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

RESEARCH ARTICLE
10.1002/2013GC005148
Serpentinization of mantle-derived peridotites at mid-ocean
ridges: Mesh texture development in the context of tectonic
exhumation
St ephane Roum ejon
1
and Mathilde Cannat
1
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 oli-
vine relicts and forms a mesh texture. We present a petrographic study (SEM, EBSD, and Raman) of the ser-
pentine 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 33 619 mm corresponding to serpentiniza-
tion extents of 70–80%. Published laboratory experiments suggest that mesh rims formation could be com-
pleted 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.,
1992, 1995b, 1997; Kelemen et al., 2004; Gr acia et al., 2000], the Gakkel ridge [Michael et al., 2003], and the
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 consist-
ent with rotations of 46–80in the footwall of Mid-Atlantic Ridge detachments [Garc
es and Gee, 2007; Mor-
ris 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 serpentiniza-
tion are active.
Serpentinization has significant geodynamic and environmental consequences. It modifies the rheology of
the oceanic lithosphere [Reinen et al., 1994; Escart ın 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 CO
2
[Berndt et al., 1996] to produce meth-
ane 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
Key Points:
Serpentinization occurs along
thermal, tectonic, and reaction-
induced fractures
Serpentinization may occur quasi
instantly at exhumation time scales
The depth and extent of
serpentinization are variable in space
and time
Supporting Information:
ReadMe
Table S1_v3
text_Auxiliary_material
Fig_annexe_opx_reviews
Fig_annexe_EBSDmaps
Correspondence to:
S. Roum ejon,
[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
ROUM EJON AND CANNAT V
C2014. American Geophysical Union. All Rights Reserved. 2354
Geochemistry, Geophysics, Geosystems
PUBLICATIONS

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 mid-
ocean 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 spread-
ing 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 intru-
sions [Cannat, 1993]. Increasing serpentinization results in a linear decrease of both seismic velocity and
density, from nearly 8 kms
21
and 3300 kg m
23
at 0% of serpentinization to 5 kms
21
and 2500 kg m
23
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 serpenti-
nized 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]). Ser-
pentinized 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 repre-
sentative of the deeper levels of the fault’s footwall (Figure 1).
Vp
Vp
S
2468Vp (km/s) 0
50 100
Serpentinization (%)
0
exhumation
crust
mantle
ridge axis
~5 km
2-4km
8 km
dredge
drill hole
footwall
rotation
domain of axial
hydrothermal
circulation
Fig12
detachement fault
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).
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The study presented here is based on a large set of serpentinized peridotites samples from slow and ultra-
slow 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 [Can-
nat 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 prop-
erties 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 subse-
quent 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 pre-
dates 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 typi-
cally 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 pro-
gresses from microfractures, individualizing the mesh cells, toward their centers [Maltman, 1978]. These ser-
pentinizing 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
Figure 2. Location of the studied ultramafic samples. For the Mid-Atlantic Ridge: ODP Site 920 at 23210N[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 typi-
cal serpentinized harzburgite.
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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 serpentiniza-
tion [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 detach-
ment 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 com-
posed 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 sam-
ples 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 Phy-
sique 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 occa-
sionally occur as intergrowths in the serpentine [Bach et al., 2004]. The iron released during the olivine ser-
pentinization 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 crystal-
lographic 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 simi-
lar configuration in serpentinized peridotites from the Ligurian and Oman ophiolites. Systematic observa-
tion 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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Table 1. Characteristics of the 12 Serpentinized Peridotites Selected (Out of 278 Samples) for Detailed Analyses of the Serpentine Mesh Texture
a
Serpentinization %
Apparent Thickness of Serpentine Along
Microfractures (mm)
Apparent Spacing of
Microfractures (mm)
Whole TS Mesh Cells
Area/
Initial Hierarchical Initial Hierarchical
Sample Protolith
Primary
Texture
Mesh
Texture Subdomains
EBSD
Maps
Visual
Estimate
Perimeter
6std ‘‘Yellow’’ ‘‘Blue’’ ‘‘Red’’ ‘‘Green’’ ‘‘Yellow’’ ‘‘Blue’’ ‘‘Red’’ ‘‘Green’’
ODP 920 (2320.30N45
01.00W)
B 11R2 118–127
(piece 16)
Harzburgite Porphyroclastic Equant/ribbon
shaped
42 75 20 613
D 14R2 114–120
(piece 7C)
Harzburgite Coarse-grained
equigranular
Equant 63 46 15 68
D 18R2 113–118
(piece 13)
Harzburgite Porphyroclastic Ribbon shaped/
equant
30 45–85 15 611 45 620 80 635 45 615 30 610 320 6110 135 660 170 695 170 685
ODP 1274 (1443.30N44
53.30W)
A 05R1 119–122
(piece 10)
Harzburgite Coarse-grained
equigranular
Equant/ribbon
shaped
60
A 08R1 114–117
(piece 16)
Dunite Very coarse
grained
Equant 75
A 17R1 119–121
(piece 20)
Harzburgite Coarse-grained
equigranular
Equant 95
SE_DR08 (1444.8’N 4458.1’W)
07–02 Harzburgite Porphyroclastic Equant 90 100 25 615 130 660 140 650 100 650 80 635 315 6150 400 6165 325 6175 255 6190 07–32 Harzburgite Porphyroclastic Equant 87 74 26 615 SMS_DR17 (2833.4’N 6221.2’E)
04-04
b
Harzburgite Coarse-grained
equigranular
Equant/ribbon
shaped
76 81 21 613 115 655 100 645 75 635 70 625 360 6170 295 6120 265 6140 265 6150
26 615 19 613
04–41 Harzburgite Coarse-grained
equigranular
Equant 31 30 15 69
04–62 Harzburgite Porphyroclastic Equant/ribbon
shaped
66 70 15 69
04–66
b
Dunite Very coarse
grained
Equant 67 75 17 610 55 615 50 615 40 615 55 620 370 6170 320 6160 150 675 135 670
13 610 58 624 57 622 41 619 40 616 237 6107 282 6169 153 678 183 6100
17 612 60 619 71 631 42 611 47 621 366 6166 324 6136 164 669 160 685
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.
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