2/25/2009
1
Dolomitization
www.aoqz76.dsl.pipex.com/ Landscapes.htm
A Short Course VU March, 2009 Peter Swart University of Miami
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
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.
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
2/25/2009
2
1980
1994
"Dolomite is a complicated mineral. It exhibits a wide range in the concentration of major elements jand is as complex as Feldspar" (Land, 1980)
2/25/2009
3
CaMg(CO3)2
CARBON
OXYGENDolomites appear to be between 3 to 4 per mille heavier (more O-18) than co-occuring carbonates.
Dolomite
Calcite
+3
+1Calcite
Dolomites are about 1 per mille heavier in carbon
+1
Stable Isotopes
1 2 3 4 5
200
400
600
DolomitesBulk
UNDA
(fb
mp
) o/oo3
18Oo/oo -8 -6 -4 -2 0 2 4 6
Natural Variations
800
1000
1200
1400
Dep
th
2/25/2009
4
Experimental
CaMgCO3+H3PO4=CO2+H2O+CaMgHPO4
= [CO2]1/2[H2O][CaMgHPO4]1/4 /[CaMgCO3][H3PO4]
2/25/2009
5
20
25
30
35
40
45
a m
iner
al-w
ater
0
5
10
15
0 50 100 150 200
Temperature oC
De
lta
Northrup and Clayton (1966)
O.Neil and Epstein (1966)
Sheppard and Schwarcz, (1970)
O'Neil et al, (1969)
Vasconcelos et al (2005)
Water or Temperature?
30
35
40
45
a m
iner
al-w
ater
Northrup and Clayton (1966)
O.Neil and Epstein (1966)
Sheppard and Schwarcz, (1970)
O'Neil et al, (1969)
Vasconcelos et al (2005)
20
25
0 10 20 30 40 50
Temperature oC
Del
ta
2/25/2009
6
60
80
100
120140
160
180
200
Te
mp
era
ture
-8
-4
0
4
8
0
20
40
-12 -10 -8 -6 -4 -2 0 2Oxygen
After Land (1980)
10
15
20
25
30
35
40
45
De
lta
min
era
l-w
ate
r
0
0.5
1
1.5
2
2.5
3
3.5 De
lta
ca
lcit
e-d
olo
mit
e
0
5
0 50 100 150 200 250
Temperature oC
4
4.5
Northrup and Clayton (1966) O.Neil and Epstein (1966)
Sheppard and Schwarcz, (1970) O'Neil et al, (1969)
Delta calcite-dolomite
Swart, Cantrell, Handford, Kendall & Westphall 2005
QATAR
SAUDI ARABIA
ARABIANGULF
GhawarField
0 50 0.1 10 1000 0.0 10.0 20.06550
6555
0.50 1.50 2.50 -7.00 -5.00 -3.00Mineralogy % Porosity % 18 O o
/ooPermeability mD 13 C o/oo
-1 1 dolomite-calcite
50 100 150 200Sr (ppm)
1 5R value
6560
6565
6570
6575
6580
6585
6590
6595
6600
Dep
th (
ft)
2/25/2009
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 ( p )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 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.
2/25/2009
8
Dolomitization2CH2O + SO42- = 2CO2 + H2O + H2S
H2S = H+ + HS-
CaCO3 = Ca2+ + CO32-
Decomposition of organic material in either oxic or anoxic environments favours dissolution of carbonates and promotes dolomitization by increasing the concentration of carbonate ions
Ca2+ + Mg2+ + 2CO32- = CaMg(CO3)2
Stoichiometry of Dolomitization
• Ca2+ + Mg2+ + 2CO32- = CaMg(CO3)2
• 2CaCO3 + Mg 2+ = CaMg(CO3) + Ca2+
• Ca2++Mg2++2HCO32- = CaMg(CO3)2 + 2H+
•2CaCO3 + Mg 2+ = CaMg(CO3) + Ca 2+
200 186
2.71 2.83
186/2.83 / 200/2.71 = 0.89
11% increase in porosity
Beumont 1837
2/25/2009
9
What is Needed for Dolomitization?
• Supply of Magnesium – Seawater
• Seawater is about 1000x oversaturated with respect to dolomitep
– High-Mg Calcite
• Mechanism of supplying Magnesium
Dolomitization Models
• Mixing Zone– Chemical model
– Supply model
• Reflux of Hypersaline fluidsyp
• Thermal Convection– Low Temperature
– High temperature
• Bacterial
• Special Geochemical Environments
2/25/2009
10
Mixing-zone driven flow
Mixing Zone
Figure from Moore 2001
2/25/2009
11
Figure from Moore 2001
Thermal Convection
Hot
ColdCold
Wilson et al. (1992)
2/25/2009
12
Wilson et al. (1992)
Wilson et al. (1992)
2/25/2009
13
60
80
100
120140
160
180
200
Te
mp
era
ture
-8
-4
0
4
8
0
20
40
-12 -10 -8 -6 -4 -2 0 2Oxygen
After Land (1980)
Reflux
Evaporation
Figure from Moore 2001
2/25/2009
14
Figure from Moore 2001
Figure from Moore 2001
Dolomitization Models
• Mixing Zone– Chemical model
– Supply model
• Reflux of Hypersaline fluidsyp
• Thermal Convection– Low Temperature
– High temperature
• Bacterial
• Special Geochemical Environments
2/25/2009
15
1-Hardgrounds with increased dolomite content below2- Background dolomite
1. Burrowing, formation of firmground, pause in sedimentation
SeawaterOpenburrows Firmground
surface
Mudstone-W ackestone
Mineralogy along the Bahamas Transect
10 km
2/25/2009
16
4000
3 000
200 0
1000
500
200
127°E 129°E128°E
34°S
33°S
Eyre
Terrace
10 0
0 50 km
1131
4500
1127
1126
1128
1129
1130
11331134
1132
W estern TransectEastern Transect
Precam-brian
Eyre Terrace
1127 1131 1129
200 km
2/25/2009
17
Other Reactions
• Oxidation of Organic Material– 2CH2O + SO4
2- = 2HCO3- + H2S
• DissolutionC S M CO CO 2 C 2+ S 2+ M 2+– Ca(1-x-y)SrxMgyCO3 = CO3
2- + Ca2+ + xSr2+ yMg2+
• Precipitation– CO3
2- + Ca2+ + xSr2+ yMg2+ = Ca(1-x-y)SrxMgyCO3
• Alkalinity– Alkalinity= CO3
2-+ HCO3-
2/25/2009
18
Mineralogy along the Bahamas Transect
10 km
0
1
2
3
N
km
4
5
6
CenozoicSequences
Mesozoic
Precambrian
Eyre Terrace
1127 1131 1129
200 km
Mineralogy in Unda and Clino within Sequence Stratigraphic Framework
2/25/2009
19
Dolomite from 628
1000
500
200
127°E 129°E128°E33°S
Eyre
Terrace
100
11311127
1126
1129
1130
11331134
1132
W estern TransectEastern Transect
4000
3000
200034°S
0 50 km4500
1128
2/25/2009
20
0
1
2
3
N
km
4
5
6
CenozoicSequences
Mesozoic
Precambrian
Eyre Terrace
1127 1131 1129
200 km
0 1 2 3 km
water bottom
Site 1127 Site 1131 Site 1129
M esozoic sediments
Zone of hydrate formation
Dolomite isotopes
2/25/2009
21
0 1 2 3 km
water bottom
Site 1127 Site 1131 Site 1129
Sulfate/Cl
2 0
3 0
4 0
5 0
6 0
Sulfate /Chloride
Mesozoic sediments
Sulfate /chloride ratio in western transect
0
1 0
2 0
0 1 2 3 km
water bottom
Site 1127 Site 1131 Site 1129
Alkalinity (mM)
40
60
80
100
120
Alkalinity
Mesozoic sediments
Alkalinity in western transect
20
0
500
sea level
s
Saline Brines Develop on Continent
1000
1500Precambrian crystalline basementMesozoic syn-rift
terrigenous clastic sediment
met
ers
2/25/2009
22
0
500
sea level
met
ers
Salty water diffuses out of sediment
1000
1500Precambrian crystalline basementMesozoic syn-rift
terrigenous clastic sediment
0
500
met
ers
During subsequent sea-level falls further saline ponds develop
1000
1500Precambrian crystalline basementMesozoic syn-rift
terrigenous clastic sediment
m
Dolomitization Models
• Mixing Zone– Chemical model
– Supply model
• Reflux of Hypersaline fluidsyp
• Thermal Convection– Low Temperature
– High temperature
• Bacterial
• Special Geochemical Environments
2/25/2009
23
2/25/2009
24
Geochemical Geochemical StratigraphyStratigraphy
NEXT: