1

Marbles and Metaperidotites;

Geothermobarometry

GEOL 13.53

Metamorphic Lecture 5

Typical Composition of Peridotitesand

Carbonate Rocks 0.05

0.24

0.34

K2O

0.03

0.07

0.49

Na2O

32.87

53.06

5.05

CaO

21.88

1.59

31.24

MgO

0.05

0.07

0.41

MnO

0.60

1.15

11.19

FeO

0.13

0.98

4.23

Al 2O3

0.41

3.64

42.26

SiO

2

Dolostone

Limestone

Peridotite

Metaperidotitesand Marbles

Low Grade Reactions in

Metaperidotites

2

Metamorphism of Calcareous Rocks

Figure 29-1. Winter (2001) An Introduction to Igneous and M

etamorphic Petrology. Prentice Hall.

Isogradsin Marbles

Figure 29-5. Metamorphic zones developed in regionally m

etamorphosed dolomitic rocks of the Lepontine Alps, along the Swiss-Italian border. After

Trommsdorff (1966) Schweiz. Mineral. Petrogr. Mitt., 46, 431-460 and (1972) Schweiz. Mineral. Petrogr. Mitt., 52, 567-571. Winter (2001) An

Introduction to Igneous and M

etamorphic Petrology. Prentice H

all.

Figure 29-2. A portion of the Alta aureole in Little Cottonwood Canyon, SE of Salt Lake City, UT, where talc, tremolite, forsterite, and periclase isograds were

mapped in m

etacarbonates by M

oore and K

errick(1976) Amer. J. Sci., 276, 502-524. Winter (2001) An Introduction to Igneous and M

etamorphic Petrology.

Prentice Hall.

3

Figure 29-4. After Spear (1993) Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths

Isogradsin Marbles

Mixed

Volatile

Reactions

Mixed

Volatile

Reactions

Internal vsExternal Buffering

4

Intersecting Isograds

Figure 29-7b. Isograds mapped in the field. Note that isograd (5) crosses the others in a m

anner sim

ilar to that in part (a). This behavior is attributed to infiltration

of H

2O from the syn-m

etamorphic pluton in the area, creating a gradient in X

H2Oacross the area at a high angle to the regional temperature gradient, equivalent to

the T-X

diagram. After Carmichael (1970) J. Petrol., 11, 147-181.

1

234

Bt+Cc+Qz=

Ca-Amph+Kf

Intersecting Isograds

Figure 29-7a. T-X

H2Odiagram illustrating the shapes and relative locations of the reactions for the isograds mapped in the W

hetstone Lake area. Reactions 1, 2,

and 4 are dehydration reactions and reaction 3 is the Ky = Sil transition, all in m

etapelites. Reaction 5 is a dehydration-decarbonation in calcic rocks with a

temperature maxim

um at X

H2O= 0.25. After Carmichael (1970) J. Petrol., 11, 147-181,

Intersecting Isograds

Figure 29-7a. T-X

H2Odiagram illustrating the shapes and relative locations of the reactions for the isograds mapped in the W

hetstone Lake area. Reactions 1, 2,

and 4 are dehydration reactions and reaction 3 is the Ky = Sil transition, all in m

etapelites. Reaction 5 is a dehydration-decarbonation in calcic rocks with a

temperature maxim

um at X

H2O= 0.25. b. Isograds mapped in the field. Note that isograd (5) crosses the others in a m

anner similar to that in part (a). This behavior

is attributed to infiltration of H

2O from the syn-m

etamorphic plutonin the area, creating a gradient in X

H2Oacross the area at a high angle to the regional

temperature gradient, equivalent to the T-X

diagram. After Carmichael (1970) J. Petrol., 11, 147-181.

Geothermobarometry

5

Geothermobarometry

•Metamorphic facies

and petrogenetic

grids allow us to

estimate PT

conditions based on

mineral assemblages

•Only requires field

and/or petrographic

data

Continuous Reactions

Figure 27-6. AFM

projections showing

the relative

distribution of Fe and

Mg in garnet vs.

biotite at

approxim

ately 500oC

(a) and 800oC (b).

Winter (2001) An

Introduction to

Igneous and

Metamorphic

Petrology. Prentice

Hall.

Ion Exchange

Reactions

Geothermobarometry

•Continuous and ion-

exchange reactions

require chemical

composition data from

coexisting minerals

•Can further limit PT

estimates

6

Distribution Coefficient (K

D)

•The distribution coefficient (K

D) is the ratio

by which two elements are partitioned

between two minerals at equilibrium

•For example, the distribution coefficient of

Fe and Mg in garnet and biotite is:

KDGt-Bt = (Mg/Fe)Gt/ (Mg/Fe)Bt

The Garnet -Biotite Geothermometer

T oC

Initial

X(Fe-Bt)

Final

X(Fe-Bt)

Final

X(Fe-Grt)

Final

(Mg/Fe)Grt

Final

(Mg/Fe)Bt

KT

Kelvins

1/T

Kelvins

lnK

799

1.00

0.750

0.905

0.105

0.333

0.315

1072

0.00093

-1.155

799

0.50

0.710

0.896

0.116

0.408

0.284

1072

0.00093

-1.258

749

0.50

0.695

0.896

0.116

0.439

0.264

1022

0.00098

-1.330

738

1.00

0.730

0.906

0.104

0.370

0.281

1011

0.00099

-1.271

698

0.75

0.704

0.901

0.110

0.420

0.261

971

0.00103

-1.342

698

0.50

0.690

0.896

0.116

0.449

0.258

971

0.00103

-1.353

651

0.75

0.679

0.901

0.110

0.473

0.232

924

0.00108

-1.459

651

0.50

0.661

0.897

0.115

0.513

0.224

924

0.00108

-1.497

599

0.75

0.645

0.902

0.109

0.550

0.197

872

0.00115

-1.623

599

0.50

0.610

0.898

0.114

0.639

0.178

872

0.00115

-1.728

550

0.75

0.620

0.903

0.107

0.613

0.175

823

0.00122

-1.741

550

0.50

0.590

0.898

0.114

0.695

0.163

823

0.00122

-1.811

601

0.50

0.500

0.800

0.250

1.000

0.250

874

0.00114

-1.386

601

0.25

0.392

0.797

0.255

1.551

0.164

874

0.00114

-1.807

697

0.75

0.574

0.804

0.244

0.742

0.329

970

0.00103

-1.111

697

0.25

0.468

0.796

0.257

1.137

0.226

970

0.00103

-1.487

Table 27-2. Experimental results of Ferry and Spear (1978) on a Garnet-Biotite Geothermometer

The Garnet -Biotite Geothermometer

Figure 27-6. AFM projections showing the relative distribution of Fe and M

g in garnet vs. biotite at approxim

ately 500oC (a) and 800oC

(b). From Spear (1993) Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths. M

ineral. Soc. Amer. M

onograph 1.

The Garnet -Biotite Geothermometer

Figure 27-7. Pressure-temperature diagram sim

ilar to Figure 27-4 showing lines of constant K

Dplotted using equation (27-35) for the

garnet-biotite exchange reaction. The Al 2SiO

5phase diagram is added. From Spear (1993) Metamorphic Phase Equilibria and Pressure-

Temperature-Time Paths. M

ineral. Soc. Amer. M

onograph 1.

7

The GASP

Geobarometer

Figure 27-8. P-T phase diagram

showing the experimental results of

Kozioland New

ton (1988), and the

equilibrium curve for reaction (27-37).

Open triangles indicate runs in which

An grew, closed triangles indicate runs

in which G

rs + K

y + Q

tz grew, and

half-filled triangles indicate no

significant reaction. The univariant

equilibrium curve is a best-fit

regression of the data brackets. The

line at 650oC is Kozioland New

ton’s

estimate of the reaction location based

on reactions involving zoisite. The

shaded area is the uncertainty

envelope. After Kozioland New

ton

(1988) Amer. Mineral., 73, 216-233

The GASP Geobarometer

Figure 27-8. P-T diagram contoured for equilibrium curves of various values of K for the GASP geobarometer reaction: 3 An = G

rs + 2

Ky + Q

tz. From Spear (1993) Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths. M

ineral. Soc. Amer. M

onograph 1.

Geothermobarometry

Figure 27-11. P-T phase diagram calculated by TQW 2.02 (Berman, 1988, 1990, 1991) showing the internally consistent reactions between garnet, muscovite,

biotite, Al 2SiO

5and plagioclase, when applied to the mineral compositions for sample 90A, Mt. M

oosilauke, NH. The garnet-biotite curve of Hodges and Spear

(1982) Amer. Mineral., 67, 1118-1134 has been added.

PTtPaths

8

Figure 27-12. Chemically zoned

plagioclase and poikiloblastic

garnet from m

eta-pelitic sample 3,

WopmayOrogen, Canada. a.

Chem

ical profiles across a garnet

(rim

→ →→→rim). b. An-content of

plagioclase inclusions in garnet

and corresponding zonation in

neighboring plagioclase. After St-

Onge(1987) J. Petrol.28, 1-22 .

PTtPaths

PTtPaths

Figure 27-13. The results

of applying the garnet-

biotite geothermometer

of Hodges and Spear

(1982) and the GASP

geobarometer of Koziol

(1988, in Spear 1993) to

the core, interior, and

rim composition data of

St-Onge(1987). The

three intersection points

yield P-T estim

ates

which define a P-T-t

path for the growing

minerals showing near-

isothermal

decompression. After

Spear (1993).