Can we Distinguish Between Serpentinised Peridotites and ...

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


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


Mag

netic

Sus

cept

ibili

ty (

10-3

SI)

Grain Density (g/cm3)

100

1000

104

0.1 1 10


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


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


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


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


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


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


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


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%



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)







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



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 ).

Can we Distinguish Between Serpentinised Peridotites and ...

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