C. R.

Geoscience 335 (2003) 825852

Geomaterials (Petrology)

Serpentinization of abyssal peridotites at mid-ocean ridges

Catherine Mvel
Laboratoire de Gosciences marines, CNRS UMR 7097, IPGP, 4, place Jussieu, 75252 Paris cedex 05, France

Received 31 March 2003; accepted 28 July 2003

Written on the invitation of the Editorial Board

Abstract
Serpentinites are an important component of the oceanic crust generated in slow to ultraslow spreading settings. In this
context, the MOHO likely corresponds to a hydration boundary, which could match the 500 C isotherm beneath the ridge axis.
Textures from serpentinites sampled in ridge environments demonstrate that most of the serpentinization occurs under static
conditions. The typical mineralogical association consists of lizardite chrysotile + magnetite tremolite talc. Despite
the widespread occurrence of lizardite, considered as the low temperature serpentine variety, oxygen isotope fractionation
suggests that serpentinization starts at high temperature, in the range of 300500 C. The fluid responsible for serpentinization
is seawater, possibly evolved by interaction with the crust. Compared with fresh peridotites, serpentinites are strongly hydrated
(1015% H2 O) and oxidized. Serpentinization, however, does not seem to be accompanied by massive leaching of major
elements, implying that it requires a volume increase. It results in an increase in chlorine, boron, fluorine, and sulfur, but
its effect on other trace elements remains poorly detailed. The presence of serpentinites in the oceanic crust affects its
physical properties, in particular by lowering its density and seismic velocities, and modifying its magnetic and rheological
properties. Serpentinization may activate hydrothermal cells and generate methane and hydrogen anomalies which can sustain
microbial communities. Two types of hydrothermal field have been identified: the Rainbow type, with high temperature (360 C)
black smokers requiring magmatic heat; the low temperature (4075 C) Lost City type, by contrast, can be activated by
serpenintization reactions. To cite this article: C. Mvel, C. R. Geoscience 335 (2003).
2003 Acadmie des sciences. Published by ditions scientifiques et mdicales Elsevier SAS. All rights reserved.
Rsum
Serpentinisation des pridotites abysales aux dorsales ocaniques. Les serpentinites reprsentent un constituant important
de la crote ocanique forme aux dorsales lentes et ultralentes. Dans ce contexte, le MOHO pourrait correspondre un front
dhydratation et ventuellement lisotherme 500 C sous laxe. Lobservation des textures des serpentinites chantillonnes
aux dorsales montre que lessentiel de la serpentinisation a lieu en rgime statique. Lassociation minrale type comprend
lizardite chrysotile + magnetite tremolite talc. Malgr la prsence de lizardite considre comme lespce serpentineuse
de basse temprature, le fractionnement isotopique de loxygne montre que la serpentinisation commence temprature leve,
de lordre de 300 500 C. Le fluide responsable de la serpentinisation est majoritairement leau de mer, qui a pu voluer
par raction avec la crote. Par rapport aux pridotites fraches, la serpentinisation se traduit par une hydratation massive
et loxydation du fer. Elle ne semble pas tre accompagne dchanges importants en lments majeurs, ce qui implique
quelle entrane une augmentation de volume. Elle saccompagne dun enrichissement en chlore, bore, fluor, soufre. Son
effet sur les autres lments en traces reste mal contraint. La prsence de serpentinites dans la crote ocanique affecte ses

E-mail address: mevel@ipgp.jussieu.fr (C. Mvel).

1631-0713/$ see front matter 2003 Acadmie des sciences. Published by ditions scientifiques et mdicales Elsevier SAS. All rights
reserved.
doi:10.1016/j.crte.2003.08.006
826 C. Mvel / C. R. Geoscience 335 (2003) 825852

proprits physiques, en particulier en abaissant la densit et les vitesses de propagation des ondes sismiques, et en modifiant
ses proprits magntiques et rhologiques. La serpentinisation peut activer des cellules de convection hydrothermales et
provoquer des anomalies de mthane et dhydrogne, capables dalimenter des communauts microbiennes. Deux types de
champs hydrothermaux ont t mis en vidence : le type Rainbow , fumeurs noirs de haute temperature (360 C), ncessite
un apport de chaleur magmatique ; le type Lost City de basse temprature (4075 C) peut tre activ par la seule raction
de serpentinisation. Pour citer cet article : C. Mvel, C. R. Geoscience 335 (2003).
2003 Acadmie des sciences. Published by ditions scientifiques et mdicales Elsevier SAS. All rights reserved.

Keywords: serpentinization; oceanic lithosphere; hydrothermalism

Mots-cls : serpentinisation ; lithosphre ocanique ; hydrothermalisme

Version franaise abrge de laxe (Fig. 1). Aux extrmits des segments, elle
est constitue de pridotites plus ou moins serpentini-
La prsence de pridotites serpentinises sur le ses recoupes par des intrusions gabbroques (Fig. 2).
plancher ocanique est connue depuis longtemps mais Dans ce contexte, le MOHO, discontinuit sismique
ce nest que rcemment, grce une meilleure connais- qui spare la crote du manteau, pourrait correspondre
sance de la gologie des fonds ocaniques, que lon un front de serpentinisation. Dans un contexte de li-
a commenc comprendre les mcanismes respon- thosphre paisse, les pridotites serpentinises peu-
sables de leur mise laffleurement. Les serpenti- vent tre galement mises laffleurement par le jeu
nites sont un constituant majeur de la crote oca- de grandes failles faible pendage. Cette nouvelle
nique forme aux dorsales lentes et ultralentes. Elles perspective sur le fonctionnement des dorsales lentes
affleurent galement la transition continent-ocan, montre que les serpentinites ny sont pas cantonnes
o leur prsence est le rsultat de la dchirure oca- aux grandes failles transformantes, mais reprsentent
nique dans un contexte de faible production magma- un constituant majeur de la crote. Au contraire, aux
tique (par exemple la marge Ibrique). Enfin, on les dorsales rapides, le modle PENROSE semble sap-
rencontre au niveau des zones de subduction comme pliquer. Les pridotites serpentinises naffleurent que
la fosse des Mariannes. Elles rsultent alors de la re- dans des contextes tectoniques particuliers.
monte diapirique de manteau hydrat provenant soit Les nombreux travaux effectus sur les pridotites
de la plaque plongeante, soit du coin mantellique sus- serpentinises collectes aux dorsales, et en particu-
jacent. Dans cette revue ne seront prises en compte que lier dans le cadre du programme de forages ocaniques
les serpentines des dorsales ocaniques, dont la forma- ODP, montrent des caractres constants dans les tex-
tion est lie aux processus de laccrtion. tures et la minralogie. Le taux de serpentinisation
Aux dorsales lentes et ultralentes, la prsence dune est gnralement lev (80100%). La serpentinisa-
lithosphre paisse rend plus difficile la remonte vers tion a eu lieu en rgime statique (Fig. 3A), mme si
la surface du magma produit par la fusion partielle. les veines sont nombreuses (Fig. 3B et 3G). Les tex-
De nombreuses tudes, tant dans la dorsale mdio- tures originelles des pridotites sont prserves : les
Atlantique que la dorsale sud-ouest-Indienne et plus olivines sont remplaces par de la serpentine maille
rcemment la ride de Gakkel dans lArctique, ont (Fig. 3C et 3D) et les pyroxnes par de la serpentine
montr que, dans ce contexte, le magma tend se foca- en bastites (Fig. 3E). Amphibole et talc sont parfois
liser au centre des segments, et les serpentinites affleu- associs aux bastites (Fig. 3F). Les serpentines schis-
rent frquemment dans la valle axiale aux extrmits teuses (Fig. 3H) sont rares.
des segments. La crote ocanique forme dans cet en- La serpentine, phyllosilicate de silice et magnsium
vironnement ne rpond pas au modle PENROSE de hydrat, est le minral le plus abondant. Sa formation
lithosphre lite, caractris par une superposition de rsulte de laltration de lolivine et des pyroxnes par
couches (Couche 2 = basaltes, Couche 3 = gabbro un fluide hydrat, raction qui produit galement de
et, sous le MOHO, Couche 4 = pridotites mantel- la magntite et de lhydrogne. Brucite, divers types
liques). Son architecture est htrogne et varie le long damphiboles, talc, chlorites, carbonates peuvent ga-
 C. Mvel / C. R. Geoscience 335 (2003) 825852 827

lement tre prsents. Des trois varits de serpentine en dehors de lhydratation et de loxydation du fer,
(Fig. 4), la lizardite est la plus frquente. Le chryso- ainsi que de mtasomatisme local (formation de talc,
tile lui est parfois associ, mais est surtout prsent en rodingitisation), et militent donc plutt en faveur de
veines. Lantigorite est rare. Labondance de la lizar- la seconde hypothse. En revanche, certains lments
dite semble rsulter de sa capacit accepter les sub- traces sont trs affects. En particulier, les serpenti-
stitutions. nites sont enrichies en Cl, B, S, F. Le comportement
Les conditions de la serpentinisation aux dorsales des terres rares est encore mal contraint, de mme que
ocaniques sont difficiles dterminer en labsence celui de la plupart des isotopes radiogniques. Ces l-
dindicateurs prcis, et parce que les roches mon- ments sont surtout concentrs dans les pyroxnes et en
trent la superposition de diffrents stades. La pres- consquence, les rsultats sont certainement fonction
sion est ncessairement faible puisque la profondeur de leur comportement au cours de la serpentinisation.
du MOHO, au-del duquel les serpentinites ne doi- Une tude systmatique reste faire pour bien caract-
vent plus exister, ne dpasse gure les 6 km. Des don- riser les flux de matire entre pridotites et eau de mer
nes exprimentales et thermodynamiques suggrent au cours de la serpentinisation.
des tempratures dcroissantes pour les limites de sta- La serpentinisation a des consquences majeures
bilit respectives de lantigorite, du chrysotile et de sur les proprits physiques. Par rapport une pri-
la lizardite (Fig. 5). Cependant, labondance de la li- dotite frache, la chute de densit provoque une baisse
zardite est en dsaccord avec les tempratures le- des vitesses sismiques. Il existe une relation inverse
ves dduites dautres indicateurs. Les autres min- entre le taux de serpentinisation et la densit ainsi que
raux associs aux serpentines suggrent gnralement
la vitesse de propagation des ondes sismiques (Fig. 8).
des conditions compatibles avec le facis schiste vert
Ceci explique lexistence du MOHO, mme dans le
(300500 C). Des donnes plus prcises peuvent tre
cas o des roches dorigine mantellique (pridotites
obtenues avec le fractionnement des isotopes de loxy-
serpentinises) affleurent sur le plancher ocanique.
gne (Fig. 6). Mais les courbes de calibration sont en-
Dans ce cas, le MOHO doit correspondre un front
core mal contraintes. Le Tableau 1 rsume les rsultats
dhydratation voire, dans la mesure o la temprature
obtenus. Ils montrent un grand ventail de tempra-
maximum de stabilit de la serpentine est de 500 C,
tures (de 500 C moins de 100 C), qui suggre une
lisotherme 500 C. Les proprits magntiques des
grande diversit dans les conditions de serpentinisa-
tion, mais souligne que la serpentinisation commence pridotites sont galement affectes. tant donn que
haute temprature (350500 C). Les serpentinites la serpentinisation produit de la magntite secondaire,
contiennent 1015% deau et la phase fluide respon- les serpentinites acquirent une magntisation rma-
sable de la serpentinisation est constitue principale- nente naturelle (RMN) et donc peuvent contribuer aux
ment deau de mer. Mais les compositions isotopiques anomalies magntiques du plancher ocanique. Une
de loxygne, de lhydrogne et du strontium (Fig. 7) tude de dtail des proprits magntiques des serpen-
suggrent quil sagit deau de mer qui a volu par tinites a cependant montr que lintensit de la sus-
raction avec la crote. ceptibilit magntique naugmentait pas linairement
La serpentinisation, qui saccompagne dune forte avec le taux de serpentinisation. En effet, elle dpend
hydratation, provoque une baisse de la densit qui de capacit des minraux serpentineux incorporer du
chute de 3,3 2,5. Cette constatation a engendr de fer, qui contrle la quantit de magntite produite. En-
nombreux dbats concernant les consquences de la fin, la prsence de serpentine modifie les proprits
serpentinisation sur les flux dlments chimiques : rhologiques et donc a une influence sur lactivit tec-
si la baisse de densit a lieu volume constant, elle tonique.
est ncessairement accompagne dun lessivage mas- Aux dorsales ocaniques, la serpentinisation peut
sif dune partie des lments chimiques ; si elle est ac- activer une circulation hydrothermale. Les premires
compagne dune augmentation de volume, la compo- manifestations de cette activit ont t dcouvertes
sition chimique originelle peut tre prserve. Les tra- sous forme danomalies de mthane dans la colonne
vaux rcents effectus sur les serpentinites ocaniques deau, systmatiquement associes des affleurements
suggrent que les lments majeurs sont peu affects de serpentinites. La production de mthane est lie
828 C. Mvel / C. R. Geoscience 335 (2003) 825852

des ractions de type Fisher-Trops : sales lentes et ultralentes. La prsence de ces serpen-
tinites a des consquences importantes sur la compo-
CO2 + 4 H2 = CH4 + 2 H2O sition chimique de la lithosphre, mais les flux chi-
qui font intervenir lhydrogne produit pendant la miques globaux rsultant de la serpentinisation sont
serpentinisation. encore mal contraints. Elle modifie galement ses pro-
Plus rcemment, de vritables champs hydrother- prits physiques et peut contribuer aux anomalies ma-
maux ont t dcouverts sur les pridotites serpenti- gntiques du plancher ocanique. Elle implique de r-
nises. Les champs de Logatchev et de Rainbow, res- valuer les budgets magmatiques aux dorsales, dans la
pectivement localiss 14 45N et 36 14N sur la dor- mesure o la crote ocanique peut comprendre une
sale mdio-Atlantique, sont assez semblables ceux certaine proportion de roches dorigine mantellique et
installs sur les basaltes : des fumeurs noirs mettent donc rsiduelles. Des cellules de convection hydro-
des fluides chargs en particules des tempratures thermale peuvent tre actives par la serpentinisation,
de lordre de 360 C. Ils construisent des chemines soit avec la contribution de chaleur dgage par des
de sulfures et abritent des colonies biologiques. Les intrusions magmatiques, soit sous le simple effet de la
fluides ont un faible pH et sont chargs en H2 S et en raction exothermique de serpentinisation. Enfin, sur
mtaux. Cependant, par rapport aux fluides mis en le plan global, la prsence de serpentinites riches en
contexte basaltique, ils ont des teneurs leves en H+ , eau et en halognes dans la lithosphre subducte doit
rsultat des ractions de serpentinisation (Tableau 2). avoir des consquences dans les processus de dshy-
Ils contiennent galement des molcules dhydrocar- dratation de la plaque plongeante et dans les mca-
bures complexes, considres comme des molcules nismes de fusion des magmas darcs insulaires.
prbiotiques, qui font de ces sites hydrothermaux des
endroits potentiels pour lapparition de la vie sur Terre.
Mais un nouveau type dactivit hydrothermale vient 1. Introduction
dtre dcouvert au site de Lost City, 30 N hors axe
de la dorsale mdio-Atlantique. Les fluides sont mis The presence of serpentinites on the seafloor has
des tempratures beaucoup plus faibles (4075 C), been documented for a very long time. On the basis of
ont un fort pH et de faibles concentrations en H2 S this occurrence and of the seismic structure, Hess pro-
et sont chargs en MgO (Tableau 2). Ils construisent posed [73] that oceanic layer 3 consists of a hydrated
dnormes chemines constitues de brucite et de car- mantle, i.e. serpentinites, and that the uniform thick-
bonates. ness of the ocean crust corresponded to the depth of
Diffrents auteurs ont tent destimer si la chaleur the 500 C isotherm at mid-ocean ridges. The increas-
produite par la raction de serpentinisation qui est exo- ing knowledge of the oceanic lithosphere has demon-
thermique peut suffire activer une circulation hydro- strated that this picture of the ocean crust was certainly
thermale. Il ressort des calculs que la seule chaleur d- extreme, but that serpentinites do represent a signifi-
gage par la serpentinisation ne peut suffire rchauf- cant portion of the crust.
fer des fluides des tempratures de plus de 300 C All the abyssal peridotites that crop out on the
et quen consquence, de la chaleur dgage par des seafloor are more or less serpentinized. They com-
intrusions magmatiques est certainement implique monly occur in slow to ultraslow spreading ridge
Rainbow comme Logatchev. En revanche, dans le environments. In the early days, when the topogra-
cas de Lost City, la seule chaleur de serpentinisation phy of ridges was not well known, peridotites were
pourrait suffire. Les champs hydrothermaux de ce type dredged mostly from major scarps, which were es-
devraient tre frquents dans la valle axiale ou hors sentially transform fault walls [22,23,27,28,100], al-
axe, condition que leau de mer puisse pntrer dans though some also came from the axial valley [7]. It
des pridotites fraches. was also recognized that these ultramafic occurred in
En conclusion, il est maintenant dmontr que les the Atlantic and Indian oceans, i.e. at slow spread-
serpentinites, rsultat de linteraction des pridotites ing ridges, but this could be attributed to the higher
ocaniques avec leau de mer, sont un constituant im- density of fracture zones in this environment. It was
portant de la lithosphre ocanique forme aux dor- argued that hydration of the underlying mantle peri-
 C. Mvel / C. R. Geoscience 335 (2003) 825852 829

dotites was made possible by the presence of large sure of serpentinites at the ocean floor in this con-
faults, driving seawater at depth and the outcropping text are discussed. A general description of typical
of serpentinites was commonly ascribed to diapiric ocean floor serpentinites is presented. The conditions
emplacement, due to the low density of serpentine of serpentinization (temperature, composition of the
(2.5 g/cm3) with respect to the average density of the fluid phase) are evaluated. Our present knowledge of
ocean crust (2.8 g/cm3 ) [7,27,28]. It is only during the the consequences of serpentinization on the chemical
last 15 years, when the knowledge of the mid-ocean composition of the crust, and on the physical proper-
ridge structure improved, that a general understanding ties of the ocean lithosphere is presented. Finally, the
for the location of peridotites outcrops has emerged, two major types of serpentinized peridotite hosted hy-
and it has been recognized that at slow to ultraslow drothermal fields are described and their relation with
spreading ridges, serpentinized peridotites are an im- serpentinization processes are discussed.
portant component of the crust.
Serpentinites also occur in other tectonic settings at
the seafloor. They have been recorded at passive mar- 2. Occurrence of serpentinites at mid-ocean ridges
gins. Serpentinized peridotites were first documented
at the Galicia margin where they form a few km wide It is now well established that, in the global ridge
and a hundred km long ridge, at the continent-ocean system, serpentinites crop out essentially in slow to ul-
boundary [18]. Further work showed that this ridge is traslow spreading ridges [34]. In this environment, not
a major feature, which continues south to the Iberian only the amount of melting is low, but the lithosphere
margin [12]. This serpentinite ridge is interpreted as tends to be thicker. Melt delivery to the surface is
resulting from mantle exhumation due to crustal thin- episodic and tends to be focused at segment centres
ning during the initial stage of rifting. At the continent- [49]. Moreover, the thick lithosphere allows tectonic
ocean transition, the tectonic activity is inferred to activity and the development of deep rooted faults.
favour seawater penetration and serpentinization [20, Serpentinized peridotites are clearly not restricted to
21]. The serpentinites exposed at the Galicia and Iberia transform fault scarps, but also occur along the axial
margins have been drilled by the Ocean Drilling Pro- valley walls and even at the axial valley floor. A num-
gram during Leg 103 [19] and 149 [134] respec- ber of field studies suggests that, at least at segment
tively. A similar mechanism has been invoked to ex- ends, oceanic layer 3 may consist of more or less ser-
plain the occurrence of serpentinized peridotites at the pentinized peridotites intruded by gabbro pockets and
Tyrrhenian margin, drilled during ODP leg 107 [31]. directly overlain by basalts. This has been documented
A similar occurrence of serpentinized peridotites [102] at the slow spreading Mid-Atlantic Ridge (MAR) [36,
at the southwest Australian margin is interpreted as re- 71,81,88], at the ultraslow spreading South West In-
sulting from the rifting between Australia and Antarc- dian Ridge (SWIR) [8,49,97] and more recently at the
tica [13]. However, serpentinites also occur at active slowest ridge of the global system, the Gakkel ridge in
margins. They have been dredged at the Tonga trench the Arctic [98]. A thick gabbro layer as well as a dike
[61] and the Mariana trench [60]. Subsequent work at complex are often missing. At a Penrose conference
the Mariana trench showed that sheared serpentinites in 1972 [110], combining seismic studies, results from
crop out all along the trench in a chain of seamounts. seafloor dredging and observations in ophiolite com-
The exposure of these serpentinites is clearly associ- plexes, a layered structure was proposed for the ocean
ated with subduction processes and is thought to result lithosphere (Fig. 1). According to the Penrose model,
from diapiric emplacement, either from material from beneath a sediment cover, the oceanic crust is com-
the subducting plate [16] or from the mantle wedge posed of magmatic rocks. Layer 2 consists of basaltic
[67]. One of these seamounts was drilling during ODP rocks, either extruded on the seafloor as pillow-lavas
leg 125 [68]. or intruded as dikes. Layer 3 consists of gabbroic rocks
This review paper concentrates on serpentinites crystallized at depth. The seismically defined MOHO,
collected at, and in the vicinity of mid-ocean ridges, located at about 6 km beneath the seafloor, separates
i.e. where their formation is related to accretionary the crust from the underlying mantle (Layer 4) con-
processes. The mechanisms responsible for the expo- sisting in peridotites, residues of partial melting. The
830 C. Mvel / C. R. Geoscience 335 (2003) 825852

Fig. 1. Seismic velocity profiles and geological interpretation of a Penrose-type layered crust [110] and a heterogeneous crust [34]. Note that
the layered crust consists of magmatic rocks (basalts and gabbros) only, while the heterogeneous crust contains residual rocks (serpentinized
peridotites).
Fig. 1. Profils de vitesses sismiques et interprtation gologique dune crote lite de type Penrose [110] et dune crote htrogne [34].
La crote lite est constitue uniquement de roches magmatiques (basaltes et gabbros), alors que la crote htrogne comprend des roches
rsiduelles (pridotites serpentinises).

Fig. 2. Hypothetical along axis cross section of a slow-spreading ridge, from [36]. Based on geological observations at the Mid-Atlantic Ridge,
this sketch suggests a different magma supply between the centre and the ends of the segments. At the centre, the crust is supposed to be layered
and magmatic while its ends are heterogeneous, with small magma pockets enclosed in residual mantle-derived serpentinized peridotites.
Fig. 2. Coupe hypothtique le long de laxe dune dorsale lente, daprs [36]. Bas sur des observations gologiques la dorsale
mdio-Atlantique, ce schma suggre un apport magmatique diffrent entre le centre et les extrmits des segments. Au centre, la crote est
suppose lite et magmatique, alors quaux extrmits elle est htrogne, constitue dintrusions gabbroques dans des pridotites rsiduelles
serpentinises.

crust formed at a slow and ultraslow spreading ridge ping of residual peridotites at segment ends is likely to
clearly does not fit the Penrose model. It does not have result from the melt focusing at segment centres. In the
a layered structure but is heterogeneous and its archi- absence or paucity of basaltic melt, plate divergence is
tecture varies along its axis [36] (Fig. 2). The outcrop- accommodated by advection of asthenospheric man-
 C. Mvel / C. R. Geoscience 335 (2003) 825852 831

tle to the lithosphere and by tectonic processes that peridotites do not represent a significant portion of the
result in the exposure of rocks from the mantle. The crust. The presence of serpentinized ultramafics be-
tectonic activity also favours seawater penetration and neath the gabbro layer cannot be completely ruled out,
alteration of the mantle peridotites. The layered and but serpentinization must then occur off axis, when the
homogeneous seismic structure that is observed every- temperature has dropped below 500 C.
where can be explained by the fact that gabbros and Sample suites of serpentinized peridotites from
partially serpentinized peridotites have the same seis- all these environments have led to various types of
mic velocities [34] (Fig. 1, see Section 6). This sug- studies (mineralogy, petrology, geochemistry, physical
gests that when serpentinized peridotites crop out at properties). The most comprehensive data sets come
the surface, the MOHO does not correspond to the from sites drilled by the Ocean Drilling Program
magmatic crust/residual peridotite boundary, but could (ODP). Two adjacent sites (670 and 920) are located
be a hydration boundary, i.e. the boundary between at the Mid-Atlantic Ridge, in the western wall of the
serpentinized peridotites and fresh peridotites [73]. axial valley of the MARK area (23 N) and represent
More recently, very low angle faults, i.e. megamul- serpentinites from a slow spreading environment [35,
lions or detachment faults, that accommodate spread- 48]. Site 895 is located on the intrarift ridge of
ing for long periods of time (i.e. over a million years), Hess Deep, which exposes rocks generated at the fast
have been discovered at slow spreading ridges [33,91, spreading East Pacific Rise [70]. The other samples
123,127]. These long lived faults expose large portions which are referred to in this paper have been either
of the oceanic lithosphere, and serpentinites as well as dredged or collected with submersibles.
talcschists commonly crop out on their surfaces.
This new perspective on accretionary processes
at slow spreading ridges suggests that serpentinized 3. Mid-ocean ridge serpentinite textures and
peridotites make up a significant portion of the so- mineral assemblages
called oceanic crust at slow to ultraslow spreading
ridges, although at this stage, it is still difficult to Abyssal peridotites collected from the seafloor
quantify. On the basis of geological observations, consist primarily of harzbugites, with minor dunites
Cannat et al. [36] tentatively suggest that exposure and lherzolites. They are always between partly and
of serpentinites could represent 20% of the seafloor, completely altered. Altered peridotites display various
but in volume they would not exceed 515% of the appearances in colour: black, dark or light green, red,
crust. On the basis of seismic data, Carlson [37] comes yellow. Serpentinites are generally black or green,
to the estimate of 5% maximum. Further work is while yellowish to reddish rock contain abundant clay
undoubtedly needed for a better understanding. minerals. This section concentrates on serpentinites
By contrast, in fast spreading ridges, most evidence sensu stricto, and synthesizes observations made on
suggests that the crust is layered and consists of ex- different sample sets [7,14,47,50,65,74,84,87,96,100,
trusive basalts, a dike complex and a massive gabbro 101,111].
layer overlying mantle peridotites (Fig. 1). In this case,
the MOHO is interpreted as the transition between 3.1. Textures
a magmatic crust and the residual mantle peridotites
[117]. Peridotites occur only below the MOHO, i.e. Serpentinization is always extensive. The extent of
at depths > 6 km. Given this general structure, ma- serpentinization, i.e. the percentage of secondary min-
jor tectonic movements are necessary to expose them erals, generally varies between 80 and 100%, but peri-
at the seafloor. Outcrops of serpentinized peridotites dotites with as little as 50% serpentinization are oc-
have been documented only in very particular areas: casionally found. Dunites are generally more heav-
major transform faults (Garrett [14]), trenches open- ily serpentinized than harzburgites. Most of the de-
ing at the boundary of rotating microplates, such as scribed serpentinites are undeformed, i.e. have been
Easter and Juan Fernandez [46], and the tip of prop- altered under static conditions: in hand specimen and
agators such as Hess Deep [63]. In this case, as op- under the microscope, primary textures are still iden-
posed to slow spreading environments, serpentinized tifiable (Fig. 3A). However, the samples are typically
832 C. Mvel / C. R. Geoscience 335 (2003) 825852

Fig. 3. Examples of serpentinite textures in hand specimen and photomicrographs. (A) Hand specimen of serpentinized peridotite.
Serpentinization occurred under static conditions and preserved the primary textures. Areas of mesh texture (m) pseudomorphing olivine
crystals are juxtaposed to bastites (b) pseudomorphing orthopyroxene crystals. Sample SDM DR34-1-1, M.A.R [36]. (B) Hand specimen of
serpentinized peridotite. The sample is cross-cut by a dense network of parallel serpentine-filled veins. Sample EDUL DR23, SWIR [97].
 C. Mvel / C. R. Geoscience 335 (2003) 825852 833

cross-cut by vein networks, recording a complex his- tinites are likely emplaced by gravity sliding or slump-
tory of fracturing, crack opening and fluid circula- ing [28].
tion [50,65]. The abundance and orientation of veins In undeformed oceanic serpentinites, microscopic
is very variable. A dense network of parallel veins textures are quite similar to those described in conti-
may generate an apparent schistosity, even though the nental serpentinites (see review in [104]). A first de-
vein filling minerals have grown perpendicularly to the tailed description of ophiolite and ocean floor serpen-
vein walls, clearly under static conditions (Fig. 3B). tinite textures [111] has been confirmed by subsequent
Scarce sheared veins have also been reported [111] studies. The primary mineralogy of abyssal peridotites
and dredged blocks often display slickensides on their consists of olivine orthopyroxene (opx) clinopy-
outer surfaces. roxene (cpx) spinel. Thin section observations dis-
Although not as common, deformed serpentinites play a different sensitivity to alteration for the primary
phases. Resistance typically increases from olivine
have also been described [7,28,100]. In general, dredge
through orthopyroxene to clinopyroxene. High tem-
collections typically contain a few percent of serpen-
perature of serpentinization (see Section 4) may how-
tine schist fragments, even if those are generally not
ever modify this general rule. Olivine is replaced by
studied. Recently discovered detachment surfaces may
serpentine in mesh textures, while ortho- and clinopy-
also yield tremolite-talc-serpentine schists. Drill cores roxenes are replaced by bastite. In the mesh texture,
of one of these surfaces at 15 45N, off axis the MAR broken olivine fragments are replaced by concentric
show that these schists make up the fault zone [91]. layers of serpentine aggregates associated with mag-
Serpentinite breccias have also been reported. A first netite grains. A fresh olivine kernel might be preserved
type consists of serpentinite clasts cemented by a car- at the center (Fig. 3C and 3D). Alternatively, the cen-
bonate matrix (generally calcite or aragonite). These tre may consist of brucite or carbonate. Bastite is made
rocks are similar to what is called ophicalcite in ophi- up of serpentine fibers which mimic the orthopyroxene
olites, and may have, at least partly, a hydrothermal shape (Fig. 3E). Because clinopyroxene is more resis-
origin [26,28]. A second type consists of indurated ser- tant to alteration, clinopyroxene exsolution lamellae
pentinite fragments of sedimentary origin, often dis- are often preserved in bastites replacing orthopyrox-
playing graded bedding. These sedimentary serpen- ene [7]. Talc and/or amphibole may be associated with

Fig. 3. (C) Photomicrograph of a mesh texture replacing olivine in a serpentinized peridotite. Parallel nicols. Fresh olivine kernels (olv) are
preserved in serpentine cells (ser) surrounded by magnetite layers (mt). Sample HS13-1, M.A.R [124]. (D) Photomicrograph of a mesh texture
completely replacing olivine. Crossed nicols. Each cell of the mesh texture consists of concentric layers of serpentine. Sample ODP Leg
153, 920D 5R2 #12, M.A.R. [35]. (E) Photomicrograph showing a bastite replacing orthopyroxene isolated in serpentine in a mesh texture
(pseudomorphs after olivine). Crossed nicols. Late veins cross-cut both the bastite and the mesh. Sample ODP Leg 153, 920D 3R2 #7, M.A.R.
[35]. (F) Photomicrograph showing a bastite consisting of serpentine and minor tremolite fibers (trem). Crossed nicols. Sample HS13-7, M.A.R
[124]. (G) Photomicrograph showing a serpentinite with a mesh texture cross-cut by two perpendicular sets of serpentine veins. Crossed
nicols. Sample ODP Leg 153, 920D 7R2 #4, M.A.R. [35]. (H) Photomicrograph of a talc-serpentine schist. The primary texture is completely
obliterated by recrystallization under deformation conditions. Crossed nicols. Sample BR28, pc3. M.A.R. [58].
Fig. 3. Exemples de textures de serpentinites en macro-chantillons et au microscope. (A) Echantillon de pridotite serpentinise. La
serpentinisation a eu lieu en rgime statique et a prserv les textures primaires. Des zones texture maille (m) remplaant les cristaux
dolivine sont juxtaposes des bastites (b) remplaant les orthopyroxnes. Echantillon SDM DR34-1-1, M.A.R [36]. (B) Echantillon de
pridotite serpentinise, recoup par un dense rseau de veines parallles remplies de serpentine. Echantillon EDUL DR23, SWIR [97].
(C) Photographie au microscope dune texture maille remplaant lolivine dans une pridotite serpentinise. Nicols parallles. Des coeurs
dolivine frache (olv) sont prservs dans des cellules de serpentine (ser) entoures de magntite (mt). Echantillon HS13-1, M.A.R [124].
(D) Photographie au microscope dune texture maille remplaant compltement lolivine. Nicols croiss. Chaque cellule consiste en couches
concentriques de serpentine. Echantillon ODP Leg 153, 920D 5R2 #12, M.A.R. [35]. (E) Photographie au microscope dune bastite remplaant
un orthopyroxne isol dans la serpentine maille remplaant lolivine. Nicols croiss. Des veines tardives recoupent la fois la serpentine
maille et la bastite. Echantillon ODP Leg 153, 920D 3R2 #7, M.A.R. [35]. (F) Photographie au microscope dune bastite forme de serpentine
et de fibres de trmolite (trem). Nicols croiss. Echantillon HS13-7, M.A.R [124]. (G) Photographie au microscope dune serpentine texture
maille, recoupe par deux gnrations de veines orientes perpendiculairement. Nicols croiss. Echantillon ODP Leg 153, 920D 7R2 #4,
M.A.R. [35]. (H) Photographie au microscope dun schiste talc et serpentine. Nicols croiss. La texture originelle est compltement oblitre
par les recristallisations en rgime de dformation. Echantillon BR28, pc3. M.A.R. [58].
834 C. Mvel / C. R. Geoscience 335 (2003) 825852

blage resulting from peridotite hydration is simple.

A typical mid-ocean ridge serpentinite is made up of
serpentine minerals + magnetite talc brucite
tremolite. The most common accessory minerals con-
sist of ferrit-chromite, chlorite, native metals and sul-
fides, clay minerals, diopside, secondary olivine, horn-
blende. Textural relationships, however, suggest that
not all these phases are in equilibrium: they record
phases that pre- or postdate the serpentinization event.

3.2.1. Serpentine minerals

A number of general reactions are often used to
describe the formation of serpentine. In a purely
magnesian system, the most commonly referred to are
the following:
Fig. 4. Serpentine species crystallographic structures: A = lizardite; 2 Mg2 SiO4 + 3 H2 O = Mg3 Si2 O5 (OH)4 + Mg(OH)2
B = chrysotile; C = antigorite. Triangles represent 4-coordinated
(1)
Si and more or less deformed squares represent 6-coordinated Mg
(from B. Devouard, pers. comm.).
olivine + water = serpentine + brucite
Fig. 4. Structures cristallographiques des espces serpentineuses :
A = lizardite ; B = chrysotile ; C = antigorite. Les triangles Mg2 SiO4 + MgSiO3 + 2 H2 O = Mg3 Si2 O5 (OH)4
reprsentent des ttrahdres de Si et les carrs plus ou moins (2)
dforms des octahdres de Mg (B. Devouard, comm. pers.).
olivine + orthopyroxene + water = serpentine

the alteration of orthopyroxene (Fig. 3F). In these sta- The serpentine species, however, may vary. Serpentine
tically altered rocks, it is generally easy to reconstruct minerals are trioctahedral 1:1 layer silicates. They
the primary assemblages because the secondary min- consist of alternating infinite sheets of 4-coordinated
erals perfectly pseudomorph the original phases, even Si and 6-coordinated Mg. The three dominant species
if cross-cutting veins are normally abundant (Fig. 3G) are lizardite, chrysotile and antigorite, which differ
Some rocks, however, show evidence of recrystalliza- by their crystal structure. Lizardite consists of planar
tion which may obliterate the primary textures [121]. layers (Fig. 4A). Chrysotile consists of scrolled layers,
In serpentine schists, the original texture has been which tend to form cylinders (Fig. 4B). In antigorite,
completely modified by deformation and recrystalliza- the 1:1 layer periodically reverses, resulting in the loss
tion (Fig. 3H). In talcschists, the ultramafic precur- of a octahedral coordinated sites (Fig. 4C). The three
sor can be inferred from the mineral assemblage (i.e. species can be distinguished by their XRD patterns;
the presence of spinel grains), or from the bulk chem- however, the crystal structures are best illustrated by
istry [58]. TEM images (see review in [104,105]).
The fabrics of mid-ocean ridge serpentinites clearly The general formula of serpentines is Mg5 {Si2 O4 }
demonstrate that most of the serpentinization occurs (OH)4 . Substitution of Al and Fe3+ for Si may occur
under static conditions, by reaction of the peridotite in tetrahedral sites, and that of Fe2+ , Fe3+ , Cr, Al,
with a fluid phase driven through fractures and cracks. Ni and Mn for Mg in octahedral sites. It has been
These undeformed textures are in agreement with a shown that lizardite tends to accept more substitution
non-diapiric emplacement. than chrysotile, and is typically more enriched in
Al, although their compositions overlap [101,133].
3.2. Mineralogy Antigorite, however, because it lacks octahedral sites,
is not strictly a polymorph. This results in the loss of
Because the primary mineral assemblage of abyssal Mg and (OH)4 with respect to Si, and antigorite is
peridotites is relatively constant, the secondary assem- therefore systematically enriched in silica [9,128,132].
 C. Mvel / C. R. Geoscience 335 (2003) 825852 835

All the serpentinites collected from the seafloor netite often underlines the concentric layers of serpen-
display lizardite as a dominant phase ([100,101,111] tine in the mesh texture. Its grain size is generally very
and references therein). It is found in the mesh texture small (in the order of a few microns or below), al-
as well as in the bastites. Chrysotile may be associated though occasionally euhedral crystals may reach over
with lizardite. Chrysotile is the major phase occurring 10 (Fig. 3C).
in veins. Antigorite has also been reported, but is
definitely more scarce, associated with veins as well 3.2.3. Talc
shear zones [100]. Recrystallizations generally result Talc is commonly associated with the alteration
in the replacement of lizardite by chrysotile [121]. of orthopyroxene. It generally occurs along cracks,
It is difficult to assess a specific composition to progressively pseudomorphing the large crystals. It
the different serpentine species in ocean floor serpen- results from the following reaction:
tinites, as the serpentine variety is generally not de-
termined when the chemical analyses are available. 6 MgSiO3 + 3 H2 O
Moreover, the coexistence of lizardite and chrysotile = Mg3 Si2 O5 (OH)4 + Mg3 Si4 O10 (OH)2 (4)
in undetermined proportions is common in specific
orthopyroxene + water = serpentine + talc
sites such as mesh after olivine or bastites. However,
the microtextural site where the serpentine has crystal- Talc is also present in deformed rocks, as a major
lized clearly influences its chemistry [74,96]. Serpen- constituent of serpentine-talc schists.
tine replacing olivine is typically devoid of aluminium
and chromium but contains some nickel, matching the 3.2.4. Amphiboles
composition of olivine. By contrast, serpentine from Amphiboles often occur in serpentinites, although
bastites contains aluminium and chromium. The Mg# they are generally not very abundant.
is not as consistent, because it may be controlled by The most common variety is tremolite. Texturally,
external factors such as the oxygen fugacity which it forms needles, generally associated with the alter-
influences the Fe3+ /Fe2+ ratio; the amount of iron ation of pyroxenes (Fig. 3F). It may result from the
in serpentines varies with the abundance of associ- following reaction (in a purely magnesian system):
ated magnetite, and therefore reflects oxidixing con-
ditions [108]. When recrystallization occurs, the sec- 6 MgSiO3 + 2 CaMgSi2 O6 + 3 H2 O
ondary serpentine is often chrysotile, replacing pri- = CaMg5 Si8 O22 (OH)2 + Mg3 Si2 O5 (OH)4 (5)
mary lizardite [121]. Its composition shows less sub-
stitution, as if recrystallisation resulted in serpentine opx + cpx + water = tremolite + serpentine
minerals closer to the ideal formula. Serpentine in Tremolite may contain some iron, as well as some
veins also tends to have less substitutions, which is aluminium and chromium.
consistent with the fact that Al, Cr, Ni are relatively Other types of amphiboles have also been de-
immobile during alteration and therefore remain in scribed, although less frequently. In particular, magne-
their original microstructural site, and with the pre- sio-hornblende, edenitic hornblende, cummingtonite
dominance of chrysotile. have been identified in serpentinites. However tex-
tural relationships suggest that they predate the serpen-
3.2.2. Magnetite tinization event [14,84].
Magnetite is ubiquitous in serpentinites and is the
product of the serpentinization reaction of the iron 3.2.5. Brucite
end member. Incorporating iron, reaction (1) can be Brucite is an Mg-hydroxide which is only occa-
modified in the following way: sionally found in oceanic serpentinites. At Hess Deep,
6 (Mg, Fe)2SiO4 + 7 H2 O however, it occurs as a major phase [65,96]. Its ideal
formula is Mg(OH)2, but an Mg is often substituted
= 3(Mg, Fe)3 Si2 O5 (OH)4 + Fe3 O4 + H2 (3) by Fe2+ . It occurs either in the core of mesh texture,
It is important to note that the oxidation of iron to form or more often in veins. Its formation is generally de-
magnetite generates hydrogen (see Section 7). Mag- scribed by reaction (1).
836 C. Mvel / C. R. Geoscience 335 (2003) 825852

3.2.6. Other minerals tallization, i.e. the replacement of olivine by the mesh
Ferrit-chromite results from the alteration of alu- textured serpentine and replacement of pyroxenes by
minous spinel. This reaction provides aluminium, and bastite. Determining the temperature and pressure of
a rim of chlorite is generally present around the al- serpentinization is not an easy task, because serpentine
tered spinel grains [74]. Secondary sulfides (pent- minerals are stable over a wide range of temperatures
landite, marcasite, haezlewoodite, etc.) and native (between 500 C and room temperature) and because
metals and/or alloys (native Cu, arawuite, taenite) have reaction producing serpentines are relatively insensi-
also been described [4]. They are inferred to result tive to pressure.
from the recrystallization of primary sulfides during
serpentinization. Carbonates occur essentially in veins 4.1. Pressure of serpentinization
and as breccia matrix, and are generally aragonite and
calcite. They correspond to very late stages and are not The maximum pressure at which serpentinisation
in equilibrium with the serpentine minerals. occurs is reasonably well constrained by the geody-
namic setting. The depth of MOHO generally does not
3.2.7. Conclusions exceed 6 km. It may attain a maximum of 10 km and
This review of the mineral assemblages in serpen- can be shallower at slow spreading ridges (< 3 km
tinites from mid-ocean ridges points out the predom- occasionally). High seismic velocities beneath the
inance of lizardite and minor chrysotile, associated MOHO indicate that the mantle is composed of fresh
with magnetite. Other phases are much less abun- peridotites. Therefore, serpentinites form at a maxi-
dant and are associated with particular mineral do- mum depth of 10 km but more generally at 6 km and
mains. It also suggests that the chemistry of serpen- even less when the MOHO is shallower. This trans-
tine species is largely controlled by the microstructural lates into a maximum pressure in the order of 3 kb,
site. It should be pointed out that the presence of pri-
and more generally < 2 kb, quite low compared to oro-
mary mineral relics, the juxtaposition of primary min-
genic belts. Precise indicators, unfortunately, are lack-
eral pseudmorphs and the occurrence of cross-cutting
ing, to better constrain the pressure, and therefore the
vein networks are not in favour of equilibrium at the
actual depth of serpentinization within the lithosphere.
sample scale.
4.2. Temperature of serpentinization
4. Conditions of serpentinization
To estimate the temperature, a number of indica-
The serpentinization of abyssal peridotites results tors can be used, but their precision remains poor, as
from their interaction with large volumes of a hydrous will be discussed. Not only the stability field of ser-
fluid, at temperatures below 500 C, maximum stabil- pentine minerals is not well constrained, but also the
ity of serpentine minerals [32], although a number of attainment of equilibrium is not really demonstrated.
studies suggests that seawater penetration in the peri- Finally, as evidenced by the numerous generations of
dotites may start at a higher temperature [14,84]. veins, serpentinization does not occur in a single event.
Although the serpentinites are presently exposed
on the seafloor, a number of factors indicate that at 4.2.1. Stability of serpentine species
least part of the serpentinization occurred at depth, Based on experimental work, the general consen-
when the peridotites were cooling down. In this sec- sus is that chrysotile and lizardite are stable at temper-
tion, the conditions of serpentinization, i.e. the pres- atures lower than antigorite [59,101]. The first phase
sure, temperature, oxygen fugacity as well as the na- diagram of serpentine species was constructed by
ture and composition of the fluid phase will be dis- Evans [59], based on experimental work. It showed
cussed. It is important to keep in mind that serpen- that forsterite can be stable down to 400 C, and
tinites are polyphased rocks that record successive that antigorite is the serpentine species stable at high
stages of crystallization. This section will essentially temperatures (up to 500 C). This phase diagram is
consider processes associated with the peak of crys- consistent with his field observation in the central
 C. Mvel / C. R. Geoscience 335 (2003) 825852 837

Fig. 5. Phase relationships of serpentine species and related minerals in a temperaturepressure diagram. Simplified from [104]. Continuous
lines represent stable reaction curves, dashed lines, metastable reaction curves. Atg = antigorite; Brc = brucite; Ctl = chrysotile; Fo = forsterite;
Lz = lizardite; Tc = talc.
Fig. 5. Relations de phase des espces serpentineuses et minraux associs dans un diagramme pressiontemprature. Simplifi de [104]. Les
lignes continues reprsentent les courbes de raction stables, les lignes tiretes les courbes de raction mtastables. Atg = antigorite ; Brc =
brucite ; Ctl = chrysotile ; Fo = forstrite ; Lz = lizardite ; Tc = talc.

Alps. An independent calculation of the tempera- currence of lizardite is its ability to depart from the
ture for the serpentine species was made using 18 O ideal formula by incorporating Fe2+ , Fe3+ and Al.
(see below). The formation temperature for antigorite, Some authors have shown that these substitutions en-
chrysotile and lizardite were estimated at 235, 180 and large the stability field towards higher temperatures
125 C, respectively [129]. More recently, a more pre- [38,94,106]. Crystallization of lizardite in olivine and
cise phase diagram was constructed on the basis of pyroxene could also be structurally controlled by the
thermodynamic data [104] and shows the sequence preexisting phases. In any case, it is obvious that the
antigorite>chrysotile>lizardite with decreasing tem- nature of the serpentine species remains a poor indica-
perature (Fig. 5). It also shows that forsterite is stable tor of the temperature for oceanic serpentinites.
down to 350 C.
In ocean floor serpentinites, reactions are retro- 4.2.2. Mineral assemblages of serpentinites
grade (progressive hydration of a cooling peridotite), Besides serpentine minerals, other phases occur in
as opposed to the western Alps serpentinite section serpentinites or associated cross-cutting dikelets and
[59]. The peak of serpentinization is characterized by provide some constraints on the temperature.
a predominance of lizardite minor chrysotile. Antig- The significance of talc in serpentinites has been
orite is extremely scarce. This would suggest rela- largely debated because talc is stable over a wide range
tively low temperatures ( 200 C). Other indicators, of temperatures. It does not seem to be a good tem-
such as associated minerals and 18 O fractionation, dis- perature indicator because its formation is controlled
agree with this estimate and suggest high tempera- by other factors, particularly by silica activity in the
tures (> 300 C) (see below). Moreover, the exper- fluid phase [74]. This is confirmed by the experimen-
imental reaction between peridotite and seawater at tal reaction between peridotites and seawater at 300 C
300 C and 500 bars produced lizardite as a major which shows that talc formation is favoured by high
phase [114]. This discrepancy again raises the ques- activities of SiO2 , MgO and H+ [78]. Further experi-
tion of equilibrium attainment in oceanic serpentinites. ments at 400 C [3] suggest that at high temperatures,
Another possible explanation for the widespread oc- the preferential alteration of pyroxenes (with respect
838 C. Mvel / C. R. Geoscience 335 (2003) 825852

to olivine) results in fluids enriched in dissolved Ca,

SiO2 , Fe and H+ which favour the formation of Si-
rich silicates (talc and tremolite). These results are in
agreement with the observed association of talc with
pyroxenes. The presence of talc would therefore indi-
cate higher temperatures of serpentinization, as well as
high silica activity.
The common presence of chlorite and tremolite
suggest temperatures consistent with greenschist fa-
cies conditions, i.e. in the range of 300500 C. For
example, in serpentinites at Hess Deep [65,96] as well
as Garrett Fracture zone [14], the association with al-
tered gabbros dikes or impregnations containing chlo-
rite, albite, actinolite as secondary phases suggest tem-
perature conditions between 300 and 500 C. Minor
phases may also be informative. In Hess Deep ser-
pentinites, the presence of a FeNi-alloy, taenite even Fig. 6. Compilation of 18 O measurements in serpentinites and
serpentine minerals from mid-ocean ridge environments. The values
suggests temperatures as high as 450 C [72]. Another span a wide range, with a peak at +3 to +5. Data from: [1,2,30,
indication of temperature is given by the respective 47,65,87,129,130].
dissolution rates of olivine and orthopyroxene. Exper- Fig. 6. Compilation des donnes de 18 O dans les serpentinites et
imental reactions have shown that olivine is preferen- minraux serpentineux des environnements de dorsales ocaniques.
tially dissolved at temperatures around 250300 C, Donnes de [1,2,30,47,65,87,129,130].
while orthopyroxene is rapidly dissolved at tempera-
tures above 400 C [93]. If all the orthopyroxene has closing the mantle value. This suggests that serpen-
been serpentinized in a given sample, while olivine re- tinization reactions occur over a wide range of temper-
licts are still present, this indicates reactions at high ature. The 18 O is also influenced by the water/rock
temperature (> 400 C). This case has been reported ratio (W/R). The minimum W/R required to convert
in samples from Hess Deep [121]. fresh peridotites to serpentinites stoichiometrically is
0.13 [113]. As a rule, at higher W/R, isotopic composi-
4.2.3. Oxygen isotopes tions are more strongly modified [113]. For the MARK
18 O fractionation also allows us to estimate serpen-
area serpentinites, a W/R in the range of 0.55 to 1 has
tine crystallization temperatures. The starting material been calculated at 400 C [2], depending on the fluid
is a peridotite with a well defined mantle value ( 18 O composition.
= 56). The reacting fluid phase is inferred to be More precise temperatures can be tentatively eval-
seawater ( 18O = 0). Empirical fractionation curves uated. The serpentine/water fractionation curve [129]
between serpentine and water as well as serpentine and allows the calculation of the temperature if the 18 O
magnetite have been proposed [129,130] and recently of the fluid phase is constrained. It is reasonable to
refined [65]. However, because these curves have not assume that seawater is the major component of the
been experimentally constrained, the uncertainty on reacting fluid phase. However, as it penetrates into the
temperature estimates remain very high, up to 50 crust, it reacts to form secondary minerals and its 18 O
120 C [2,65]. A first result is to show that the frac- is consequently modified. An obvious example of this
tionation between serpentine and water depends on the compositional evolution of the reacting fluid phase is
temperature: the 18 O of the serpentine is heavier than the 18 O of fluids venting at black smokers. These flu-
the starting material at low temperatures, lighter at ids, which are the end product of seawater convecting
high temperatures, the critical change appears around cells in the crust, yield systematic positive 18 O val-
100200 C. Fig. 6 shows a compilation of 18 O mea- ues, intermediate between seawater ( 18 O = 0) and
sured in mid-ocean ridge serpentinites. It shows a large the magmatic rocks ( 18 O = 56) with which it has
dispersion in the values, between 0 and +12, en- reacted see the review in [115]. For instance, at the
 C. Mvel / C. R. Geoscience 335 (2003) 825852 839

Table 1
Temperature estimates for serpentine formation from oxygen isotopes
Tableau 1
Estimations de temprature des conditions de serpentinisation partir des compositions isotopiques de loxygne
Location Reference Temperatures Indicator
MAR [130] 118 (lizardite)
1 N and 43 N 180 (chrysotile) 18 O fractionation between serpentine and magnetite
235 (antigorite)
Vema and [30] 30177 C (1) 18 O fractionation between serpentine and fluid
Romanche FZ, MAR 45233 C (2) reacting fluid = (1) seawater, 18 O = 0 ; (2) hydrothermal fluid, 18 O = 1.6
ODP Site 670 [87] 113246 (1) 18 O fractionation between serpentine and fluid
MARK 160385 C (2) reacting fluid = (1) seawater, 18 O = 0 ; (2) hydrothermal fluid, 18 O = 2.4
ODP site 920, [2] 430590 18 O fractionation between serpentine and magnetite
MARK 120 C
EPR 325 50 C 18 O fractionation between serpentine and magnetite
Hess Deep [1] 55135 (1) 18 O fractionation between serpentine and fluid
80190 reacting fluid = (1) seawater, 18 O = 0 ; (2) hydrothermal fluid, 18 O = 2
115285 (3) (3) hydrothermal fluid, 18 O = 4
ODP Site 895 [65] 297386 50 C 18 O fractionation between serpentine and magnetite
Hess Deep 410533 50 C
SWIR [47] 116275 (1) 18 O fractionation between serpentine and fluid
5269 E 160427 (2) reacting fluid = (1) seawater, 18 O = 0 ; (2) hydrothermal fluid, 18 O = 2.4

Snake Pit vents on the Mid-Atlantic Ridge, hydrother- of 100 C are also reported (see Table 1 and refer-
mal fluids yield a 18 O of 2.30 [80]. The composition ences therein). It is interesting to point out that sam-
of fluids involved in serpentinization reactions remains ples yielding the highest temperature come from drill
unknown, but is likely to have evolved with respect holes (MARK, Hess Deep). The other samples, col-
to seawater. In the absence of a precise composition lected from the seafloor, and often off axis in fracture
for the fluid phase, a number of authors have used dif- zone, may have been subjected to later re-equilibration
ferent 18 O values varying between 0 (pure seawater) at or near the seafloor.
and 4 (evolved seawater), and calculated a tempera-
ture range. Table 1 summarizes the temperatures ob- 4.3. Nature and composition of the altering fluid
tained and shows a wide spectrum, ranging from 30 C phase
to over 400 C.
Calculations based on empirical oxygen isotope An ocean floor serpentinite typically contains 10
fractionation curve between serpentine and magnetite 15% of water or even higher, depending on the ex-
[65,129] help avoid assumptions on the composition tent of serpentinization. The formation of serpen-
of the fluid phase. Equilibrium temperatures have been tinites from an essentially anhydrous peridotite cer-
calculated for only a few examples, because separating tainly heavily involves seawater. However, as already
magnetite from serpentine is not often possible due to pointed out, some tracers suggest that the fluid may
the very small grain size of magnetite. The results are have evolved from pure seawater by reaction with the
again very variable (Table 1) and reach up to 500 C. ocean crust and/or mixing with other fluids.
In conclusion, despite the uncertainties in the tem- Stable isotopes are good tracers for the nature of
perature estimates due to the lack of experimental cal- the altering fluid. The 18 O values measured in ser-
ibration as well as the fact that equilibrium attainment pentinites suggest that the fluid phase has a 18 O 
may not be reached, this review clearly demonstrates 5.5, the average value of fresh peridotites and con-
that serpentinization starts at a high temperature, in the sistent with dominant seawater. As discussed above,
range of 300500 C, but temperatures in the range as seawater penetrates into the crust, however, it re-
840 C. Mvel / C. R. Geoscience 335 (2003) 825852

acts with the rocks and its composition evolves. This native explanation could be the D/H fractionation dur-
evolution results in heavier 18 O. A possible way of ing the production of molecular hydrogen associated
estimating the 18 O of the fluid phase is to constrain with serpentinization [65].
the temperature using the fractionation between ser- At Hess Deep, the existence of a magmatic com-
pentine and magnetite, and then calculate the compo- ponent is also suggested by 13 C signatures [65]. The
sition of the fluid phase using the partition coefficient thermal decomposition of the serpentinites produces
between serpentine and fluid at a given temperature. CO2 with a 13 C consistent with magmatic values
At MARK (ODP Site 920), the calculated range of (4.57.8), as opposed to aragonite veins which are
18 O for the fluid phase varies between 3.3 and 4.4 consistent with oceanic carbon.
at 350 C, and between 4.3 and 5.4 at 450 C. At A very low oxygen fugacity during serpentinization
Hess Deep, consistent values of 4 for a temperature can be inferred from the presence of sulfides and
of 325 C have been calculated for a sample collected metal alloys [64,101]. In Hess Deep serpentinites, the
by submersible [1], and 3.34.4 at 450 C for the paragenesis of opaque phases consisting of magnetite
drilled samples [65]. Even with the uncertainty, it can + awaruite + pentlandite heazlewoodite, stable at
be stated that 18 O suggests evolved seawater rather temperatures ranging from 400 to 100 C, indicates
than unaltered seawater. The relatively heavy value of highly reducing conditions and low f O2 [4]. These
6.6 found at Hess Deep could result from mixing extreme conditions are attributed to the production of
with magmatic fluid [65]. H+ during silicate reactions (see reaction (3)) which
A possible mixing of unaltered seawater with a maintains low f O2 and produces H2 S from primary
fluid of deep origin has also been suggested from D sulfides.
values. Ocean floor serpentinites are characterized by Strontium isotopes are also a good tracer for the
very light D (< 60), which cannot be explained interaction with seawater. A number of studies have
by simple interaction with interaction with seawater, shown that the 87 Sr/86 Sr span a range between man-
and their origin has been debated [65,116,130]. The tle (0.7023) and present-day seawater (0.7092) values
mixing with magmatic or metamorphic fluids (result- with a peak around 0.7080.709, therefore arguing for
ing from dehydration reaction of previously altered a seawater dominated fluid phase (Fig. 7). The large
ocean crust) has been invoked [116,130]. However, at interval is interpreted in terms of the mixing of three
Hess Deep, at least 20% of magmatic fluid is required components: MORB, mantle peridotites and seawa-
to explain the very light D values. Given the very low ter [85]. MORBs and oceanic peridotites have simi-
water content of MORB magmas, this percentage does lar strontium isotopic compositions, but the concen-
not seem realistic. Moreover, there is no petrological tration is much higher in MORBs than in peridotites
evidence for intensive dehydration reactions. An alter- [85]. MORBs and peridotite have the same strontium

Fig. 7. Compilation of 87 Sr/86 Sr ratios in serpentinites and serpentine minerals from mid-ocean ridge environments. Data from [25,47,85,120].
The data from [118] are not shown because they correspond to magnetic fraction separates.
Fig. 7. Compilation des rapports 87 Sr/86 Sr dans les serpentinites et minraux serpentineux des environnements de dorsales ocaniques. Donnes
de [25,47,85,120]. Les donnes de [118] ne sont pas figures, car elles correspondent la fraction magntique.
 C. Mvel / C. R. Geoscience 335 (2003) 825852 841

isotopic compositions, but their strontium concentra- dotite (2.5 g/cm3 versus 3.3 g/cm3 ). If serpentiniza-
tions vary from above 100 ppm in basalts to less than tion occurs at constant volume, the decrease in density
10 ppm in peridotites. Because the strontium con- must be accompanied by a loss of chemical elements.
tent of peridotites is very low compared with sea- By contrast, if the chemical elements remain constant,
water, interaction with seawater is dominated by the then there must be a volume increase to account for
87 Sr/86 Sr ratio of the fluid, and serpentinites should the decrease in density.
rapidly be in equilibrium with seawater, depending on The problem of volume increase due to hydra-
the water/rock ratio. Intermediate values may result ei- tion has not yet been completely solved. Most ocean
ther from low water/rock ratios, or from interaction floor serpentinites have been formed under static con-
with the overlying basaltic crust during penetration at ditions; serpentine pseudomorphing primary minerals
depth. Fig. 7 also points out that some whole rock preserve the details of the original texture, and does
analyses of serpentinites yield puzzling 87 Sr/86 Sr ra- not favour a volume increase at the crystal scale. At
tios, higher than seawater. These values were first doc- the sample scale, however, vein networks are com-
umented for serpentinized peridotites from the equato- monly reported and can account for a volume ex-
rial M.A.R. [25]. The proposed explanation for these pansion. From observations in continental serpen-
values was that the peridotites were remnants of conti- tinites, OHanley [103] proposes that progressive ser-
nental mantle left behind during the breakup of Pangea pentinization produces a kernel pattern than can ac-
[24]. More recently, such high 87 Sr/86 Sr ratios were count for volume expansion. This model could apply
found again in serpentinized abyssal peridotites from to oceanic serpentinites, but, unfortunately, no such
other localities [118]. On the basis of mineral separates detailed outcrop observations are available.
and leaching experiments, these authors argue that the The problem in evaluating chemical fluxes is that
high ratios are due to detrital sediment particles of con- the fresh protolith is never available for comparison
tinental origin infiltrated through the cracks of the ser- with the altered rock. Although peridotites are rela-
pentinites. Although this process is not related to ser- tively homogeneous in terms of mineral assemblages,
pentinization, it should be kept in mind because it may the mineral compositions may vary widely, reflecting
affect the 87 Sr/86 Sr ratios of abyssal serpentinites. the amount of melting as well as possible reaction
The combined use of stable and radiogenic isotopes with percolating melts. Therefore there is no standard
as tracers leads to the conclusion that the fluid phase composition that could be used as a reference.
involved in the serpentinization reaction at mid-ocean
ridges consists of evolved seawater, with a possible 5.1. Major elements
minor magmatic component.
On the basis of correlated variations between el-
ements (CaO, Al2 O3 , FeO*/MgO) that can be inter-
5. Elemental fluxes during serpentinisation preted in terms of primary processes, it has been sug-
gested that most of the chemical variations among
The first and major consequence of serpentinization samples are due to the primary heterogeneity of the
is hydration. Serpentine minerals contain over 12% peridotites, even though some mobilization during
water. In serpentinites, they represent the major phase serpentinization may also occur (i.e. loss of CaO)
and the amount of water has often been used as a proxy [100]. A more quantitative attempt to evaluate chemi-
for the degree of serpentinization. The second conse- cal fluxes was made in serpentinized peridotites from
quence is oxidation of iron due to the formation of the Franciscan formation [45]. In this example, ser-
magnetite by reaction (3). However, there has been a pentinization, besides hydration, does not seem to af-
number of debates on whether, besides these two ef- fect major element concentrations, except for calcium.
fects, serpentinization has a chemical consequence on The consequence is a calculated volume increase of
the bulk chemistry of the rocks. A good summary of 35% for harzbugites and 48% for dunites. A study of
these debates is given by OHanley [104]. The essence serpentinized dunites and wehrlites from the Bay of Is-
of the problem is the question of volume, because ser- land ophiolites comes to the same conclusion [86]. On
pentinite has a much lower density than a fresh peri- the basis of modal reconstructions, a volume increase
842 C. Mvel / C. R. Geoscience 335 (2003) 825852

of 32% is estimated and mass balance calculation indi- olivine is stable down to 400 C. Serpentinization of
cated evidence for appreciable metasomatism for CaO, pyroxenes definitely results in a loss of silica. An in-
Al2 O3 , FeO and MgO. No mass balance calculations creased silica activity in the fluid results in the sta-
have been attempted on serpentinized abyssal peri- bility of talc and tremolite over serpentine [3]. If sil-
dotites but the general consensus is that the major ele- ica rich fluids are channelled in fault zones, the al-
ments are not severely affected, which implies an over- teration of peridotites occurs under dynamic condi-
all volume increase [74,87,96]. The fact that serpen- tions and results in the formation of serpentine-talc-
tine minerals pseudomorphing primary phases com- tremolite schists. However, this evidence for metaso-
monly retain the minor elements such as Al, Cr and Ni matism remains volumetrically small compared with
is also in favour of immobility during serpentinization. the bulk of massive serpentinites.
The only evidence for a loss in magnesium has been
documented in peridotites altered at low-temperature 5.2. Trace elements
and is likely to be related to the formation of clay
minerals [119]. In this case, an average of 5% MgO
loss is estimated as a result of weathering (tempera- The behaviour of trace elements during serpen-
ture  100 C), probably when the rocks are exposed tinization is not well constrained yet.
on the seafloor. Serpentines are enriched in chlorine. In situ analy-
Experimental reactions between peridotite and wa- ses of individual serpentine crystals document values
ter have been conducted at temperatures between 200 between 90 and 14 800 ppm [74,84,96], see review in
and 400 C and a pressure of 500 bars, with the aim [6], with an average value of 1850 ppm. Bulk rock
of constraining chemical fluxes [3,78,114]. The results analyses confirm these numbers: 10651470 ppm in
remain inconclusive on the final chemical budget be- serpentinites from the 15 N detachment surface at the
cause equilibrium is not reached, but they help us to M.A.R. (Mvel, unpublished data) and 1682211 ppm
understand reaction pathways. In most experiments, in serpentinites from the SWIR (Decitre, unpublished
SiO2 initially increases in the solution while MgO and data). Anselmi et al. [6] favour the location of Cl as
pH decrease. The low pH favours the dissolution of isomorphic substitution for (OH) groups in serpen-
Fe, Mn, Zn. Subsequently, the pH increases and the tines as opposed to secondary formed hydroxichlo-
dissolution rates decrease. The data show that reaction ride. Frh-Green et al. [65] also report 6% chlorine
pathways are dependant on the mineral assemblages of in brucite veins, but brucite is not a common phase,
the peridotites, the solution chemistry, in particular the except at Hess Deep. Further work is certainly neces-
MgO concentration which affects the pH, and there- sary to better constrain which mineral partitions chlo-
fore the dissolution of minerals and the water/rock ra- rine, but in any case, serpentinites are clearly a sink for
tios. chlorine.
There is evidence for local metasomatism associ- Serpentinites are enriched in boron (100 ppm) com-
ated with serpentinization. The occurrence of rodin- pared with fresh peridotites (1525 ppm) [125]. Values
gites, gabbroic dikes recrystallized to secondary as- ranging from 24 to 110 ppm (corrected for the per-
semblages rich in calsilicates (hydrogrossular, prehni- centage of serpentinization) have subsequently been
te, epidote, pumpellyite), implies the input of calcium, measured [30] on another suite of samples, also doc-
generally considered to have been leached from the umenting an inverse correlation between B and tem-
peridotites during serpentinization [7,14,76]. At Hess perature. The uptake of B is favoured at low tempera-
Deep, however, it has been argued that the incipi- tures (< 50 C). This behaviour was confirmed by ex-
ent formation of rodingites could simply reflect a re- periments [114] which show that at 300 C, there is
distribution of sodium and calcium within the gab- no change in the B concentration of the reacting fluid,
broic bodies during the serpentinization event [96]. while it drastically decreases during cooling to 25 C,
The other evidence for mass transfers during serpen- suggesting that the boron uptake occurs during retro-
tinization is the presence of tremolite-talc schists often grade reactions with seawater.
associated with serpentinites [58,66,91]. Pyroxene is Fluorine also seems to be taken-up during serpen-
serpentinized at temperatures as high as 500 C, while tinization. A bulk rock value of 204 ppm [122] of fluo-
 C. Mvel / C. R. Geoscience 335 (2003) 825852 843

rine is confirmed by in situ nuclear microprobe analy- petrologists because it is important to determine whet-
ses [107] which yield an average of 206 ppm. her they retain their primary values; however, no sys-
Decitre et al. [47] report bulk rock serpentinite con- tematic studies have been conducted on oceanic ser-
centrations for lithium ranging from 0.6 to 8.2 ppm, pentinites. A review of studies on continental/ophiolitic
while the mantle value is 13 ppm. They conclude that peridotites [95] shows that there is no consensus on
serpentinized peridotites can be only a minor sink for the behaviour of REE. The authors conducted exper-
Li. The 6 Li suggest that Li is recycled from the al- iments to better constrain this behaviour, by reacting
tered ocean crust rather than seawater. a lherzolite, a harzburgite and a dunite with seawa-
The behaviour of sulfur during serpentinization has ter at 300 C. Again, the results remain ambiguous.
been studied at Hess Deep in comparison with the For lherzolite and dunite, the REE remain immobile,
Iberian margin [4]. At Hess Deep, high temperature while for the harzburgite, an uptake in LREE (La >
serpentinization results in an slight uptake of sulfur Ce > Nd) occurs. The authors attribute this difference
(mean = 355 ppm versus 250 ppm for the mantle to the role of secondary Ca-bearing phases. It should
value). It reflects the combined effect of a decrease be pointed out that some of the experiments produce
in sulfide sulfur (trace to 165 ppm) due to the break- anhydrite which is never observed in natural serpen-
down of primary sulfides (replaced by metal alloys and tinites because it has likely been redissolved by seawa-
sulfide phases containing less S) and an increase in ter at temperatures < 150 C. These experiments also
sulfate sulfur (50 to 790 ppm) due to the uptake of constrain the behaviour of some radiogenic isotopes,
sulfate from seawater. It is suggested that the wide 143 Nd/144 Nd and 87 Sr/86 Sr. As discussed in Section 4,

spectrum of 34 S (21 to 3.3) for sulfates re- the 87 Sr/86 Sr ratio is strongly affected during serpen-
flects a mixing of seawater sulfates (high values) with tinization. But a 87 Sr/86 Sr versus 143 Nd/144 Nd binary
sulfates derived from oxidation of sulfides. The 34 S diagram shows that, by contrast, neodymium isotopes
of sulfides varies from 1.5 to 23.7. The high- are little affected. The representative points form a
est values are consistent with magmatic values, while horizontal array at constant 143 Nd/144Nd values of
the lowest are best explained by the microbial reduc- 0.513100.51314, consistent with mantle values. Only
tion of seawater sulfates during the late stage of ser- at very water/rock ratios could the 143 Nd/144Nd ratio
pentinization (at temperature < 120 C). The combi- be also affected [119].
nation of these processes results in a net increase in
34 S (mean = 8) for bulk serpentinites. This study 5.3. Discussion
also indicates a difference with the Iberian margin ser-
pentinites, formed at lower temperatures and higher This review shows that the chemical consequences
fluid fluxes, where a higher microbial activity results of serpentinization are still not very well understood.
in a higher uptake of total sulfur (mean S content = It seems that serpentinization does not occur at con-
3800 ppm) and a decrease in 34 S (mean = 5). stant volume and that major elements, besides hydra-
A subsequent study of MARK area serpentinites also tion and oxidation, are not severely affected. Local
documents high sulfur contents (up to 1 wt%) and high metasomatism may result in the formation of rodin-
34 S in sulfides (3.7 to 12.7) [5]. In this case, how- gites and/or talc and tremolite schists. Serpentinites
ever, the authors ascribe the enrichment in sulfur to a are clearly a sink for Cl, B, F and S. The behaviour
multistage reaction process. Aqueous fluids enriched of REE and some radiogenic elements seems to be
in sulfur through hydrothermal reactions with subja- dependent on the water/rock ratio, on the extent of
cent gabbroic intrusions at temperatures in the range serpentinization, and on the behaviour of pyroxene,
of 350400 C are subsequently involved in serpen- since this phase concentrates most of these elements.
tinization reactions at temperatures < 300 C. In any Further work is clearly necessary to better understand
case, serpentinization obviously influences the global the chemical fluxes associated with serpentinization
cycle of sulfur and results in a net increase of sulfur processes. Now that it is recognized that serpentinites
and a modification in the 34 S. are a important component of the crust, it becomes
The behaviour of REE as well as radiogenic iso- crucial to constrain these fluxes. They may have a
topes during serpentinization is a major issue for global impact on the composition of seawater. On the
844 C. Mvel / C. R. Geoscience 335 (2003) 825852

other hand, when heterogenous oceanic lithosphere is 44], all the seismic velocities measured on serpen-
entrained in subduction zones, there is a potential for tinites have shown a strong dependence with the de-
recycling chemical elements into the mantle. gree of serpentinization. A compilation [99] shows
an inverse relation between the amount of serpen-
6. Consequences of serpentinization on physical tinization and seismic velocities (Fig. 8B). A partially
properties of peridotites serpentinized peridotite may therefore have the same
seismic velocities as a gabbro [37,77], and a seis-
Due to massive hydration and crystallization of mically layered oceanic crust may be composition-
secondary phases, the physical properties of abyssal ally heterogeneous (Fig. 1) [34]. Because observations
peridotites are strongly modified by serpentinization. and sampling on the seafloor are necessarily localized,
A summary of the major consequences are given in evaluating the proposition of serpentinites in the crust
this section. is an arduous ask [37]. If serpentinized peridotites,
The average density of a fresh peridotite is approx- which are residual rocks from the mantle, crop out
imately 3.3 g/cm3 . Serpentine minerals have a den- at the seafloor, this raises the question of the signifi-
sity of approximately 2.5 g/cm3 . Serpentinization is cance of the MOHO, classically interpreted as the tran-
therefore responsible for a strong decrease in density. sition between magmatic, mafic rocks and residual, ul-
A compilation of density measurements of serpen- tramafic rocks (see Section 2) [34]. This sharp seis-
tinites [99] shows that, as expected, there is an inverse mic boundary could correspond to a hydration front,
correlation between density and the extent of serpen- i.e. the transition between partially serpentinized peri-
tinization (Fig. 8A). As discussed in Section 5, serpen- dotites and fresh peridotites, corresponding to an in-
tinization does not seem to be accompanied by major crease in density. Because serpentinization occurs at
chemical fluxes, and the consequence is an increase temperatures below 500 C, this hydration front may
of volume. In the extensional environment of a mid- also correspond to a thermal boundary, i.e. the 500 C
ocean ridge, this volume increase is likely accommo- isotherm [73].
dated by tectonic activity, as evidenced by vein net- The magnetic properties of ocean floor peridotites
works. are also strongly affected by serpentinization because
The decrease in density affects the seismic veloci- it produces secondary magnetite [52]. Therefore, com-
ties. Following the pioneer work of Christensen [43, pared to a fresh peridotite, a serpentinite has a high

Fig. 8. Compilation of densities (A) and seismic velocities (B) measured on ocean floor serpentinites. Adapted from [99].
Fig. 8. Compilation des densits (A) et vitesses sismiques (B) mesures dans les serpentinites ocaniques. Adapt de [99].
 C. Mvel / C. R. Geoscience 335 (2003) 825852 845

magnetic susceptibility and a ferromagnetic behav- of magnetite to be able to predict the magnetic proper-
iour [126]. Because of the crystallization of secondary ties of the serpentinites peridotites in the ocean crust.
magnetite, it also acquires a natural remanent mag- A last major effect of serpentinization concerns
netization (NRM) which can contribute to the mag- rheology which in turn affects the tectonic activity
netic anomalies of the seafloor [52]. At slow spread- of the ocean lithosphere. Deformation experiments
ing ridges, it has been recognized that the off-axis show that serpentinites, particularly when made of
traces of inside corner highs massifs located at the lizardite, are weaker than the other components of
intersection between a fracture zone and the ridge axis the oceanic lithosphere and display brittle deformation
are commonly marked by more positive magneti- without dilatancy [55,56,112]. A small degree of
zation, attributed to the occurrence of serpentinites serpentinization (< 15%) reduces the strength of the
[109,127]. Moreover, the skewness of magnetic anom- peridotite to that of serpentine, while displaying the
alies on the seafloor has been ascribed to the contri- characteristic behaviour of pure serpentinites (e.g. non
bution of serpentinized peridotites [53,54]. To inter- dilatant brittle deformation). The presence of lizardite-
pret magnetization maps of the seafloor it is therefore rich serpentinites in the crust may therefore strongly
essential to consider the possible presence of serpen- influence its strength and tectonics [55,57].
tinites.
A detailed study of the magnetic properties of
oceanic serpentinites [108] has revealed the complex- 7. Serpentinization driven hydrothermal activity
ity of magnetite formation and how it affects the mag-
The fact that serpentinization could affect the chem-
netic properties. The magnetic susceptibility is di-
istry of the water column was first documented in the
rectly correlated to the amount of magnetite. However,
1990s at the Mid-Atlantic Ridge. Specific anomalies,
this amount does not increase linearly with the rate of
characterized by high methane concentrations together
serpentinization as initially proposed [15] because the
with low TDM (total dissolved manganese) contents
partitioning of iron between serpentine minerals and
and turbidity were identified and shown to be asso-
magnetite varies with the extent of serpentinization. ciated with serpentinite outcrops [39,40,42]. Serpen-
The authors have shown that it remains modest at ser- tinization produces H2 because of the oxidation of iron
pentinization rates below 75% because the iron partly during the formation of magnetite using the oxygen
enters serpentine minerals (up to 6% FeO). At higher from the aqueous fluid phase [64,101]. Calculated ul-
serpentinization rates, the iron content of serpentine tramafic hosted vent fluid compositions predict abun-
minerals decreases (23%) and more magnetite forms. dant H2 concentrations, as opposed to basalt hosted
They also show that the size of magnetite grains in- vent fluids [131], in agreement with the observations.
fluences the NRM. The small sized magnetite grains The production of methane is attributed to Fischer
associated with serpentine in mesh texture produce Tropsch type reactions such as:
NRM as high as basalts. By contrast, in the case of
large magnetite crystals, their occurrence as irregular CO2 + 4 H2 = CH4 + 2 H2 O
aggregates likely promotes strong magnetostatic in- The formation of methane as well as other hydro-
teractions between the grains and as a result reduces carbon components during serpentinization have been
their coercitivity. Finally, low temperature oxidation reproduced experimentally at 300 C, 500 bars [10].
of magnetite produces maghemite, and drastically de- Further evidence for the existence of hydrother-
creases the magnetic susceptibility and NRM of ser- mal activity associated with serpentinites is the dis-
pentinites. In conclusion, this study confirms that ser- covery of black smokers hosted on serpentinized peri-
pentinites have generally high magnetic susceptibility dotites. The Logatchev field was first discovered at
and NRM and can contribute to magnetic anomalies. 14 45 N latitude [17], followed by the Rainbow field
It demonstrates, however, that the modification in the at 36 14 N latitude [62] on the Mid-Atlantic Ridge.
magnetic properties of serpentinites is not linearly re- Although they are built on serpentinites, these two
lated to the rate of serpentinization. It is still neces- hydrothermal fields share a number of characteristics
sary to better understand what controls the formation with hydrothermal fields hosted in basalts (Table 2),
846 C. Mvel / C. R. Geoscience 335 (2003) 825852

Table 2
Comparison of fluid chemistry between hydrothermal vent fields from various ridge environments and seawater
Tableau 2
Comparaison de la chimie des fluides mis des endroits diffrents
Logachev Rainbow Lost City TAG Seawater
1996 1997 2001 1993
Host rock serpentinites serpentinites serpentinites basalts
Temperature 353 365 C 4075 C 363 C 2 C
pH 2.8 3.3 8.0 3.1 7.8
H2 S mM 0.8 1.0 0.064 34 0
Mg mM 0 0 919 0 53
Na mM 438 553 479485 550 464
Fe mM 2500 2400 5170 0.0045
Mn mM 330 2250 710 0.0013
Cl mM 515 750 546549 650 546
Co mM <2 13 <2 <2
Ni mM <2 3.0 <2 <2
SO4 mM 5.912.9 28.6
CH4 mM 2.2 0.130.28 0.150.16 4 107
H2 mM 13 0.250.43 0.180.23 4 104
[51] [51] [82] [51] except
CH4 and H2 [41]

and support biological communities. The fluids vent and turbidity documented in the water column [39] in
at high temperatures (> 300 C) and they build up sul- the vicinity of serpentinite exposures.
fide chimneys. They have low pH, high H2 S and metal The most recent discovery was the Lost City
contents. However, they are characterized by high H+ hydrothermal field [82]. This field is located off
contents, consistent with the production of hydrogen axis (1.5 Myear old crust) at 30 N, at the corner
during serpentinization [51]. Moreover, the presence intersection between Atlantis fracture zone and the
of complex hydrocarbon molecules [75] similar to M.A.R. The fluids vent at low temperature (4075 C)
those produced experimentally [10] has been doc- and build spectacular hydrothermal chimneys (up to
umented at Rainbow. This could be of great biolog- 60 m high) made up essentially of carbonates and
ical importance. Not only can methane support mi- brucite. The fluid compositions contrast with those
crobial communities [79,92], but also complex hydro- from the Rainbow and Logachev sites (Table 2).
carbons are considered as prebiotic molecules which In particular, they are characterized by high MgO
could have played a role in the appearance of life on and high pH, and low H2 S contents. The associated
Earth. There are some differences between the two serpentinites indicate crystallization temperature of
fields, however. Hydrothermal fluids from the Rain- around 200 C [66]. This field definitely differs from
bow field yield very high chlorinities, metal and REE the two others both in temperature and chemistry of
concentrations, which could reflect phase separation the fluids and associated deposits. Abundant microbial
[51]. Another field, the Saldanha hydrothermal field activity is associated with fluid venting and mineral
has also been discovered at 36 30N latitude on the precipitation [83]. In this case, the venting fluids could
Mid-Atlantic ridge, at the southern tip of the FA- generate the methane anomalies not associated with
MOUS segment. Active vents discharge clear warm turbidity that are commonly in the water column, in
fluids through sediments, but no data on fluid compo- association with serpentinite outcrops.
sitions are yet available [11]. It should be pointed out Another evidence for hydrothermal activity was
that the black smoker type of vents generate turbidity first documented by Bonatti et al. [29]. It consists
in the water column. Therefore, they cannot account of massive sepiolite deposits which are ascribed to
for the methane anomalies not associated with TDM the precipitation from a fluid phase resulting from
 C. Mvel / C. R. Geoscience 335 (2003) 825852 847

serpentinization. The same type of deposits occurs at These results suggest that black smoker vent fields
the western portion of the SWIR [8]. The presence of (Rainbow type) can occur only at the ridge axis, where
sepiolite on the ocean floor could therefore be used as magmatic instrusions can provide heat to activate the
an indicator of serpentinization at depth. hydrothermal circulation. Because the heat generated
Hydrothermal activity requires heat and different by serpentinization reactions can activate the Lost City
possible heat sources can be invoked [8]: (i) latent type of vent field, such a type of hydrothermal activ-
heat released by crystallization of basaltic magma in- ity can theoretically occur everywhere, either beneath
trusions in the lithospheric mantle; (ii) cooling heat the axial valley or off-axis. It only requires that sea-
mined from the lithospheric mantle; (iii) heat released water penetrates into unaltered peridotites. This can
by exothermic serpentinization reactions [69]. The happen off-axis if tectonic activity generates perme-
question is whether the only heat released by serpen- ability though fractures and faults. At this stage, we
tinization reactions can activate hydrothermal circula- know that serpentinization is active at the axis since all
tion and can account for fluid temperatures as high as the peridotites collected in the axial valley are serpen-
350 C. Lowell and Rona [89] show that the heat re- tinized. However, we lack appropriate data to evaluate
leased depends both on the serpentinization rate and the intensity of off-axis serpentinization. Searching for
methane and/or hydrogen anomalies in the water col-
the mass flow rate. They calculate that, in the absence
umn off-axis would probably be the best strategy.
of a regional background heat flux, serpentinization
rates need to be very high (103 kg/s) and flow rates
low (10 kg/s) to generate hydrothermal temperatures 8. Conclusions
above 100 C. Based on experimental diffusion rates,
Mac Donald and Fyfe [90] evaluated that serpentiniza- Recent developments on the understanding of the
tion rates are likely in the order of 0.1 kg/s. Even structure of mid-ocean ridges have demonstrated that
if this number can be increased if permeability gen- serpentinites make up a significant portion of the
erated by tectonic activity is considered, it remains seismically-defined crust. Therefore, the MOHO does
extremely low compared to the value required. If a not systematically correspond to the boundary be-
regional background heat flux is considered, the nec- tween a magmatic crust and a residual mantle. In
specific areas, particularly at the end of ridge seg-
essary rate of serpentinization decreases and becomes
ments generated in slow to ultraslow spreading envi-
more realistic. But to generate hydrothermal tempera-
ronments, it may instead correspond to a hydration
tures of 300 C, a high temperature regime, typical of
boundary, matching the 500 C isotherm under the
magmatic systems at mid-ocean ridges is necessary.
ridge axis. This has a major implication when eval-
Lowell and Rona conclude that high temperature hy-
uating the magmatic fluxes at mid-ocean ridges. The
drothermal fields such as Rainbow require magmatic
latter are clearly overestimated when the whole crustal
heat to drive the system. By contrast, the Lost City thickness is considered. It is therefore of major impor-
field can be driven by the heat released by serpen- tance to be able to quantitatively evaluate the propor-
tinization reactions. Provided that seawater can pene- tion of serpentinized peridotites in the crust. Besides
trate at depth, hydrothermal systems generated by ser- direct geological observation and sampling through
pentinization reactions are likely to be a significant dredging, diving and ocean drilling, it is necessary
component of hydrothermal activity at slow spread- to develop tools, using indirect measurements such as
ing ridges and generate anomalies in the water column. seismic velocity profiles and/or magnetic properties.
Bach et al. [8] came to the same conclusion. Assum- Serpentinite fabrics show that most of the serpen-
ing a heat capacity of seawater of 4 J g1 K1 and a tinization occurs under static conditions. Determina-
water/rock ratio of 1, they calculate that serpentiniza- tions of serpentine species show the predominance of
tion can heat up the circulating water by 25 to 150 C. lizardite. By contrast, mineral associations as well as
This can account for the range of temperatures mea- oxygen isotope fractionation demonstrate that serpen-
sured at the Lost City type of hydrothermal field as tinization reactions start at high temperature, in the
well as for the formation of sepiolite deposits, but not range of 300500 C, even if lower temperatures are
for the Rainbow type of field. also documented. This suggests that the nature of the
848 C. Mvel / C. R. Geoscience 335 (2003) 825852

serpentine species may be controlled by factors other bow type hydrothermal vent field with high tempera-
than temperature. High temperature serpentinization ture black smokers, however, seems to require a mag-
likely starts at depth, at the ridge axis, by interaction of matic heat source. In contrast, the Lost City-type hy-
seawater or seawater derived fluids with cooling peri- drothermal vent field could be generated by serpen-
dotites. The lack of evidence for a massive leaching tinization reaction heat only. Biological activity, sup-
of major elements favours a compensation of the de- ported by the production of hydrogen and methane, is
crease in density by a volume increase. Because ser- associated with active venting. The recent discovery of
pentinization requires a major flux of water, this raises these hydrothermal fields stresses that serpentinization
the question of fluid penetration. The extensional en- at mid-ocean ridges must be considered in thermal and
vironment must generate permeability through normal geochemical budgets.
faulting, fissuring and cracking. At temperatures be-
low 500 C, the availability of water must be the lim-
iting factor for extensive serpentinization. Acknowledgements
The serpentinization of essentially anhydrous peri-
dotites results in a massive hydration and oxidation. This review paper benefited from discussions with
Major elements do not seem to be strongly affected by a number of colleagues, in particular P. Agrinier,
serpentinization, although the local rodingitization of M. Cannat, J. Escartin, G. Frh-Green and W.E.
associated gabbroic dikes and formation and talcshists Seyfried. I thank J. Honnorez for encouraging me to
require the mobilization of calcium and silicium. The write this synthesis and for his constructive remarks.
global chemical fluxes between peridotite and seawa- This is IPGP contribution number 1949.
ter, however, remain poorly constrained. They seem
to be dependent on the temperature and the mineral
association. In particular, the presence and degree of References
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