dolomitization - university of miamimgg.rsmas.miami.edu/groups/sil/dolomitization.pdfdolomitization...

2/25/2009

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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

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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)

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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

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Experimental

CaMgCO3+H3PO4=CO2+H2O+CaMgHPO4

= [CO2]1/2[H2O][CaMgHPO4]1/4 /[CaMgCO3][H3PO4]

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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

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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)

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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.

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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

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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

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Mixing-zone driven flow

Mixing Zone

Figure from Moore 2001

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Thermal Convection

Hot

ColdCold

Wilson et al. (1992)

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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


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– Supply model




• Bacterial


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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

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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

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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-

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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

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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

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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

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0 1 2 3 km

water bottom


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


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

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0

500

sea level

met

ers

Salty water diffuses out of sediment

1000



0

500

met

ers

During subsequent sea-level falls further saline ponds develop

1000



m



– Supply model




• Bacterial


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Geochemical Geochemical StratigraphyStratigraphy

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