can we distinguish between serpentinised peridotites and
TRANSCRIPT
Can we Distinguish Between Serpentinised Peridotites and Gabbros from their Physical Properties ?Insights from the GEOman ExperimentB. Ildefonse(1), P. Pezard (1), D. Mainprice (1), W.S.D. Wilcock (2), D.R. Toomey (3), S. Constable (4)
(1) Laboratoire de Tectonophysique, Université Montpellier II, France ([email protected])
(2) School of Oceanography, University of Washington, Seattle, USA ([email protected])
(3) Geological Sciences, University of Oregon, Eugene, USA ([email protected])
(4) Scripps Institution of Oceanography, La Jolla, USA ([email protected])
Introduction Discussion & Conclusions
• Ben Ismail, W., G. Barruol, and D. Mainprice, in press. The Kaapvaal craton seismic anisotropy: A petrophysic analysis of upper mantle kimberlite nodules, Geophys. Res. Lett.• Christensen, N.I., 1966. Elasticity of ultrabasic rocks. J. Geophys. Res., 71: 5921-5931.• Clennell, M.B., 1997. Tortuosity: a guide through the maze. In: M. A. Lovell & P. K. Harvey (ed.), Developments in petrophysics. Geol. Soc. Spec. Pub., London, pp. 299-344.• Guéguen, Y. and Palciauskas, V., 1992. Introduction à la physique des roches. Hermann, Paris.• Horen, H., Zamora, M. and Dubuisson, G., 1996. Seismic waves velocities and anisotropy in serpentinized peridotites from Xigaze ophiolite: Abundance of serpentine in slow spreading ridge. Geophys. Res. Lett., 23: 9-12.• Ildefonse, B., Einaudi, F., Pezard, P.A. and Hermitte, D., 2001. Physical properties of crustal oceanic rocks, with implications in terms of nature of seismic boundaries. EGS 2001, Nice, • Ildefonse, B. and Pezard, P.A., 2001. Electrical properties of slow-spreading ridge gabbros from ODP site 735, Southwest Indian Ridge. Tectonophysics, 330: 69-92.• Katsube, T.J. and Hume, J.P., 1987. Permeability determination in crystalline rocks by standard geophysical logs. Geophysics, 52: 342-352.• Mainprice, D. and Humbert, M., 1994. Methods of calculating petrophysical properties from lattice preferred orientation data. Surveys in Geophysics, 15: 575-592.• Nicolas, A., Boudier, F., Ildefonse, B. and Ball, E., 2000. Accretion of Oman and United Arab Emirates ophiolite. Discussion of a new structural map. Mar. Geoph. Res., 21: • Pezard, P.A., 1990. Electrical properties of Mid-Ocean Ridge basalt and implications for the structure of the upper oceanic crust in Hole 504B. J. Geophys. Res., 95: 9237-9264.• Revil, A., Cathles, L.M., Losh, S. and Nunn, J.A., 1998. Electrical conductivity in shaly sands with geophysical applications. J. Geophys. Res., 103: 23925-23936.• Revil, A. and Glover, P.W.J., 1998. Nature of surface electrical conductivity in natural sands, sandstones, and clays. Geophys. Res. Lett., 25: 691-694.• Walsh, J.B. and Brace, W.F., 1984. The effect of pressure on porosity and the transport properties of rocks. J. Geophys. Res., 89: 9425-9431.• Waxman, M.H. and Smits, L.J.M., 1968. Electrical conductivities in oil-bearing shaly sands. Soc. Pet. Eng. J., 8: 107-122.
References
A classical problem in marine geophysics is to assign a lithological significance to the seismic Mohorovicic discontinuity; is it the mantle crust boundary or is it a serpentinization front in the mantle?This still unresolved question mostly arises from the similar seismic velocities of gabbros and serpentinised peridotites, and from a lack of a complete petrophysical dataset for these rock types. In February 1998, we conducted a series of small-scale (100m - 7km) seismic, electrical and electromagnetic experiments in the Oman ophiolite. We also studied the microstructures and the physical properties (P-wave velocity, electrical conductivity, porosity,…) of serpentinized harzburgites,serpentinised wehrlites and gabbro samples from the field stations. This poster mostly reports on the laboratory dataset, and on its implications for the in-situ petrophysical signature of the oceanic lithosphere.
Laboratory measurements(electrical properties, porosity, density, Vp, magnetic susceptibility)3 orthogonal minicores (1" Ø) in XYZ reference frame
The GEOman Experiment
Samples
10
20
30
40
50
60
70
80
90
100
Peridotites
2 kb
GabbrosDolerites
9000
8000
7000
6000
5000
40002000 2500 3000 3500 4000 4500 5000
Vs (m s-1)
Vp
(m s
-1)
(Horen et al., 1996;after Christensen, 1966, 1972, 1978; Salisbury & Christensen, 1978;
Christensen & Smewing, 1981; Iturrino et al., 1991)
The objectives were to determine whether the crust/mantle boundary, inferred to be the paleoMoho, could be imaged with seismic and electrical techniques and to compare velocity/resistivity and anisotropy measurements at outcrop and hand-sample scales. Data were collected for sheeted dikes, gabbros and harzburgites in Wadi Hilti, Wadi Al Abyad and near the village of Mahram. Seismic refraction and reflection data were obtained with a sledgehammer and a 60-channel Geometrics acquisition package at offsets up to 500 m. A 600-kg hydraulic weight drop and RefTek recorders were used to collect refraction data at larger ranges. Schlumberger resistivity soundings were conducted in the same experiment sites.
Feb 1998
W.S.D. Wilcock (Univ. of Washington, Seattle)D.R. Toomey (Univ. of Oregon, Eugene)S. Constable (Scripps Instit. of Oceanography, La Jolla)B. Ildefonse (ISTEEM, Université Montpellier II)L.M. MacGregor (Dpt. of Earth Sciences, Univ. of Cambridge)T.L. Pratt (U.S. Geological Survey, Univ. of Washington, Seattle)
Small scale seismic (refraction/reflection)Electrical resistivityTransient electromagneticMagnetotelluric
Layered gabbros
Wehrlites
foliated gabbros
Extrusives
Sheeted dikes
Harzburgites
MOHO ?Dunites
Metamorphic sole
Continental margin & shelf
5 km
Geophysical Experiment in Oman
Electrical Properties
Compressional Velocities
harzburgites19 samples
(53 minicores)31 to 60 % serpentine(from Christensen, 1966)
wehrlites5 samples
(12 minicores)61 to 76 % serpentine(from Christensen, 1966)
gabbros14 samples
(39 minicores)
Co = + CsCwF
F = Electrical Formation FactorCs = Electrical Surface Conductivity (at the pore/mineral interface)Cw = Pore Fluid Conductivity
Waxman & Smits (1968), Revil & Glover (1998)
O -
OH
OH 2+
O -
Na+
O -
O -
Na+
Na+
Na+
Na+Cl -
Cl -
Cl-
Min
eral
Sur
face
(>S
i-; >
Al-
)
DiffuseLayer
0β
Shear(Zeta potential)
O -
SternLayer
Na+
Cl-
Electrolyte
Saturated samplesmeasurements at 7 salinities (0.05-100g/l)and various frequencies (20-100 kHz)
Electrodes
Sample
ImpedanceBridge
0,0001
0,001
0,01
0,1
0,01 0,1 1 10 100Mea
sure
d co
nduc
tivity
(m
S/m
)
Cw (Fluid conductivity mS/m)
Increasing saturating fluid salinity
Porosity Structure
F = φ-m m = Cementation Index(constant, from 1.5 to 2.5)
Archie (1942)
F =τφ
τ = Pore volume electrical tortuosity (~ 10 for low-porosity cristaline rocks)
Walsh & Brace (1984), Katsube & Hume (1987) Pezard (1990), Ildefonse & Pezard (2001)
F is measured or modelled to increase as the porosity decreases or the conduction path becomes more tortuous.
At high fluid salinity, or else if no alteration phases :
F = f (φ, structure)Cs << Cw
m describes the non-uniformity of the section of the conductive channels, τ relates to the complexity of the path followed by the electrical current (e.g. Guéguen and Palciauskas, 1992) or, in a more general sense, the efficiency of electrical flow processes (Clennell, 1997).
Altered Fraction
ßs : ionic mobilityρm : mineral phase average densityCECalt : Cation Exchange Capacity
Revil et al. (1998)
Cs = ϕw ßs ρm CECalt2
3
The altered weight fraction ϕω of the matrix contributing to electrical conductioncan be estimated from Cs and CEC measurements
0
10
20
30
40
50
0.1 1
gabbrosharzburgiteswehrlites
Porosity ø (%)
Ele
ctri
cal T
ortu
osity
τ
2.6
2.7
2.8
2.9
3
3.1
3.2
3.3
3.4
0.1 1
gabbrosharzburgiteswehrlites
Gra
in D
ensi
ty (
g/cm
3 )
Porosity ø (%)
0.1
1
10
100
1000
2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4
gabbrosharzburgiteswehrlites
Mag
netic
Sus
cept
ibili
ty (
10-3
SI)
Grain Density (g/cm3)
100
1000
104
0.1 1 10
gabbrosharzburgiteswehrlites
Ele
ctri
cal F
orm
atio
n F
acto
r F
Porosity ø (%)
F=1/ø 2 (Archie ,1942)
F ~ 10/ø
0.001
0.01
0.1
1
100 1000 104
gabbrosharzburgiteswehrlites
Ele
ctri
cal S
urfa
ce C
ondu
ctiv
ity (
mS/
m)
Electrical Formation Factor
1
1.2
1.4
1.6
1.8
2
0.1 1
gabbrosharzburgiteswehrlites
Ele
ctri
cal C
emen
tatio
n In
dex
m
Porosity ø (%)
0
10
20
30
40
50
0.001 0.01 0.1 1
gabbrosharzburgiteswehrlites
Ele
ctri
cal T
ortu
osity
τ
Electrical Surface Conductivity Cs (mS/m)
0
10
20
30
40
50
0.001 0.01 0.1 1
gabbrosharzburgiteswehrlites
Ele
ctri
cal T
ortu
osity
τ
Altered Fraction ϕw (%)
100
1000
10000
0.001 0.01 0.1
Porosity
Ele
ctri
cal f
orm
atio
n F
acto
r
F=1/ø 2 (Archie ,1942)
F ~ 10/ø
0.001
0.01
0.1
1
10
100
100 1000 10000
Electrical Formation Factor
Ele
crtic
al S
urfa
ce c
ondu
ctiv
ity (
mS/
m)
Compilation by Florence Einaudi (Ildefonse et al., 2001) Compilation by Florence Einaudi (Ildefonse et al., 2001)
Saturated samplesmeasurements at room pressure/temperature(Panametrics Epoch III-2300)
SWIR GabbrosOman Gabbros
Oman HarzburgitesOman Wherlites
x Oman BasaltsMAR Basalts
5000
5500
6000
6500
7000
7500
8000
8500
9000
0.1 1
gabbros (measured)gabbros (calculated)harzburgites (measured)harzburgites (calculated)wehrlites (measured)
Com
pres
sion
al v
eloc
ity V
p (m
/s)
Porosity ø (%)
5000
5500
6000
6500
7000
7500
8000
8500
9000
2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4
gabbros (measured)gabbros (calculated)harzburgites (measured)harzburgites (calculated)wehrlites (measured)
Com
pres
sion
al v
eloc
ity V
p (m
/s)
Grain Density (g/cm3)
gabbro - 98OB8b
plag
iocl
ase
clin
opyr
oxen
eol
ivin
e
[UVW] =100
N = 467
Max.Density = 3.48
[UVW] = 010
Max.Density = 4.75
[UVW] = 001
Max.Density = 3.04
N = 320
Max.Density = 2.18 Max.Density = 2.71 Max.Density = 2.38
N =56
Max.Density = 5.10 Max.Density = 4.56 Max.Density = 5.13
Vp (km/s)
Max. = 7.35 Min. = 7.18
- Anisotropy = 2.4%- in minicore ref. frame :measured anisotropy = 1.6%calculated anisotropy = 1.5%
7.35
7.32 7.30 7.28 7.26 7.24 7.22 7.20
7.18
harzburgite- 98OB1b
oliv
ine
orth
opyr
oxen
e
[UVW] =100
N = 354
Max.Density = 5.09
[UVW] = 010
Max.Density = 5.89
[UVW] = 001
Max.Density = 3.85
N = 74
Max.Density = 8.43 Max.Density = 5.99 Max.Density = 10.27
Vp (km/s)
Max. = 8.74 Min. = 8.00
- Anisotropy = 8.9%- in minicore ref. frame :measured anisotropy = 7.2%calculated anisotropy = 5.3%
8.74
8.64 8.56 8.48 8.40 8.32 8.24 8.16 8.08
8.00
measured Vp (m/s)
anisotropy of measured Vp (%)
calculated Vp (m/s)
anisotropy of calculated Vp in
minicore reference frame (%)
maximum anisotropy of
calculated Vp (%)
gabbrosmean 6887.7 2.4 7395.6 1.9 3.2standard deviation 258.1 1.8 220.2 1.3 1.5median 6950.0 2.0 7415.0 2.0 3.0harzburgitesmean 6267.0 5.7 8317.7 6.2 8.2standard deviation 507.3 2.5 265.6 1.4 1.0median 6299.0 5.2 8380.0 5.6 8.1wehrlitesmean 5732.5 3.2 - - - standard deviation 322.0 3.1 - - - median 5652.5 1.4 - - -
100
1000
10000
4000 4500 5000 5500 6000 6500 7000 7500 8000
Vp (m/s)
Ele
ctri
cal f
orm
atio
n F
acto
r
SWIR GabbrosOman Gabbros
Oman HarzburgitesOman WherlitesOman Basalts
Compressional velocities were also calculated from the crystallographic fabrics of the primary phases (Mainprice and Humbert, 1994), measured using the EBSD technique;i.e., the contribution of cracks and alteration phases (including serpentine) is then not considered, and can be estimated by comparing the calculated values to the measured ones.
Laboratory measurements and calculations suggest that the various rocks composing the oceanic lithosphere, especially the gabbros and peridotites with various degrees of serpentinization, show characteristic and distinct petrophysical signatures. The laboratory model data show a seismic anisotropy stronger (a few %) on average in harzburgites than in gabbros. There is no clear correlation between the anisotropy and the degree of serpentinization in harzburgites. The compared laboratory data on electrical properties also show differences between gabbros and harzburgites, the latter being generally more conductive, with a higher contribution of surface electrical conductivity (related to higher alteration). Combining electrical and seismic properties allows a fairly good discrimination of the different rock types. Extrapolation of these data to other scales/contexts is not straightforward, as shown by the compared laboratory and field (100-1000 m) results in the GEOman experiment.
0
2
4
6
8
10
12
0.1 1
gabbros (measured)gabbros (calculated, max)harzburgites (measured)harzburgites (calculated, max)wehrlites (measured)
Vp
anis
otro
py (
%)
Porosity ø (%)
2
4
6
8
10
12
30 35 40 45 50 55 60 65mea
sure
d V
p an
isot
ropy
in h
arzb
urgi
tes
(%)
% serpentine (calculated after Christensen, 1966)
Example of comparison withfield electrical data
line1 (harzburgites, // lineation)line2 (harzburgites, lineation)
line3 (gabbros - road)line4 (gabbros - wadi)
AB/2 (m)
appa
rent
res
isti
vity
(Ω
m)
Electrical resistivity field measurements
103
102
1 10 100
• Electrical resistivities are about an order of magnitude lowerin field measurements than in hand samples• No anisotropy• Gabbro resistivities (~ 500 Ωm) < harzburgite resistivities (~ 1000 Ωm ; ≠ from hand samples)• Wadi water ~ 25-50 Ωm --> bulk ø ~ 30% at depth >30m, or wadi water resistivity is lower ?
Mantle
Crust
Hayl al Mashakim
Hyad
Al Muta'arishah
Sharjah
Abu Khatutin
Halahil
Bani Ghayth
Al Khabt Al 'Ablah
Al 'Abaylah
Ar RikahWadi al Hilti
Wadi as Suhayli
experiment sites in Wadi Hilti
10 10
25
30
19
24
15
17 13
10
40
4012
31 2530
54
1718
22
3
3
9
14
11
38
15
28
54
1 km
2660
2666
440
2660
435 440 445
low-temperature
shear zone
SUMAIL
WADI TAYIN
NAKHL - RUSTAQ
HAYLAYN
SARAMI
WUQBAH
MISKIN
BAHLAH
D J E B E L A K H D A R
S A I H H A T A T
KHAWR FAKKAN
FIZH
HILTI
ASWAD
Oman & UAE ophioliteafter Nicolas & Boudier (2000)
gabbros & extrusivesperidotitesmetamorphic sole
0 20 40km
muscat
6 7
26
25
54
6 754
27
26
27
25°00'
24°00'
56°00'
23°00'
57°00'
24°00'
58°00'
25
experiment sites
Example of comparison withfield seismic data
LPO measurements5 thin sections
oliv
ine
orth
opyr
oxen
e
[UVW] =100 N = 2243
Max.Density = 2.72
[UVW] = 010
Max.Density = 4.48
[UVW] = 001
Max.Density = 2.24
N = 407
Max.Density = 4.75 Max.Density = 2.79 Max.Density = 4.62
field crackmeasurements
average ellipsoid1.35/1.56/1
2.32
2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6
0.24
N =1272
E1 - best axis 174.29 88.09 0.42 E2 279.85 0.51 0.31 E3 - best pole 9.86 1.84 0.27 Max.Density 25.00 60.00 2.32 Mean orientation 230.41 50.71
Vp calculated from LPO
8.49
8.40
8.32
8.24
8.16
8.08
7.94
olivine/orthopyroxene LPOolivine/orthopyroxene LPO+ 44% serpentine (isotropic)
7.23
7.15
7.10
7.05
6.96
Max. = 8.49 Min = 7.94Anisotropy = 6.8%
Anisotropy XY = 1.55%
Max. = 7.23 Min = 6.96Anisotropy = 3.8%
Anisotropy XY = 0.84%
X
Z
X
Z
Vp calculated from LPO + cracksolivine/orthopyroxene LPO+ 44% serpentine (isotropic)+ average crack (sub-vertical, E-W,10% volume, 50% serpentine + 50% water)
Max. = 6.97 Min = 6.72Anisotropy = 3.6%
Anisotropy XY = 0.9%
Max. = 6.99 Min = 6.75Anisotropy = 3.6%
Anisotropy XY = 3.4%
Max. = 7.06 Min = 6.59Anisotropy = 7.0%
Anisotropy XY = 6.7%
6.99
6.90
6.85
6.80
6.75
1/1/1 1.35/1.56/1
2/2/1
6.97
6.90
6.85
6.80
6.72
7.06
6.95
6.80
6.65
6.59
Max. = 7.11 Min = 6.01Anisotropy = 16.7%
Anisotropy XY = 16.5%
5/5/17.11
6.90
6.60
6.20
6.01 refraction profiles
minicore measurements
petrofabric calculation
E-W subparallel to lineation 5.6 km/s 6.41 km/s 8.55 km/s
N-S subperpendicular to lineation 5.2 km/s 6.26 km/s 8.40 km/s
horizontal anisotropy 8 % 2.5 % 1.8 %
The properties at the scale 100-1000m strongly depend on site effects (local fault distribution, ...). Each type of oceanic rocks appears to have an intrinsic petrophysical signature, but..Identifying these signatures in-situ may still be difficult (poor resolution data, poorly known scale/site effects). One can expect, however, that the effect of fracturation will be weaker a few km deep in the ocean crust than in the Oman ophiolite outcrops.
What do we need?• more petrophysical data• better resolution in experiments at sea (high-resolution, 3D seismic experiments, ...)• coupled seismic/electrical experiments• more high-resolution measurements of the various lithologies in boreholes (ODP-IODP)• more samples from the same boreholes (we want some pieces of 6 to 8 km/s!)
In wadi Hilti (site on the map), the measured azimuthal (parallel to the foliation plane XY)anisotropy is 8%. At the sample scale, the XY anisotropy (measured and calculated) is much smaller. The field observation is best modelled taking the local fracture network into account, asmeasured in the field (1272 measured cracks in site ).