Geochem Geophys Geosyst - 2014 - Roum jon - Serpentinization of mantle‐derived peridotites at mid‐ocean ridges Mesh

PUBLICATIONS
Geochemistry, Geophysics, Geosystems
RESEARCH ARTICLE
10.1002/2013GC005148
Key Points:
Serpentinization occurs along
thermal, tectonic, and reactioninduced fractures
Serpentinization may occur quasi
instantly at exhumation time scales
The depth and extent of
serpentinization are variable in space
and time
Supporting Information:
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Table S1_v3
text_Auxiliary_material
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Fig_annexe_EBSDmaps
Correspondence to:
S. Roumejon,
[email protected]
Citation:
Roum
ejon, S., and M. Cannat (2014),
Serpentinization of mantle-derived
peridotites at mid-ocean ridges: Mesh
texture development in the context of
tectonic exhumation, Geochem.
Geophys. Geosyst., 15, 2354–2379,
doi:10.1002/2013GC005148.
Received 13 NOV 2013
Accepted 19 MAY 2014
Accepted article online 23 MAY 2014
Published online 10 JUN 2014
Serpentinization of mantle-derived peridotites at mid-ocean
ridges: Mesh texture development in the context of tectonic
exhumation
phane Roume
jon1 and Mathilde Cannat1
Ste
1
Equipe de G
eosciences Marines, Institut de Physique du Globe de Paris, Sorbonne Paris Cit
e, UMR7154 CNRS, Paris, France
Abstract At slow spreading ridges, axial detachment faults exhume mantle-derived peridotites and
hydrothermal alteration causes serpentinization in a domain extending more than 1 km next to the fault. At
the microscopic scale, serpentinization progresses from a microfracture network toward the center of olivine relicts and forms a mesh texture. We present a petrographic study (SEM, EBSD, and Raman) of the serpentine mesh texture in a set of 278 abyssal serpentinized peridotites from the Mid-Atlantic and Southwest
Indian Ridges. We show that serpentinization initiated along two intersecting sets of microfractures that
have consistent orientations at the sample scale, and in at least one studied location, at the 100 m scale. We
propose that these microfractures formed in fresh peridotites due to combined thermal and tectonic
stresses and subsequently served as channels for serpentinizing ﬂuids. Additional reaction-induced cracks
developed for serpentinization extents <20%. The resulting microfracture network has a typical spacing of
60 mm but most serpentinization occurs next to a subset of these microfractures that deﬁne mesh cells
100–400 mm in size. Apparent mesh rim thickness is on average 33 6 19 mm corresponding to serpentinization extents of 70–80%. Published laboratory experiments suggest that mesh rims formation could be completed in a few years (i.e., quasi instantaneous at the plate tectonic timescale). The depth and extent of the
serpentinization domain in the detachment fault’s footwall are probably variable in time and space and as a
result we expect that the serpentine mesh texture at slow spreading ridges forms at variable rates with a
spatially heterogeneous distribution.
1. Introduction
The oceanic lithosphere formed at slow (<4 cm/yr) [e.g., Small, 1998] spreading ridges contains a variable
amount of serpentinized peridotite in the ﬁrst kilometers. These rocks, associated with gabbroic bodies,
have been widely observed and sampled along the Mid-Atlantic Ridge [Karson et al., 1987; Cannat et al.,
cia et al., 2000], the Gakkel ridge [Michael et al., 2003], and the
1992, 1995b, 1997; Kelemen et al., 2004; Gra
Southwest Indian Ridge [Dick, 1989; Dick et al., 2003; Seyler et al., 2003; Sauter et al., 2013]. They are exhumed
along large offset normal faults, also called detachment faults [Karson, 1990; Cannat et al., 1992, Cann et al.,
1997; Lavier et al., 1999; Smith et al., 2006]. Microseismicity distribution [Toomey et al., 1985; deMartin et al.,
2007] reveals that the detachment faults extend down to at least 8 km on axis and geological observations
show that the emergence angle of these faults at the seaﬂoor is locally <10 [Smith et al., 2006]. Together,
these results suggest that axial detachments have a concave-downward shape, corresponding to a ﬂexure
of the footwall as it is exhumed (Figure 1). Paleomagnetic data acquired on drilled samples are also consistent with rotations of 46 –80 in the footwall of Mid-Atlantic Ridge detachments [Garces and Gee, 2007; Morris et al., 2009; MacLeod et al., 2011]. Axial detachment faults can therefore be sketched as conveyor belts for
the exhumation of mantle-derived peridotites (Figure 1): these rocks are progressively uplifted from the
base of the brittle lithosphere toward shallower levels in which hydrothermal circulation and serpentinization are active.
Serpentinization has signiﬁcant geodynamic and environmental consequences. It modiﬁes the rheology of
the oceanic lithosphere [Reinen et al., 1994; Escartın et al., 1997] and may help localizing the deformation
along fault planes. Serpentinization also consumes large volumes of water and produces hydrogen [Frost,
1985; Charlou et al., 2002], part of which combines with dissolved CO2 [Berndt et al., 1996] to produce methane anomalies in the water column [Charlou and Donval, 1993; Charlou et al., 1998]. These hydrogen and
methane ﬂuxes provide energy to sustain microbial systems within the substratum and at hydrothermal
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Serpentinization (%)
0
50 100
dredge
10.1002/2013GC005148
ridge axis
~5 km
drill hole
crust
2-4km
Vp
mantle
footwall
rotation
lt
fau
nt
me
he
tac
de
S
Fig12
8 km
domain of axial
hydrothermal
circulation
exhumation
0 2 4 6 8 Vp (km/s)
Figure 1. Conceptual sketch of detachment faulting and serpentinization of mantle-derived peridotites at a slow spreading ridge, drawn
after Cannat et al. [2010]. The detachment fault acts as a conveyor belt bringing peridotites up from the base of the brittle lithosphere and
into the domain of hydrothermal circulation and serpentinization. Serpentinization is accompanied by a decrease in density and seismic
velocities. (left) Due to large footwall rotation [e.g., Morris et al., 2009], crustal thickness determined by seismic experiments in the ridge
ﬂanks [after Canales et al., 2000] translates into on-axis distance from the fault. Available sampling of the exhumed ultramaﬁcs is restricted
to near-seaﬂoor levels and includes dredges, submersible sampling, and drilling (open rectangles).
vents [Shock and Holland, 2004]. The heat released on axis by serpentinization reactions may also partly fuel
hydrothermal circulations [Kelley et al., 2001], although this effect is probably limited to low-temperature
hydrothermal systems [Lowell and Rona, 2002; Allen and Seyfried, 2004]. Serpentinization developed at midocean ridges or later on in the hinge zone of subducting oceanic plates [Ranero et al., 2003; Contreras-Reyes
et al., 2008] also inﬂuences the rheology of subduction zones [Hilairet et al., 2007; Hirauchi et al., 2010] and
prograde metamorphism of subducted serpentine induces ﬂuid and chemical transfers between the slab
and the mantle wedge [Ulmer and Trommsdorff, 1995].
The intensity and the spatial distribution of serpentinization reactions in the crust accreted at slow spreading mid-ocean ridges are therefore important parameters. Seismic velocities and gravity data indicate that
domains of ultramaﬁc outcrops correspond to thin but nonabsent low seismic velocity-low density crust. In
domains of ultramaﬁc seaﬂoor, this geophysically deﬁned crust is typically 1–4 km thick [Hooft et al., 2000;
Canales et al., 2000; Cannat et al., 1995a]. It probably corresponds to partially serpentinized mantle-derived
peridotites (as initially proposed by Hess [1962]), with a variable proportion of gabbroic and doleritic intrusions [Cannat, 1993]. Increasing serpentinization results in a linear decrease of both seismic velocity and
density, from nearly 8 kms21 and 3300 kg m23 at 0% of serpentinization to 5 kms21 and 2500 kg m23 at
100% of serpentinization [Christensen, 1972; Miller and Christensen, 1997]. Seismic velocities models can
therefore be used to predict the degree of serpentinization at depth (Figure 1). However, this geophysical
approach has signiﬁcant limitations [Cannat et al., 2010]: (1) it does not uniquely determine the extent of
serpentinization: gabbros, dolerites, and basalts may have seismic properties similar to partially serpentinized peridotites [Miller and Christensen, 1997]; (2) the spatial resolution of seismic velocity models is poor
(200–500 m) so that homogeneous and moderate serpentinization could have the same seismic signature
as heterogeneous and locally complete serpentinization; and (3) seismic velocities determined in the ﬁrst
kilometer of the crust are typically less than values for fully serpentinized peridotites indicating that another
mechanism, probably fracturing, further reduces the velocities [Detrick et al., 1994; Spudich and Orcutt, 1980;
Korenaga et al., 2002; Behn and Kelemen, 2003].
Another method to constrain the intensity and spatial distribution of serpentinization at slow spreading
mid-ocean ridges is to study samples of serpentinized peridotites [Dilek et al., 1997; Ouﬁ et al., 2002; Bach
et al., 2004; Fr€
uh-Green et al., 2004; Andreani et al., 2007]. The limitation of this approach is that most
dredged and submersible samples represent the uppermost footwall of the detachment faults and that the
deepest hole drilled so far in these terranes is only 200 m deep (ODP hole 920D [Cannat et al., 1995b]). Serpentinized peridotite samples that are currently available from slow spreading ridges therefore probably
come from no more than a few hundred meters below the exhumation fault and are not necessarily representative of the deeper levels of the fault’s footwall (Figure 1).
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Figure 2. Location of the studied ultramaﬁc samples. For the Mid-Atlantic Ridge: ODP Site 920 at 23 210 N [Cannat et al., 1995b]; IODP Site
1274 [Kelemen et al., 2004]; and dredges of the Serpentine cruise [Picazo et al., 2012] at 13 N and 15 N. For the Southwest Indian Ridge:
dredges of the Smoothseaﬂoor cruise [Sauter et al., 2013] between 62 E and 65 E. The depth distribution of our samples from ODP Site
920 is shown in the left-hand side of ﬁgure. Sample (SMS_DR17-04-04) from the Southwest Indian Ridge is shown as an example of a typical serpentinized harzburgite.
The study presented here is based on a large set of serpentinized peridotites samples from slow and ultraslow ridges (Figure 2 and Table 1), including dredged samples from the Mid-Atlantic Ridge at 13 N and
15 N (setting described in Picazo et al. [2012]), from the Southwest Indian Ridge at 62 E–65 E [Sauter et al.,
2013] and a set of drilled samples from ODP Site 920 at 23 N on the Mid-Atlantic Ridge (ODP Leg 153 [Cannat et al., 1995b]), and from ODP Site 1274 at 15 N also on the MAR (ODP Leg 209 [Kelemen et al., 2004]).
Some of these drilled samples have been partially oriented using paleomagnetic measurements [Hurst
et al., 1997]. Previous sample studies of serpentinization at slow ridges have focused on mineralogical and
geochemical aspects to unravel successive phases of serpentinization and veining [Dilek et al., 1997;
Andreani et al., 2007; Fr€
uh-Green et al., 2004; Bach et al., 2004], to constrain element mobility and water-rock
exchanges during serpentinization [Paulick et al., 2006; Boschi et al., 2008] and to study the magnetic properties of variably serpentinized peridotites [Ouﬁ et al., 2002; Bach et al., 2006]. In this paper, we address the
question of ﬂuid penetration into the fresh peridotite at the initial stages of serpentinization and the subsequent development of the serpentine mesh texture, in the context of axial exhumation and detachment
faulting (Figure 1).
The mesh texture characterizes most serpentinized peridotites independent of their geodynamic setting
(abyssal, ophiolites, subduction wedges, orogens [e.g., Francis, 1956; Prichard, 1979; Wicks and Whittaker,
1977; Viti and Mellini, 1998]). In serpentinized samples from slow spreading ridges, this mesh texture predates complex veining and local serpentine replacement [Dilek et al., 1997; Andreani et al., 2007]. Mesh cells
(Figure 3) comprise a mesh core made of relict olivine, or of isotropic serpentine and occasional brucite and
a mesh rim made primarily of lizardite [Wicks and Whittaker, 1977; Viti and Mellini, 1998]. Mesh cells are typically 100–500 mm in size [Wicks et al., 1977; Prichard, 1979; Ouﬁ et al., 2002; Andreani et al., 2007; Pl€
umper
et al., 2012]. Early studies of ophiolites and orogenic serpentinites have shown that serpentinization progresses from microfractures, individualizing the mesh cells, toward their centers [Maltman, 1978]. These serpentinizing microfractures therefore act as a pathway for the hydrous serpentinizing ﬂuids from the early
stages of serpentinization. The origin of these microfractures is the focus of our study, speciﬁcally for the
case of serpentinization at slow spreading mid-ocean ridges.
Two mechanisms have been proposed to form these serpentinizing microfractures. One hypothesis invokes
anisotropic thermal contraction of the peridotite and particularly of its primary mineral constituent olivine.
This process is capable of generating residual stresses sufﬁcient to produce microcracks in mantle rocks at
slow spreading ridges [deMartin et al., 2004]. Boudier et al. [2010] have proposed this mechanism for the
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Geochemistry, Geophysics, Geosystems
10.1002/2013GC005148
formation of serpentinizing microfractures in a sample from the Oman ophiolite. The other hypothesis
invokes reaction-induced hierarchical fracturing of the olivine due to volume increase during serpentinization [Iyer et al., 2008; Jamtveit et al., 2009; Rudge et al., 2010; Kelemen et al., 2011; Kelemen and Hirth, 2012;
Pl€
umper et al., 2012]. Microstructural observations in support of these hypotheses were made mostly on
ophiolitic samples (although the most recent work by Pl€
umper et al. [2012] includes one sample from ODP
Site 920).
Our study is the ﬁrst to consider a comprehensive set of serpentinized peridotites samples from slow
spreading ridges. We investigate the morphology and distribution of serpentinizing microfractures at a
range of scales from optical microscopy and Scanning Electron Microscopy (SEM) observations (<1 mm),
to sample (>10 cm) and to drill hole scale (>100 m) and we test the control of the olivine crystallography
using Electron Back Scattering Diffraction (EBSD) maps and pole ﬁgures. Our results lead us to propose
that thermal contraction and tectonic stresses combine to initiate the tight network of microfractures that
allows the onset of serpentinization in fresh peridotites in the footwall of slow spreading ridges detachment faults.
2. Sample Description
For this work, we examined thin sections of 278 samples of mesh-textured serpentinized peridotites from
slow spreading ridges (Figure 2 and Table 1): 45 from ODP Site 920 (MAR 23 N [Cannat et al., 1995b]), 4
from ODP Site 1274 (MAR 15 N [Kelemen et al., 2004]), 23 dredged samples from the MAR axial valley walls
at 13 N and 15 N [Picazo et al., 2012], and 201 samples dredged on and off axis the Southwest Indian Ridge
between 62 E and 65 E [Sauter et al., 2013].
We ﬁnd that mesh textures are similar at all sampling sites and belong to one of the two types that
have been described in early studies of serpentinized peridotites: the equant and the ribbon-shaped
mesh textures [Francis, 1956; Maltman, 1978; Wicks, 1984]. The equant mesh texture (Figure 3a) is composed of polygonal mesh cells with mesh rims of rather homogeneous widths [Wicks et al., 1977]. In the
ribbon-shaped texture (Figure 3b), one orientation of mesh rims is more developed. This ribbon-shaped
texture was interpreted by Francis [1956] and Maltman [1978] as due to deformation of an initially
equant mesh texture. However, Wicks [1984] argued that there was no clear evidence for deformation of
the ribbon-shaped serpentine mesh and proposed instead that these two distinct textures derived from
two different types of initial microfracture networks in the olivine prior to serpentinization. The equant
mesh texture is the most widespread. It is expressed alone in 140 of our samples. The other 138 samples contain either a combination of equant and ribbon-shaped mesh domains (132 samples) or only
ribbon-shaped mesh (6 samples).
Raman analyses (Olympus FV1000 microscope coupled with a Renishaw InVia spectrometer, Institut de Physique du Globe de Paris, France) on nine selected samples indicate that, as observed in previous studies of
the meshwork in serpentinized peridotites [Rumori et al., 2004; Boudier et al., 2010], lizardite is the dominant
serpentine species in the mesh rims (Figure 3e). We did not identify brucite in our samples but it may occasionally occur as intergrowths in the serpentine [Bach et al., 2004]. The iron released during the olivine serpentinization crystallizes as magnetite in the mesh rims. Most magnetite grains are smaller than a few
micrometers and are concentrated along the trace of serpentinizing microfractures (Figures 3f and 3g).
More pronounced concentrations of magnetite also commonly delineate a broader polygonal pattern that
includes up to 10 individual mesh cells (Figures 3a and 3b).
Optical microscopy using a wave plate under polarized light (Figures 3c and 3d) shows that the fast crystallographic axes of serpentine in the mesh rims tend to be perpendicular to the initial microfractures. This is
consistent with lizardite plates being stacked in pseudocolumns [Wicks and Zussman, 1975; Cressey, 1979]
along their [001] (fast) crystallographic axis. Rumori et al. [2004] and Boudier et al. [2010] documented a similar conﬁguration in serpentinized peridotites from the Ligurian and Oman ophiolites. Systematic observation of our samples with the wave plate shows that lizardite [001] axes are indeed strongly oriented in all
samples, subperpendicular to the serpentinizing microfractures (Figures 3c and 3d). In the next section, we
will take advantage of this strong crystallographic preferred orientation to visualize and map the traces of
serpentinizing microfractures at the scale of thin sections.
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0
0
Protolith
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V
Dunite
Very coarse
grained
Equant/ribbon
shaped
Equant
Equant
Equant/ribbon
shaped
Coarse-grained
equigranular
Coarse-grained
equigranular
Porphyroclastic
Equant
Equant
67
66
31
76
90
87
75
70
81
74
30
100
95
60
75
Visual
Estimate
Equant
45–85
46
EBSD
Maps
75
30
63
42
Subdomains
Equant/ribbon
shaped
Equant
Ribbon shaped/
equant
Equant/ribbon
shaped
Equant
Mesh
Texture
Porphyroclastic
Porphyroclastic
Coarse-grained
equigranular
Very coarse
grained
Coarse-grained
equigranular
Coarse-grained
equigranular
Porphyroclastic
Porphyroclastic
Primary
Texture
55 6 15
58 6 24
60 6 19
13 6 10
17 6 12
115 6 55
130 6 60
45 6 20
‘‘Yellow’’
80 6 35
‘‘Blue’’
57 6 22
71 6 31
50 6 15
100 6 45
140 6 50
Initial
17 6 10
15 6 9
26 6 15
19 6 13
15 6 9
21 6 13
25 6 15
26 6 15
15 6 11
15 6 8
20 6 13
Mesh Cells
Area/
Perimeter
6 std
41 6 19
42 6 11
40 6 15
75 6 35
100 6 50
45 6 15
‘‘Red’’
40 6 16
47 6 21
55 6 20
70 6 25
80 6 35
30 6 10
‘‘Green’’
Hierarchical
Apparent Thickness of Serpentine Along
Microfractures (mm)
237 6 107
366 6 166
370 6 170
360 6 170
315 6 150
320 6 110
‘‘Yellow’’
‘‘Blue’’
282 6 169
324 6 136
320 6 160
295 6 120
400 6 165
135 6 60
Initial
153 6 78
164 6 69
150 6 75
265 6 140
325 6 175
170 6 95
‘‘Red’’
183 6 100
160 6 85
135 6 70
265 6 150
255 6 190
170 6 85
‘‘Green’’
Hierarchical
Apparent Spacing of
Microfractures (mm)
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a
The three values given for the serpentinization extent correspond to the proportion of fresh olivine in the microscopic domains of Figure 8, in EBSD maps (Figure S1), or are estimates made visually for the whole thin
section. The mesh cells area/perimeter data are obtained as shown in Figure 8. The dimensions listed for apparent thickness of serpentine along microfractures and apparent spacing of microfractures are explained in
the text and correspond to the histograms in Figure 9.
b
Three orthogonal thin sections.
04–66
Harzburgite
04–62
b
Harzburgite
04–41
ODP 920 (23 20.3 N 45 01.0 W)
B 11R2 118–127 Harzburgite
(piece 16)
D 14R2 114–120 Harzburgite
(piece 7C)
D 18R2 113–118 Harzburgite
(piece 13)
ODP 1274 (14 43.30 N 44 53.30 W)
A 05R1 119–122 Harzburgite
(piece 10)
A 08R1 114–117 Dunite
(piece 16)
A 17R1 119–121 Harzburgite
(piece 20)
SE_DR08 (14 44.8’N 44 58.1’W)
07–02
Harzburgite
07–32
Harzburgite
SMS_DR17 (28 33.4’N 62 21.2’E)
b
Harzburgite
04-04
Sample
Whole TS
Serpentinization %
Table 1. Characteristics of the 12 Serpentinized Peridotites Selected (Out of 278 Samples) for Detailed Analyses of the Serpentine Mesh Texturea
Geochemistry, Geophysics, Geosystems
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Figure 3. Typical equant and ribbon-shaped serpentinization mesh textures. Abbreviations: olivine (Ol), pyroxene (Px), lizardite (Liz), magnetite (Mgt). (a) Photomicrograph of equant
mesh texture in natural light (sample SE_DR08-07-02). (b) Photomicrograph of ribbon-shaped mesh texture in natural light (sample 920D-18R2, 113–118 cm, piece 13). Concentrations of
magnetite along some microfracture planes underline a polygonal pattern that includes up to 10 individual mesh cells. (c and d) Detail of Figures 3a and 3b in polarized light with the
addition of a wave plate revealing the preferred orientation of fast [001] crystallographic axes of lizardite subperpendicular to serpentinizing microfractures. (e) Detail of Raman spectra
for serpentine in the mesh rims of Figure 3a in pale gray and Figure 3b in dark gray. Peak positions are typical of lizardite. (f and g) Scanning Electron Microscope (backscattered electron)
images located by insets in Figures 3a and 3b and showing details of the mesh structure: olivine relicts, lizardite mesh rims, and serpentinizing microfractures outlined by variable
amounts of magnetite. Raman spots for Figure 3e are shown as circles.
Our samples show a variety of preserpentinization mineralogies: dunites (17 samples), harzburgites, or lherzolites (261 samples), some (82 samples) with evidence for magmatic impregnations and veins that have
been described in previous studies [Cannat et al., 1997; Kelemen et al., 2004; Picazo et al., 2012]. Preserpentinization textures, identiﬁable in samples that are not fully serpentinized, range from very coarse grained
with olivine grains up to 0.5 cm in size (12 samples), to coarse-grained equigranular with mosaics of equant
olivine grains up to 1 mm in size (34 samples), to porphyroclastic textures with variably elongated
millimeter-sized olivine grains and local recrystallization in neoblasts <0.2 mm in size (77 samples). Ceuleneer and Cannat [1997] have described these textures and the corresponding mineral preferred orientations
for the ODP Site 920 samples. Most samples from the Southwest Indian Ridge show locally tight and irregular subgrain boundaries in olivine, kink-bands, and fractures in pyroxenes, local recrystallization in smaller
neoblasts and occasional brittle microshear zones, suggesting plastic to semibrittle deformation in higher
stress-lower temperature lithospheric conditions [Cannat et al., 2012]. Samples of mylonitic peridotites are
rare at the ridge locations considered here and therefore not representative of the bulk of the serpentinizing mantle material. We chose not include these rare mylonites in our study.
In most samples, pyroxenes are less altered than the olivine, and even in samples with pervasively
serpentinized olivine, relics of pyroxene are frequent. Pyroxenes are altered into bastites (serpentine pseudomorphs) with occasional amphibole (tremolite). The 159 samples also show signiﬁcant overprinting of
the serpentine mesh texture by microscopic to mesoscopic serpentine veins. Four main veining episodes
have been described in the serpentinized peridotites of ODP Site 920 [Dilek et al., 1997; Andreani et al.,
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2007] and preliminary observations in the Southwest Indian Ridge samples suggest a comparable chronology (S. Roumejon et al., manuscript in preparation, 2013). Oxygen-isotope constraints suggest that the serpentine mesh texture formed at temperatures in excess of 350 C and 280 C, respectively, in the ODP Site
920 samples [Agrinier and Cannat, 1997] and in the Southwest Indian Ridge samples [Roumejon et al., 2013].
In order to perform detailed microstructural, Raman, SEM, EBSD, and 3-D analyses of the mesh texture, we
selected a set of representative samples based on four criteria: (1) samples from each studied area (Figure
2); (2) well-developed mesh textures with limited veining and adequate representation of the ribbon and
equant mesh textures; (3) variable pyroxene content; and (4) a wide range of serpentinization degrees.
Among our 12 selected samples (Table 1), three come from the ODP Leg 153 holes 920B and 920D, three
from the ODP Leg 209 hole 1274A, two from dredges in the MAR13 N–15 N area, and four from dredges at
the Southwest Indian Ridge (62 E–65 E). Because mesh textures are three-dimensional structures [e.g.,
Wicks et al., 1977], we studied two of the selected Southwest Indian Ridge samples for which we had sufﬁcient material in three orthogonal thin sections.
The selected samples are moderately to fully serpentinized and include 10 serpentinized harzburgites and 2
serpentinized dunites. The equant mesh texture (Figure 3a) is observed in all samples with zones of ribbonshaped mesh in four of them. The ribbon-shaped mesh dominates only in one selected sample (920D-18R2,
113–118 cm, piece 13; Figure 3b). It is associated with advanced serpentinization (85% based on image
analyzes of EBSD maps; Table 1) and is in sharp contact with a domain of nearly fresh harzburgite. Serpentinization in this sample is therefore strongly heterogeneous. In most samples, serpentinization is more even,
yet we observe variations at the thin section scale, with less serpentinized domains a few millimeter-wide
that are commonly associated with greater concentrations of pyroxenes.
The two selected serpentinized dunites exhibit very coarse-grained olivine, up to 0.5 cm in size. The harzburgitic samples have coarse-grained equigranular (ﬁve samples; Table 1) or porphyroclastic initial textures
(ﬁve samples). Additional features crosscut some of the selected samples: SMS_DR17-04-62 contains brittle
microshear zones up to 400 mm thick, characterized by local cataclasis with angular olivine and pyroxene
clasts; SMS_DR17-04-41 is cut by a tremolite vein; and three samples selected from ODP Site 920 contain
altered magmatic veins [Cannat et al., 1997].
The selected samples are representative of our larger sample set in terms of serpentine texture and mineralogy: lizardite is dominant in the mesh rims, with occasional chrysotile (identiﬁed by Raman analysis using
the peak characterization of Auzende et al. [2004] and Schwartz et al. [2013]; e.g., Figure 3e) in the mesh rims
of samples 920B-11R2, 118–127 cm, piece 16 and 920D-18R2, 113–118 cm, piece 13. Magnetite is found
associated with mesh rim serpentine in every sample. Two samples selected from ODP Site 920 also contain
needles of iron-nickel sulﬁde [Dilek et al., 1997].
3. Geometry and Distribution of Serpentinizing Microfractures at Thin Section
Scale
Optical microscopy images of equant serpentine mesh texture in natural or polarized light may give the
impression that serpentinizing microfractures are randomly oriented. This impression is reinforced by the
polygonal pattern outlined by more pronounced magnetite concentrations that commonly encloses up to
dozens of mesh cells (Figure 3a). However, optical microscopy images obtained under polarized light with
the addition of a wave plate systematically reveal that the lizardite in the mesh rims, that tends to orient
with its [001] fast optical axis perpendicular to serpentinizing fractures (Figures 3c and 3d), has a pronounced crystallographic preferred orientation (CPO) at the scale of the whole thin section (Figure 4). In
these images, polarization colors change from yellow to blue when the preferred orientation of the [001]
fast axis of lizardite is, respectively, parallel and perpendicular to the fast axis of the wave plate (Figures 3c
and 3d). Intermediate orientations lead to magenta colors. This method therefore underlines serpentinizing
microfractures, including those with low magnetite concentrations that are less visible otherwise.
Using this approach, we have been able to map the trace of serpentinizing microfractures in the 12 selected
samples, and in 21 additional partially oriented drilled samples from ODP Site 920 (Table 1 and Table S1 in
supporting information). In every case, we found four orientations of microfractures, forming two sets of
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Geochemistry, Geophysics, Geosystems
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Figure 4. Mapping the traces of serpentinizing microfractures. Sample SMS_DR17-04-66 (serpentinized dunite, equant mesh texture). (a)
Scan of the whole thin section, natural light. (b and c) Thin section maps with the serpentine mesh texture in pale gray, pyroxenes, bastites, and late serpentine veins in darker gray, and the traces of serpentinizing microfractures in yellow, blue, red, or green. For design
clarity, only two orientations of microfractures are represented in each map. (d and e) Photomicrographs of the 2.5 mm by 3.5 mm area
shown as inset in Figures 4a–4c). Polarized light with wave plate inserted. Lizardite with fast [001] axis parallel or perpendicular to wave
plate axis appears, respectively, in yellow or in blue. These colors are used to trace microfractures that have lizardite rims > 10 mm in thickness and draw their average orientation in the thin section maps of Figures 4b and 4c. The ‘‘yellow’’ and ‘‘red’’ orientations in Figure 4b correspond to the blue and yellow colors in Figure 4d; the ‘‘blue’’ and ‘‘green’’ orientations in Figure 4c correspond to the blue and yellow
colors in Figure 4e. By convention, we use blue or yellow colors to draw microfractures that can be traced over a few millimeters (see text).
mutually perpendicular orientations. We now use four of the selected samples to describe this microfracture
pattern.
Our approach is illustrated in Figure 4 in the case of a serpentinized coarse-grained dunite (Southwest
Indian Ridge sample SMS_DR17-04-66). We explore the whole thin section (Figure 4a) at regular steps
(about 5 mm) using a graduated stage holder and turn the stage relative to the wave plate (in Figures 4d
and 4e, we show the fast axis of the wave plate turning relative to the thin section instead) until one set of
serpentinizing microfractures turns blue. We note that lizardite along perpendicular microfractures then
turns yellow. We measure the apparent strike of these microfractures and draw them in the thin section
map (Figure 4b). We then turn the stage again until another set of serpentinizing microfractures turns blue.
In the thin section considered in Figure 4, we note that this occurs after a rotation of the stage by 40 ,
and that again lizardite associated with another orientation of perpendicular microfractures turns yellow
(Figures 4d and 4e). We measure and draw the apparent strike of this other set of mutually perpendicular
microfractures in the thin section map (Figure 4c). We then check for other orientations of the stage that
would correspond to a clear lizardite preferred orientation, and move the stage to the next step. This
method produces about 20 apparent strike measurements per thin section for each microfracture orientation, with standard deviations 65 to 615 . We also checked the local variability of these apparent strikes
in 3.5 mm by 2.6 mm microphotographs using a regular grid of lines spaced by 200 mm and making measurements at the intersection with each grid line. This method yields more than 60 measurements per microphotograph for each microfracture orientation, and standard deviations are 67 to 617 .
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Geochemistry, Geophysics, Geosystems
A
A
B
a SMS_DR17-04-66
10.1002/2013GC005148
B
C
b SMS_DR17-04-04
C
Figure 5. Three-dimensional reconstruction of the ‘‘blue’’ and ‘‘yellow’’ serpentinizing microfracture planes mapped in three orthogonal thin sections for (a) sample SMS_DR17-04-66 and
(b) sample SMS_DR17-04-04. As sketched in the insets, these microfractures exhibit an overall planar morphology, despite thin section-scale variations of apparent strikes with standard
deviations up to 617 . The angle 6 standard deviation between the ‘‘blue’’ and ‘‘yellow’’ microfractures for sections A, B, and C are, respectively, 42 6 11 , 55 6 15 , and 54 6 11 in
Figure 5a, and 43 6 14 , 37 6 8 , and 41 6 9 in Figure 5b. The arrow in Figure 5b means that thin section A corresponds to the lower face of the cube.
In the serpentinized coarse-grained dunite presented in Figure 4, the traces of two families of microfractures
that make an apparent angle of 40 (42 6 11 ; the yellow and blue traces in Figures 4b and 4c) are
actually directly visible on the hand-held thin section (Figure 4a). The ‘‘wave plate’’ method allows to show
Figure 6. Mapping the traces of serpentinizing microfractures in (a) sample SE_DR08-07-02 (serpentinized harzburgite, equant mesh texture) and (b) sample 920D-18R2, 113–118 cm,
piece 13 (serpentinized harzburgite, ribbon-shaped mesh texture). As in Figure 4, for each sample, the ﬁrst ﬁgure is a scan of the whole thin section, the two ﬁgures below that are maps
of the traced microfractures, shown in pairs (yellow and red, and blue and green) for clarity, and the two larger ﬁgures to the right are detailed microphotographs in polarized light with
a wave plate inserted. Additional explanations may be found in the caption for Figure 4.
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that these two families are associated with strongly oriented lizardite rims and are perpendicular to two
other orientations of serpentinizing microfractures that are only visible at the thin section scale (the red and
green traces in Figures 4b and 4c). The step-by-step mapping approach also permits to assess the continuity
of each family of microfractures at thin section scale. This allows us to show that two orientations of microfractures, ‘‘red’’ in Figure 4b and ‘‘green’’ in Figure 4c, have shorter traces, while the other two, ‘‘yellow’’ in
Figure 4b and ‘‘blue’’ in Figure 4c, commonly link and extend up to a few centimeters. In ﬁgures following
this one, we retain the same colors for serpentine in the same sets of microfractures, with the more continuous sets in blue and yellow, and the shorter sets in red and green. For this sample and for Southwest Indian
Ridge serpentinized harzburgite sample SMS_DR17-04-04, we mapped three orthogonal thin sections (Figure 5) and show that the traces of the more continuous ‘‘blue’’ and ‘‘yellow’’ serpentinizing microfractures in
each orthogonal section are consistent with a dominantly planar morphology (inset cubes in Figure 5).
The orientation of the traces of serpentinizing microfractures in the serpentinized harzburgite thin section
shown in Figure 5b is more variable than in the serpentinized dunite of Figure 4, particularly in the vicinity
of partially serpentinized pyroxenes. Microfractures with traces parallel to the edge of a pyroxene show
greater continuity and have thicker associated mesh rims. The two serpentinized harzburgites presented in
Figures 3 and 6 display similar results. Sample SE_DR08-07-02 (Figure 6a) is fully serpentinized and has an
equant mesh texture. Sample 920D-18R2, 113–118 cm, piece 13 (Figure 6b) presents a ribbon-shaped mesh
texture in a fully serpentinized region, in sharp contact with much less serpentinized peridotite. In both
cases, we ﬁnd the two sets of mutually perpendicular serpentinizing microfractures. Each set comprises serpentinizing microfractures that tend to be more continuous at an apparent angle of 38 6 8 and 43 6 11 ,
respectively (in yellow and blue; Figure 6), and shorter perpendicular serpentinizing microfractures (in red
and green). In the ribbon texture, one orientation of the more continuous microfractures is strongly predominant (represented in blue in Figure 6b).
In each of our 40 mapped samples, we found a similar pattern with a maximum apparent angle of 55 between
the traces of the ‘‘yellow’’ and ‘‘blue’’ serpentinizing microfractures and standard deviations lower than 617 .
Although we could not make orthogonal thin sections in most samples because they were too small, this apparent angular relationship indicates that the ‘‘yellow’’ and ‘‘blue’’ microfractures make true angles 55 .
In order to further characterize the geometry of serpentinizing microfractures, we also tried the U-stage
method described by Boudier et al. [2010]. In this method, the section is tilted so as to align the two edges
of the microcrack over the thickness on the thin section (30 mm). As a consequence, microfractures that
have a rim of lizardite thicker than 10 mm cannot be measured if their dip in the thin section is less than
60 . This method is therefore ﬁt to explore the ﬁne-scale geometry of microfractures in weakly serpentinized peridotites [Boudier et al., 2010] but it introduces a bias for thin section-scale statistical studies of
microfractures in signiﬁcantly serpentinized samples.
4. SEM Images of Serpentinizing Microfractures
The SEM image (Zeiss Supra 55P SEM, Universite Pierre et Marie Curie, France) in Figure 7a shows a detail of
the ribbon-shaped mesh texture described in Figures 3b and 6b (920D-18R2, 113–118 cm, piece 13). Serpentinizing microfractures that control the ribbon texture (‘‘blue’’ microfractures in Figure 6b) are spaced by
50 mm or less and extend over more than the height of the image. Serpentinizing fractures with the orientation of the ‘‘yellow’’ family in Figure 6b are less abundant and appear shorter. Both families are associated
with short perpendicular serpentinizing microfractures (the red and green families in Figure 6b) creating Tjunctions and limiting four-sided olivine domains. Some of these short fractures develop from etch pits in
the olivine (Figure 7a) and are therefore posterior to the onset of serpentinization. Following Iyer et al.
[2008], Kelemen and Hirth [2012], and Pl€
umper et al. [2012], we interpret these textural characteristics as indicators of a hierarchical microfracturing process associated with volume increase during serpentinization.
This reaction-induced fracturing process produces the red and green orientations of microfractures and initiates along the more continuous ‘‘blue’’ and ‘‘yellow’’ microfractures that we will therefore describe as initial
serpentinizing microfractures.
The SEM image in Figure 7b shows a broader view of the same sample, in the poorly serpentinized domain
visible in the left part of the thin section in Figure 6b. It shows that the initial serpentinizing microfractures
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Figure 7. Scanning Electron Microscopy (backscattered electron) images of serpentinizing microfractures. Abbreviations: olivine (Ol), orthopyroxene (Opx), serpentine (Serp), microfracture
(microfract). Insets show the overall orientation of the traces of ‘‘blue’’ and ‘‘yellow’’ serpentinizing microfractures at thin section scale. (a) Detail of the ribbon-shaped mesh texture in sample
920D-18R2, 113–118 cm, piece 13. Maps of serpentinizing microfractures for this thin section are shown in Figure 6b. The ‘‘blue’’ and less common ‘‘yellow’’ initial microfractures (see text)
extend over more than the image width. The ‘‘red’’ and ‘‘green’’ hierarchical microfractures are shorter and at right angles to the ‘‘blue’’ and ‘‘yellow’’ microfractures. Some hierarchical microfractures seem to be related to etch pits in the olivine. (b) A broader view of the same sample in a less serpentinized domain, showing initial microfractures cutting olivine-pyroxene contacts
with no apparent displacement. (c) Detail of the equant mesh texture in sample SE_DR08-07-02. Maps of serpentinizing microfractures for this thin section are shown in Figure 6a. Segments
of the ‘‘blue’’ and ‘‘yellow’’ initial serpentinizing microfractures branch with no apparent displacement. (d) Detail of the equant mesh texture in sample SMS-DR17-04-62. This serpentinized
harzburgite displays several brittle shear zones <100 mm in width, containing angular clasts of the primary mineralogy (olivine and pyroxene). One of these is outlined by arrows. The ‘‘blue’’
and ‘‘yellow’’ initial serpentinizing microfractures cut across these shear zones. (e) Detail of Figure 7d showing segments of the ‘‘blue’’ initial serpentinizing microfractures that cut across olivine and pyroxene clasts in microshear zone. (f) Detail of the equant mesh texture in sample SMS_DR17-04-04. Maps of serpentinizing microfractures for this thin section are shown in Figure
5b. The thickness of serpentine varies between <2 mm along microfractures in the olivine relict and >100 mm along magnetite-lined microfractures that bound the mesh cells.
also cut into pyroxenes and that the olivine-pyroxene contacts are not displaced. We make similar observations in Figure 7c that shows a SEM detail of sample SE_DR08-07-02, a serpentinized harzburgite with an
equant mesh texture (Figures 3a and 6a). In this sample, the two families of initial fractures are equally
developed, forming anastomosing relays and delimiting elongated 50–200 mm wide lozenge-shaped
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domains, with not visible displacement. Orthogonal hierarchical microfractures are observable in the more
serpentinized right part of the SEM image. Lizardite rims developed along these initial and hierarchical
microfractures outline the overall equant meshwork.
Figures 7d and 7e show SEM images of sample SMS-DR17-04-62, a serpentinized harzburgite with welldeveloped brittle microshear zones. Most of these shear zones are less than 100 mm wide and contain angular clasts of the primary mineralogy (olivine and pyroxene). They are distinct from the anastomosing network of initial serpentinizing microfractures. In Figure 7e, we present a detail of a less serpentinized part of
the sample in which initial serpentinizing microfractures of the ‘‘blue’’ family, with spacing of 50 mm, are
continuous across the olivine and pyroxene clasts that form the shear zone. These crosscutting relations
indicate that the formation of the brittle microshear zones predated the formation of the serpentinizing
microfractures.
Our SEM study also reveals that the amount of serpentine associated with the different orientations of serpentinizing microfractures may be variable: in samples with a ribbon-shaped mesh, those initial serpentinizing microfractures that are parallel to the ribbons tend to have thicker serpentinized rims (Figure 7a). We
also note that in all of our samples some serpentinizing microfractures have a very thin serpentine coating.
This is true in moderately serpentinized domains (Figures 7b and 7c) but this is also observed in extensively
serpentinized samples where some microfractures are coated by <2 mm of serpentine (the thinnest fractures in the olivine relict of Figure 7f), while other are lined by serpentine rims 100 mm thick or more (the
broad lizardite rims in Figure 7f). In the next section, we further examine the scales of initial and hierarchical
microfracturing and their relation to the characteristic length scales of the mesh cells.
5. Distribution and Spacing of Serpentinizing Microfractures
In order to investigate the characteristic scales of the mesh cells, and to link these scales with the advancement of the serpentinization reactions, we used microphotographs (magniﬁcation 1003) from 39 selected
1.4 mm by 1 mm domains in nine samples (Table 1 and Figure 8). We included microphotographs from
orthogonal thin sections when available. In each domain, we traced manually all the visible serpentinizing
microfractures and we used image processing to delineate the contours of olivine relicts (Figures 8a and
8b). SEM images made at yet greater magniﬁcation reveal a few additional cracks with very thin (<2 mm)
serpentine rims that are not visible with the optical microscope but we ﬁnd that the 1.4 mm by 1 mm
microphotographs adequately capture the geometry and scales of the serpentinizing microfracture network
in our samples.
We derived a local serpentinization extent for each domain from the ratio of olivine surface to the total
surface enclosed by the mapped microfractures. These serpentinization extents range between 18% and
100% for the 39 selected domains and may vary signiﬁcantly within each sample. We could not ﬁnd
domains with serpentinization extents <18%. Figures 8a and 8b show examples of the microfracturing
patterns obtained for two serpentinized harzburgites with an equant mesh texture (Table 1). The size of
the mapped domains is close to that of a typical olivine grain in these samples and the microfracture network at this scale may be at a small angle to the average orientations measured for the thin section (inset
in Figures 8a and 8b shows average orientation of the ‘‘blue’’ and ‘‘yellow’’ initial serpentinizing microfractures in each sample).
In Figure 8c, we plot the mean area over perimeter ratio for the cells deﬁned by the microfracture network
in each domain, as a function of the local serpentinization extent. This ratio is equivalent to the half radius
of a circle of same area than the cell. Between 18% and 70% of serpentinization, the mean value of this ratio
is around 15 mm in all of our samples (suggesting mean diameters around 60 mm for the unit cells of this
microfracture network), with large but consistent standard deviations. This stability of unit cell dimensions
for serpentinization rates between 18% and 70% indicates that little additional microfracturing of the protolith, reaction-induced or otherwise, occurred after 20% of serpentinization.
For serpentinization extents >70%, the mean ratio of cell area over cell perimeter increases to >25 mm,
yielding mean diameters >100 mm for the microfracture unit cells at >90% serpentinization (Figure 8c).
This increase is due to the overgrowth of serpentine rims from nearby cells over the smaller microcracks
and to the full alteration of the smaller olivine relicts.
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Figure 8. Characteristic dimensions of the serpentinizing microfractures network in selected samples. (a and b) Examples of the microphotographs and line drawings used to obtain the
statistical data plotted in (c) and (d). Photomicrographs are taken under natural or polarized light (magniﬁcation 1003) and cover 1.4 mm by 1 mm domains. Several domains are analyzed in each sample. Microfractures are drawn manually. Local serpentinization extents (S) are derived from the surface proportion of olivine over the total surface enclosed by the
microfractures. Insets show the overall orientation of the traces of ‘‘blue’’ and ‘‘yellow’’ serpentinizing microfractures at thin section scale. (c) Mean area over perimeter ratio for the cells
deﬁned by the microfracture network in each 1.4 mm by 1 mm domain, as a function of the local serpentinization extent. Vertical error bars: one standard deviation. (d) Complementary
cumulative distribution function for the cell areas in eight samples for which we measured >150 cells. Symbols highlight distribution functions for three representative samples: 920D
14R2, 114–120 cm, piece 7C (534 measured cells); SMS_DR17-04-66 (1151 measured cells); and SE_DR08-07-32 (186 measured cells). Mesh cell areas in these samples follow lognormal
distributions with ﬁtting parameters (mean 6 one standard deviation) of, respectively, 8.07 6 0.99, 8.32 6 1.1, and 8.81 6 1.15 mm2.
The complementary cumulative distribution function for the mesh cell areas (measured on microphotographs as shown in Figures 8a and 8b) is shown in Figure 8d for eight samples. For samples SMS_DR17-0466 and SMS DR17-04-04, the cumulative distributions are similar for cell areas measured in the three orthogonal thin sections. Figure 8d highlights three samples (920D 14R2, 114–120 cm, piece 7C; SMS_DR17-04-66;
and SE_DR08-07-32) with serpentinization extents of 63%, 67%, and 87%, respectively (Table 1).
The y axis in Figure 8d gives the percent of mesh cells having an area larger than the value indicated on the
x axis (as underlined by the 50%, 10%, and 1% lines), showing that different samples contain different proportions of medium to large mesh cells. For example, only 10% of the mesh cells have areas larger than 104
mm2 (equivalent to a 60 mm diameter for an ideal circular cell) in sample 920D 14R2, 114–120 cm, piece 7C,
whereas this proportion is of 20% and 40%, respectively, in samples SMS_DR17-04-66 and SE_DR08-07-32.
The proportion of larger mesh cells is positively correlated with the average serpentinization extent as also
seen in Figure 8c and consistent with the overgrowth of serpentine rims at the expense of the smallest
microcracks in the more serpentinized samples.
The measured distributions in each sample show a close ﬁt to lognormal cumulated distribution curves
(Figure 8d). The mean and standard deviation of the natural logarithm of the measured cell areas are similar
in the samples represented in Figure 8d: 8.07 6 0.99, 8.32 6 1.1, and 8.81 6 1.15 mm2, respectively, in samples
920D 14R2, 114–120 cm, piece 7C, SMS_DR17-04-66, and SE_DR08-07-32. Converting these values back into
units of distance yield the following areas: 3197, 4105, and 6701 mm2. These lognormal distributions are similar
to the distribution found by Pl€
umper et al. [2012], who used a similar method for measuring mesh cell areas in
a serpentinized harzburgite from ODP Site 920 Hole D.
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Geochemistry, Geophysics, Geosystems
“yellow” initial
microfractures
15
nb. of data
65 measures
average = 50±15 µm
av. spacing = 320±160 µm
15
25
20
91 measures
average = 40±15 µm
av. spacing = 150±75 µm
40
147 measures
average = 55±20 µm
av. spacing = 135±70 µm
30
10
20
10
5
5
0
20
28 measures
average = 45±20 µm
av. spacing = 320±110 µm
6
15
4
10
2
5
0
“green” hierarchical
microfractures
“red” hierarchical
microfractures
15
0
8
b
920D 18R2 #13
20
38 measures
average = 55±15 µm
av. spacing = 370±170 µm
10
nb. of data
SMS_DR17-04-66
a
“blue” initial
microfractures
10.1002/2013GC005148
25 65 105 145 185 225
0
10
5
118 measures
average = 80±35 µm
av. spacing = 135±60 µm
0
15
38 measures
average = 45±15 µm
av. spacing = 170±95 µm
10
0
10
8
19 measures
average = 30±10 µm
av. spacing = 170±85 µm
6
4
5
2
25 65 105 145 185 225
0
25 65 105 145 185 225
0
25 65 105 145 185 225
apparent thickness of lizardite rims along microfractures (µm)
Figure 9. Histograms of the apparent width of serpentine measured across initial and hierarchical serpentinizing microfractures in samples (a) SMS_DR17-04-66 (equant mesh texture;
microphotographs and microfractures maps in Figure 4) and (b) 920D-18R2, 113–118 cm, piece 13 (ribbon-shaped mesh texture; microphotographs and microfractures maps in Figure
6b). Measurements were performed along regularly spaced grid lines on several 3.5 mm by 2.6 mm microphotographs taken under polarized light with the wave plate inserted. Microfractures associated with serpentine width <10 mm are not taken into account. The plots also give the total number of measurements per sample, the average apparent serpentine thickness, and the average apparent spacing of the corresponding microfractures.
6. Thickness and Spacing of Serpentine Mesh Rims
The wave plate method is effective to produce thin section-scale maps of those serpentinizing microfractures that correspond to apparent lizardite thickness >10 mm (Figures 4–6). We measured this apparent
thickness of lizardite and the apparent spacing of these microfractures in the four samples illustrated in Figures 5 and 6 (Table 1). For two of these samples, measurements were carried out in three orthogonal sections, yielding comparable results. Measurements were made in 3.5 mm by 2.6 mm microphotographs
taken under polarized light with the wave plate inserted (partly shown in Figures 4d and 46). We measured
the apparent serpentine thicknesses at the intersections with a regular grid of lines spaced by 400 mm. This
method allows us to measure lizardite rims with apparent thickness >10 mm. The measured values (Figure
9) correspond to the full apparent thickness of the serpentine domains associated with serpentinizing
microfractures, approximately equivalent to twice the apparent thickness of the mesh rims that surround
intervening mesh cores (Figure 3). If we assume that the rims replace preexisting olivine, these apparent
rim thickness values provide an approximation for the extent of the serpentinization reactions associated
with each serpentinizing microfracture.
Histograms of the apparent thickness of lizardite along fractures in the serpentinized dunite sample with an
equant mesh texture (SMS_DR17-04-66; Figure 9a) display similar 20–100 mm ranges across the four sets of
serpentinizing microfractures with a mean value around 50 mm. By contrast, in the serpentinized harzburgite
with a ribbon-shaped mesh texture (920D-18R2, 113–118 cm, piece 13; Figure 9b) those initial microfractures
that parallel the ribbons (the ‘‘blue’’ microfractures; Figure 6b) develop wider serpentine rims, ranging
between 20 mm and more than 200 mm in apparent thickness with a mean value of 80 mm. Serpentine thicknesses along the other three orientations of serpentinizing microfractures in this sample are thinner (20–90
mm with a mean value of 40 mm). In samples SMS_DR17-04-04 and SE_DR08-07-02 (Figures 5b and 6a) that
have a dominantly equant mesh texture, the widest mesh rims are found in the vicinity of pyroxenes and are
associated with those microfractures that are subparallel to the former olivine-pyroxene contacts. Depending
on the orientation of these contacts, this leads to larger mesh rim thicknesses associated with one or the other
of the four families of serpentinizing microfractures and produces ribbon-shaped textured domains. The
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average apparent thickness of serpentine along microfractures in our four selected samples (Table 1) is
66 6 38 mm. This corresponds to an average thickness of the mesh rims of 33 6 19 mm.
The apparent spacing of the serpentinizing microfractures that correspond to these well-developed serpentine rims is also variable. In serpentinized dunite sample SMS_DR17-04-66 (equant mesh texture; Figure 8a),
the average apparent spacing is similar for the ‘‘blue’’ and ‘‘yellow’’ families of initial microfractures
(370 6 170 mm and 320 6 160 mm, respectively). It is more than twice the spacing between the shorter hierarchical (red and green) microfractures (150 6 70 and 135 6 75 mm; Figure 9a). Measurements of the apparent spacing of the serpentinizing microfractures that correspond to well-developed mesh rims in two
other equant mesh-textured samples (SMS_DR17-04-04 and SE_DR08-07-02; equant serpentine mesh textures) give similar results for initial microfractures (Table 1), and higher values (250–350 mm) for the spacing
of hierarchical microfractures.
In the ribbon-shaped textured serpentinized harzburgite sample 920D-18R2, 113–118 cm, piece 13 (Figure
9b), the average spacing of microfractures that correspond to well-developed serpentine rims is 135 6 60
mm for the orientation paralleling the ribbons (‘‘blue’’ orientation) and 320 6 110 mm for the ‘‘yellow’’ orientation. For hierarchical (red and green) microfractures, the spacing is similar (170 6 95 and 170 6 85 mm),
and similar to that measured in the dunitic equant mesh-textured sample (Figure 9a).
Ribbons are thus found to form along tightly spaced initial microfracture planes, consistently with the
hypothesis of a link between ribbon-shaped mesh texture and a different microfracture pattern in the fresh
olivine prior to serpentinization [Wicks, 1984]. It should also be noted that microscopic domains of ribbonshaped mesh develop next to pyroxenes or bastites in samples that have a dominantly equant mesh texture (Figures 5b and 6a). In this case, it is possible that the pyroxenes create local perturbations either
before or during serpentinization.
7. Serpentinizing Microfractures and Olivine Preferred Orientation
In this section, we explore the relationship between olivine crystallography and the geometry of initial serpentinizing microfractures. Olivine has anisotropic thermal expansion coefﬁcients, with the largest value
along the [010] crystallographic axis and the lowest along the [100] axis [Suzuki, 1975; Bouhifd et al., 1996].
The [010] axis also corresponds to the direction of minimal energy required to fracture the olivine grains
[Swain and Atkinson, 1978]. Thus, microfractures that form in response to stresses generated by thermal
contraction of the exhumed mantle material are expected to preferentially form parallel to the easy (010)
olivine cleavage planes, as observed in a sample from the Oman ophiolite by Dewandel et al. [2003] and
Boudier et al. [2010]. Mapping serpentinizing microfractures at thin section scale in our abyssal samples
(Figures 4–6) has already revealed a more complicated pattern, with two intersecting sets of initial microfractures. We used Electron Back Scattering Diffraction (EBSD) in six selected samples in order to check
whether these families of initial serpentinizing microfractures formed at a systematic angle to the olivine
fabric. The analyses were carried out at the SEM-EBSD facility at Geosciences Montpellier (Universite Montpellier 2, France), using a JEOL JSM 5600 SEM.
The EBSD maps (Figure S1) cover between 2.63 and 5.78 cm2. They are based on up to 500,000 data points
attributed to olivine, pyroxene or serpentine and spaced by 35 or 50 mm. The proportion of points that correspond to primary minerals and to serpentine is used to derive a serpentinization extent at the EBSD map
scale. This extent varies between 45% and 85% (Table 1). The EBSD data reveal a moderate to strong olivine
crystallographic preferred orientation (CPO) in each sample (Figure 10). The interpretation of these CPOs in
terms of plastic slip systems in olivine is detailed in the supporting information. In order to compare the
pole ﬁgures of olivine CPOs with the orientation of initial serpentinizing microfractures in each sample, we
consider the average strike of the traces of these microfractures in each thin section (as determined using
the wave plate method; Figures 4–6). In most samples, we can only compare these strikes with the olivine
CPO because we do not have measurements of the microfracture dips. We only have this information for
the two samples (SMS-DR17-04-04 and SMS-DR17-04-66; Figures 10a and 10c) in which we have been able
to cut and study three orthogonal sections.
In sample SMS-DR17-04-04 (Figure 10a), we ﬁnd that while the ‘‘blue’’ family of serpentinizing microfractures
is close to parallel to the preferred orientation of the olivine (010) crystallographic planes (i.e., close to perpendicular to the preferred orientation of the [010] axes), there is a signiﬁcant angle between olivine (010)
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Geochemistry, Geophysics, Geosystems
a SMS_DR17-04-04
[010]ol
d SE_DR08-07-32
[100]ol
10
Area = 3.32 cm
N = 133536
nol = 12886
8
[001]ol
b SMS_DR17-04-62
[010]ol
6
8
6
[001]ol
4
2
2
e 920D 14R2 #7C
10
8
[001]ol
Area = 2.63 cm
N = 105984
nol = 14156
[100]ol
4
[100]ol
Area = 5.78 cm
N = 473800
nol = 102168
[010]ol
10.1002/2013GC005148
6
[010]ol
[100]ol
8
Area = 2.63 cm
N = 105984
nol = 42188
6
[001]ol
2
2
c SMS_DR17-04-66
Area = 4.12 cm
N = 165600
nol = 34100
[010]ol
[100]ol
f 920D 18R2 #13
15
[001]ol
4
4
10
Area = 4.24 cm
N = 510600
nol = 110176
5
[010]ol
[100]ol
5
4
[001]ol
3
2
1
0
Figure 10. Electron Backscattered Diffraction (EBSD) results showing polar Crystallographic Preferred Orientation (CPO) diagrams for olivine in six selected samples (lower hemisphere
projection). Mean apparent strikes and dips (standard deviations < 617 ) of the ‘‘blue’’ and ‘‘yellow’’ initial serpentinizing microfractures at thin section scale are shown as either yellow
and blue planes in (a) and (c) for samples in which we analyzed three orthogonal thin sections (Figure 5), or as traces in the plane of the studied thin section: yellow and blue rectangles
in (b), (d), (e), and (f). The trace of elongated spinel grains is shown by open rectangles. For each sample, we specify the analyzed area, the total number of EBSD data points (N), and the
number of data points attributed to olivine (nol). Corresponding EBSD maps and orthopyroxene CPOs are shown in Figures S1 and S2, respectively.
and the microfractures of the ‘‘yellow’’ family. In sample SMS-DR17-04-66 (Figure 10c), ‘‘blue’’ initial serpentinizing microfractures tend to be nearly orthogonal to the (010) planes in olivine, and there is a signiﬁcant
angle between olivine (010) and the ‘‘yellow’’ family. In sample SE-DR08-07-32 (Figure 10d), the preferred
strike of the olivine (010) planes is parallel to the average strike of the ‘‘blue’’ microfractures. In the two analyzed ODP site 920 samples, the average strikes of the ‘‘blue’’ and ‘‘yellow’’ serpentinizing microfractures
tend to make small angles with the girdles outlined by olivine [010] axes (Figures 10e and 10f). This indicates that these two families of initial microfractures preferentially make large angles with the olivine (010)
planes. Sample SMS-DR17-04-62 shows a similar pattern (Figure 10b).
Therefore, in the six samples considered for this study, we did not ﬁnd a consistent geometrical relationship
between the olivine CPO and the initial serpentinizing microfractures. The lack of a systematic relationship
between olivine CPO and the geometry of initial serpentinizing microfractures is also observed at the scale
of individual olivine grains. In section 3, we described the use of a U-stage to measure thin serpentinizing
microfractures within olivine relicts. We did not ﬁnd a systematic relationship between the strike and dip of
these microfractures and the crystallography of the host olivine.
8. Partial Reorientation of the Initial Serpentinizing Microfractures at ODP Site
920, Using Paleomagnetic Constraints
Long pieces of core drilled at ODP Site 920 are parallel to the drilling direction and therefore close to vertical.
However, these pieces can rotate about the vertical axis during recovery. A reorientation of these long pieces
of core at ODP site 920 was proposed based on paleomagnetic data [Cannat et al., 1995b; Hurst et al., 1997].
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It has signiﬁcant uncertainties in terms of absolute reorientation in the geographic frame [Hurst et al., 1997],
but provides robust constraints on the reorientation of long pieces of core relative to one another.
Harzburgites drilled at ODP site 920 are crosscut by successive generations of nearly planar serpentine veins
[Dilek et al., 1997; Andreani et al., 2007]. Among them, the so-called ‘‘V2 veins’’ are the most widespread. They
are white on the core cut surfaces (Figure 11a), commonly sigmoidal in shape, and ﬁlled with chrysotile ﬁbers
[Dilek et al., 1997; Andreani et al., 2007] that grow perpendicular to the vein contacts (Figure 11c). The reorientation of these V2 veins based on core demagnetization data [Hurst et al., 1997] shows that they have a statistically consistent geometry, with a 30 average dip throughout the 200 m deep Hole 920D. Hole 920B, drilled
only 20 m away, shows a similar pattern but is only 100 m deep. Hurst et al. [1997] further proposed that if the
magnetic declination was 0 when the magnetization was acquired, V2 veins in the cored section can be reoriented to a dominant north to northwest strike and an eastward 30 dip (Figure 11d).
In this section, we propose a reorientation, at the drill hole scale, of the initial serpentinizing microfractures
at ODP site 920. For this we examined the geometrical relations between the V2 veins and the microscopic
serpentinizing microfractures. We did this in 19 samples that contain V2 veins and correspond to core
pieces sufﬁciently long to have preserved the orientation of the vertical (Table S1). Onboard ODP Leg 153,
cores were preferentially cut orthogonal to macroscopic V2 vein planes so that the dip of these veins in the
cut plane of the long pieces of core is the true dip. Our 19 selected samples are serpentinized harzburgites
with coarse-grained equigranular or porphyroclastic primary textures and equant or ribbon-shaped serpentine mesh textures. Serpentinization extents are >50%. Figure 11a to c shows one of these samples, with a
ribbon-shaped serpentine mesh texture. In thin section, it is clear that the V2 veins are parallel to the dominant (‘‘blue’’) set of serpentinizing microfractures (Figure 11c). We found this to be the case in all samples,
with the V2 veins parallel to one of the two orientations of initial serpentinizing microfractures (‘‘blue’’ or
‘‘yellow’’). Since V2 veins have consistent dips and orientations at the 200 m scale of ODP Hole 920D [Hurst
et al., 1997] (Figure 11d), this must be also the case for at least one set of initial serpentinizing
microfractures.
9. Discussion
In this study, we examined the serpentine meshwork in 278 samples of serpentinized peridotites from several locations along slow spreading ridges (Figure 2). We then carried out a detailed microstructural, SEM,
EBSD, and Raman study of the serpentine mesh in a selection of representative samples (Table 1). We ﬁnd
that the characteristic dimensions of the serpentine mesh and the geometry of the network of microfractures that controls the formation of this mesh are similar at all studied locations. This suggests that mechanisms of microfracturing and serpentine mesh texture development are also similar. The observed
microfracture network is consistently made of two sets of initial serpentinizing microfractures that make an
angle of <55 and commonly extend across several adjacent grains of olivine and pyroxene. Each set of initial microfractures is associated with shorter orthogonal microfractures that we interpret, following [Iyer
et al., 2008; Jamtveit et al., 2009; Kelemen and Hirth, 2012; Pl€
umper et al., 2012], as hierarchical cracks due to
serpentinization-induced volume increase. Another characteristic of all samples is that, although serpentinizing microfractures form tight networks (typical spacing 60 mm for microfractures that have a rim of serpentine >2 mm; section 5 and Figure 8), those microfractures that have serpentine thicknesses >10 mm and
thus may have experienced larger ﬂuid/rock ratios, due to higher time-integrated ﬂuid ﬂux, are more widely
spaced (100–400 mm; section 5 and Figure 9). In this discussion, we address two questions: (1) what processes cause the initial microfracturing of the peridotite? (2) What are the characteristic scales of the ﬂuid
pathways from the onset of serpentinization to the fully developed mesh texture? For both questions, we
consider how the processes involved may ﬁt in with the tectonic and hydrothermal evolution of mid-ocean
ridge detachment faults.
9.1. Initial Microfracturing of the Peridotite
9.1.1. Observational Constraints and Possible Processes
We consider the four following observational constraints: (1) The initial and the hierarchical serpentinizing
microfractures are pervasive and closely spaced in all samples: 60 mm based on the statistical analysis illustrated in Figure 8c, consistent with SEM observations (Figure 7). This spacing is independent of, and much
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Figure 11. Relative reorientation of initial serpentinizing microfractures at ODP site 920. (a) Example of a long piece of core (sample 920D 13R3, 129–135 cm, piece 19; see Table S1) cut
perpendicular to macroscopic V2 serpentine veins. (b) Thin section located in Figure 11a. Scan, photographed in polarized light, showing a well-developed ribbon-shaped mesh texture.
(c) Microphotograph located in Figure 11b, photographed in polarized light with the wave plate inserted. Inset shows the overall orientation of the traces of ‘‘blue’’ and ‘‘yellow’’ serpentinizing microfractures at thin section scale. Serpentine ribbons (fast axis of lizardite subperpendicular to axis of wave plate) and the corresponding ‘‘blue’’ microfractures are parallel to V2
veins (fast axis of chrysotile subparallel to axis of wave plate). (d) Reoriented azimuths and dips of V2 veins in ODP hole 920D based on paleomagnetic data [Hurst et al., 1997]. The proposed absolute reorientation has signiﬁcant uncertainties [Hurst et al., 1997]. However, the relative reorientation is robust and indicates that V2 veins have a consistent orientation and
dip over the 200 m of the drilled section. We have examined seven of these reoriented V2-bearing samples (shown as diamonds in Figure 11d) and conﬁrmed that V2 veins are parallel
to one orientation of initial serpentinizing microfractures.
less than, the size of the original olivine grains. (2) Initial serpentinizing microfractures develop in two orientations that intersect at angles < 55 . SEM images (Figure 7) show that there is neither displacement across
these fractures nor any indication for a chronology in the development of the two orientations. We therefore propose that they formed at the same time (conjugate microfractures). (3) Initial serpentinizing microfractures have consistent geometries over thin section and sample scales. In the one location where we had
access to suitable sampling (ODP drill site 920), these microfractures appear to have consistent trends and
dips over a 200 m thick downhole section. (4) We did not ﬁnd a systematic relationship between the trend
and dip of initial serpentinizing microfractures and the olivine CPO. Our ﬁndings therefore differ from the
results of Boudier et al. [2010] who found a preferred orientation of serpentinizing microfractures perpendicular to the olivine [010] axis (highest value of thermal expansion coefﬁcients [Suzuki, 1975; Bouhifd et al.,
1996]) in a sample from the Oman ophiolite.
We now examine how these constraints ﬁt with the two proposed methods for formation of serpentinizing
fractures in peridotites: anisotropic thermal contraction of olivine [deMartin et al., 2004; Boudier et al., 2010]
and reaction-induced hierarchical fracturing due to volume increase during serpentinization [Iyer et al.,
2008; Jamtveit et al., 2009; Rudge et al., 2010; Kelemen and Hirth, 2012; Pl€
umper et al., 2012]. Because the
abyssal samples we are considering come from the footwall of large offset faults (Figure 1), we will also
examine microfracturing due to tectonic deviatoric stresses as a third possible mechanism.
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9.1.2. Reaction-Induced Microfracturing
Initial serpentinizing microfractures are not mutually perpendicular and do not form T-junctions such as
expected for local reaction-induced hierarchical microfracturing [Iyer et al., 2008; Jamtveit et al., 2009;
Pl€
umper et al., 2012]. They could be extensional cracks caused by serpentinization of neighboring peridotite
domains as described by O’Hanley [1992] and many other references cited in the previous paragraph. However, they are very closely spaced which would suggest that these neighboring domains would be both
small and nearby. Such closely spaced cracks are, for example, observed in plagioclase-rich microdomains
next to serpentinized olivine grains in troctolitic rocks [e.g., Jamtveit and Hammer, 2012], but they are radial
at thin section scale and therefore not consistent with observational constraint 3 (constant orientations of
initial serpentinizing microfractures at sample and, at least at ODP Site 920, 100 m scale). Initial serpentinizing microfractures are also conjugate which do not ﬁt well with them being purely extensional cracks.
9.1.3. Thermal Cracking
The observation that serpentinizing microfractures do not have a systematic preferred orientation relative
to olivine CPOs (observational constraint 4) is different from observations in Oman that Boudier et al. [2010]
suggested were consistent with their thermal cracking hypothesis. The close spacing and pervasive distribution of initial serpentinizing microfractures are, however, potentially consistent with a role for stresses
induced by thermal contraction. Fredrich and Wong [1986] published a thorough review of thermal cracking
experiments on a variety of rock types and developed a 2-D micromechanical interpretation. Another extensive review including more recent experimental work may be found in deMartin et al. [2004] who speciﬁcally
addressed thermal cracking of peridotites at slow spreading mid-ocean ridges. Micromechanical models
that adequately predict experimental results propose that stresses due to thermal contraction mismatch (in
polymineral aggregates or in monomineral aggregates with thermal contraction anisotropy) build up at
grain boundary and intragranular ﬂaws as temperature decreases and viscous stress dissipation slows.
Cracking occurs when a critical stress intensity factor is reached at these local stress singularities. Crack density is expected to increase with the differential of temperature applied and crack length increases with
grain size. Although crack initiation is modeled as occurring preferentially at grain boundaries, intragranular
and transgranular cracks (as observed in our samples, e.g., Figure 7) are frequent in the experimental
material.
9.1.4. Tectonic Cracking
A role for tectonic deviatoric stresses is consistent with the observation that initial serpentinizing microfractures form two sets intersecting at less than 55 . This would ﬁt an interpretation of these microfractures as
conjugate Riedel structures formed in a dominantly non coaxial deviatoric stress ﬁeld [e.g., Katz et al., 2004].
The absence of displacement along these microfractures would then argue for a formation at stress levels
lower than the yield stress [e.g., Tapponnier and Brace, 1976]. Peridotites cooling into the brittle regime in
the region closest to the detachment (Figure 1) may undergo such pervasive cracking while deviatoric
stresses build up to ultimately equal the rock strength and cause localized failure. We ﬁnd, however, that
this far ﬁeld tectonic stresses hypothesis is difﬁcult to reconcile with initial serpentinizing microfractures
being so closely and pervasively spaced in all the samples we studied. This characteristic seems more consistent with a crystal-scale mechanism for stress build up, which points back to anisotropic thermal contraction as a suitable mechanism. In order to reconcile these seemingly contradictory constraints, we propose
as a working hypothesis that initial serpentinizing microfractures in our slow spreading ridge samples
formed due to local stress concentrations caused by a combination of anisotropic thermal contraction and
deviatoric tectonic stresses lower than the rock strength.
9.1.5. Formation of Initial Serpentinizing Microfractures Next to Mid-Ocean Ridge Detachments
The addition of tectonic and thermal stresses proposed in the previous paragraph is actually a predictable
consequence of tectonic exhumation. As exhumation proceeds, the axial detachment fault acts as conveyor
belt bringing newly fractured peridotites from the mantle to the domain of active hydrothermal circulation
(Figure 1). Strain localizes in the fault, but the footwall is subjected to signiﬁcant shear stresses and to bending stresses [e.g., Lavier et al., 1999], while the temperature of a given volume of peridotite decreases, leading to thermal contraction. In Figure 12, we follow deMartin et al. [2004] and infer that the base of the
elastic domain in peridotites on axis corresponds with the 800 C isotherm. We further infer that the temperature interval for thermal stress build up is between 800 C and 400 C, the maximum temperature for initial serpentinization [e.g., Agrinier and Cannat, 1997; Fr€
uh-Green et al., 2004]. Contraction due to cooling over
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onset of serpentinization,
hierarchical microfracturing,
mesh texture development
oth
ui
ds
fault
r
hyd
Domain of
active
serpentinization
~500 µm
er
mal fl
crust
initial microfracturing
Brittle
domain
400°C
mantle
Ductile
domain
800°C
exhumation
onset of microfracturing
Figure 12. Conceptual sketch of microfracturing and development of a serpentine mesh texture in the footwall of a mid-oceanic detachment fault. The sketch corresponds to the domain within a few hundred meters of the fault, as indicated in the inset in Figure 1. Fresh peridotite is progressively exhumed and passes from the ductile domain into the brittle domain. There, the combination of anisotropic
thermal contraction and tectonic stresses leads to pervasive microfracturing. At temperatures <400 C, ﬂuid inﬁltration along these initial
microfractures causes serpentinization and the formation of additional, hierarchical microfractures. Provided that ﬂuid supply is sufﬁcient,
the serpentine mesh texture may then be fully developed (i.e., to serpentinization extents of 70–80%) in times that could be as short as a
few years (see text). Alternatively, local heterogeneities in ﬂuid ﬂuxes may cause domains of fresher peridotites to be preserved. Stresses
due to serpentinization-induced volume increase and/or tectonics may cause cracks to develop and allow enhanced ﬂuid ﬂuxes and
hydrothermal cooling in previously less permeable rocks. As a result, we expect that the mesh-texture formation stage of serpentinization
at slow spreading ridges occurs in a nonsteady state and spatially heterogeneous fashion during tectonic exhumation of the peridotites.
the 800 C–400 C temperature interval should generate 1% porosity [Bouhifd et al., 1996]. If our hypothesis
is correct, this will be complemented by a component of tectonic dilatancy. Initial serpentinizing microfractures that have thin serpentine rims (and have therefore retained their original geometry; e.g., Figure 7) typically extend a few hundred mm and their dominant sets commonly crosscut each other. This would generate
a connected porosity open to ﬂuid transport once the cracked peridotites get into the hydrothermal domain.
This permeable network of conjugate microfractures has a preferred orientation, at least at scales of a few
decimeters, and possibly of hundreds of meters (c.f. ODP Site 920). If our interpretation is correct, this preferred orientation would be inﬂuenced both by preexisting olivine CPOs, and by the orientation of tectonic
stress, in proportion of the misorientation and of the relative magnitude of tectonic versus thermal stresses.
The reorientation proposed (Figure 11) for initial serpentinizing microfractures at ODP Site 920 is therefore
not necessarily representative of what happens at mid-ocean ridge detachment faults in general. However,
since it is our only example, in which the present-day dip of one set of initial microfractures is known, it is
worth noting that in this particular instance this dip is moderate. If the absolute reorientation of the ODP
site 920 cores discussed by Hurst et al. [1997] is correct, this set of microfractures would have a dominantly
north to north-northwest strike and an eastern dip, near parallel to the emerging detachment fault at this
location [Karson and Lawrence, 1997].
The samples from the Southwest Indian Ridge present crosscutting relationships (Figures 7 d and 7e) that
suggest a separation in time between the formation of discrete brittle shear zones in the peridotite and the
later formation of the initial serpentinizing microfractures. A working hypothesis is that the observed brittle
shear zones formed at the ﬁnal stages of plastic deformation. Subsequent deformation would then focus in
the detachment fault, and tectonic stresses would decrease below yield strength in the bulk of the footwall,
allowing for thermal stresses to accumulate with progressive cooling as sketched in Figure 12.
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The thickness of the layer, over which cracking is inferred (Figure 12), is expected to be variable, depending
on the efﬁciency and geometry of hydrothermal heat extraction. Its depth is at least 8 km because microseismicity typically extends to this depth at the axis of the Mid-Atlantic Ridge [Toomey et al., 1985; Wolfe et al.,
1995; deMartin et al., 2007]. A recent microseismicity study at the Knipovich Ridge [Schlindwein et al., 2013]
suggests that the brittle to ductile transition could be as deep as 20 km along some portions of ultraslow
spreading ridges. deMartin et al. [2004] estimated a maximum depth of 4–6 km for thermal cracking at a slow
spreading mid-ocean ridge based on the balance between the thermal stress intensities they calculated with
the micromechanical model of Fredrich and Wong [1986] and conﬁning pressure (that counteracts extensional
thermal stresses). This approach, however, does not take detachment faulting and the associated tectonic
stresses into account. The yield strength of rocks in the brittle domain increases with depth [Byerlee, 1978]
and exceeds the conﬁning pressure (Goetze’s criterion) [Kohlstedt et al., 1995]. Tectonic loading in the footwall
of the detachment does not reach the yield strength except in the fault zone(s), yet peridotites that are being
exhumed into the brittle domain (Figure 12) experience deviatoric stresses that can partially counteract conﬁning pressure. Introducing tectonic stresses into the picture therefore offers a way to account for the formation of initial serpentinizing fractures on axis at depths greater than estimated by deMartin et al. [2004].
The region sketched in Figure 12 represents only the portion of the detachment’s footwall that is closest to
the fault: i.e., the domain that will become the shallow seismically deﬁned crust as exhumation proceeds
(Figure 1). We do not have the samples to document serpentinization-related processes in the footwall far
from the fault, because ultramaﬁc samples from slow spreading ridges have so far been recovered either at
the seaﬂoor, or in shallow drill holes (Figure 1).
9.2. From the Onset of Serpentinization to the Fully Developed Mesh Texture
As serpentinization initiates, volume change will induce additional, reaction-driven cracking, producing an
increasingly interconnected, three-dimensional network of closely spaced microfractures for ﬂuid inﬁltration. Our results suggest that, in the case of abyssal serpentinized peridotites from slow spreading ridges,
hierarchical reaction-driven microfractures combine with the initial microfracture network. The average
dimension (60 mm) of the domains (or cells) deﬁned by these combined microfractures does not appear
to vary for serpentinization extents between 18% (the minimum local serpentinization extents found in our
set of samples) and 70% (Figure 8c). This indicates that reaction-induced microfracturing mainly occurs
between 0 and 20% serpentinization. Since the mesh rim formation stage of serpentinization is probably
isochemical for major elements [Coleman and Keith, 1971; Shervais et al., 2005; Bach et al., 2006; Paulick
et al., 2006] there should still be an increase of volume as the reaction proceeds past 20%. A possible explanation is that hierarchical fracturing of olivine relicts is suppressed, when the extent of serpentinization
exceeds 20%, because the stresses induced in the small olivine relicts (diameter < 60 mm on average) by
the ongoing reaction are dissipated by deformation of the adjacent serpentine rims and by frictional sliding
along the abundant network of microfractures.
Not all the initial and hierarchical microfractures are coated by well-developed serpentine rims. Quite a few
have very thin rims (2 mm), and the network of microfractures with well-developed rims (i.e., >10 mm)
deﬁnes larger olivine domains, or cells, typically between 100 and 400 mm in diameter as reported in the literature for serpentine mesh in samples from a variety of geodynamic contexts [Wicks et al., 1977; Prichard,
1979; Viti and Mellini, 1998]. Using mesh rim width as a proxy for the local extent of serpentinization, this
suggests that all microfractures do not act as equally efﬁcient ﬂuid pathways during meshwork-forming serpentinization. This may be because they represent poorly connected portions of the permeability network
from the start, or because they close due to reaction-induced volume increase after incipient serpentinization [e.g., Emmanuel and Berkowitz, 2006]. In any case, the relevant mesh cell dimensions (100–400 mm in
diameter) during meshwork-forming serpentinization are 2–4 times greater than the scales of initial and
hierarchical microfracturing (60 mm in diameter).
Another characteristic of mesh-textured serpentinites is that lizardite pseudocolumns forming the mesh
rims rarely reach the mesh cell centers (Figure 3). In section 6, we measured the apparent thickness of serpentine along microfractures in several well-serpentinized samples (Table 1 and Figure 9) and found an
average value of 66 6 38 mm, corresponding to serpentinization extents of 70–80%. Isotropic serpentine
ﬁlls the mesh cores at a later stage, replacing the olivine relicts [Dungan, 1977; Wicks et al., 1977; Viti and
Mellini, 1998]. Viti and Mellini [1998] proposed that mesh rims form in conditions of low ﬂuid/rock ratio
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Geochemistry, Geophysics, Geosystems
10.1002/2013GC005148
and cease to grow because the available ﬂuid has been consumed, while the replacement of the olivine
cores by small serpentine crystals forming isotropic assemblages occurs later at conditions of higher
ﬂuid/rock ratio.
Given that lizardite pseudocolumns grow from the microfractures into the serpentinizing olivine grains
[e.g., Rumori et al., 2004; Boudier et al., 2010], and assuming that none of the lizardite grew in an open microfracture via precipitation from a ﬂuid bringing Mg, Fe and Si into the rock from some other rock volume,
the widths of serpentine mesh rims represent twice the mean distance from permeable microfracture to
the reaction front where lizardite replaces olivine. If some of the lizardite did precipitate directly from ﬂuid
in an open fracture, and/or if the lizardite volume is signiﬁcantly larger than the volume of olivine that has
been replaced in a given sample, then the widths of the serpentine mesh rims represent upper bounds on
the thickness of the olivine that has been replaced. In turn, these upper bounds, divided by the time interval
of lizardite formation, will yield upper bounds on the rate of olivine replacement. Experiments carried out at
250 C–350 C by Malvoisin et al. [2012] with aggregates of 50–65 mm sized grains of San Carlos olivine have
reached 80% serpentinization in about 500 days. Using these rates for natural samples would lead, if ﬂuid
supply is adequate, to the formation of a well-developed mesh texture (i.e., full width of serpentine rims
between 30 and 140 mm, Table 1, and serpentinization extents 70–80%) in 500 to 1100 days. Allowing for
some direct precipitation of lizardite would not change the order of magnitude of these rates that are fast
compared to exhumation rates: 17 mm yr21 (Southwest Indian Ridge) or 24 mm yr21 (northern MidAtlantic Ridge) in the conveyor belt conﬁguration of Figure 1. At these plate tectonic timescales, the formation of the serpentine mesh texture can therefore be modeled as a quasi-instantaneous process occurring
when the microfractured peridotites come into contact with hydrous ﬂuids in the domain of hydrothermal
circulation (Figure 12). Alternatively, the serpentine could have formed over a longer time period, controlled
by small and intermittent supplies of aqueous ﬂuids.
9.3. Time and Space Heterogeneity of Fluid Circulations and the Development of the Serpentine
Mesh Texture
In our study, we considered evidence for ﬂuid availability at the microscopic and sample scales (<30 cm). We
show that microfracture densities are more or less constant for serpentinization extents between 18% and
70% (Figure 8c), indicating that microfracturing alone does not guarantee a ﬂuid supply sufﬁcient for mesh
texture formation to proceed to completion. Sample 920D-18R2, 113–118 cm, piece 13 is a good example,
with an extensively serpentinized ribbon-shaped mesh texture on one side of the thin section (Figure 6b) in
sharp contact with a less serpentinized domain that displays similarly spaced microfractures (Figures 7a and
7b). This indicates that at the spatial scales documented here (i.e., <30 cm), ﬂuid ﬂux is variable. Variations of
the serpentinization extents at the larger scale of the drill hole at ODP Site 920 [Dilek et al., 1997] also suggest
heterogeneities in ﬂuid ﬂux. The limit of the hydrothermally cooled domain where serpentinization occurs
(Figure 12) is therefore probably complex, with strong topography at a variety of scales.
Spatial and temporal heterogeneity of the ﬂuid circulation patterns is predicted by porous ﬂow models of
hydrothermal circulation in detachment faults and in their footwall [e.g., Fontaine et al., 2009]. These effects
will be enhanced by complexities of the permeable structure, such as due to tectonic stresses keeping speciﬁc
orientations of microfractures open. Time and space heterogeneity of ﬂuid distribution should also be
enhanced by serpentinization-induced volume increase in ﬂuid-rich domains. This will promote cracking at a
range of scales [O’Hanley, 1992; Jamtveit et al., 2008; 2009; Kelemen and Hirth, 2012; Jamtveit and Hammer,
2012] and create channels for ﬂuids into previously impermeable domains. In this way, the base of the serpentinization domain interface sketched in Figure 12 will evolve dynamically as serpentinization proceeds. Finally,
it is probable that tectonic stresses that help to create the initial microfractures in the fresh peridotites are
lower in regions of the footwall that are away from the detachment fault (Figure 1). As a result, thermal
stresses there would be more effectively counteracted by conﬁning pressure and the initial microfracture network may be less pervasive. All these factors combined argue for large time and space heterogeneity of the
mesh texture forming serpentinization in the mantle-derived peridotites exhumed at slow spreading ridges.
10. Conclusions
Our microstructural and petrographic study of the serpentine meshwork was carried out on 278 variably
serpentinized abyssal peridotites sampled at or near the seaﬂoor at the slow spreading Mid-Atlantic and
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Geochemistry, Geophysics, Geosystems
10.1002/2013GC005148
Southwest Indian ridges. Serpentinization of these samples occurred in the footwall within a few hundred
meters of axial detachment faults.
Our study reveals that the serpentine mesh texture developed from a network of microfractures that has
consistent characteristics and dimensions in all samples. This network comprises intersecting sets of initial serpentinizing microfractures that commonly extend across several grains of olivine and into adjacent pyroxenes. These initial microfractures have a preferred orientation at sample scale (and at 100 m
scale at ODP Site 920). We propose that they form due to stresses caused by a combination of anisotropic thermal contraction and tectonic stresses. These microfractures generate a connected porosity
capable of transporting hydrothermal ﬂuids. The onset of serpentinization at temperatures <400 C leads
to additional reaction-driven (hierarchical) cracking, forming a crack network with a typical spacing of
60 mm.
This study shows that all microfractures do not act as equally efﬁcient ﬂuid pathways during meshworkforming serpentinization. This is illustrated by differences in the thickness of the associated serpentine
rims. Those microfractures that have serpentine thicknesses >10 mm are spaced by 100–400 mm. The relevant cell dimensions during meshwork-forming serpentinization are therefore 2–4 times greater than the
scales of initial and hierarchical microfracturing. Serpentine thickness, representing twice the distance
between the permeable microfracture and the reaction front, can be used as an upper bound for the
advancement of mesh-forming serpentinization. In the most serpentinized samples, lizardite selvages on
microfractures have an average apparent thickness of 33 6 19 mm, corresponding to a serpentinization
extent of 70–80%.
Based on published laboratory experiments, the formation of these mesh rims could be completed in a few
years and therefore be quasi instantaneous at the plate tectonic timescale. We propose that formation of
the serpentine mesh texture in ultramaﬁc rocks exhumed at slow spreading ridges occurs near the base of
the hydrothermally cooled domain adjacent to the detachment faults (Figure 12). We envision that the
domain of active serpentinization is limited by the 400 C isotherm and that this limit is complex, with
strong topography at a variety of scales. We expect that it is also highly variable in time, due both to timedependant hydrothermal circulation in the detachment footwall, and to reaction-induced cracking allowing
for enhanced ﬂuid ﬂux into previously less permeable domains. As a result, we expect that mesh-texture formation during serpentinization at slow spreading ridges occurs in a nonsteady state and spatially heterogeneous fashion during tectonic exhumation of the peridotites.
Acknowledgments
The authors thank Françoise Boudier,
David Mainprice, Benedicte M
enez,
Mark Van Zuilen, Gianreto Manatschal,
and Javier Escartın for their help and
insight. This manuscript beneﬁted
from constructive reviews from Peter
B. Kelemen, Bruno Reynard, and an
anonymous reviewer. This work was
supported by the Agence Nationale de
la Recherche (Rift2Ridge project). This
is IPGP contribution number 3502.
ROUMEJON AND CANNAT
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