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Earth and Planetary Science Letters 212 (2003) 417^432
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Serpentinization of the forearc mantle
Roy D. Hyndman a; , Simon M. Peacock b
a
Paci¢c Geoscience Centre, Geological Survey of Canada, 9860 W. Saanich Rd., Sidney, BC, Canada V8L 4B2
b
Department of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404, USA
Received 15 July 2002; received in revised form 5 May 2003; accepted 9 May 2003
Abstract
A wide range of geophysical and geological data indicate that extensive serpentinization in the forearc mantle is
both expected and observed. Large volumes of aqueous fluids must be released upwards by dehydration reactions in
subducting oceanic crust and sediments. Subduction of oceanic lithosphere cools the overlying forearc such that low
temperature hydrous serpentine minerals are stable in the forearc mantle. Over several tens of millions of years
estimated fluid fluxes from the subducting plate are sufficient to serpentinize the entire forearc mantle wedge.
However, fluid infiltration is probably fracture controlled such that mantle serpentinization is heterogeneous.
Geological evidence for hydration of the forearc mantle includes serpentine mud volcanoes in the Mariana forearc
and serpentinites present in exposed paleo-forearcs. The serpentinization process dramatically reduces the seismic
velocity and density of the mantle while increasing Poisson’s ratio. Serpentinization may generate seismic reflectivity,
an increase in magnetization, an increase in electrical conductivity, and a reduction in mechanical strength.
Geophysical evidence for serpentinized forearc mantle has been reported for a number of subduction zones including
Alaska, Aleutians, central Andes, Cascadia, Izu-Bonin^Mariana, and central Japan. Serpentinization may explain
why the forearc mantle is commonly aseismic and in cool subduction zones may control the downdip limit of great
subduction thrust earthquakes. Flow in the mantle wedge, induced by the subducting plate, may be modified by the
low density, weak serpentinized forearc mantle. Large volumes of H2 O may be released from serpentinized forearc
mantle by heating during ridge subduction or continent collision.
Crown Copyright 2003 Elsevier Science B.V. All rights reserved.
Keywords: forearc; mantle; serpentine; subduction
1. Introduction
Large volumes of aqueous £uids are released
from subducting plates (e.g., [1]) and at depths
* Corresponding author. Tel.: +1-250-363-6428;
Fax: +1-250-363-6565.
E-mail addresses: [email protected] (R.D. Hyndman),
[email protected] (S.M. Peacock).
of about 100 km such £uids may trigger partial
melting in the overlying mantle wedge, the source
of arc volcanism (e.g., [2]). Subducting sediments
and altered oceanic crust contain free water in
pore spaces and bound water in hydrous minerals.
At shallow depths, free water is expelled by compaction of subducted sediments and collapse of
porosity in the upper oceanic crust. At greater
depths, extending to at least 200 km, aqueous
£uids are produced by progressive metamorphic
0012-821X / 03 / $ ^ see front matter Crown Copyright 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0012-821X(03)00263-2
418
R.D. Hyndman, S.M. Peacock / Earth and Planetary Science Letters 212 (2003) 417^432
conductivity,
strength.
and
reduction
in
mechanical
2. Serpentinization
Fig. 1. Schematic cross section illustrating £uid expulsion
from subducting oceanic crust and sediments, and serpentinization of the overlying forearc mantle.
dehydration reactions involving numerous hydrous minerals (e.g., [3,4]). Fluid production generally decreases with depth; most £uid is released
beneath the forearc, whereas smaller amounts are
expected to be released at depths of arc magma
generation and beneath the backarc. Note that in
this text we use ‘water’ for H2 0, although the
conditions may be such that it is a supercritical
£uid.
In this article we document the wide range of
geological and geophysical evidence and modeling
that give a consistent picture of extensive serpentinization of the forearc mantle between the
trench and volcanic arc (Fig. 1). To our knowledge, serpentinization of the forearc mantle was
¢rst proposed by Fyfe and McBirney [5] to explain the uplift of coast ranges that commonly
parallel subduction trenches. In subduction zones,
the forearc mantle is exceptionally cool such that
serpentine and related hydrous minerals are stable. In contrast, the backarc mantle is usually too
hot for hydrous minerals to be stable. The forearc
is underlain by a subducting plate that releases
H2 O-rich £uids that may migrate into and hydrate the overlying mantle. The presence of serpentine and other hydrous minerals has signi¢cant
e¡ects on the physical and mechanical properties
of the forearc mantle, including a decrease in seismic velocity, increase in Poisson’s ratio, generation of seismic re£ectivity, increase in magnetization, reduction in density, increase in electrical
Dry forearc mantle is inferred to be composed
of depleted ultrama¢c rocks consisting primarily
of olivine and orthopyroxene with lesser amounts
of clinopyroxene and spinel. Harzburgites (olivine
+orthopyroxene rocks) and dunites (olivine-rich
rocks) are the most abundant ultrama¢c rocks in
supra-subduction zone ophiolites (e.g., [6]). Protoliths of serpentinized ultrama¢c rocks recovered
from Mariana forearc serpentine seamounts on
Ocean Drilling Project Leg 125 consist mostly of
harzburgite with lesser amounts of dunite [7].
At temperatures less than 700‡C, a number of
hydrous minerals are stable in ultrama¢c bulk
compositions. The hydration of depleted mantle
can be described using the simple MgO^SiO2 ^
H2 O system (e.g., [8,9]). The addition of H2 O to
a mantle wedge composed of olivine and orthopyroxene may generate a variety of hydrous minerals including serpentine (antigorite, chrysotile,
lizardite), talc, and brucite, with the speci¢c mineral assemblage depending on temperature (T),
pressure (P), and bulk composition [10,11] (Fig.
2).
Serpentine minerals, VMg3 Si2 O5 (OH)4 , are the
most abundant hydrous minerals in altered ultrama¢c rocks because the Mg:Si ratio of serpentine
(1.5) lies between that of olivine (2) and orthopyroxene (1). Antigorite is the stable serpentine mineral in ultrama¢c rocks metamorphosed under the
moderate temperatures of blueschist and greenschist facies conditions [10] and is stable to 620‡C
at 1 GPa [12] (Fig. 2). At lower temperatures
( 6 V350‡C), chrysotile and lizardite are stable
in the lower grade zeolite and pumpellyite facies
[10].
Brucite, Mg(OH)2 , coexists with serpentine in
olivine-rich compositions at T 6 500‡C (Fig. 2).
Brucite is a common mineral in serpentinites
[13], but its ¢ne-grained nature makes it di⁄cult
to estimate modal abundances. The maximum
amount of brucite in hydrated mantle will occur
for a dunite protolith consisting of 100% olivine.
R.D. Hyndman, S.M. Peacock / Earth and Planetary Science Letters 212 (2003) 417^432
Fig. 2. P^T diagram showing important reactions in the
MgO^SiO2 ^H2 O system and calculated P^T conditions of
cool forearc mantle in oceanic (shallow dark shaded area)
and continental (deeper light shaded area) subduction zones.
Antigorite serpentine is stable over a wide range of forearc
conditions. Ternary diagrams show stable three-phase mineral assemblages in hydrated ultrama¢c bulk compositions;
note brucite is not stable in hydrated mantle at T s 500‡C
(shaded triangles). Chrysotile reactions from [10]; talc reactions from [12]; antigorite+brucite breakdown reaction calculated using Holland and Powell’s (1998) thermodynamic database. Stability ¢eld for anthophyllite at 600 6 T 6 800‡C
and P 6 1.2 GPa omitted for clarity. Mineral abbreviations:
antig, antigorite; bru, brucite; en, enstatite (orthopyroxene);
fo, forsterite (olivine); per, periclase; qtz, quartz.
Complete hydration of pure forsterite (olivine) at
400‡C and 1 GPa will yield a rock consisting of 78
vol% antigorite and 22 vol% brucite ; however,
only smaller amounts of brucite are usually observed (e.g., [17]). Talc, Mg3 Si4 O10 (OH)2 , coexists
with serpentine in pyroxene-rich compositions at
T 6 700‡C. Talc also may form in the forearc
mantle along the plate interface (Fig. 1) [14]
where in¢ltrating £uids derived from the underlying subducting crust are silica-saturated [15] and
mechanical mixing of mantle with siliceous sediments may occur [16].
Additional minerals, which lie outside the simple MgO^SiO2 ^H2 O system, may form in hydrated forearc mantle depending on bulk composition. Hydration of Al-bearing mantle rocks will
419
form chlorite. At T 6 V500‡C, diopside is the
stable Ca-bearing mineral ; tremolite is the stable
Ca-bearing mineral at higher temperatures [10].
Fe partitions into serpentine, brucite, and talc to
a limited extent, but the serpentinization process
invariably produces magnetite (e.g., [17]). Most
serpentinite is strongly magnetic, to the degree
that commonly there are strong deviations in
magnetic compass directions near surface outcrops. Thus, cold serpentinized mantle might be
detected by magnetic anomalies caused by magnetite as discussed below.
Except for unusual metasomatic rocks, serpentine is the most abundant hydrous mineral in hydrated ultrama¢c rocks at T 6 500‡C. Hacker et
al. [18] present normative calculations, assuming
full hydration, for three di¡erent ultrama¢c compositions: lherzolite (enriched upper mantle), depleted lherzolite, and harzburgite (depleted upper
mantle). In their calculation, at P = 1 GPa and
T 6 500‡C, antigorite makes up s 50 vol% of
the rock in each of the three bulk compositions.
3. Thermal structure of the forearc mantle
The thermal structure of a subduction zone is
the primary control on the location of slab dehydration reactions that produce aqueous £uids and
the region where hydrous minerals are stable in
the forearc mantle. In addition, temperature is
important because of the thermal dependence of
the physical properties of the forearc mantle
rocks. Interpretation of geophysical data in terms
of forearc mantle hydration requires separation of
the e¡ects of temperature from the e¡ects of hydration.
Numerous general subduction zone thermal
models have been presented in the past (see review
by [19]). More recently, two-dimensional ¢nite element models have been constructed for speci¢c
subduction zones that more accurately predict
slab and forearc temperatures down to V100
km or more (e.g., [20,21]). In these models the
thermal structure of the forearc mantle is found
to be most sensitive to the age of the incoming
oceanic plate, the convergence rate, and the geometry of the plate interface (slab dip). Also im-
420
R.D. Hyndman, S.M. Peacock / Earth and Planetary Science Letters 212 (2003) 417^432
Fig. 3. Calculated thermal structure of the warm N. Cascadia continental subduction zone and associated surface heat
£ow data (after [20]) (BSR, bottom simulating re£ector;
ODP, Ocean Drilling Program). Numerical two-dimensional
model results extend to 200 km seaward from the trench.
Deep temperatures near the arc and abrupt rise are from
simple one-dimensional models (dashed isotherms).
portant are the thickness of insulating sediment
on the incoming oceanic crust, and the radioactive
heat generation of the overlying accretionary
prism and forearc crust [22]. Most studies have
concluded that the thermal e¡ect of frictional
shear heating and metamorphic reactions are
small (e.g., [19,23,24]), but these processes remain
a source of uncertainty in the model temperatures.
Advective heat transport by the expelled £uids
also should not be important except perhaps at
the very start of underthrust sediment consolidation. At least 1 mm/yr is needed (e.g., [26]).
Heat £ow pro¢les across the forearc provide an
important check on the thermal models. Most
subduction zone forearcs are characterized by observations of very low heat £ow (30^40 mW m32 )
as predicted by the models. Forearcs are very cool
as a consequence of the heat removed by the
underthrusting of the cool near-surface rocks of
the oceanic lithosphere. Extensive marine and
land heat £ow data are available for comparison
with model predictions, such as Cascadia [22,25]
(Fig. 3). On this margin, a correction is needed for
the heat £ow reduction e¡ect of rapidly thickening large accretionary prisms (e.g., [26]). After this
correction, good agreement has been found between the subduction thermal model and observed
heat £ow, within the uncertainties in the heat £ow
and other thermal data. Heat £ow decreases landward for young hot subduction zones, whereas
heat £ow is nearly constant across the forearc
for margins subducting old and cold oceanic lithosphere [27].
Calculated temperatures in the forearc mantle
are signi¢cantly di¡erent for subduction zones
where old cold lithosphere is being underthrust
compared to young hot oceanic lithosphere (e.g.,
[21]) (Fig. 4). Calculated forearc temperatures are
400^600‡C for warm continental subduction
zones with young incoming oceanic lithosphere
such as Cascadia (Fig. 3), SW Japan (Fig. 4),
Mexico [28], and S. Chile near the subducting
Chile ridge [27]. In cool continental subduction
zone forearcs such as NE Japan (Fig. 4), Alaska,
and N. Chile, where the subducting slab intersects
the forearc Moho at 30^50 km depth, the calculated temperatures in the uppermost forearc mantle are 150^250‡C. Uppermost forearc mantle
temperatures are especially low for island arcs
with thin forearc crust (10^15 km) such as the
Izu-Bonin subduction zone where the tempera-
Fig. 4. Calculated thermal structure of (A) warm continental
subduction zone (SW Japan; Cascadia is similar), (B) cool
continental subduction zone (NE Japan), and (C) cool oceanic subduction zone (Izu-Bonin) [21,99]. Forearc mantle where
serpentine is stable is shaded.
R.D. Hyndman, S.M. Peacock / Earth and Planetary Science Letters 212 (2003) 417^432
421
tures in the mantle wedge corner are as low as
V100‡C (Fig. 4C).
The region of the forearc upper mantle in
which hydrous minerals such as serpentine are
stable is commonly 50^100 km wide for warm
continental subduction zones (Fig. 4), and 100^
150 km wide for cool continental subduction
zones (Fig. 4). The hydrated mantle region may
be as much as 200 km wide for cool oceanic subduction zones, where the forearc crust is thin and
the forearc mantle is reached by the underthrusting oceanic crust at shallow depth within about 50
km of the trench. However, the steeper slab angles
for some West Paci¢c subduction zones such as
the Mariana, reduce the trench^arc distance.
4. Dehydration of the downgoing plate and upward
£uid expulsion rates
Large amounts of aqueous £uid are expelled
from the downgoing oceanic crust and overlying
sediments with increasing pressure and temperature. Estimated £uid production rates of V0.1
mm/yr or 100 m/Myr (Fig. 5) suggest that over
several tens of Myr enough water is released from
the subducting oceanic crust and sediments to hydrate the entire forearc mantle [4,29]. The main
factors controlling £uid production beneath the
forearc mantle are the convergence rate, the thickness of the forearc crust (i.e., forearc Moho
depth), and the amount of water in the incoming
crust and sediments. The rate at which free and
bound water enters a subduction zone is approximately proportional to the convergence rate. In
oceanic subduction zones where the forearc crust
is thin, the subduction thrust intersects the forearc
mantle at a shallow depth where larger amounts
of £uid are driven o¡ the subducting sediments
and oceanic crust. For the thicker continental
crust, more of the water goes into the forearc
crust, rather than the forearc mantle. Thus,
more £uid may be available for hydration of the
forearc mantle of oceanic compared to continental subduction zones. The degree of serpentinization will be controlled by the amount of H2 O that
actually chemically interacts with the forearc
mantle. Field observations indicated that £uid
Fig. 5. Estimated amounts of £uid released upward from porosity collapse and dehydration reactions of subducted sediments and oceanic crust for (1) a cool subduction zone (N.
Japan), and (2) a warm subduction zone (SW Japan). Locations of the main progressive dehydration reactions are
marked; £uid expulsion is probably more smoothed with
depth. Note (a) the scales are a factor of two di¡erent to accommodate the approximately factor of two di¡erence in
subduction rate, (b) the amount of £uid for 1, 2, 3 is approximately proportional to the amount of sediment subducted. See text for discussion.
£ow during serpentinization tends to be fracture
controlled rather than pervasive [30]. Serpentinization of the forearc mantle is probably very heterogeneous and some £uid may escape to the surface. In higher temperature forearcs, faster
reaction rates and more rapid di¡usion should
permit greater £uid penetration from fractures
and channels. However, as noted above, in higher
temperature subduction zones there may be less
£uid available because a larger fraction of the slab
dehydration will have occurred before the subducting slab reaches the forearc mantle.
422
R.D. Hyndman, S.M. Peacock / Earth and Planetary Science Letters 212 (2003) 417^432
Fig. 5 shows estimates of the amounts of £uids
that may be expelled from the subducting oceanic
crust and overlying sediments as a function of
distance from the trench for examples of cool
(N. Japan) and warm (SW Japan) continental
subduction zones. In this simple ¢rst order approximation, an extreme example of 1000 m of
50% porosity sediment is assumed to be underthrust. The H2 O expelled at shallow depths comes
primarily from this source and the amount of £uid is nearly proportional to the thickness of subducted sediments (components 1, 2, 3, in Fig. 5).
Therefore, the £uid expelled from subducted sediments may be roughly scaled by the thickness of
subducted sediment section. The upper 1000 m of
the oceanic crust is assumed to contain an average
of 7% porosity based upon Ocean Drilling Program core and downhole measurements (e.g.,
[31]). The porosity deeper in the oceanic crust is
smaller and is ignored in this ¢rst order discussion. Also, the porosity may decrease with aging
of the crust. In the forearc, we assume the 500 m
of fully compacted, zero-porosity sediments consists of 400 m of terrigenous sediments containing
7.5 wt% bound H2 O, 50 m of siliceous sediments
containing 11 wt% H2 O, and 50 m of anhydrous
calcareous sediments [32,33]. The basaltic and
gabbroic sections of the oceanic crust are assumed
to contain 2 and 1 wt% bound H2 O, respectively
[3]. The bound H2 O estimates have an uncertainty
of about a factor of two. The dehydration of serpentinized peridotite in the uppermost mantle of
the incoming plate or incorporated into the oceanic crust have not been included, but they could
be a signi¢cant additional source of £uids (e.g.,
[34,35]). At shallow depths in subducting plates,
there are two competing processes in the underthrusting sediments and oceanic crust: £uid loss
from the free water released through porosity collapse and £uid incorporated into low temperature
hydrous minerals, such as zeolites. Thus, the
amount of £uid expelled from the slab is quite
uncertain. However, for the underthrusting sediments and porous uppermost oceanic crust, there
is more than enough water for complete hydration, and in our calculations we assume the remaining water is expelled. We assume vertical upward £uid £ow. Some £uid may be expelled
seaward along the subduction thrust, but we are
not aware of any direct evidence of such £ow or
of an associated heat £ow anomaly.
In both the cool and warm continental subduction zones, most H2 O released beneath the forearc
mantle comes from the subducted oceanic crust
(elements 4^6 in Fig. 5). H2 O released from compaction and dehydration of subducted sediments
occurs mostly beneath the forearc crust (Fig. 5).
In an oceanic subduction zone (not shown), where
the forearc mantle is shallower, the release of H2 O
from subducted sediments should extend downdip
to beneath the forearc mantle. However, in many
oceanic subduction zones there is very little incoming sediment.
5. Geological evidence for serpentinization of the
forearc mantle
5.1. Serpentine mud volcanoes
Active and inactive serpentinite mud volcanoes
observed in the Mariana and Izu-Bonin forearcs
provide direct evidence for hydration of the forearc mantle by slab derived £uids (e.g., [36]). Geochemical arguments that £uids emanating from
the mud volcanoes rise from the subducting plate
are summarized by [37^39]. These serpentinite
mud volcanoes may occur just arcward of the
intersection of the subduction thrust with the
forearc mantle. The Mariana forearc crust is
V10 km thick [40] and Fryer et al. [39] estimate
depths of 15^25 km to the subducting slab beneath the Mariana mud volcanoes. In the Izu-Bonin forearc, the seismic structure model of Suyehiro et al. [41] suggests that the serpentine mud
volcanoes lie above the intersection of the plate
interface and forearc Moho.
Serpentine mud volcanoes may be particularly
well exposed in the Mariana and Izu-Bonin oceanic subduction zones because there the forearc
crust is especially thin and sediment cover is
sparse. In oceanic subduction zones, the forearc
mantle is reached by the subducting plate at shallow depths of V10 km where the largest amounts
of £uid are being driven o¡ the subducting sediments and upper oceanic crust. Although still sig-
R.D. Hyndman, S.M. Peacock / Earth and Planetary Science Letters 212 (2003) 417^432
ni¢cant, smaller amounts of £uid are being driven
o¡ at the V40 km depths of continental forearcs.
An example of possible serpentine masses reaching the surface in a continental forearc is described by Kido et al. [42] for Shikoku in southwest Japan.
5.2. Forearcs of paleo-subduction zones
Exposed ultrama¢c rocks, interpreted to have
formed in paleo-subduction zones, are extensively
hydrated. In many cases, it is concluded that the
hydration predates exposure, i.e., serpentinization
is not the result of meteoric water in¢ltration subsequent to exposure at the surface (e.g., [17]). The
base of the ultrama¢c section of Cordilleran-type
ophiolites is commonly serpentinized by £uids derived from underthrust rocks (e.g., [43]). In several well studied regions the ultrama¢c hanging
wall of a paleo-subduction zone is extensively hydrated, including Santa Catalina Island, California [16], the Trinity peridotite and Josephine
ophiolite in the Klamath Mountains of California
[30,44], and the Shuksan suite of Washington [45].
423
tensen and Mooney [51]. The temperature correction from room temperature to 400^500‡C of
about 30.2 km s31 gives good agreement with
that commonly observed for stable continental
uppermost mantle (average 8.13 km s31 [51]). A
Vp of 7.2^7.6 km s31 in the forearc mantle thus
represents 15^30% serpentinization (Fig. 6). The
Vp for 100% serpentinization is about 5.1 km s31 .
The temperature coe⁄cient of velocity for serpentinite is small up to temperatures between 550 and
800‡C for di¡erent hydrous minerals [52].
The Poisson’s ratio (related to Vp /Vs ) also is
diagnostic of the degree of serpentinization. In
the compilation of laboratory data of Fig. 6
[46,47], the Poisson’s ratio for peridotite of
0.26^0.28 (Vp /Vs = 1.76^1.81) increases to about
0.30 (1.87) at 15% serpentinization, and to 0.38
6. Seismic properties of serpentinite
Hydrated mineral assemblages generally have
substantially lower seismic velocities than their
parent rocks; this e¡ect is well known for serpentinite. Fig. 6 shows compressional wave velocity
(Vp ), shear wave velocity (Vs ), and Poisson’s ratio
for hydrated mantle peridotite samples as a function of degree of serpentinization from [46] (see
also [47^49]). The data were obtained at P = 1
GPa appropriate for the base of V35 km thick
continental crust. Seismic velocities are about
0.3 km s31 slower for the 200 MPa con¢ning
pressure appropriate for the base of thinner oceanic crust. However, at 200 MPa there may be
some residual cracks generated by sampling remaining in the laboratory samples, so the latter
velocities should be taken as minimum values.
The average zero-serpentine Vp for this suite of
laboratory data is V8.4 km s31 , similar to that
from larger compilations of unaltered ultrama¢c
rocks by Rudnick and Fountain [50] and Chris-
Fig. 6. A compilation of laboratory mantle peridotite Vp , Vs ,
Vp /Vs , and Poisson’s ratio as a function of degree of serpentinization [46], with best ¢t linear relations. Vp also is shown
corrected to the temperature of normal stable uppermost
mantle.
424
R.D. Hyndman, S.M. Peacock / Earth and Planetary Science Letters 212 (2003) 417^432
(2.27) at 100% serpentinization. The best ¢t Poisson’s ratio versus degree of serpentinization line
for these data gives an unaltered peridotite value
of 0.28 (1.81). This is slightly higher than that for
larger compilations of unaltered ultrama¢c rocks
of 0.26 (Vp /Vs = 1.76) [48], but within the data
scatter. Also, Poisson’s ratio is sensitive to the
olivine composition [48]. An o¡set of about
30.02 for Poisson’s ratio (30.05 for Vp /Vs ) may
be needed for the application of the laboratory
data best ¢t line to averages from seismic data.
Christensen [48] and Rudnick and Fountain [50]
conclude that the temperature e¡ect on Poisson’s
ratio is usually very small. We note that the Poisson’s ratio for ma¢c deep crustal rocks, 0.29^0.30
(Vp /Vs = 1.84^1.87) (e.g., [48]), is higher than that
for unhydrated ultrama¢c mantle rocks of 0.26
(1.76). Therefore, both the Vp and Poisson’s ratio
of moderately hydrated mantle compositions
(V30% serpentinization) are not much di¡erent
from unaltered ma¢c crustal rocks. However, at
higher degrees of serpentinization (over 40%,
Vp 6 6.5 km s31 ), the Poisson’s ratio for serpentinized mantle rocks ( s 0.32) (Vp /Vs s 1.94) is
much higher than that for ma¢c rocks with a
similar Vp (e.g., [53]).
7. Geophysical evidence for forearc mantle
serpentinite
7.1. Upper mantle reference velocities with no
serpentinite
The uppermost mantle refracted or head wave
(Pn ) velocity is usually one of the best determined
parameters in wide-angle seismic refraction studies. The relation between Pn velocity and Moho
temperature has been estimated from heat £ow
data by Black and Braile [54] (Fig. 7). These
data show the large temperature e¡ect on Pn velocity. For cratons and other stable continental
areas, the estimated Moho temperature is generally 350^450‡C and the Pn velocity is 8.15^8.25
km s31 . For continental backarc areas the Pn velocity is commonly 7.8^7.9 km s31 , in agreement
with that predicted for ultrama¢c composition at
high temperatures with no serpentinization. An
Fig. 7. Uppermost mantle Pn velocity as a function of (a)
surface heat £ow, and (b) inferred Moho temperature (lower). Range of velocity^temperature relations from laboratory
data also is given for comparison. Pn velocity for cold forearcs is unusually low, deviating substantially from the predictions of both relations.
excellent example is the detailed series of Lithoprobe seismic refraction lines in the southern Canadian Cordillera backarc summarized by Clowes et
al. [55]. The Moho temperature is estimated to be
800^1000‡C [56], and the Pn velocity range of 7.8^
7.9 km s31 expected for that temperature is observed (Fig. 7). Fig. 7 also shows the correlation
of Pn velocity with heat £ow for continental crustal areas. Except for uncommon areas of very
high crustal heat generation, uppermost mantle
temperatures usually correlate closely with surface
heat £ow. Low heat £ow areas (V40 mW m32 )
R.D. Hyndman, S.M. Peacock / Earth and Planetary Science Letters 212 (2003) 417^432
exhibit Pn velocities of about 8.2 km s31 and most
high heat £ow areas (80^100 mW m32 ) exhibit
Pn velocities of 7.8^7.9 km s31 .
Other important controls of uppermost mantle
seismic velocities where there is no hydration are
composition variations and anisotropy, but we
believe that temperature is the most important
control. This conclusion is in contrast to that
for deep crustal velocities where composition appears to be the dominant control of seismic velocity. The other two e¡ects remain important uncertainties for upper mantle velocity. Fliedner and
Klemperer [57] discuss the e¡ect of upper mantle
composition on velocity ; unusual compositions
are required for the Vp to be as low as 7.8 km
s31 at low temperatures. In extreme cases the continental mantle velocity anisotropy may be as
large as 0.2 km s31 (e.g., [58]), but other well
studied continental areas exhibit almost no Pn
velocity anisotropy in the upper mantle (e.g.,
[55]).
7.2. Low seismic velocities and high Poisson’s
ratios in forearc mantle
Low seismic velocities and high Poisson’s ratios
observed in subduction zone forearcs strongly
suggest that forearc mantle is partially serpentinized. Several types of data provide information
on forearc mantle velocities : (1) Both Vp and Vs
in the uppermost mantle can be obtained by seismic tomography studies using a suite of earthquake sources, (2) Vs contrasts across the forearc
Moho can be determined by receiver function
studies, (3) forearc Pn arrivals provide Vp information for the uppermost forearc mantle,
although they are often weak, (4) that forearc
Pn arrivals are weak or absent is also diagnostic
of low mantle velocities such that the Moho contrast is lowered (e.g., [59]), and may provide an
important mapping tool for hydration. We note
the compilation of Zhao [60] who summarized
low upper mantle velocities in forearc of the Mariana, Izu-Bonin, northeast Japan, Alaska, Chile,
and northern New Zealand.
Where observed, forearc P-wave velocities are
commonly 7.8 km s31 or less, similar or lower
than the adjacent backarc. Surface heat £ows in
425
forearcs are 30^40 mW m32 , indicating forearc
mantle T 6 400‡C, so the expected Pn velocity
for anhydrous mantle is greater than 8.2 km
s31 . At these low temperatures, P-wave velocities
of less than 7.8 km s31 can be explained by more
than 15% average serpentinization of the uppermost mantle.
7.2.1. Cascadia
An excellent compilation of the evidence for
low P-wave and S-wave velocities in the Cascadia
forearc has been presented recently by Brocher et
al. [71], and we provide only a short summary.
Four types of evidence are available : P- and
S-wave velocities from seismic tomography, Pn
velocities from wide-angle active source experiments, receiver function S-wave contrast at the
Moho (e.g. [69]), and lack of or weak uppermost
mantle Pn velocities. The most striking evidence
for serpentinization in the Cascadia forearc mantle is the receiver function data recorded by a
dense broadband array. Scattered teleseismic
waves reveal a normal continental Moho at
36 km depth in the Cascadia volcanic arc. Seaward, as the trench is approached, the velocity
contrast at the Moho ¢rst disappears and then
reappears with reverse polarity [70]. The reverse
polarity indicates a Vs for the uppermost forearc
mantle that is less than for the overlying crust.
Near the wedge corner, the maximum Vs downward perturbation is V10% suggesting mantle
serpentinization levels could be as high as 50^
60% in this region [70]. In the northern Cascadia
forearc, Pn velocities are usually less than 7.8 km
s31 [67,68]. Brocher et al. [71] mapped low mantle
velocities from controlled source and earthquake
tomography and weak wide-angle re£ections from
the top of the forearc upper mantle in a narrow
region along the margin. The forearc upper mantle velocities range from 7.2 to 7.7 km s31 representing up to 30% serpentinization.
7.2.2. Central Andes
The shallow mantle beneath the western Cordillera of Bolivia (18^21‡S) exhibits slightly reduced average Vp ( 6 8 km s31 ), slightly reduced
average Vs ( 6 4.4 km s31 ), and high Vp /Vs
(V1.83) [61]. Locally strong seismic velocity
426
R.D. Hyndman, S.M. Peacock / Earth and Planetary Science Letters 212 (2003) 417^432
anomalies are observed with Vp V7.8 km s31 ,
Vs V4.3 km s31 , and Vp /Vs V1.86 [61]. The low
seismic velocity, high Vp /Vs region lies above the
subducted Nazca plate beneath the forearc and
arc region and is interpreted to re£ect extensive
hydration [61]. These authors also suggest possible local partial melting in the mantle, but this is
more likely beneath the arc, rather than in the
very cool forearc. Beneath northern Chile
(V33‡S), the Andean forearc mantle is characterized by anomalously low Vp and high Vp /Vs (1.79
to s 1.87), based on three-dimensional tomographic images derived from simultaneous inversions of local earthquake data [62]. The zone of
high Vp /Vs occurs between 70 and 120 km depth,
overlies the subducted Nazca plate, and is interpreted to re£ect hydration of the mantle wedge
caused by £uids released from the subducting
plate [62].
7.2.3. Central Japan
Kamiya and Kobayashi [63] detected serpentinized forearc mantle beneath the Kanto district of
central Japan. They observed low seismic velocities (Vp = 6.9 km s31 , Vs = 3.4 km s31 ) 20^70 km
deep in the forearc mantle adjacent to the subducted Philippine Sea plate. Between 20 and
45 km depth, the Poisson’s ratio is greater than
0.3 (up to 0.34), as compared to surrounding
rocks with normal Poisson’s ratios of V0.25. Kamiya and Kobayashi [63] interpreted the forearc
mantle to be 50% serpentinized in this region. The
high Poisson’s ratio region also corresponds approximately to the downdip limit of thrust earthquakes, to the rupture limit of the Kanto 1923
great earthquake, and to the base of the subduction thrust locked zone estimated from GPS
data.
7.2.4. Izu-Bonin^Mariana
A marine seismic re£ection^refraction survey of
the Izu-Bonin subduction zone (32‡N) by Suyehiro and colleagues [41,64] found that the well
de¢ned Moho beneath the arc disappears beneath
the forearc because of low seismic velocities in the
mantle (Vp V7.2 km s31 ) inferred to result from
hydration. Very low Vp (V5 km s31 ) are observed beneath a forearc serpentinite diapir [41].
An extreme case may be the Mariana forearc
where Hussong and Uyeda [40] concluded that
the uppermost mantle velocities are 6.1^7.2 km
s31 , although the data for this region are limited.
These velocities represent 40^60% serpentinization
(Fig. 6).
7.2.5. Alaska^Aleutians
A tomographic inversion beneath southern
Alaska revealed a pronounced low Vp anomaly
(at least 10% reduction) in the forearc mantle
[65]. The anomaly beneath the Cook Inlet extends
upward from the subducting Paci¢c plate into the
overlying mantle wedge from 20 to 120 km depth
[65]. In the same area, a tomographic study by
Zhao et al. [66] revealed a V50 km thick zone
of low Vp located V50 km above the subducted
slab that may correspond, in part, to the anomaly
seen in Kissling and Lahr’s [65] study. Fliedner
and Klemperer [57] found average Vp of V7.7
km s31 , and as low as 7.4 km s31 , for the eastern
Aleutian forearc mantle. Vp increases downward
and away from the mantle wedge corner. Fliedner
and Klemperer [57] proposed that the low seismic
velocities could be explained by pyroxene-rich cumulates, but also suggested that serpentinization
could account for the low seismic velocities in the
forearc mantle, and might also explain the observed mantle re£ectivity. These velocities correspond to 10^20% serpentinization (Fig. 3). The
limited Vs data of Fliedner and Klemperer [57]
also suggest an unusually high Poisson’s ratio in
the Aleutian forearc mantle consistent with serpentinization.
7.3. Other geophysical expressions of forearc
mantle serpentinite
Magnetic susceptibilities increase up to 30 times
with increasing serpentinization, from about 1033
to about 3U1032 S.I. [72]. Thus, cold serpentinized mantle where the temperature is below the
magnetite Curie temperature may be detected by
magnetic anomalies caused by magnetite. A magnetic source located in the uppermost mantle
would yield a long spatial wavelength anomaly,
although tectonic processes often emplace serpentinite into the crust. Strong trench-parallel mag-
R.D. Hyndman, S.M. Peacock / Earth and Planetary Science Letters 212 (2003) 417^432
netic anomalies are ubiquitous just seaward of the
volcanic arcs in present and former subduction
forearcs, including NE Japan [73], Alaska [74],
and along much of western North America from
the southwestern British Columbia coast range
plutonic complex, through Washington and Oregon to the Great Valley of California [75,76].
Coles and Currie [75] and others show that the
short spatial wavelengths of the Cascadia magnetic anomalies require that at least part of the
source must lie in the upper to middle crust. Further study is required to determine whether a signi¢cant part of the magnetic signal is coming
from the forearc mantle.
Some serpentinites are characterized by high
electrical conductivity interpreted to be due to
interconnected magnetite [77], the same source
material suggested for the high magnetizations
of serpentinite. We have not examined this e¡ect
in detail, but one example of high conductivity
that probably lies in the forearc mantle is from
the MSLAB Cascadia magnetotelluric and magnetovariation study [78,79]. High conductivities
are found at a depth below about 25 km for
about 100 km seaward of the volcanic arc. In
contrast, only moderate conductivity was estimated for the Andes forearc by Echternacht et
al. [80].
Density also decreases substantially with increasing degree of serpentinization, from about
3200 kg m33 for unaltered ultrama¢c rocks to
about 2500 kg m33 for 100% serpentinization
[47^49]. Such a large density reduction may result
in substantial gravity anomalies, as inferred for
the Aleutian arc [81]. However, a large-scale 15^
30% serpentinization as suggested by the P-wave
velocity data in several subduction zones, only
reduces the upper mantle density from 3200 to
about 3000 kg m33 , which is still higher than
normal crustal densities. Where there is a very
high degree of serpentinization, the reduction of
density and seismic velocity are such that forearc
mantle serpentinite will not be easily distinguished
from greater crustal thickness in gravity modeling.
The e¡ect of dynamic topography in subduction
zones also complicates gravity modeling which
makes detection of reduced density forearc mantle
di⁄cult.
427
8. Consequences of forearc serpentinization
8.1. Aseismic forearc mantle
In subduction zones, the forearc mantle appears
to be aseismic. The maximum temperature for
earthquake behavior in continental crustal compositions is about 350‡C, based on laboratory
and ¢eld data (e.g., summary by Hyndman and
Wang [22]) which encompasses most forearc
crusts. The brittle^ductile transition for dry mantle rocks is 600^800‡C (e.g., [82]) which exceeds
the maximum temperature of most forearc upper
mantles. Dry mantle rocks in cool forearcs should
be much stronger than crustal rocks (e.g., [82]).
Thus, earthquakes might be expected to occur to
considerable depth in forearc mantle. However,
few if any earthquakes are reliably located within
the forearc mantle (e.g., for Cascadia, [83]; for
South America [84]). Serpentinite is dramatically
weaker than dry mantle rocks and may deform
aseismically under forearc P^T conditions.
8.2. Downdip limit of subduction thrust
earthquakes
For cool subduction zones, the downdip limit
for subduction great thrust earthquakes often corresponds to the intersection of the thrust with the
forearc mantle (Fig. 1) (e.g., [24,85]). This limit
may be explained by aseismic hydrous minerals
such as serpentinite present in the forearc mantle
wedge that exhibit stable sliding [14,27,86]. Just
above the slab interface itself, talc-rich rocks
may form in the mantle by the addition of silica
transported by rising £uids and by mechanical
mixing of underthrust siliceous rocks with the
overlying mantle [14]. In the laboratory, the behavior of serpentine minerals is complex, but they
generally exhibit stable-sliding aseismic behavior
(e.g., [87,88]). Recent experiments demonstrate
that brucite, a layered hydroxide, has a very low
coe⁄cient of friction (WV0.30 at room temperature decreasing to 0.20 at 300‡C and 0.23 at
450‡C) [89]. Serpentinites containing disseminated
brucite will have lower coe⁄cients of friction (10^
15% lower) than pure serpentinites [89]. Further
data are required to con¢rm this behavior under
428
R.D. Hyndman, S.M. Peacock / Earth and Planetary Science Letters 212 (2003) 417^432
forearc P^T conditions. We have not found data
on the sliding behavior of talc, but its layered
silicate structure also suggests very weak stablesliding behavior. Slow earthquakes (silent slip
events) that rupture the plate interface downdip
of the subduction thrust have recently been recognized in Cascadia [90]. This phenomenon may
be controlled by the rheologic behavior of forearc
serpentinite (e.g., [71]). The depth to the forearc
mantle (i.e., forearc Moho) and inferred maximum depth for subduction thrust earthquakes
varies from nearly 50 km in some continental subduction zones with a thick forearc crust to as little
as 10^15 km in oceanic subduction zones such as
the Mariana, that have thin forearc crust,
although the depth of the forearc Moho intersection is usually poorly known.
8.3. Flow in the mantle wedge
In subduction zones, mantle wedge convection
may be driven primarily by viscous coupling between the subducting plate and mantle wedge
(forced convection) [91] and by horizontal thermal gradients (free convection) (e.g., [92]). For
the thermal models presented in Fig. 4, mantle
wedge convection was simulated using an analytical corner £ow solution [93] in which a no-slip
velocity boundary condition is employed along
the plate interface and the wedge material has a
constant viscosity. More realistic models of mantle wedge convection incorporate the strong temperature dependence of mantle viscosity and
buoyancy forces (e.g., [94,95]).
Serpentinized forearc mantle, because of its
weak rheology and low density, will likely alter
the pattern of convection in the mantle wedge.
In almost all mantle wedge convection models,
the subducting slab and mantle wedge are assumed to be fully coupled such that the base of
the mantle wedge moves downward at the same
velocity as the subducting plate. Where the temperature is low, serpentinite, present at the base of
the mantle wedge seaward of the arc, should decrease the coupling between the subducting plate
and mantle wedge. Rather than deformation
being distributed throughout the mantle wedge,
much of the shear deformation will be focused
in the weak, serpentinized mantle adjacent to
the subducting plate. In addition, the positive
buoyancy resulting from serpentinization will act
to counter the negative thermal buoyancy that
drives the free convection component of mantle
wedge £ow. We propose that the weak rheology
and positive buoyancy of serpentinite will act to
isolate hydrated forearc mantle from the overall
mantle wedge £ow system. Models of mantle
wedge convection linking arc magmatism to
downdragged hydrous mantle wedge material
(e.g., [94,96]) need to consider the possible rheological e¡ects of forearc serpentinization.
8.4. Heating and dehydration of previously
hydrated forearc mantle
The dehydration of forearc mantle serpentinite
can occur by a number of processes. Simple stopping of subduction will allow temperatures to
slowly rise for some tens of Myr resulting in
slow dehydration of at least the deeper portions
of the forearc mantle wedge. More rapid dehydration and large upward £uid £uxes may occur from
high temperatures associated with other changes
in margin tectonics. Forearc temperatures are
high where spreading ridges are being subducted
[97] and there are slab windows (e.g., [98]), in
contrast to normal low forearc temperatures. In
such special areas the forearc upper mantle may
be largely dehydrated. Over tens of millions of
years, many active continental margins have
been subjected to a short period of high temperatures caused by the subduction of a spreading
ridge that swept along the margin. The consequence may be a period of forearc mantle dehydration and strong upward £uid expulsion that
migrates along the margin. Rehydration may follow ridge subduction as older oceanic lithosphere
is once again subducted and the forearc cools.
Although not as dramatic as ridge subduction, a
signi¢cant decrease in convergence rate can lead
to forearc warming and dehydration. A trenchward shift in the axis of arc volcanism, perhaps
due to slab steepening, might also trigger dehydration of the landward portion of previously hydrated forearc mantle.
The petrologic and thermal structure of the two
R.D. Hyndman, S.M. Peacock / Earth and Planetary Science Letters 212 (2003) 417^432
converging continental margins prior to continental collision is an important control for the nature
of orogeny. We have not examined the role of
hydrous minerals in the forearc mantle during
collision, but we point out several important expected consequences. First, although forearcs
have very low temperatures and might be taken
to be strong (in contrast of hot weak backarcs),
they may be weaker than expected due to the
presence of weak hydrous minerals in the forearc
upper mantle. A weak upper mantle may allow
detachment just below the base of the forearc
crust. The hydrated forearc mantle also may provide substantial aqueous £uid for orogenic magmatism during continental collision if there is
forearc heating, such as may occur due to underthrusting of high radioactive heat generation continental crust beneath the forearc.
9. Conclusions
We conclude that serpentinization of the forearc mantle is a global phenomenon linked to the
large volumes of free and bound water released
from subducted oceanic crust and sediments. Subduction chills the forearc mantle such that serpentine and related hydrous minerals are stable. Hydrous £uids derived from the subducting plate will
react strongly with ultrama¢c rocks in the overlying forearc mantle to produce serpentine minerals, brucite, and talc. There is direct geological
evidence of forearc mantle hydration in serpentine
mud volcanoes of the Mariana and Izu-Bonin
subduction zones and indirect evidence in exposed
paleo-subduction zones. Geophysical observations, including reduced seismic velocities and
high Poisson’s ratio, suggest forearc mantle may
commonly be V20% serpentinized; locally, serpentinization may reach 50%. Serpentinized forearc mantle may provide an explanation for the
aseismic nature of the forearc mantle and the
downdip limit of subduction thrust earthquakes.
The weak rheology and positive buoyancy of serpentinite will act to isolate hydrated forearc mantle from the mantle wedge £ow system. Dehydration of forearc serpentinite may provide a £uid
429
source to £ux magmatism during ridge subduction
and continental collision.
Acknowledgements
Geol. Survey of Canada Publication Number:
2001017. R.D.H. acknowledges support from the
Geological Survey of Canada, Natural Sciences
and Engineering Research Council of Canada
and the U.S. Geol. Survey National Earthquake
Hazards Reduction Program. S.M.P. acknowledges NSF support through grants EAR9725406 and 98-09602.[SK]
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