water redistribution in the continental crust

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550. 600. 650. 700. 0.2. 0.4. 0.6. 0.8. 1.0. 400. 450. 500. 550. 600. 650. 700. 0. 1. 2. 3. 4. Phanerozoic shale. Proterozoic shale. Archean shale. Pre ss ure. (G. Pa. ).
Granulites & Granulites 2018 - Ullapool, Scotland

[email protected] @geolegologist

WATER REDISTRIBUTION IN THE CONTINENTAL CRUST Gautier Nicoli - Department of Earth Sciences, University of Cambridge, UK Brendan Dyck - Department of Earth Sciences, Simon Fraser University, Brunaby, Canada

INTRODUCTION Fluid circulation in the lithosphere (e.g. H 2O, CO2) exerts a direct control on a large range of processes (e.g.partial melting mechanisms, magma crystallisation and magma ascent). Low to medium-grade metamorphosed sedimentary rocks comprise mineral assemblages dominated by water-rich minerals such as biotite, muscovite, staurolite and amphibole. They represent important crustal water reservoirs, which, when buried within collisional orogenic settings, will influence melt fertility, and mass transfer. IMPLICATIONS rheology or the crust, metamorphic fluid production, thermal state of the crust THEORITICAL MODELLING Precambrian and Phanerozoic shales and greywakes compositions along 2 Barrovian geotherms (A: 1300 °C/GPa & B: 800 °C/GPa) Archean shale

10

ou t

0.4

0.2 500

550

600

650

700

ou t

0.8

M

s

8

/GPa 30 °C B 13

Wt %H2O

4

0.2 400

450

500

550

600

650

700

400

450

500

550

600

650

0.9

850 °C 8 kbar

7

0.7

-1

m

C.k

0° B' 4

6

3

700

-1

Phanerozoic greywacke

Proterozoic greywacke

1.0

2

Archean greywacke

1.0

5

0.2

0.2

0.2 400

700

450

500

550

600

650

700

400

450

500

Temperature (C)

550

600

650

700

Temperature (C)

30

Results Subsolidus conditions

A

Vol%

{ Sh.

A

{

B

Sh.

{

1.5

B

Sh.

Sh.

Gr.

Gr.

Phanerozoic

Proterozoic

Archean

25

A

B

B Crd Ms

Ms

A

A

Ms

20

Bt Bt

Bt Phanerozoic

B

Crd

Proterozoic

Archean

0

15

B

10

B

5

Bt

Bt

Proterozoic

Archean

0.5

50 40

50 40

30

30

20

20

10

10

0

0

650

700

750

800

850

900

950 1000

650

700

750

T (OC)

850

900

950 1000

T (OC) 25

50

Bt-out

Ms-out

Crd-out

Bt-out

60

30 20

kbar 8

6.75

0.05

B/SU = 7.4

800

750

Pl

6.75 4.5

6

850

Cpx

0

3 2 Maximum CO2 (wt%)

1

β

0.15

5

4

Kfs

Grt

0.1

0.3

Grt+Opx

0.2 0.05

β Cpx Pl

Kfs

0.1

0.0

0.3

0.2

0.4

0.0

0.1

0.0

Fe+Mg Supracrustal Unit Basement

10

0.2

0.3

0.4

Fe+Mg

Post-tectonic granitoid

Modelled melt

Supracrustal - Basement mixing

Closepet-type

Crust - Mantle mixing

EDC

Mineral entrainment

WDC

Fig. 7. Composition of the the posttectonic intrusions in the Western and Eastern Dharwar Craton compared to modelled melt compositions.

700

750

800

850

900

950 1000

650

700

750

800

850

900

950 1000

T (OC)

T (OC) FeO+MgO

FeO+MgO

Summary: Fluid budget in the crustal column FeO+MgO

FeO+MgO

Geotherm A

Geotherm B

a -1 1000 C O

50%

50%

1000 C O

H 2O 0.55 wt%

O

O

BASEMENT Green schists facies

50%

50%

10

450°C

Anatectic granites

O

Tsolidus

Secular trend

50%

CaO

A/NK

650°C

Tsolidus

50%

50%

CaO

Basement melt

H 2O 0.11 wt% Migmatites

25

Solidus

FPPM

0.44 wt% H 2O Granulite facies

FAPM

melt 20 vol% Basement melt + Supracrustal melt

melt 15 vol%

30

REFERENCES Nicoli & Dyck (2018) Geoscience Frontiers; Condie (1993) Chemical Geology, 104, 1-37; Brown (2006) Geology, 34, 961-964 ; Nicoli (in review) GSL Spec. Pub; Jayananda et al., (2018) Earth Sciences Reviews

H 2O

Ms-out

35

Highland Charnockite

Fig. 4. Evolution of melt chemistry generated by water-absent partial melting of shale and greywacke along geotherm A and B between Tsolidus and 1000 °C.

20

Chl-out

700 C O

Ep-out

Transition zone

15

50%

CO2 diffusion front

Amphibolite facies

Secular trend

700 C

Closepet granite

SUPRACRUSTAL UNIT

.M

5

1000 C

1000 C

Basement melt + Mantle melt

F=0.07 0.8 wt%

F=0.13 1.0 wt%

F=0.04 0.6 wt%

Basement melt + Supracrustal melt

F=0.30 2.6 wt%

850°C METASOMATIZED MANTLE DERIVED MELT

CO2

Geotherm B

GREYWACKE

Geotherm A

Dyke

Km

Phanerozoic Proterozoic Archean

km

650

A/NK

B/SU = 3.2

B/SU = 7.4

0.1

0

0

SHALE

700

0.0

15

5

10

0.2 0.15

0.1

20

40

Vol%

Vol%

800

30

70

GREYWACKE

Bt-out

60

unit

8

0.25

0.4

Fig. 3. Modal proportion of melt along geotherms A and B for shales and greywackes under water saturated solidus. Vertical boxes indicate the disappearance of hydrousphases. Bt: biotite Ms: muscovite Crd: cordierite

Ca/(Ca+Na)

70

Vol%

Vol%

SHALE

80

Phanerozoic Proterozoic Archean

60

Ms-out

Bt-out

Crd-out

90

70

kbar

Fig. 6. Volume of anatectictic melt for different B/Su volumetric ratios and flux of carbon dioxide from the mantle through the basement and the supracrustal unit.

Geotherm B

90

CO2 in the lower crust

Supracrustal Unit Basement

T (°C) Phanerozoic

700

650

0

0 650

Suprasolidus conditions Geotherm A

tal

rus

rac

Bt

Bt

3.2

Sup

Bt

Fig. 2. Total H2O loss during devolatilization from 400 °C to Tsolidus for geotherms A and B (Sh–Shales, Gr–Greywackes) and Volume % of hydrous phases at Tsolidus

80

B/SU =

Ms

Bt Bt

5

Bt

A

Crd

Ms

10 Bt

A A

15

Bt

20 Bt-out

B

600

0.3

CO2

1.0

Gr.

Sh. Gr.

Sh. Gr. Gr.

0.35

2.1

2.0

45 40 35 30 25 20 15 10 5 0

550

K

2.5

Anatectic melt

F = CO2 flux (m3.Ma-1)

{

B

{

H2O loss (vol%)

{

A

3.0

GREYWACKE Vol %

3.5

500

T (°C)

950

25

SHALE

450

Supracrustal Unit: 58 % greywacke, 21% pelite, 21% mafic schist

At middle and lower crustal levels

Fig. 1. Heatmaps showing mineral-bound water content (wt%)

Opx-in

650

Bt-out

600

Opx-in

550

Temperature (C)

850

750

Bt-out

500

T (°C)

Opx-in

450

650

Grt-in

400

550

0 400

Fig. 5. P-T conditions along crustal transects in the Western nd Eastern Dharwar and bulk water content in mineral-bound water in the basement (B) and the supracrustal unit (SU).

Ch

Ms

450

/GPa 30 °C A 13

lo ut

ou t

0.4

/GPa 30 °C A 13

N-S Crustal transect A-A' Western Dharwar Craton B-B' Eastern Dharwar Craton

450 °C 2.5 kbar

2

in

0.4

out

Bt

/GPa 30 °C A 13

in

0.4

Ms

B

Pa °C/G 800

Basement B/SU = 7.4

0.1

3

Melt

Melt

0.6

ou t

B

Pa °C/G 800

Ms

Melt

0.6

Melt B/SU = 3.2

0.4

0.2

B

Bt

Pressure (GPa)

in

B

Pa °C/G 800

0.6 0.5

0.3

A

0.8

ut lo

Bt

ut lo

0.6

Ch

Ch

0.8

.km

°C

4

0 0.8

30

1

1.0

Hydrated Solidus

Melt

A'

Metamafic rock

P (kbar)

M

s

/GPa 30 °C A 13

M s

ou t

0.4

0.2 450

Chl ou t

Melt

out

in

out

/GPa 30 °C A 13

400

Pa °C/G 00 B8

0.6

1

Metasediment

Bt

0.4

Pa °C/G 00 B8

0.6 Chl

Chl

Pa °C/G 00 B8

0.6

B/SU = Basement /Supracrustal Unit volumetric ratio

Peninsular gneiss

9

Supracrustal Unit

0.8

Melt

in

Pressure (GPa)

Bt

0.8

Basement

0.8

Geothermal gradient (30-40 °C/km) and subsolidus conditions

Bt in

1.0

Chl-out

Proterozoic shale 1.0

H2O content (wt%)

Phanerozoic shale 1.0

CASE STUDY: THE DHARWAR CRATON, INDIA 2.5 Gyrs old N-S crustal section from greenschist facies condtions to granulite facies condtions (migmatites + charnockites)

METASOMATIZED MANTLE DERIVED MELT