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
