Experimental study of deformation processes within
an aggregate under isotropic conditions
M. Rossi (1), O. Vidal (1) and B. Wunder (2)
(1) LGCA-OSUG, Université Jospeh Fourier, BP 53, 38 041 Grenoble CEdex, France (mrossi@ujf-grenoble.fr)
(2) GFZ-Postdam, Telegrafenberg, 14473 Potsdam, Germany
Introduction:
Experimental setting:
This study is part of a larger project dealing with
mineralogical evolution and associated mass transfer resulting from LOCAL DISEQUILIBRIUM.
- Cold-sealed vessels experiment :
(isoptropic confining pressure, no compaction)
P = 100 - 200 - 300 - 400 MPa
T = 25 - 150 - 250 - 350 - 450°C
In contrast to classical deformation experiments
generaly conducted applying a strong deviatoric
stress (pure or simple shear, rotary experiments),
the aim of this study is to investigate the efficiency of deformation processes under isotropic midcrustal P-T conditions.
Samples
Closure nut
Pressure vessel
Gland
Thermocouple
T(°C)
Furnace
- Calibrated glass spheres (45-90μm) + micas
glass: kinetic of dissolution is higher than
that of quartz
Support rod
spheres: initial geometry is well known, easy
to quantify deformation and dissolution of
individual grains
- Quartz grains (rounded aeolian sand, D50=200μm) Glass spheres chemical composition
This poster thus presents the results of HYDROSTATIC experiments perfomed to study deformation of a spheres aggregate and mass transfer
triggered by pressure gradients at the grain scale.
+ different amount of fluid:
dry, 1vol.%, 5vol.%
SiO2
Na2O
72%
P(bars)
Pressure supply connection
Cap nut
Sample: glass spheres + 3% micas
± H2O
CaO
14%
Collar
MgO
10%
starting material
3%
gold capsule
Peff = Ptot − Pfluid
Experiments with glass spheres:
dry : flat contacts - viscous flow
1% H2O : binding of the spheres (sintering)
C
B
mean contact diameter (µm)
Evolution of deformation:
D
(P = 200 MPa, T = 350°C)
rc
28
(A)
20
150°C
350°C
1% H2O
1% H2O
dry
5% H2O
18
16
?
14
12
0
200
400
600
0.08
rs
(B,C)
22
dry
ε=−
(D)
rb − h − rb
h
=−
rb
rb
time (h)
0.07
0.06
0.05
0.04
0.03
0.00
0
100
200
Effect of Temperature on deformation: (6h run)
5
0.08
0.05
Exponentiel (1% H2O)
0.04
2
R = 0.9044
0.03
Exponentiel (dry)
Chemical reaction
(dissolution-recrystallization)
4
% fluid
dry
3
Mechanical
deformation
(ductile flow)
2
1
0.02
0.01
+ sintering
0
50
0
100 150 200 250 300 350 400 450 500
100
200
300
400
500
600
Deformation is strongly temperature dependant: ε(T)=A.exp BT
Increasing the fluid content at constant T has the same effect as increasing T at low fluid content: there is
a
switch from mechanical deformation processes (ductile flow, sintering) to chemical reaction with the fluid
(dissolution of the spheres and precipitation in the porosity).
free surface (pore)
800
(6h run)
0.08
0.07
2
R = 0.9272
0.06
(P = 200 MPa, T = 350°C)
surface at the contact between quartz grains
1% H2O
dry
0.05
0.04
Linéaire (1% H2O)
2
R = 0.8974
0.03
Linéaire (dry)
0.02
0.00
0
Temperature (°C)
Temperature (°C)
Experiments with quartz grains:
700
0.01
no sintering
0
0.00
600
0.09
Deformation lεl
0.09
2
R = 0.9656
time (h)
500
0.10
6
0.06
400
Effect of Pressure on deformation:
0.10
1% H2O
300
In dry and 1vol% H2O experiments, deformation occurs principaly during the beggining
of the runs when the effective pressure is the highest. As the spheres deform, the
size of the contacts increases so that the effective pressure and therefore deformation drop rapidely.
With fluid, the spheres are bound to each other, which precludes further deformation.
The differences between the experiments performed at 150°C and 350°C and the dry
and “wet“ ones are mainly due to differences of viscosity.
E: Fractures form radialy from the contacts and appear whatever the amount of fluid
0.07
150°C
1% H2O
dry
σ eff
ε ∝
η
0.01
Effect of the amount of water:
A: without any water (highest strain): viscous flow only (flat contacts)
B.C: low amount of water (1vol%, high strain): viscous flow + spheres binding (sintering?)
D: high amount of water (5vol%, lower strain): dissolution-recrystallization, reaction with the
fluid
Deformation lεl
350°C
1% H2O
dry
stress rate (1% H2O)
stress rate (dry)
0.02
ds − d2s − dc2
ε =
db
800
E
0.09
26
24
fracturing (1% H2O)
0.10
h
Deformation lεl
A
5% H2O : dissolution and recrystallization
100
200
300
400
500
Pressure (MPa)
Deformation is also pressure dependant: ε(P)=C.P
Fracturing increases with increasing pressure
Conclusion:
- No need to apply a strong deviatoric stress to deform an aggregate
- Deformation is extremely quick (90% in a few hours at 350°C).
σ
Viscous flow is the main deformation process:
ε =
η
- Deformation is strongly dependant of P and T
ε = A. exp BT + C.P
- The presence and amount of water control deformation but not the deformation rate
- Several deformation processes are present in our very simple chemical system:
- brittle deformation: fracturing
- ductile deformation: viscous flow, sintering (with water only)
- chemical reaction with the fluid: dissolution and crystallization in the porosity
Intragranular pressure-solution (IPS) is the dominant deformation process of
a siliceous aggregate at mid-crustal conditions.
IPS is efficient even without applying any deviatoric stress.
- Glass spheres are not a good analog of natural material under the same P-T conditions but
might be valuable analogues of high temperature rocks