2/25/2009. Dolomitization

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1 Dolomitization A Short Course VU March, 29 Landscapes.htm Peter Swart University of Miami Dieudonné Sylvain Guy Tancrede de Gratet de Dolomieu usually known as Déodat de Dolomieu (Dolomieu June 23, Chateuneuf November 28, 181) 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. 1

2 "Dolomite is a complicated mineral. It exhibits a wide range in the concentration of major elements and is as complex as Feldspar" (Land, 198) 2

3 /25/29 CaMg(CO 3 ) 2 Dolomites appear to be between 3 to 4 per mille heavier (more O-18) than co-occuring carbonates. +3 OXYGEN Dolomite CARBON Calcite +1 Stable Isotopes Dolomites are about 1 per mille heavier in carbon Natural Variations UNDA 18 O o /oo Bulk Dolomites 4 (fbmp) Depth o /oo

4 Experimental CaMgCO 3 +H 3 PO 4 =CO 2 +H 2 O+CaMgHPO 4 = [CO 2 ] 1/2 [H 2 O][CaMgHPO4] 1/4 /[CaMgCO 3 ][H 3 PO 4 ] 4

5 a mineral-water Delta Northrup and Clayton (1966) O.Neil and Epstein (1966) 1 Sheppard and Schwarcz, (197) 5 O'Neil et al, (1969) Vasconcelos et al (25) Temperature oc Water or Temperature? Delta mineral-water Northrup and Clayton (1966) O.Neil and Epstein (1966) Sheppard and Schwarcz, (197) O'Neil et al, (1969) Vasconcelos et al (25) Temperature oc 5

6 dolomite-calcite 2/25/29 Temperature Oxygen After Land (198) Delta mineral-water Temperature oc Northrup and Clayton (1966) O.Neil and Epstein (1966) Sheppard and Schwarcz, (197) O'Neil et al, (1969) Delta calcite-dolomite Delta calcite-dolomite SAUDI ARABIA ARABIAN GULF Swart, Cantrell, Handford, Kendall & Westphall 25 Ghawar Field QATAR 13 C o /oo Mineralogy % Porosity % 18 O o /oo Permeability md Sr (ppm) R value Depth (ft)

7 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 (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+ Ca2+ + Mg2+ + 2CO32- = CaMg(CO3)2 1 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 2 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

8 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 Ca 2+ + Mg CO 3 2- = CaMg(CO 3 ) 2 2CaCO 3 + Mg 2+ = CaMg(CO 3 ) + Ca 2+ Ca 2+ +Mg 2+ +2HCO 3 2- = CaMg(CO 3 ) 2 + 2H + 2CaCO 3 + Mg 2+ = CaMg(CO 3 ) + Ca /2.83 / 2/2.71 =.89 11% increase in porosity Beumont

9 What is Needed for Dolomitization? Supply of Magnesium Seawater Seawater is about 1x oversaturated with respect to dolomite High-Mg Calcite Mechanism of supplying Magnesium Dolomitization Models Mixing Zone Chemical model Supply model Reflux of Hypersaline fluids yp Thermal Convection Low Temperature High temperature Bacterial Special Geochemical Environments 9

10 Mixing-zone driven flow Mixing Zone Figure from Moore 21 1

11 Figure from Moore 21 Thermal Convection Cold Hot Cold Wilson et al. (1992) 11

12 Wilson et al. (1992) Wilson et al. (1992) 12

13 Temperature Oxygen After Land (198) Reflux Evaporation Figure from Moore 21 13

14 Figure from Moore 21 Figure from Moore 21 Dolomitization Models Mixing Zone Chemical model Supply model Reflux of Hypersaline fluids yp Thermal Convection Low Temperature High temperature Bacterial Special Geochemical Environments 14

15 1-Hardgrounds with increased dolomite content below 2- Background dolomite 1. Burrowing, formation of firmground, pause in sedimentation Open burrows Seawater Firmground surface Mudstone-Wackestone Mineralogy along the Bahamas Transect 1 km 15

16 33 S 34 S Eyre E 128 E 129 E Terrace 2 1 W estern Transect Eyre Terrace Eastern Transect km /25/29 5 km Precambrian 16

17 Other Reactions Oxidation of Organic Material 2CH 2 O + SO 2-4 = 2HCO 3- + H 2 S Dissolution Ca (1-x-y) Sr x Mg y CO 3 = CO Ca 2+ + xsr 2+ ymg 2+ Precipitation CO Ca 2+ + xsr 2+ ymg 2+ = Ca (1-x-y) Sr x Mg y CO 3 Alkalinity Alkalinity= CO HCO

18 Mineralogy along the Bahamas Transect 1 km Eyre Terrace Mesozoic Precambrian km N 1 Cenozoic Sequences 2 3 km Mineralogy in Unda and Clino within Sequence Stratigraphic Framework 18

19 Dolomite from S Eyre E 128 E 129 E 1 Terrace Western Transect Eastern Transect S km 19

20 Eyre Terrace Mesozoic Precambrian km N 1 Cenozoic Sequences 2 3 km Site 1127 Site 1131 Site km water bottom M esozoi c sediments Dolomite isotopes Zone of hydrate formation 2

21 s 2/25/29 Sulfate /Chloride Site 1127 Site 1131 Site 1129 Sulfate/Cl km 6 water bottom Mesozoic sediments Sulfate /chloride ratio in western transect Alkalinity Site 1127 Site 1131 Site 1129 Alkalinity (mm) km 12 water bottom Mesozoic sediments Alkalinity in western transect sea level Saline Brines Develop on Continent 5 meters 1 Mesozoic syn-rift terrigenous clastic sediment Precambrian crystalline basement 15 21

22 sea level Salty water diffuses out of sediment 5 meters 1 Mesozoic syn-rift terrigenous clastic sediment Precambrian crystalline basement 15 During subsequent sea-level falls further saline ponds develop 5 meters 1 Mesozoic syn-rift terrigenous clastic sediment Precambrian crystalline basement 15 Dolomitization Models Mixing Zone Chemical model Supply model Reflux of Hypersaline fluids yp Thermal Convection Low Temperature High temperature Bacterial Special Geochemical Environments 22

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