottom M esozoi c sed i m en t s Dolomite isotopes Zone of hydrate formation 20 2/25/2009 Sulfate /Chloride Site 1127 Site 1131 Site 1129 Sulfate/Cl 0123 km 60 water bottom 50 40 30 2 0 10 0 Mesozoic sediments Sulfate /chloride ratio in western transect Alkalinity Site 1127 Site 1131 Site 1129 Alkalinity (mM) 0123 km water bottom 120 100 80 60 40 20 Mesozoic sediments Alkalinity in western transect Saline Brines Develop on Continent sea level 0 500 s r e t e m 1000 1500 Mesozoic syn-rift Precambrian crystalline basement terrigenous clastic sediment 21 2/25/2009 Salty water diffuses out of sediment sea level 0 500 s r e t e m 1000 1500 Mesozoic syn-rift Precambrian crystalline basement terrigenous clastic sediment During subsequent sea-level falls further saline ponds develop 0 500 s r e t e m 1000 1500 Mesozoic syn-rift Precambrian crystalline basement terrigenous clastic sediment Dolomitization Models • Mixing Zone – Chemical model – Supply model • Reflux of Hypersaline fluids • Thermal Convection – Low Temperature – High temperature • Bacterial • Special Geochemical Environments 22 2/25/2009 23 2/25/2009 NEXT: Geochemical Stratigraphy 24.
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