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Tectonophysics 370 (2003) 77–94

Seismic wave velocity and anisotropy of serpentinized peridotite in

the Oman ophiolite

Benoıt Dewandela, Franc�oise Boudiera,*, Hartmut Kernb,Waris Warsic, David Mainpricea

aLaboratoire de Tectonophysique, Universite Montpellier II, 34095 Montpellier, Franceb Institut fur Geowissenschaften, Olshausenstr., 40, D-24098 Kiel, Germany

cDepartment of Earth Science, Sultan Qaboos University, P.O. Box 36, Al-Khod, Sultanate of Oman

Accepted 31 March 2003

Abstract

Shallow seismic measurements in harzburgite from the Oman ophiolite performed in a zone where the maximum horizontal

anisotropy is expected (vertical foliation and horizontal lineation) point to a dominant dependence of seismic properties on

fracturing.

Optical microscopy studies show that microcracks are guided by the serpentine (lizardite) penetrative network oriented

subparallel to the harzburgite foliation and subperpendicular to the mineral lineation, and that serpentine (lizardite) vein filling

has a maximum concentration of (001) planes parallel to the veins walls. The calculated elastic properties of the oriented

alteration veins filled with serpentine in an anisotropic matrix formed by oriented crystals of olivine and orthopyroxene are

compared with seismic velocities measured on hand specimens.

Laboratory ultrasonic data indicate that open microcracks are closed at 100 MPa pressure, e.g. (J. Geophys. Res. 65, (1960)

1083) and (Proc. ODP Sci. Results Leg 118, (1990) 227). Above this pressure, laboratory measurements and modeling show

that P-compressional and S-shear wave velocities are mainly controlled by the mineral preferred orientation. Veins sealed with

serpentine are effective in slightly lowering P and S velocities and increasing anisotropy. The penetrative lizardite network does

not affect directly the geometry of seismic anisotropy, but contributes indirectly in the fact that this network controls the

microcrack orientations.

Comparison between seismic measurements of peridotite and gabbro in the same conditions suggest that P- and S-waves

anisotropies are a possible discriminating factor between the two lithologies in the suboceanic lithosphere.

D 2003 Published by Elsevier B.V.

Keywords: Oman ophiolite; Seismic wave velocity; Anisotropy

0040-1951/03/$ - see front matter D 2003 Published by Elsevier B.V.

doi:10.1016/S0040-1951(03)00178-1

* Corresponding author.

E-mail addresses: [email protected] (F. Boudier),

[email protected] (H. Kern).

1. Introduction

The relative contribution of serpentinized perido-

tites to the seismic properties of oceanic lithosphere

is still poorly known, due to the difficulty in

discriminating between serpentinized peridotite from

B. Dewandel et al. / Tectonophysics 370 (2003) 77–9478

the crustal constituents, gabbro and dolerite. In the

recent years, several investigators have measured the

seismic properties of serpentinized peridotites (Chris-

tensen, 1966; Kern and Tubia, 1993; Horen et al.,

1996; Iturrino et al., 1996). Christensen (1966) has

correlated the volume of serpentinization with the

decrease of P-wave velocity. Horen et al. (1996)

have explored the relationships between degree of

serpentinization and P- and S-wave velocities aniso-

tropies. Although some experimental ultrasonic

measurements on spherical samples have been per-

formed on various lithologies (Pros and Babuska,

1968; Babuska, 1972; Siegesmund et al., 1993;

Rasolofosaon et al., 2000), laboratory seismic meas-

urements published on serpentinized peridotites are

made in three orthogonal directions related to the

olivine LPO and they do not necessarily correspond

to the maximum and minimum velocities of the

rock.

Previous theoretical calculations (e.g. Baker and

Carter, 1972; Mainprice and Silver, 1993; Ji et al.,

1994; Barruol and Kern, 1996) of P- and S-waves

velocities and seismic anisotropy of mantle rocks,

have only taken into account the lattice preferred

orientation (LPO) of the primary phases in peri-

dotite. The modeling of seismic properties of ser-

pentinized peridotites is a difficult task because

the macroscopic physical properties result from the

interference of two microstructural elements, the

peridotite polycrystalline aggregate and the serpen-

tine network. The effect of oriented microcracks on

elastic wave propagation has been discussed by

several authors, for example Nur (1971) in the case

of dry cracks, by Anderson et al. (1974) for fluid-

filled cracks. Siegesmund et al. (1991) and Rasolo-

fosaon et al. (2000) discussed such effect based on

U-stage measurements of microcracks in an ultra-

mylonitic rock and in a gneiss (KTB pilot hole),

respectively. These authors assume either that the

matrix and filling cracks are elastically isotropic or

that the matrix is anisotropic and the filling cracks

isotropic.

The objective of the present study is to develop a

realistic model of seismic properties of serpentinized

peridotite, by integrating the seismic anisotropy due to

the crystals preferred orientation of the primary peri-

dotite aggregate and the anisotropy due to the sealed

crack-like serpentine network.

2. Field (hectometric scale) measurements

The area studied in the Oman ophiolite (Fig. 1a,b)

has been chosen on the basis of the structural mapping

(Nicolas and Boudier, 1995), so that the seismic

measurements could be related to the lattice preferred

orientation (LPO) of the peridotite rocks, whose

seismic anisotropy is well understood from the pre-

vious studies (e.g. Babuska, 1972; Mainprice and

Silver, 1993; Barruol and Kern, 1996; Weiss et al.,

1999). The Khafifah area in the Wadi Tayin massif

(Fig. 1b) is characterized by a steeply dipping folia-

tion striking NNE–SSW and a lineation subhorizontal

in the mantle harzburgites (Boudier and Coleman,

1981). According to the relationships of penetrative

structures in the peridotite with minerals component

(foliation subparallel to max (010)ol and lineation

subparallel to max [100]ol (Nicolas and Poirier,

1976)) the Vp maximum and Vp minimum, if con-

trolled by the peridotite LPO, are expected to lie close

to lineation and perpendicular to foliation, respec-

tively, that is along two horizontal directions striking

perpendicular to each other at the selected field site.

2.1. Structural measurements

The map of lineaments (Fig. 2a) drawn, in the

Khafifah area, on the basis of the aerial photographs

provides an image of the distribution of the fracture

network at the kilometric scale. Aerial photographs

show two sets of vertical fractures. The main sub-

vertical set trending NW–SE is parallel to a kilometer

wide shear zone marked on the structural map (Fig.

1b), the second set includes longer subvertical frac-

tures (sometimes more than 5 km length) trending

NE–SW to N–S and parallel to the regional foliation

in mantle rocks (Fig. 1b, and Nicolas and Boudier,

1995).

Field observations show that all fractures or joints

are hydrothermal veins, along which measurable dis-

placement (>1 cm) is exceptional. At the scale of field

observations, the fracture network has been measured

along two 70–80 m long seismic lines (Fig. 2c) in a

regular centimeter-spaced network of veins 1 mm

thick sealed by serpentine, and in a meter-spaced

network of 10 cm thick veins filled by fibrous

serpentine and calcium–magnesium carbonates. A

bipolar distribution of fractures, similar to that

Fig. 1. (a) Geographical location of the studied area, the box represents the area shown in (b). (b) Structural map of Wadi Khafifah area, Wadi

Tayin massif, Oman ophiolite (Nicolas and Boudier, 1995). Foliation trending NE–SW, steeply dipping SE, lineation horizontal. Box, area of

Fig. 2a; white square, Fig. 2c.

B. Dewandel et al. / Tectonophysics 370 (2003) 77–94 79

observed at the map scale, appears in these field

measurements (Fig. 2b). One set of fractures is strik-

ing NW–SE and steeply dipping northeastward,

and the second, striking N–S and vertical. Both

types of mineral filled fractures are observed in the

two sets. Despite the limited number of field measure-

ments (96), this ‘scale-similar’ organization accounts

for a common process of fracturing at different scales


in a homogeneous material. We point out another bias

of these measurements at both scales (aerial photo-

graph and field measurements), that is: horizontal

fracturing is hardly visible in aerial photograph and

measurable in the field, thus definitely underestimated

in our data.

2.2. Seismic field measurements

The selected site along Wadi Khafifah (Fig. 1) is

located in the serpentinized harzburgite (average

degree of serpentinization 50–60%) a few hundred

meters below the paleoMoho (Fig. 1b), providing a

hard flat rock surface for which topographic correc-

tions are not required. Shallow seismic refraction

profiles were carried on along two lines 70–80 m

long, oriented parallel to lineation, i.e. NNE and

perpendicular to foliation, i.e. ESE (Fig. 2c). Sledge-

hammer shots were used as the source, corresponding

to 5–10 kHz frequency. The geophones were spaced

every 10 m and the lines were shot three times.

Seismic energy was recorded from the surface to a

few tens meters.

The seismic lines parallel to lineation (Fig. 3b)

show an increase in apparent velocity with distance

from the shot point, corresponding to velocity

increase with depth. The three shots perpendicular

to foliation are less consistent. Previous studies on

oceanic drilling (Iturrino et al., 1996) and on land in

Oman have shown that the degree of serpentinization

does not vary significantly within a hundred meters

depth. Thus, the velocity increase for the two lines is

solely indicative of rapid closure of open fractures

caused by lithostatic decompression, at shallow

depth of few tens meters (e.g. Matthews et al.,

1971). A stabilization of the apparent velocities is

observed 70 m from the shot point, Vp = 5.3 km/s

perpendicular to the foliation (Fig. 3a), Vp = 4.2 km/

s parallel to the lineation (Fig. 3b). A high aniso-

tropy of 20.8% is deduced for propagation along the

two profiles.

Fig. 2. (a) Detailed lineaments (fractures) drawn from aerial

photographs. (b) Poles of fractures, field measurements on site 97

OA 129. Number of measurements: 96, in geographical reference

system, lower hemisphere of projection. S1: foliation, L1: mineral

lineation. X, Y, Z, axes of the shape preferred orientation. (c) Site 97

OA 129 (white square in Fig. 1b) with orientation of seismic lines.

Fig. 3. Shallow P-waves measurements from seismic lines shown in Fig. 2c. (a) Seismic line perpendicular to foliation. (b) Seismic line parallel

to lineation. SP1, SP2 and SP3 are three shots along the same seismic line.


The second interesting result is that a higher

velocity is measured along the line perpendicular

to the foliation compared to the line parallel to

the lineation, which is opposite to anisotropy

expected on the basis of penetrative structures in

the peridotite.

3. Sample (centimeter scale) measurements

In order to calculate seismic properties of the

peridotite, the LPO of the primary aggregate (olivi-

ne + orthopyroxene) and the altered network, serpen-

tine veins were measured on a harzburgite sample

having an averaged composition and degree of

serpentinization representative for the area of studied

site 97 OA 129. The composition of the primary

paragenesis is 68% olivine, 32% enstatite and less

than 1% spinel. The texture observed in thin section

is a high temperature porphyroclastic microstructure,

3–5 mm grain size, with a low degree of recrystal-

lization; foliation is well marked by spinel and

olivine crystal elongation. The penetrative serpentine

network represents 60% by volume, formed of veins

Fig. 4. Sample 97 OA 129a, thin section (crossed nicols) cut

perpendicular to the two sets of mineralized microcracks (see also

Fig. 10b). The crystallographic plane (001) lizardite is parallel to the

veins. Lizardite II refers to the second set of orientations of Fig. 6.

Fig. 5. Sample 97 OA 129a, cut for U-stage measurements (A3

symmetry). Sections (X), (Y), (Z) are perpendicular, respectively, to

X, Y, Z axes of the minerals shape preferred orientation. Grey

ellipses represent spinel lineation in the foliation plane.


with an average thickness of 50 Am and a spacing of

500 Am (Fig. 4). The serpentine network is quite

homogeneous, constituted of a-lizardite (Deer et al.,

1966) exhibiting a strong LPO with fast-vibration

(low optical indicatrix) perpendicular to the vein.

Thus the homogeneous crystallographic orientation is

such that (001) plane of lizardite is parallel to the

vein margin. The pseudo-fiber habit of the lizardite

veins is very common in serpentinized harzburgites

from ophiolites and from suboceanic mantle rocks

(e.g. Mevel et al., 1996). The a-lizardite veins are

unsheared; serpentine pseudo-fibers are rectilinear,

suggesting that their development is hydrostatic.

The enstatite is partly transformed to talc, in addition

to lizardite. Another secondary network, less pene-

trative, hundred micrometers thick with spacing at

millimeters scale (Fig. 4), is identified as micro-

cracking, either homogeneously filled with chryso-

tile, or heterogeneous and filled with chlorite, iron

hydroxide and calcium carbonate. This secondary

network is found in the center of penetrative lizardite

veins, suggesting a reactivation of the lizardite net-

work; we refer to this secondary network as ‘‘min-

eralized microcracks’’.

3.1. Structural measurements

3.1.1. Microfractures measurements

The orientations (strike and dip) of the penetrative

network of serpentine veins sealed with a-lizardite

were measured using an optical microscope equipped

with a five axes U-stage, on a cube cut according to

the penetrative structure, Z perpendicular to foliation,

X parallel to lineation and Y perpendicular to XZ

(Fig. 5). The limitation of dip solid angle measure-

ments using U-stage being of the order of 60j, themeasurements were performed on six thin sections

cut on the truncated corners (octahedral planes) of a

cube in order to cover the 3-D hemisphere and avoid

overlap.

Fig. 6. U-stage measurements in thin sections (X), (Y), (Z), 1, 2, 3

(Fig. 5). (a) Poles of lizardite veins. (b) Sketch delineating the two

lizardite sets. Lower hemisphere of projection.


Two sets of lizardite veins are measured in indi-

vidual olivine crystal hosts and are strongly clustered

in orientation (Fig. 6a): one set (I) is slightly oblique

to the high temperature harzburgite foliation, the

second (II) is perpendicular to the foliation, in

zone with the Z axis (Fig. 6b). Mineralized micro-

cracks are parallel to both sets of lizardite-filled veins

(Fig. 4).

3.1.2. Harzburgite lattice preferred orientation (LPO)

The lattice preferred orientations of olivine and

enstatite, the primary phases, were measured in the

harzburgite sample 97 OA 129 in order to explore the

geometrical relationships of the primary aggregate

with the serpentine network and to calculate the

seismic properties of the unaltered aggregate.

Olivine and enstatite LPOs (Fig. 7) measured

optically with the U-stage show the classic charac-

teristics exhibited by high temperature peridotites

from Oman ophiolite and from the oceanic litho-

sphere (Nicolas et al., 1980; Boudier and Coleman,

1981). Olivine has a strong [100] maximum

slightly oblique to the mineral lineation, and a

[010] girdle with a submaximum subperpendicular

to the foliation. Enstatite exhibits a weak fabric

with two [100] submaxima, a [100] maximum

subperpendicular to the foliation for large por-

phyroclasts and a second [100] maximum close

to the mineral lineation that corresponds to

small recrystallized neoblasts. This classic LPO is

interpreted as resulting from high-T (asthenospheric

conditions) simple shear on the [100] (0 kl) intra-

crystalline olivine slip system where the shear

plane is the average (010) crystallographic plane,

slightly oblique to the foliation.

3.2. Calculation of seismic velocities from lattice

preferred orientations

3.2.1. Harzburgite matrix

Seismic properties of the harzburgite were calcu-

lated, using the Voigt average method (1910), through

the Christoffel equation combining single crystal

densities (3.31 g/cm3 for olivine and 3.34 g/cm3 for

enstatite), the single crystal elasticity coefficients and

the LPOs of the constitutive mineral phases, in their

modal proportion (68% of olivine and 32% of ensta-

tite). For the details of the calculation method see e.g.

Baker and Carter (1972), Peselnik et al. (1974) and

Mainprice and Silver (1993). We used a computer

program developed by Mainprice (1990) for the

calculation and the spatial representation of the seis-

mic velocity.

The calculated P-wave velocity for the harzbur-

gite sample 97 OA 129 (Fig. 8) produces a max-

imum 8.6 km/s slightly oblique (about 20j) to the

mineral lineation, a minimum 8.1 km/s oblique 20jto the pole of the foliation and a low Vp anisotropy

6.2%. The calculated maximum S-wave splitting is

close to the maximum of olivine (001) pole. The

maximum S-wave anisotropy is 4.5% and minimum

0.3%.

3.2.2. Harzburgite and serpentinized network system

The calculation of the seismic properties of the

serpentinized system combines the calculated seismic

Fig. 7. Lattice preferred orientation (LPO) of primary phases. (a) Olivine. (b) Enstatite. U-stage measurements; lower hemisphere of projection,

foliation vertical NS and lineation horizontal NS. Pfj: fabric strength index (Bunge, 1982).


properties of the harzburgite matrix and that of the

serpentine (a-lizardite) network. The 3-D orientation

of this network is based on the 548 veins measured

in the sample 97 OA 129 (Fig. 6a), in a volume of

43 cm3. Only the penetrative serpentine network was

considered in this calculation because of its homoge-

neous mineral filling and veins spacing, the miner-

alized microcrack network was ignored due to its

inhomogeneous filling and variable spacing distribu-

tion. This provides a significant limitation of the

model’s suitability that will be discussed later. It has

been shown that mineral filling of the serpentine

vein is a-lizardite and its crystallographic orientation

is such that (001) lizardite is parallel to the vein

margins. Comparison of Figs. 6a and 7a shows that

the two sets of lizardite veins are such that pole to

(001)liz in set I is parallel to the main [010]ol

maximum, near the pole to foliation (Z), and pole

to (001)liz in set II forms a girdle in the foliation

plane (XY) which correlates with [001]ol in the same

orientation. The relative density of the two groups of

orientation is shown in the stereographic representa-

tion of Fig. 6a.

The calculation of the seismic properties of the

system formed by the anisotropic harzburgite matrix

and the oriented lizardite network is based on the

self-consistent method applied to two-phase systems

(Mainprice, 1997). Each component is treated as an

inclusion in the anisotropic homogeneous matrix.

The lizardite veins were represented by ellipsoidal

inclusions with 1:10:10 aspect ratio, with an orien-

tation given by the U-stage measurements. In the

absence of elastic single crystal constants of lizar-

dite, the elastic constants of chlorite (Aleksandrov

and Ryzhova, 1961) were used. The unit cell

parameters and structure of chlorite are similar to

those of lizardite, except for the c parameter, the

tetrahedral SiO4/octaedral Mg(OH)2 layers arrange-

Fig. 8. Seismic velocities calculation of the unserpentinized aggregate based on olivine and enstatite LPO and modal composition (68% olivine

and 32% enstatite). (a) Vp velocities. (b) Vs1 velocities. (c) Vs2 velocities. (d) Vs anisotropy. (e) Vs1 polarization plane. Referential same as Fig.

7, foliation vertical NS and lineation horizontal NS.


ment being 2:1 for chlorite vs. 1:1 for lizardite. In

such crystals marked by a strong structural aniso-

tropy, the weak interlayer forces implies very low

elastic moduli in the direction of weak bonds, i.e.

perpendicular to the layers (Aleksandrov and Ryz-

hova, 1961). Thus the bias introduced by the use of

chlorite instead of the unknown lizardite elastic

moduli will result in slightly increasing the along

c minimum elastic modulus in the calculation, thus

in a slight overestimation of the seismic anisotropy.

Calculation for a 100% chlorite aggregate with a

random preferred orientation agreed to within 0.1

km/s with mean velocity of an almost pure serpen-

tine specimen (Kern et al., 1997) averaged from

measurements in three perpendicular directions, sug-

gesting that the approximation of using chlorite to

model serpentine is acceptable. The modal compo-

sition was determined from thin section to be 60%

lizardite, 27% olivine, 13% enstatite. The relative

orientation between minerals is such that the set I

has [001]liz (//Vpmin) parallel to max [010]ol (//Vpmin)

and set II has [001]liz parallel to [001]ol (//Vpint)

or [100]ol (//Vpmax) in the foliation plane (Figs. 6

and 7).

The result of calculation (Fig. 9) shows that

velocity of P waves is slightly lower when com-

pared to the calculated values for the unserpenti-

nized aggregate (Fig. 8). In this case, Vpmax has

been reduced from 8.6 to 7.3 km/s and Vpmin from

8.1 to 6.7 km/s, whereas the Vp anisotropy has

increased from 6.2% to 8.6% in the serpentinized

aggregate. Similarly the S-wave velocities have

been reduced, Vsmax from 4.9 to 3.9 km/s and

Vsmin from 4.7 to 3.6 km/s. The maximum S-wave

anisotropy is 4.6% in unaltered rock which in-

creases to 6.1% in the serpentinized peridotite.

The symmetry or distribution of the maxima and

minima of the velocity surfaces remains the same

for both unaltered and altered peridotite; the main

differences are the magnitude of the velocities and

the anisotropy, the first decreasing and the second

increasing. S-wave velocity anisotropy increases and

the polarization geometry is not changed compared

to the unserpentinized model. The bias induced by

Fig. 9. Seismic velocities calculation of the serpentinized harzburgite: 40% solid matrix and 60% serpentine with (001) lizardite parallel to veins;

aspect ratio of fractures 10:10:1, short direction normal to crack. (a) Vp velocities. (b) Vs1 velocities. (c) Vs2 velocities. (d) Vs anisotropy. (e)

Vs1 polarization plane. Referential same as Fig. 7, foliation vertical NS and lineation horizontal NS.


using chlorite elastic moduli results in slightly over-

estimate of the anisotropy increase due to the

serpentine network, but does not affect the geo-

metrical result.

4. Seismic velocity laboratory measurements

4.1. Experiments

Experimental measurements of Vp and Vs were

conducted in order to compare the results with the

calculated seismic properties, in the case where the

a-lizardite penetrative network and the mineralized

microcrack network are integrated as well. The

velocities were measured on oven-dried cubes, in a

multianvil apparatus using the ultrasonic pulse trans-

mission technique (Kern, 1982) with transducers

operating at 2 MHz. A state of near-hydrostatic

stress is achieved by pressing six pyramidal pistons

in the three orthogonal directions of the cube pro-

ducing increasing confining pressure (up to 600

MPa) at room temperature.

We have cut two cubes 43 mm sized of the same

sample, based on reference frame of primary minerals

and serpentine network, respectively.

– Cube k (Fig. 10a) was cut following the penetrative

structures of the peridotite aggregate where Xol is

parallel to mineral lineation, Zol is perpendicular to

foliation and Yol is perpendicular to lineation in the

foliation plane.

– Cube j (Fig. 10b) was cut with respect to the two

sets of regularly spaced (at the sample scale)

microcracks (1–2 mm). The first set (correspond-

ing to set I lizardite, Fig. 6) is planar, close to

foliation, filled with g-serpentine (chrysotile) and

magnetite. The second (corresponding to set II

lizardite, Fig. 6) is more sinuous and filled with

calcium carbonate, magnetite and serpentine;

microscopic observations suggest that a 100 Amsized porous network is associated with the

carbonate filling. In cube j, the C axis is

perpendicular to the set I, the B axis is

perpendicular to the set II and the A axis is

perpendicular to C and B. Fig. 10c shows for the

Fig. 10. Sample 97 OA 129a, cut for laboratory Vp and Vs measurements (two cubes). (a) Cube k cut with respect to penetrative structure of

peridotite aggregate: X, Y, Z are axes of shape preferred orientation. (b) Cube j cut according to the two sets of microcracks: A parallel to the two

sets intersection, B perpendicular to carbonate/lizardite network, C perpendicular to chrysotile/lizardite network. The number of arrowheads

graduates the velocity. (c) Represents the orientation of the cubes j and k in shape preferred orientation referential (X, Y, Z) of the peridotite

aggregate, lower hemisphere of projection.



relative orientation of the two reference frames kand j oriented in the geographic reference

system.

4.2. Results

P-wave velocities vs. pressure (Figs. 11 and 12)

are linear above 100 MPa, indicating the closure of

microcracks at pressure corresponding to a depth

about 3 km. P-wave velocity anisotropy is higher

for the cube j (A-Vp: 6.2–7%) than for the cube

k (A-Vp: 3.2–5%). Note that X and A are only

14j apart, which explains the similar velocities in

Fig. 11. Vp and Vs laboratory measurements and Vp anisotropy (A�Vp (%

of up to 600 MPa and room temperature. (a) Vp-X, Vp-Y,Vp-Z. (b) Pois

and Vs-YZ.

these directions for P waves. In cube k the max-

imum Vp (6.3 to 6.6 km/s) corresponds to X as

expected, Vp-Y and Vp-Z have similar velocities.

These observations correlate with the crystal fabric

of the aggregate (Fig. 7a) and the calculated veloc-

ities (Figs. 8 and 9). For the cube j, the minimum

velocity is along the B, perpendicular to the set

II carbonate-filled microcracks (5.8 to 6.2 km/s);

maximum velocities are along A and C (6.3 to 6.5

km/s).

In both cubes, S-wave velocities measurements

are low and tend to confirm previous observations

(Kern, 1982; Barruol and Kern, 1996). In the LPO

) = Vpmax�Vpmin/Vpmean�100), on cube k at increasing pressures

son’s ratio. (c) Vs-YX and Vs-ZX. (d) Vs-XY and Vs-ZY. (e) Vs-XZ

Fig. 12. Vp and Vs laboratory measurements and Vp anisotropy (A�Vp (%) = Vpmax�Vpmin/Vpmean�100), on cube j at increasing pressures

of up to 600 MPa and room temperature. (a) Vp-A, Vp-B,Vp-C. (b) Poisson’s ratio. (c) Vs-BA and Vs-CA. (d) Vs-AB and Vs-CB. (e) Vs-AC

and Vs-BC.


reference frame (cube k), X (Fig. 11c) is a direc-

tion of minimum S-wave splitting, in accordance

with the crystal fabric of the aggregate (Fig. 7a)

and calculated velocities (Figs. 8 and 9). In the

serpentine reference frame (cube j), the minimum

and maximum S-wave splitting is observed along B

and C, respectively. B corresponds to the normal to

the carbonate filled set II. C is normal to the

serpentine filled set I. A direction shows more

splitting than X, showing that the difference of

14j between the directions is significant for S

waves.

5. Discussion

5.1. Comparison of laboratory measurements with

modeling: contribution of the altered network

The complementarity of both studies relies on

that our modeling accounts for added contribu-

tions to anisotropy of the primary aggregate and

the lizardite network only, whereas the laboratory

measurements integrate the microcrack system

which orientation is controlled by the lizardite

network.


The calculated effect of the serpentinization is the

lowering of P- and S-wave velocities and increasing

Vp and Vs anisotropies. The increase of velocity

anisotropy is due to the imposed crystallographic

relationship of olivine and lizardite, based on micro-

structural observations, with (001)liz parallel to veins,

and veins subparallel or perpendicular to (010)ol

maximum. The calculated velocity distribution (Fig.

8a) is marked by a Vpmax axial symmetry, which

corresponds to the strong [100] maximum of olivine

LPO (Fig. 7a); this characteristic is only slightly

modified in the model by inclusion of the altered

network (Fig. 9a), more generally comparison of Figs.

8 and 9 exhibits similar symmetry for the primary and

the altered aggregates.

The measured velocities on cube k cut along X, Y,

Z shows consistent results, with slightly higher P- and

S-wave velocities for the calculated data (Table 1).

Conversely, measured anisotropies for P and S waves,

along X, Z are slightly higher than calculated. Sim-

ilarly to the model, the measured velocities distribu-

tion on cube k (Fig. 11a) exhibit a Vpmax axial

symmetry (VpxHVpy =Vpz) whereas Vs patterns

(Fig. 11c) indicate that X is the direction of low shear

wave splitting (Fig. 11c). It could be concluded that,

due to the geometrical relationships between the

primary aggregate and the a-lizardite penetrative

altered network, the altered network in the harzburgite

does not modify the wave velocity symmetry.

We have seen that two types of microcracks are

formed in the preexisting penetrative lizardite net-

work, one is sealed with chrysotile, the second has a

porous carbonate fill. The second cube (j), has been

Table 1

Comparison of P- and S-waves velocities calculated, and measured (at 6

microcrack framework A, B, C (cube j) (see text)

Vp (km/s)

Vpmax Vpx Vpmin Vpz A-Vp (%)

Vpmax�Vpmin

A

V

Calculated LPO primary

aggregate

8.6 8.6 8.1 8.1 6.2 6

LPO primary

aggregate +

serpentine

network

7.3 7.0 6.7 6.7 8.6 4

Measured Cube k – 6.6 – 6.3 – 4

Cube j 6.6 – 6.2 – 6.2

cut in relation with the identified microcracks in order

to specify the actual contribution of these networks. In

the present case, the two reference frames k and jhave close orientations (Fig. 10c). The comparison of

Vp velocity patterns in k and j cubes (Figs. 11a and

12a) gives more precise information on the two

alteration networks. The total Vp anisotropy is higher

in j-cube (6%). The Vpmax axial symmetry is not

observed on j-cube.The Vs measurements are more informative. Shear

wave splitting: VsCA>VsBA, VsAB =VsCB, VsACH

VsBC (Figs. 10b and 12b,c,d) indicate (1) that S waves

propagating along B (i.e. perpendicular to the carbo-

nate veins) are the slowest. The highest Vs velocities

are for AC and CA, lying in the calcium carbonate

plane. This observation meets with previous results of

Babuska (1981), indicating high mean velocity values

for calcite: Vp and Vs = 6.5 and 3.7 km/s, respectively.

Polarization along B reveals also the effect of the

serpentine network. In the B direction, there is no

splitting between the AB and the CB polarizations,

indicating that there is almost no effect of the chrys-

otile network on splitting. These results suggest that

compared to the chrysotile-sealed network, the micro-

cracks network has important effect on S-waves prop-

agation, and that the carbonate network is more

efficient in producing an anisotropy.

In conclusion, calculated seismic velocities, inte-

grating the effect of the anisotropic matrix represented

by the primary harzburgite aggregate and the meas-

ured lizardite network shows that this network slightly

lowers P and S velocities, and fairly increase P and S

anisotropies (Table 1), but due to preservation of axial

00 MPa) in the finite reference system X, Y, Z (cube k) and in the

Vs (km/s)

-Vp (%)

px�Vpz

Vsmax Vsx Vsmin Vsz A-Vp (%)

Vsmax�Vsmin

A-Vp (%)

Vsx�Vsz

.2 4.9 4.9 4.7 4.7 4.5 4.5

.4 3.9 3.8 3.6 3.6 6.1 5.4

.7 – 3.5 – 3.3 – 5.9

– 3.5 3.1 –


symmetry, has no effect on the directions of Vpmax

and Vpmin. Alternatively, measured seismic velocities

integrate the effect of the microcrack system. This

effect is consistently lowering the P- and S-waves

velocities, and drastically increasing Vs anisotropy

(Table 1). Hence, the microcracks network controls

the maximum and minimum velocity directions. In

our study, the role of the calcium carbonate fill is

dominant.

5.2. Field experiment: scale transfer

The first remark resulting from the comparison of

our field and microstructural measurements (Figs.

2a,b and 6) is the similarity of the distribution of

fractures at both scales, one set subparallel to the

foliation and the other perpendicular to it with a

submaximum, stronger at the field scale (Fig. 2a,b),

perpendicular to lineation. How this scale-similar

organization has general significance will be dis-

cussed later.

Field seismic measurements have much lower Vp

velocities (5.3 km/s along Z perpendicular to folia-

tion, and 4.2 km/s along X parallel to lineation) than

ultrasonic laboratory measurements. Field measure-

ments have a Vp anisotropy of 20.8% compared to

4–6% in laboratory measurements. Finally, the major

difference is that the fast Vp velocity is normal to

foliation and low Vp is parallel to the lineation, an

inversion of the laboratory measurements. This dis-

crepancy may be explained by consideration of the

geometrical organization of fracture network meas-

ured at the seismic site (Fig. 2c). It happens that the

seismic line parallel to lineation crosscuts the two

sets of fractures although the seismic line perpendic-

ular to foliation is subparallel to one set of fractures.

We observe that the seismic line crosscutting the two

sets of fractures corresponds to the slowest velocities.

These observations emphasize the dominant control

of fractures on the seismic properties of serpentinized

peridotite at the hectometric scale suggesting that at

this scale, the mesoscopic anisotropy (sample scale),

primary aggregate, and penetrative serpentine are

obliterated.

This rises the question of the role of fracture filling:

open fractures, carbonate-filled and serpentine-filled

fractures. Clearly, open fractures provide a major

control on seismic properties at shallow level. The

maximum penetration of shallow seismic measure-

ments has been evaluated at some tens of meters,

which is shallow compared to the depth range of

fracture closure due to lithostatic pressure, although

little constraints on this limit are available. In serpenti-

nized peridotite from the Oman ophiolite, a minimum

depth of 300 m is evaluated for meteoric water

circulation, based on the temperature of the hydro-

thermal system (Stanger, 1985). At deeper levels,

microcracks have been considered closed at 3 km

depth (c 100 MPa) see e.g. Birch (1960) and Iturrino

and Christensen (1990) to refer to oceanic lithosphere

lithologies. Our seismic lab measurements indicate a

progressive closure of microcracks at c 100 MPa,

confirming these data. Concerning the fracture filling,

carbonate deposits are associated with serpentine in

the open fracture system at metric scale, in contrast

with sealed fracture filled with serpentine. Taking into

account that at mesoscopic scale only the carbonate

network has a noticeable effect on the seismic proper-

ties of the peridotite aggregate, we may reasonably

consider that the contribution of carbonate-filled net-

work is dominant at large scale in addition to the role

of open fractures, and thus explain the discrepancy

between large and mesoscopic scale seismic proper-

ties.

5.3. How representative is the studied case

The major contribution of this study is to state

geometrical relationships of primary peridotite aggre-

gate with the altered network at different scales and

infer or explain anisotropic seismic properties. We

have seen that a scale-similar organization controls the

fracture system from the scale of the map to that of the

exposure and finally to that of the sample (mineralized

microcracks). At the crystals scale, we have seen that

the microcracks system is guided by the penetrative a-

lizardite network. In the studied case, the three-dimen-

sional relationships are such that the serpentine pen-

etrative network (lizardite) is geometrically related to

the internal structure of the peridotite aggregate, one

set subparallel to the foliation and the second set

perpendicular to it. This relationship implies that the

seismic properties of the peridotite aggregate and the

altered network interfere constructively. The geomet-

rical relationships determined may not be fortuitous

and are in the course of investigation. At once, we

Table 2

Comparison of P and S mean waves velocities and anisotropy in

harzburgite and in gabbro, measured up to 600MPa, (1) this study,

and (2) Barruol and Kern (1996)

Referential X, Y, Z Vp A-Vp

(%)

Vs A-Vs

(%)

Peridotite (1) 8.3 6.2 4.8 4.5

Serpentinized

peridotite 60% (1)

6.45 4.7 3.4 5.9

Gabbro (2) 6.1 1.75 3.86 2.3


assume that the measured relationships of the three-

dimensional a-lizardite penetrative framework with

olivine fabric are representative of a common situation

in peridotites from oceanic lithosphere, based on

generalized observations of thin sections cut in the

XZ plane of the peridotite primary aggregate.

We have shown that the choice of three perpendic-

ular reference directions for laboratory measurements

will strongly influence the velocities and anisotropies

deduced from these directions. As a result of this

study, one must question the interpretation of previous

measurements made using the reference frame based

on the lineation and foliation of the peridotite aggre-

gate (Kern and Tubia, 1993; Horen et al., 1996). Our

laboratory measurements in cubes k and j are sum-

marized in Table 1; Vpmax of the serpentinized peri-

dotite cubes j is equal to VpX in cube k (6.6 km/s)

and Vpmin in cube j equal to 6.2 km/s (VpB) is

comparable to VpZ in cube k equal to 6.3 km/s,

inducing an anisotropy lowered in cube k: 4.7%

compared to cube j: 6.2%. Thus when the geometry

of the altered network is not determined, measure-

ments with respect to the peridotite finite strain axes

X, Y, Z provide acceptable data for harzburgites similar

to the primary fabric of our specimen, which is the

case for oceanic mantle harzburgites (Nicolas et al.,

1980).

The other parameter that has been shown to affect

the seismic properties of serpentinized peridotites is

the degree of serpentinization, increasing degree of

serpentinization lowering the averaged Vp and Vs

(Christensen, 1966; Kern and Tubia, 1993; Horen et

al., 1996). Our study is limited to the one sample

serpentinized at 60%, certainly representative of the

standard degree of alteration in the mantle section of

the Oman ophiolite. Calculated Poisson’s ratio vs.

averaged Vp for our studied sample fits in the

general trend of data by Christensen (1966) and Kern

and Tubia (1993), accounting for the effect of degree

of serpentinization on Vp and Vs. For the seismic

anisotropy that we have explored, extrapolation of

the calculated model suggests that increasing degree

of serpentinization will increase the seismic aniso-

tropy, providing that the strong fabric of the altered

network is preserved. As a confirmation, an aniso-

tropy as high as 24% has been measured by Kern et

al. (1997) on antigorite aggregate having a strong

LPO.

5.4. Implication for oceanic lithosphere

One of the proposed objectives of this study was to

discriminate the serpentinized peridotite and gabbro,

and determine the potential implications for the iden-

tification of the suboceanic Moho (base of layer 3).

As we have seen, mean values of compressional

and shear wave velocities decrease with the volume

fraction of serpentine (Christensen, 1966; Horen et al.,

1996). For degrees of serpentinization higher than

40%, seismic velocity values are generally lower than

those obtained for gabbro (Barruol and Kern, 1996;

Iturrino et al., 1996). The anisotropy may be a more

reliable parameter; however, the problem is that

published data obtained at confining pressures are

limited and correspond to measurements in different

reference systems. Results obtained for serpentinized

harzburgite and gabbro in the same conditions, i.e. at

confining pressure and same orientation are compared

in Table 2. A noticeable difference for Vp and Vs

anisotropies is evident with lower values in gabbro

(Barruol and Kern, 1996) than in 60% serpentinized

harzburgite. Other measurements performed at the

same confining pressure on peridotites with various

volume fractions of serpentine (Kern and Tubia, 1993)

fit these comparative values. On the other hand, our

shallow seismic experiments suggest that Vp aniso-

tropy is not significant at the hectometric scale. The

depth limit (3 km) for closure of microcracks is

definitely above the Moho level at a fast spreading

ridge where Moho depth is assumed to lie between 4

and 8 km, based on studies in ophiolites (Nicolas and

Boudier, 2000). Thus in this case measurements at

confining pressure provide a consistent reference for

interpreting oceanic seismic profiles. At the slow

spreading ridges the problem is different. The Moho

is more discontinuous, mantle rocks reach the ocean


floor (Cannat, 1996) and the discrimination between

serpentinized harzburgite and gabbro using intrinsic

seismic properties will be unreliable due to the strong

influence of fractures at shallow depth.

At depths below the crack-closure limit, fixed here

at 3 km, it appears that the seismic anisotropy may be

a good discriminating factor between serpentinized

harzburgite and gabbro, provided that the seismic

refraction profiles are oriented along the directions

of maximum anisotropy.

6. Conclusions

Three-dimensional relationships of altered net-

works with primary peridotite assemblages have been

studied at microscopic scale in harzburgite with 60%

serpentine and a typical mineral preferred orientation.

Measurements of mineralized microcracks at sample

scale, of fractures at outcrop and map scale have

shown a scale-similar organization of the altered

system.

The penetrative serpentine network is composed of

lizardite having a strong fabric of (001) parallel to the

veins. The two sets of penetrative lizardite veins are

related to the preferred orientation axes X, Y, Z of the

peridotite: set I subparallel to the XY plane and set II

parallel to the Z direction. This only implies a relation-

ship between olivine and lizardite lattice preferred

orientation for this setting. The penetrative lizardite

veins I and II are locally overprinted by carbonate-

filled and chrysotile-filled veins, respectively.

The measured three-dimensional relationships are

used (1) to calculate the elastic properties of the

altered harzburgite (oriented alteration network

(60%) filled with oriented lizardite in an anisotropic

matrix (40%) formed by an assemblage of oriented

olivine and orthopyroxene) and (2) to explore the role

of microcracks using laboratory 3-D seismic measure-

ments. A comparison of data shows a reasonable

consistency between calculated and measured Vp,

Vs and seismic anisotropies, when referring to X, Y,

Z axes of the shape preferred orientation of peridotite

(Table 1). The discrepancy increases notably when

comparing calculated Vpmax, Vsmax and Vpmin and

Vsmin with Vp and Vs measured in j-cube (Table 1).

Calculated and measured velocities suggest that the

serpentine network lowers velocities. Calculated ani-

sotropy suggests that serpentinization increases the

seismic anisotropy, due to the strong fabric of lizardite

in the serpentine network. Measured compressional

velocities and shear wave splitting shows that carbo-

nate-filled microcracks can strongly influence seismic

anisotropy and obliterate the serpentine network

effect.

Comparison with data obtained in the same con-

ditions on gabbro (Table 2) suggests that for depths

greater than 3 km, corresponding to microcracks

closure in our experiments, Vp and Vs anisotropies

could be a means to discriminate between serpenti-

nized peridotite and gabbro at the base of layer 3 in

oceanic lithosphere.

Shallow seismic profiles indicate that fracturing

dominates the seismic velocities and anisotropy at

shallow depth, resulting in a drastic decrease in

compressional wave velocity.

Acknowledgements

We wish to thank C. Nevado for preparing the

polished thin sections, A. Fehler for preparing the

sample cubes, and D. Schulte-Korntnack for assis-

tance in performing the velocity measurements. The

study benefited from discussions with G. Barruol and

A. Baronnet, and the manuscript from comments of N.

Christensen and G. Iturrino, and review by L. Burlini,

Y. Gueguen, J. Khazanehdari. The study has been

supported by the CNRS/INSU, action incitative No.

0693.

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