Tectonophysics 370 (2003) 77 – 94
www.elsevier.com/locate/tecto
Seismic wave velocity and anisotropy of serpentinized peridotite in
the Oman ophiolite
Benoı̂t Dewandel a, Francßoise Boudier a,*, Hartmut Kern b,
Waris Warsi c, David Mainprice a
a
Laboratoire de Tectonophysique, Université Montpellier II, 34095 Montpellier, France
b
Institut fur Geowissenschaften, Olshausenstr., 40, D-24098 Kiel, Germany
c
Department 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
1. Introduction
* Corresponding author.
E-mail addresses: [email protected] (F. Boudier),
[email protected] (H. Kern).
0040-1951/03/$ - see front matter D 2003 Published by Elsevier B.V.
doi:10.1016/S0040-1951(03)00178-1
The relative contribution of serpentinized peridotites to the seismic properties of oceanic lithosphere
is still poorly known, due to the difficulty in
discriminating between serpentinized peridotite from
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the crustal constituents, gabbro and dolerite. In the
recent years, several investigators have measured the
seismic properties of serpentinized peridotites (Christensen, 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 anisotropies. Although some experimental ultrasonic
measurements on spherical samples have been performed on various lithologies (Pros and Babuska,
1968; Babuska, 1972; Siegesmund et al., 1993;
Rasolofosaon et al., 2000), laboratory seismic measurements 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 peridotite. The modeling of seismic properties of serpentinized peridotites is a difficult task because
the macroscopic physical properties result from the
interference of two microstructural elements, the
peridotite polycrystalline aggregate and the serpentine 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 fluidfilled cracks. Siegesmund et al. (1991) and Rasolofosaon et al. (2000) discussed such effect based on
U-stage measurements of microcracks in an ultramylonitic 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 peridotite 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 previous 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 foliation 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 controlled by the peridotite LPO, are expected to lie close
to lineation and perpendicular to foliation, respectively, 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 subvertical 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 fractures (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 displacement (>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
B. Dewandel et al. / Tectonophysics 370 (2003) 77–94
79
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.
observed at the map scale, appears in these field
measurements (Fig. 2b). One set of fractures is striking 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 measurements (96), this ‘scale-similar’ organization accounts
for a common process of fracturing at different scales
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in a homogeneous material. We point out another bias
of these measurements at both scales (aerial photograph 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 corrections 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). Sledgehammer 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 anisotropy 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.
B. Dewandel et al. / Tectonophysics 370 (2003) 77–94
81
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, serpentine 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 recrystallization; foliation is well marked by spinel and
olivine crystal elongation. The penetrative serpentine
network represents 60% by volume, formed of veins
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The enstatite is partly transformed to talc, in addition
to lizardite. Another secondary network, less penetrative, hundred micrometers thick with spacing at
millimeters scale (Fig. 4), is identified as microcracking, either homogeneously filled with chrysotile, 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 network; we refer to this secondary network as ‘‘mineralized 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 measurements using U-stage being of the order of 60j, the
measurements 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. 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.
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.
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.
B. Dewandel et al. / Tectonophysics 370 (2003) 77–94
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teristics exhibited by high temperature peridotites
from Oman ophiolite and from the oceanic lithosphere (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 porphyroclasts 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) intracrystalline 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
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 individual 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 microcracks 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-
3.2.1. Harzburgite matrix
Seismic properties of the harzburgite were calculated, 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 enstatite). 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 seismic velocity.
The calculated P-wave velocity for the harzburgite sample 97 OA 129 (Fig. 8) produces a maximum 8.6 km/s slightly oblique (about 20j) to the
mineral lineation, a minimum 8.1 km/s oblique 20j
to 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
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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 homogeneous mineral filling and veins spacing, the mineralized microcrack network was ignored due to its
inhomogeneous filling and variable spacing distribution. 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 representation 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 orientation given by the U-stage measurements. In the
absence of elastic single crystal constants of lizardite, 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-
B. Dewandel et al. / Tectonophysics 370 (2003) 77–94
85
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 anisotropy, the weak interlayer forces implies very low
elastic moduli in the direction of weak bonds, i.e.
perpendicular to the layers (Aleksandrov and Ryzhova, 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 serpentine specimen (Kern et al., 1997) averaged from
measurements in three perpendicular directions, suggesting that the approximation of using chlorite to
model serpentine is acceptable. The modal composition 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 compared to the calculated values for the unserpentinized 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 increases 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
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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 overestimate of the anisotropy increase due to the
serpentine network, but does not affect the geometrical 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 transmission 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 producing 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 (corresponding 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 Am
sized 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
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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.
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relative orientation of the two reference frames k
and 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
these directions for P waves. In cube k the maximum 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 velocities (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
Fig. 11. Vp and Vs laboratory measurements and Vp anisotropy (A Vp (%) = Vpmax Vpmin/Vpmean100), on cube k at increasing pressures
of up to 600 MPa and room temperature. (a) Vp-X, Vp-Y,Vp-Z. (b) Poisson’s ratio. (c) Vs-YX and Vs-ZX. (d) Vs-XY and Vs-ZY. (e) Vs-XZ
and Vs-YZ.
B. Dewandel et al. / Tectonophysics 370 (2003) 77–94
89
Fig. 12. Vp and Vs laboratory measurements and Vp anisotropy (A Vp (%) = Vpmax Vpmin/Vpmean100), 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 direction 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 contributions 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.
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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 microstructural 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. Similarly to the model, the measured velocities distribution 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 network, one is sealed with chrysotile, the second has a
porous carbonate fill. The second cube (j), has been
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 j
have 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 carbonate 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 chrysotile network on splitting. These results suggest that
compared to the chrysotile-sealed network, the microcracks network has important effect on S-waves propagation, and that the carbonate network is more
efficient in producing an anisotropy.
In conclusion, calculated seismic velocities, integrating the effect of the anisotropic matrix represented
by the primary harzburgite aggregate and the measured 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
Table 1
Comparison of P- and S-waves velocities calculated, and measured (at 600 MPa) in the finite reference system X, Y, Z (cube k) and in the
microcrack framework A, B, C (cube j) (see text)
Vp (km/s)
Vs (km/s)
Vpmax Vpx Vpmin Vpz A-Vp (%)
A-Vp (%)
Vpmax Vpmin Vpx Vpz
Vsmax Vsx Vsmin Vsz A-Vp (%)
Vsmax Vsmin
A-Vp (%)
Vsx Vsz
Calculated LPO primary 8.6
aggregate
LPO primary 7.3
aggregate +
serpentine
network
Measured Cube k
–
Cube j
6.6
8.6
8.1
8.1
6.2
6.2
4.9
4.9
4.7
4.7
4.5
4.5
7.0
6.7
6.7
8.6
4.4
3.9
3.8
3.6
3.6
6.1
5.4
6.6
–
–
6.2
6.3
–
–
6.2
4.7
–
–
3.5
3.5
–
3.1
3.3
–
5.9
–
B. Dewandel et al. / Tectonophysics 370 (2003) 77–94
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 discussed later.
Field seismic measurements have much lower Vp
velocities (5.3 km/s along Z perpendicular to foliation, and 4.2 km/s along X parallel to lineation) than
ultrasonic laboratory measurements. Field measurements 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 discrepancy may be explained by consideration of the
geometrical organization of fracture network measured 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 perpendicular 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
91
maximum penetration of shallow seismic measurements 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 serpentinized peridotite from the Oman ophiolite, a minimum
depth of 300 m is evaluated for meteoric water
circulation, based on the temperature of the hydrothermal 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 properties of the peridotite aggregate, we may reasonably
consider that the contribution of carbonate-filled network is dominant at large scale in addition to the role
of open fractures, and thus explain the discrepancy
between large and mesoscopic scale seismic properties.
5.3. How representative is the studied case
The major contribution of this study is to state
geometrical relationships of primary peridotite aggregate 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 alizardite network. In the studied case, the three-dimensional relationships are such that the serpentine penetrative 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 geometrical relationships determined may not be fortuitous
and are in the course of investigation. At once, we
92
B. Dewandel et al. / Tectonophysics 370 (2003) 77–94
assume that the measured relationships of the threedimensional 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 perpendicular 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 aggregate (Kern and Tubia, 1993; Horen et al., 1996). Our
laboratory measurements in cubes k and j are summarized in Table 1; Vpmax of the serpentinized peridotite 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, measurements 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 anisotropy, providing that the strong fabric of the altered
network is preserved. As a confirmation, an anisotropy 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 identification 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 anisotropy 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
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)
Serpentinized
peridotite 60% (1)
Gabbro (2)
8.3
6.45
6.2
4.7
4.8
3.4
4.5
5.9
6.1
1.75
3.86
2.3
B. Dewandel et al. / Tectonophysics 370 (2003) 77–94
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 networks 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 relationship between olivine and lizardite lattice preferred
orientation for this setting. The penetrative lizardite
veins I and II are locally overprinted by carbonatefilled 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 measurements. 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-
93
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 carbonate-filled microcracks can strongly influence seismic
anisotropy and obliterate the serpentine network
effect.
Comparison with data obtained in the same conditions 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 serpentinized 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 assistance 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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