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Invited review article
Fluid and mass transfer at subduction interfaces—The field
metamorphic record
Gray E. Bebout
a,
⁎
, Sarah C. Penniston-Dorland
b
a
Department of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015, USA
b
Department of Geology, University of Maryland, College Park, MD 20742, USA
abstractarticle info
Article history:
Received 16 June 2015
Accepted 18 October 2015
Available online 27 October 2015
Keywords:
Subduction interface
Earthquakes
Fluid flow
Mass transfer
Mélange
High-pressure metamorphism
The interface between subducting oceanic slabs and the hanging wall is a structurally and lithologically complex
region. Chemically disparate lithologies (sedimentary, mafic and ultramafic rocks) and mechanical mixtures
thereof show heterogeneous deformation. These lithologies are tectonically juxtaposed at mm to km scales, par-
ticularly in more intensely sheared regions (mélange zones, which act as fluid channelways). This juxtaposition,
commonly in the presence of a mobile fluid phase, offers up huge potential for mass transfer and related meta-
somatic alteration. Fluids in this setting appear capable of transporting mass over scales of kms, along flow
paths with widely varying geometries and P–T trajectories. Current models of arc magmatism require km-scale
migration of fluids from the interface into mantle wedge magma source regions and implicit in these models is
the transport of any fluids generated in the subducting slab along and ultimately through the subduction inter-
face. Field and geochemical studies of high- and ultrahigh-pressure metamorphic rocks elucidate the sources
and compositions of fluids in subduction interfaces and the interplay between deformation and fluid and mass
transfer in this region.
Recent geophysical studies of the subduction interface — its thickness, mineralogy, density, and H
2
Ocontent—
indicate that its rheology greatly influences the ways in which the subducting plate is coupled with the hanging
wall. Field investigation of the magnitude and styles of fluid–rock interaction in metamorphic rocks representing
“seismogenic zone” depths (and greater) yields insight regarding the roles of fluids and elevated fluid pore pres-
sure in the weakening of plate interface rocks and the deformation leading to seismic events. From a geochemical
perspective, the plate interface contributes to shaping the “slab signature” observed in studies of the composition
of arc volcanic rocks. Understanding the production of fluids with hybridized chemical/isotopic compositions
could improve models aimed at identifying the relative contributions of end-member rock reservoirs through
analyses of arc volcanic rocks. Production of rocks rich in hydrous minerals, along the subduction interface,
could stabilize H
2
O to great depths in subduction zones and influence deep-Earth H
2
Ocycling.Enhancementof
decarbonation reactions and dissolution by fluid infiltration facilitated by deformation at the interface could
influence the C flux from subducting slabs entering the sub-arc mantle wedge and various forearc reservoirs.
In this paper, we consider records of fluid and mass transfer at localities representing various depths and
structural expressions of evolving paleo-interfaces, ranging widely in structural character, the rock types in-
volved (ultramafic, mafic, sedimentary), and the rheology of these rocks. We stress commonalities in styles of
fluid and mass transfer as related to deformation style and the associated geometries of fluid mobility at subduc-
tion interfaces. Variations in thermal structure among individual margins will lead to significant differences in
not only the rheology of subducting rocks, and thus seismicity, but also the profiles of devolatilization and melt-
ing, through the forearc and subarc, and the element/mineral solubilities in any aqueous fluids or silicate melts
that are produced. One key factor in considering fluid and mass transfer in the subduction interface, influencing
C cycling and other chemical additions to arcs, is the uncertain degree to which sub-crustal ultramafic rocks in
downgoing slabs are hydrated and release H
2
O-rich fluids.
© 2015 Elsevier B.V. All rights reserved.
Contents
1. Introduction.............................................................. 229
2. Rocktypesinvolvedatthesubductioninterface.............................................. 230
Lithos 240–243 (2016) 228–258
⁎ Corresponding author.
E-mail address: geb0@lehigh.edu (G.E. Bebout).
http://dx.doi.org/10.1016/j.lithos.2015.10.007
0024-4937/© 2015 Elsevier B.V. All rights reserved.
Contents lists available at ScienceDirect
Lithos
journal homepage: www.elsevier.com/locate/lithos
3. Pressure–temperatureconditionsalongthesubductioninterface...................................... 232
4. Mechanismsformasstransferatthesubductioninterface ......................................... 233
4.1. Diffusion............................................................ 233
4.2. Advection of mass in fluids(ormelts)............................................... 233
4.3. Physical/mechanicalmixing.................................................... 235
5. Sources of fluidsatthesubductioninterface................................................ 235
6. Fieldrecordsofsubductioninterfaceprocesses .............................................. 235
6.1. Depths of b 40km........................................................ 235
6.1.1. Depths of b 30km.................................................... 236
6.1.2. Depths of 30–40km................................................... 238
6.2. Depths of 40–90km....................................................... 240
6.2.1. Depths of 40–70km................................................... 241
6.2.2. Depths of 70–90km................................................... 243
7. Discussion............................................................... 246
7.1. Commonthemesinmasstransferatthesubductioninterface .................................... 246
7.1.1. Quartz±calciteveining................................................. 246
7.1.2. Othersilicametasomatism................................................ 247
7.1.3. LILEmetasomatism................................................... 247
7.1.4. Na-richmetasomaticparageneses............................................. 247
7.1.5. Othertraceelementmetasomatismalongthesubductioninterface.............................. 247
7.1.6. Likely importance of the transition from hydrous fluidtosilicatemelt............................. 248
7.2. Contrastingmetasomaticbehaviors:closedvs.opensystem..................................... 248
7.3. Contrastingdeformationbehaviors:intactvs.highlydeformed.................................... 248
7.4. It'samatterofscale....................................................... 249
7.5. Significance and impacts of interface fluidandmasstransfer..................................... 249
7.5.1. Role of fluidsandmasstransferforseismicity........................................ 249
7.5.2. Rheology and deformation-enhanced fluid infiltration.................................... 249
7.5.3. Duration of fluid infiltrationevents............................................ 250
7.5.4. Connectionwitharclavacompositions(andthedeepermantle)............................... 250
7.5.5. Stabilizationofhydrousmineralassemblagesandimportanceforvolatilescycling....................... 251
7.5.6. Deformation-enhanceddecarbonationandcarbonatedissolution............................... 251
8. Conclusionsandfuturedirections .................................................... 252
Acknowledgments.............................................................. 254
References................................................................. 254
1. Introduction
The interface between subducting oceanic lithosphere and its hang-
ing wall is generally viewed as a structurally and lithologically complex
region within and through which fluids from various slab sources are
transported (see the sketch in Fig. 1). In this region, large differences
in rheology can lead to complex structural juxtaposition of disparate li-
thologies in a fluid-rich setting, leading to mass transfer and related
metasomatic alteration (Bebout, 2013). These fluids are generated by
devolatilization reactions at greater depths along the interface and in
underlying sediment sections, the altered ocea nic crust section, and
possibly also hydrated and potentially carbonated sub-crustal ultramaf-
ic rocks in slabs (see Emry and Wiens, 2015; Faccen da et al., 2009;
Ranero et al., 2003, 2005). In forearcs, the complex interplay of deforma-
tion and fluid flow makes the interface a major channelway for fluids re-
leased by mechanical expulsion and devolatilization to migrate up-dip
toward the seafloor, often leaving in their paths complex vein networks
and other metasoma tic features (see descriptions of veining by
Bachmann et al., 2009; Bebout and Barton, 1993; Breeding and Ague,
2002; Fisher and Byrne, 1990; Fisher et al., 1995; Raimbourg et a l.,
2015; Sadofsky and Bebout, 2003). Fluids (including possible silicate
melts) emanating from the interface and entering the mantle wedge
could convey geochemical signatures of closely juxtaposed, disparate
rock types and “hybridized” rocks having chemical/isotopic characteris-
tics inherited from multiple metasomatized end-member lithologies
(sediment, basalt/gabbro, peridotite; Bebout, 2014; Bebou t and
Barton, 2002; Ki ng et al., 2006, 2007; Marschall and Schumacher,
2012; Spandler et al., 2008;seeFig. 1). If it occurs at sub-arc depths,
this hybridization could obscure efforts to deconvolute contributions
of end-member slab fluid sources (basal t, sediment, sla b peridotite)
based on the chemical/isotopic compositions of arc lavas (see Bebout
and Barton, 2002; King et al., 2006, 2007; Marschall and Schumacher,
2012; Spandler and Pirard, 2013; Spandler et al., 2008). Field studies
of HP and UHP metamorphic suites consistently demonstrate the pres-
ence of relatively intact slabs or blocks of stronger materials separated
by shear zones of varying scales and, in some cases, intensely sheared
matrices. It appears unlikely that a deeply subducted lithosphere
section would completely retain its stratigraphy from the seafloor (see
Fig. 1). In fact, it is likely th at there is even greater complexity and
structural/lithological diversity of interfaces related to (1) subduction
erosion (Clift and Vannucchi, 2004; von Huene and Scholl, 1991), (2) ac-
cretion/underplating (Angiboust et al., 2014a; Platt, 1986), (3) subduc-
tion of asperities (e.g., seamounts ; Cloos, 1992), and (4) incipient
continental subduction (Cloos, 1993; Scholl and von Huene, 2010).
Geophysical studies have increasingly focused attention on the pos-
sible seismic expressions of a subduction channel at the tops of
subducting slabs (Abers, 2005; Abers et al., 2006, 2009; Essen et al.,
2009; Friederich et al., 2014; Hacker et al., 2003a,b; Kim et al., 2013)
and the ways in which interface mechanical properties relate to fluid
generation and flow over a wide range of dept hs (see Fig. 2, from
Audet and Bürgmann, 2014;alsoseeAbers et al., 2013; Audet et al.,
2009; Bachmann et al., 2009; Calahorrano et al., 2008; Moreno et al.,
201
4; Saffer and Wallace, 2015). Recent work by field geologists has
begun to connect met amorphic rock records of subduction interface
fluid processes with the geophysical record, with emphasis on under-
standing deformation and fluid processes in seismogenic zones and at
greater depths (see Fig. 3, from Angiboust et al., 2014a; see Andersen
et al., 2014; Bachmann et al., 2009; Deseta et al., 2014; Fisher and
Brantley, 2014; John and Schenk, 2006; John et al., 2009; Plunder
et al., 2015; Rowe et al., 2005; Vannucchi et al., 2008; Wassmann and
Stöckhert, 2012, 2013; Whitney et al., 2014; Yamaguchi et al., 2012).
Fluids are known to play key roles in deformation and mass transfer in
shear zones (e.g., Axen et al., 2001; Barnes et al., 2004; Dipple and
Ferry, 1992; Goncalves et al., 2012; Keller et al., 2004; Selverstone
229G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
et al., 1991). Rocks floating as distinct blocks within large expanses of
weaker matrix (e.g., Catalina Schist, New Caledonia, Syros) may exhibit
fracturing and brecciatio n, and related fluid–rock interaction greater
than that exhibited by rocks in larger intact regions (Angiboust et al.,
2014b; Bebout, 2013).
In this review article, we provide a synthesis of geological, geochem-
ical, geophysical, and theoretical observations aimed at understanding
the processes attending metamorphism along the forearc-to-subarc
subduction interface (~15 to 120 km), with an emphasis on fluid trans-
port processes and related mass transfer in various structural settings at
the interface. We have chosen the suites discussed in this synthesis
because they are representative of the features observed, and processes
inferred, for the range of subduction depths considered along the sub-
duction interface. We emphasize that, over some depth ranges, the set
of processes operating can be reconciled as being common among the
various suites/settings and that many of the differences in conclusions
made by previous authors, regarding fluid processes, relate to the scales
at which the key observations were made and to which part of an evolv-
ing subduction interface is being investigated (see Fig. 1).
We argue that, because of the tendenc y for deformation to focus
fluid processes (and any related mass transfer), any fluids emanating
from subducting slab sections will tend to disproportionately bear geo-
chemical signatures of such intensely deformed zones and not of the
more intact expanses of relatively undeformed rock behaving largely
as closed systems with respect to fluid–rock interaction. The estimated
compositions of slab fluid components contributing to the generation of
volcanic rocks are, in general, compatible with the likely compositions
of such fluids (e.g., King et al., 2006, 2007; Marschall and Schumacher,
2012; Mi ller et al., 2009; Spandler and Pirard, 20 13; Spandler et al.,
2008). We explore the degree to which the subduction interface be-
neath arcs can be regarded as a major chemical reaction zone leading
to the release of fluids with hybridized geochemical compositions and
resulting in significant change in the compositions of rocks subducted
into deeper parts of the mantle beyond subarc depths. The combination
of this juxtaposition of disparate lithologies, in an actively deforming
and fluid-rich setting, as slab sections are heated to N 600 °C (leading
to their increased ability to transport mass), make subarc subduction in-
terfaces (distances to tops of slabs beneath arc volcanoes are 72–173;
global mean 105 km; Syracuse and Abers, 2006) a key location for un-
derstanding the evolution of major Earth reservoirs such as continental
crust, lithospheric mantle, and asthenospheric mantle. We discuss the
impacts of metamorphic reactions in this depth interval on geochemical
cycling, including the possibility that the combination of intense
mechanical mixing, fluid infiltration , and higher temperatures at the
interface at these depths could result in the release of C, via some
combination of decarbonation and carbonate dissolution, in quantities
capable of balancing outputs of C in arc volcanic gases (see the related
discussion in Collins et al., 2015).
2. Rock types involved at the subduction interface
In general, materials at the subduction interface are derived from the
subducting slab, the overlying plate, and the accretionary wedge. Mate-
rials derived from the subducting plate are crustal rocks including deep-
sea sediment ary rocks, oceanic basa lts and gabbros, and peridotitic
rocks of the underlying lithospheric mantle. Mater ials derived from
the overlying plate may include off-scraped sedimentary rocks and
mantle wedge peridotite or serpentinite, and in some cases continental
crustal rocks. Accretionary wedge materials include sedimentary rocks
f
ormed from sediments derived dominantly from the overlying plate.
Sedimentary rock types found in subdu ction metamorphic complexes
include metacherts, metapelites, metagraywackes, metaconglomerates,
Fig. 1. Sketch of ocean-continent subduction zone, showing key structural elements and scenarios for the nature of the plate interface at shallow and deeper levels. The two boxes show
general structural styles and indicate schematically some likely fluid flow geometries and are based on the published field studies of units described in this paper. Diagram is not to scale.
230 G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
and metacarbonates. These rocks are commonly disaggregated and mixed
with other lithologies due to processes occurring within subduction zones
and upon exhumation. Much of what we know about sedimentary rocks
entering subduction zones comes from studies of material drilled from
oceanic crust. The global subducting sediment (GLOSS) is estimated to
contain ~17% biogenic components (7% is marine carbonate and 10% is
biogenic silica), and it is estimated that 76% of the material is derived
from terrigenous sediments (the remaining 7% is mineral-bound H
2
O
+
;
Plank, 2014; Plank and Langmuir, 1998).
Sedimentary rocks can be enriched in large ion lithophile elements
(LILE; e.g., K, Rb, Cs, Ba) and other elements such as Li, B, Pb, C and N rel-
ative to other rock types within subduction zones. They also have high
δ
18
O values relative to the majority of the mafic and ultramafic materials
entering subduction zones. The concentrations of these elements and
isotopic signatures have been used to trace sedimentary-derived meta-
morphic fluids within the subduction interface (e.g. Bebout, 1991a,b;
Bebout and Barton, 1989, 1993; Bebout and Fogel, 1992; Bebout et al.,
1999; King et al., 2006, 2007; Penniston-Dorland et al., 2012a).
Metamorphosed mafic rocks are found in most subduction-related
metamo rphic complexes. These can include metamorphosed basalts
and diabases whether formed at a mid-ocean ridge (MORB), as part of
an oceanic island (OIB) or plateau, or derived from subducted arc volca-
nic rocks. MORB also commonly experiences hydrothermal alteration
on the seafloor, and altered MORB exhibits variable enrichments
(relative to fresh MORB) in K, Rb, Cs, Li, Cl, CO
2
and H
2
O and local enrich-
ments in Fe, Ba, Th and U, and local depletions in Mg and Ni (Alt and
Teagle, 2003; Jarrard, 2003; Staudigel, 2014). Metamorphosed gabbroic
rocks are found in some subduction-related complexes, including Fe–Ti
gabbros (Nadeau et al., 1993; Philippot and Selverstone, 1991).
Metamorphosed ultramafic rocks, including serpentinite, are found in
most subduction-related metamorphic complexes. In addition to occur-
ring as massive serpentinite bodies or serpentinite blocks, they are also
a common component of the fine-grained matrix found within many mé-
lange zones. Subduction-related peridotitic rocks could have originated in
one of two general settings: as seafloor-altered peridotite of the
subducting plate or from the hydrated mantle wedge of the overlying
plate (Hattori and Guillot, 2007). Peridotite in the subducting slab may
have been unroofed on the ocean floor at slow spreading ocean ridges
by faulting. Peridotite is also exposed in rift valleys, transform faults and
propagating rifts on fast spreading ridges, and peridotite massifs of large
areal extent on the ocean floor (Snow and Dick, 1995). In these settings,
it is subjected to serpentinization as it is exposed to and infiltrated by
seawater. Venting of serpentinizing fluids can lead to lithification of
calcareous ooze, which fills fractures in serpentinite which may lead to
the formation of ophicarbonate units (Schroeder et al., 2002). Bending-
related faulting of the subducting plate as it enters the trench cuts deep
into the mantle also promotes hydration of crust and mantle on the
downgoing plate (Ranero et al., 2003). Understanding the derivation of
subduction-related peridotitic rocks has implications for understanding
processes of interaction between plates. However, the tectonic setting of
peridotitic protoliths can be a potentially difficult question to address.
Major element chemistry alone may not be helpful in discriminating be-
tween mantle wedge and abyssal serpentinite (Deschamps et al., 2013).
Trace element chemistry, including variations in REE patterns, has been
used by some to discern between mantle wedge and oceanic lithospheric
peridotite (e.g., Barnes et al., 2013; John et al., 2010), however there is
much overlap in REE patterns among the different tectonic settings and
ultramaficrocktypes(Deschamps et al., 2013). Most studies agree that
Cr# of relict spinel appears to discriminate effectively between mantle
wedge serpentinite and abyssal serpentinite (Barnes et al., 2013;
Deschamps et al., 2013; Xie et al., 2013). It is clear that care must be
taken in geochemical interpretation of serpentinite protoliths.
Seafloor altered serpentinites can carry H, C, and S into subduction
zones. Evidence suggests that H
2
O and S may be released from
Fig. 2. Conceptual model of how silica enrichment controls slow earthquake behavior in northern Cascadia (from Audet and Bürgmann, 2014;alsoseeFisher and Brantley,
2014). k = permeability; σ
e
= normal effective stress. Low ratios of compressional (P)-wave velocities to shear (S)-wave velocities (v
P
/v
S
) me as ure d in t he forearc crust are
interpreted to reflect forearc silica enrichment. Following a slow-slip event permeability increases, allowing fluid circulation, silica precipitation, and a reducti on in pore-
fluid pressure. The strong temperature-dependence of silica solubility results in higher permeability reduction in quartz gouge, faster re-sealing and overpressure develop-
ment (shown here as decreasing effective normal stress), and thus lower recurrence times with increasing temperature. Thin dashed lines are 200 °C isotherms.
231G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
serpentinites due to devolatilization reactions during subduction meta-
morphism while C remains unaffected (Alt et al., 2013). While
serpentinized oceanic peridotites account for less than 3% of the total
subduction budget for C and S, these elements are isotopically fraction-
ated and can be recycled into the mantle (Alt et al., 2013).
3. Pressure–temperature conditions along the subduction interface
Materials entering subduction zones encounter increasing pressure
and temperature at depth along a notably cooler geotherm compared
to typical geot hermal gradients as is witnessed by the unique high-
pressure/low-temperature minerals (e.g. omphacite, glaucophane,
lawsonite) found in exhumed rocks (e.g., Ernst, 1972). The P–T condi-
tions experienced by rocks along the subduction interface have pro-
found implications for both their physical and chemical properties and
therefore for the deformation style and behavior of rocks during sub-
duction, and for elemental solubility, volatile release, and mass transfer
associated with subduction. The deformation behavior of rocks changes
as a function of pore fluid pressure, temperature, strain rate and pres-
sure. Brittle deformation, including faulting, dominates at shallow levels
within subduction zones. Ductile deformation occurs at higher temper-
atures by deformation mechanisms such as dislocation creep and disso-
lution precipitation creep. In the ductile regime, the rheology of rocks is
dominantly affected by temperature and strain rate. Kirby et al. (1996)
demonstrated a strong dependence of the earthquake records of mod-
ern subduction zones strongly tied to the thermal structure of the mar-
gins, relating this to temperature-dependent rheology.
Computational geodynamic models (e.g., Gerya et al., 2002;
Syracuse et al., 2010) use observations such as seismic data, spreading
rates and heat flow to provide predictions of the thermal structure of
subduction zones that can be tested by comparison with evidence
from subduction-related metamorphic rocks. A recent study comparing
peak-PP–T conditions recorded by HP metamorphic suites indicates
that these models produce temperatures too low for the forearc region
(Penniston-Dorland et al., 2015). In Penniston-Dorland et al. (2015),es-
timates of temperatures at the peak pressures of metamorphism were
compiled from the literature from exhumed blueschists, eclogites, and
associated subduction-related metamorphic rocks from around the
world and compared to predictions of the thermal structure of subduc-
tion zones from computational geodynamic models. While the compar-
ison is favorable at high P, at pressures for the forearc region
(P
max
b 2 GPa), the models predict temperatures that are on average
100–300 °C cooler than those recorded by exhumed rocks. This general
relationship can be seen in Fig. 4, in which peak P–T conditions estimat-
ed for the subduction interface localities considered in this paper (see
Table 1) are plotted along with prograde P–T paths calculated using
geodynamic models for the range of active subduction zones (green
lines, Syracuse et al., 2010) and for a range of parameters (plate age, ve-
locity, dip, etc.) for incoming plates (red lines, Gerya et al., 2002). These
conclusions are further supported by comparison of prograde P–T paths
recorded by exhumed metamorphic rocks in the same compilation to
the geodynamic modeling predictions (see Fig. 6 in Penniston-Dorland
et al., 2015).
The thermal structure of subduction zones provides an important con-
trol on the cycling of elements, as heat is a major driver of devolatilization
Fig. 3. Connection of field observations of the metamorphic rock record in (A), a section across the Dent Blanche Thrust (DBT, Swiss Alps) showing the geometry of Arolla gneisses
(underplated continental crustal slices) and underlying ophiolite (Ollomont area; Angiboust et al., 2014a) with large-scale coupling/decoupling at the subduction interface in (B), a
view of the subduction interface, and relationships with seismic coupling, based on field study of the DBT (Angiboust et al., 2015). Inset shows deformation features associated with par-
tially coupled transition zone.
232 G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
reactions during subduction metamorph ism (e.g., Hacker, 2008). Models
also provide the basis for quantitative estimates of geochemical cycling
among Earth reservoirs of elements and compounds of great importance
such as H
2
OandCO
2
(e.g., Gorman et al., 2006; van Keken et al., 2011). If
models underestimate temperatures experienced within subduction
zones, taking H
2
O as an example, far more H
2
O is released during subduc-
tion at shallower levels, and far less H
2
O can be recycled into the mantle
(see Hacker, 2008; Penniston-Dorland et al., 2015; van Keken et al.,
2011). Many elements/minerals would be expected to show different
solubility relations in H
2
O-rich fluids at somewhat higher temperatures,
potentially strongly influencing the mass transfer occurring along sub-
duction interfaces (and within subducting slab sections).
4. Mechanisms for mass transfer at the subduction interface
In this section, we provide a brief discussion of the processes by
which metasomatism is thought to occur in subduction zones (also
see the discussions by Bebout and Barton, 1993; Bebout, 2013). The dra-
matically contrasting geochemical compositions of rock types found in
the subduction interface (sedimentary, mafic, ultramafic, and mixtures
thereof), and any fluids that are present, are the driving forces for signif-
icant ch anges in chemical composition (metasomatism) along the
interface.
Generally speaking, alteration of the initial chemical composition of
rocks can occur in several ways during metamorphism: 1) diffusion of
elements across a chemical potential gradient (diffusion is usuall y
more efficient within a fluid along grain boundaries than with in a
solid), 2) advection of elements dissolved in an infiltrating fluid (e.g.
aqueous fluid, melt) traveling within fractures or in intergranular pore
spaces, and 3) physical/mechanical mixing of materials with different
initial compositions. The first two processes of chemical transfer rely
on fluids and depend on the evolving porosity and permeability of the
rocks during metamorphism. The third process involves a complex in-
teraction between deformation and metasomatism (and related change
in mineral assemblages). The three processes, when operating in con-
cert, can lead to the production of so-called “hybrid” rock compositions
with geochemical characteristics variably inherited from the diverse li-
thologies at the interface (i.e., sediments, altered oceanic crust, ultra-
mafic rocks), shaping the c hemical/is otopic compositions of fluids
with which these rocks interact.
4.1. Diffusion
Diffusive mass transport across a chemical potential gradient can
occur when two adjacent lithologies have significantly different chemi-
cal compositions. During “wet” metamorphism rocks are commonly a
heterogeneous medium—a rock composed of different mineral grains
that are surrounded by an intergranular fluid. In this case, the “bulk” dif-
fusion through the medium is commonly dictated by the diffusivity of
fluid-mobile elements th rough the intergranular medium (Watson
and Baxter, 2007). Usually, diffusive transport of these elements results
in a smoothly varying profile in which the distance over which the con-
centration or isotopic composition varies is a function of the diffusivity
of the element in the medium, characteristics of the medium such as po-
rosity and tortuosity, and the amount of time over which diffusion has
occurred. Evidence for diffusive transfer is found in subduction-related
metamorphic rocks generally over small scales (distances of cms) in
regions adjacent to rock layers in which fluid infiltrat ion is inferred
(Franciscan Complex; Pen
niston-Dorland et al., 2010) and adjacent to
metamorphic veins (Tianshan; John et al., 2012). The length-scale of dif-
fusive transfer yields information regarding the timescales of the diffu-
sion (John et al., 2012; Penniston-Dorland et al., 2010).
4.2. Advection of mass in fluids (or melts)
Evidence for advection of mass by an infiltrating fluid takes many
forms in subduction-related metamorphic rocks. The presence of abun-
dant veins, met asomatic reaction zones often containing significant
amounts of hydrous minerals, and deviations of major- and trace-
element chemistry as well as stable isotope compositions from likely
protolith compositions all p rovide evidence for fluid infiltration in
subduction-related metamorphic rocks (e.g., Bebout and Barton, 1993;
Breeding et al., 2004; Miller et al., 2009; Pen niston-Dorland et al.,
2012a ,b; Sorensen et al., 1997; van de r Straaten et al., 2008, 2012).
Mass transport by advecting fluids can result in enrichment (or deple-
tion) of elements being carried by the fluid in the rock along the path
of the fluid. These changes are due to exchange between the fluid and
minerals in the rock and/or crystallization of new minerals in the pres-
ence of the fluid. This mechanism of mass transfer is expected to pro-
duce the largest-scale effects, as the flow of fluids carrying dissolved
load could certainly be km-scale in this environment (see Bebout and
Barton, 1989, 1993). Much of the quantitative basis for our understand-
ing of fluxes of metamorphic fluids duri ng subduction is based on
modeling predictions, and there are few quantitative estimates of fluid
fluxes based on evidence from subduction-related metamorphic rocks
(see Ague, 2014; Zack and John, 2007). One estimate of subduction-
related fluid fluxes of 10
4
–10
5
m
3
(fluid)
/m
2
(rock)
was based on silica pre-
cipitation in veins in the Otago Schist, New Zealand (see Breeding and
Ague, 2002) and is comparable in magnitude to estimates of fluid fluxes
required to precipitate silica in veins during regional metamorphism in
New England (Ague, 1994; Ferry, 1992; Penniston-Dorland and Ferry,
2008).
Mineral stability and solubility affect mass transport by fluids, and
the subduction zone environment has a cooler geotherm relative to
other tectonic environments, resulting in differing types of fluid chem-
istry and different fluid fluxes required for mass transfer (see discussion
by Frezzotti and Ferrando, 2015). Thermodynamic modeling (Dolejs
and Manning, 2010) suggests that fluid fl uxes required for transport
Fig. 4. Peak-metamorphism P–T estimates for the subduction interface suites discussed in
this paper superimposed on P–T paths calculated by Gerya et al. (2002; red lines) and
Syracuse et al. (2010, updated van Keken pers. comm., 2014; green lines) by region. The
model average for Syracuse et al. (2010) is shown as a thicker green line. The thick red
line represents “Model A” of Gerya et al. (2002). A global mean distribution of rock-
based P–T estimates (at P
max
)fromPenniston-Dorland et al. (2015) is shown as the dark
blue curve with a paler blue ±2σ confidence envelope. Model P–T predictions in general
match the rock P–T estimates well for P N 2.5 GPa; however, at shallower depths (b 2GPa),
the P–T estimates from metamorphic rocks are for a given pressure, hotter than the P–T
pred ictions of both models. The rock-based average is hotter th an Model A of Gerya
et al. (2002) by ~100 °C and hotter than the Syracuse et al. (2010) average by more
than 200 °C. The peak metamorphic P–T conditions of units discussed in this paper are
outl ined by rectangles (see Table 1), and the boundaries between the bluesch ist and
eclogite facies (figure modified after Penniston-Dorland et al., 2015) and the boundary
of the “Forbidden Zone” (from Liou et al., 2000)areshownasgraylines.
233G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
Table 1
Summary information about HP metamorphic suites.
Locality Age (Ma) P
(GPa)
T (°C) Rock types Matrix compositions Features
Arosa ~60–35
(lower
plate)
0.2–0.7 150–350 Pelagic chert, metabasalt, metagabbro, serpentinite,
turbidite (lower plate origin) metamorphic basement,
dolomite (upper plate origin)
Shale, serpentinite Blocks in matrix, pseudotachylytes, veins, dip angle of 8°
Shimanto Belt
- coherent Hiwasa
Formation
- Mugi mélange
Tertiary
0.1–0.2
(Mugi)
150–255
(Mugi)
Hiwasa mostly terrigenous turbidites.
Mugi mélange include basalts, pelagic cherts, red shales,
hemipelagic shales in black, scaly shale matrix
Black, scaly shale matrix Sandstone and tuff blocks in scaly shale matrix. Pseudotachylyte and
ultracataclasites
Quartz veins
Chichibu Belt Jurassic 0.5–0.6 230–290 Psammitic and pelitic rocks, chert, greenstones,
limestones
Pelitic schist Mélange, veins
Kodiak Complex
Uyak
Kodiak
Ghost Rocks
b 55 0.2–0.4 200–300 Uyak—mélange with greenstone, graywacke, gabbro,
ultramafic blocks in chert-argillite matrix
Kodiak—thin-bedded turbidites and massive sandstone
only locally disrupted
Ghost Rocks—mélange with sandstone, shale and pillow
basalt
Chert-argillite Blocks in matrix, crack-seal veins pseudotachylite, cataclasite
Franciscan Complex
- Blueschist blocks
- Eclogite blocks
- Intact Metasedimentary
140–170
0.5–0.6
2.0–2.5
0.2–0.8
300–350
550–620
~200
Blueschist, eclogite, garnet amphibolite, metagraywacke,
chert
Shale, serpentinite Veins, blocks in matrix, reaction zones around mafic blocks
Catalina Schist ~115 to 95 0.6–1.2 ~200
–700
Metasedimentary (including metaconglomerate),
metamafic, and metaultramafic rocks
Amphibolite facies matrix in
places is chlorite schist and
in places amphibole-,
talc-schist
Veins, mélange blocks in matrix, reaction zones around mafic and
ultramafic blocks, metasomatically altered igneous cobbles in
metaconglomerates
Mount Hochwart
- Metasomatic conditions
330
b 1.2 660–700
Garnet-spinel peridotite lenses within garnet-kyanite
gneiss of the Ulten Zone
Garnet-kyanite gneiss Variable thickness metasomatic bands at contact between peridotite and
felsic gneiss
Dent Blanche Thrust
- Arolla gneiss
- Tsaté Complex
43–48
37–42
1.2
1.0–1.2
~450
360–490
Gneiss
Calcschists, metagabbro pods in serpentinite
Serpentinite Serpentinite thrust zone containing metagabbro blocks in serpentinite,
cataclastic layers containing quartz-epidote veins
Sesia Zone Cretaceous–
Tertiary
1.0–2.0 500–600 Continental lithologies—pelitic, calcareous pelitic,
metagranitoids, metabasites, carbonates, quartz-rich rocks
Relatively weakly deformed Mombarone Unit separated from other units by
blueschist-facies mylonitic shear zone
New Caledonia
- Diahot Blueschists
- Pouebo Eclogite Mélange
Eocene 1.6–1.9 550–600 Diahot Blueschists—thick sequence of metamorphosed
blueschist sedimentary rocks with intercalated basalts
and rhyolites
Pouebo Eclogite Mélange—eclogite, garnet blueschist,
garnet amphibolite, metamorphosed ultramafic rocks,
quartz-garnet-mica schists
Talc-chlorite-phengite-
quartz-amphibole matrix
Blocks in matrix, reaction zones rich in chlorite and talc
Tianshan HP 343–346 1.8–2.3 480–580 Metasedimentary rocks, including marble, metamaficand
metaultrama ficrocks
Blueschist metasedimentary
matrix—felsic
pelitic schist
Veins, reaction zones around veins, blocks in matrix, fluid inclusions
Syros and Tinos Cretaceous–
Eocene
1.3–2.0 470–520 Eclogites, blueschists, metagabbros, serpentinites,
metaplagiogranites, metasedimentary rocks
Serpentinite,
metasedimentary
Reaction zones developed at contacts between mafic blocks,
metasedimentary
rocks, and/or serpentinite.
Erro-Tobbio and Voltri
- peridotite
- eclogite
Alpine
2–2.5
1.3–2.0
550–600
450–500
Spinel lherzolites, serpentinites, eclogitized metagabbro,
metarodingite, metabasalt, metasedimentary rocks
Serpentinite, chlorite-rich
schist
High-strain serpentinite shear zones and low-strain zones with peridotite,
high-pressure veins
Sivrihisar 93–80 1.5–2.6 450–550 Layers of metacarbonate, metachert and metabasalt
(eclogite and blueschist). Serpentinite lenses
Blueschist, quartzite,
calc-schist.
Elliptical, folded lawsonite eclogite pods within blueschist, quartzite and
calc-schist. Pods and lenses of metatufffs, metagabbro and serpentinite.
Serpentinite lenses surrounded by reaction zones of
chlorite-lawsonite ± talc ± oxides ± carbonate.
Monviso 50–40 2.2–2.6 480–550 Mafic eclogites, Fe–Ti metagabbros, serpentinites,
calc-schists
Serpentinite Relatively preserved oceanic lithosphere cut by two major shear zones in
which blocks are embedded in serpentinite, high-pressure veins in Fe–Ti
metagabbros, eclogite breccias
Cima di Gagnone Alpine 2.5–3 750–800 Chlorite harzburgite, garnet peridotite Pelitic schists and gneiss Ultramafic–mafic rocks occur in lenses 10–100 s of m in size within pelite
and gneiss
234 G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
and deposition of quartz and calcite in veins are 1 to 2 orders of magni-
tude lower than those required for geothermal gradients typical of oro-
genic belts. At the high pressure conditions in subduction zones,
“intermediate” fluids, with properties between H
2
O and hydrous silicate
melt may play a role in the transport of mass, however this phenome-
non is like ly less important than originally thought beca use critical
curves for chemically complex fluids occur at higher P than for simple
systems (see Manning, 2004).
4.3. Physical/mechanical mixing
Bebout and Barton (1989, 1993, 2002) and Sorensen and Grossman
(1989) describe d a process by which rocks of strongly “hybridized”
compositions can be derived in subduction interface settings in which
several highly disparate rock endmembers are availabl e (also see
Marschall and Schumacher, 2012; Penniston-Dorland et al., 2012b,
2014; Spandler et al., 2008). In such settings, physical attenuation due
to deformation, and in some cases shearing into the matrix, of rind ma-
terials with abundant sheet silicates, can result in juxtaposition of dispa-
rate compositions at hand-sample scales (see the discussi on by
Penniston-Dorland et al., 2014). Be bout and Barton (2002) observed
that larg e volumes of the matrix of the amphibolite-gra de mélange
unit in the Catalina Schist resemble geochemically the reaction rind ma-
terial developed at the boundaries of variably dis aggregated tectonic
block material and surround mélange matrix. The matrix compositions
of relatively fluid-immobile elements are consistent with mixing of ma-
terials derived from mafic and ultramafic blocks in the mélange (e.g. Cr,
Al, Os, Ir, Ru; Bebout and Barton, 2002; Penniston-Dorland et al., 2014).
The process by which the matrix and reaction rinds form are likely a
complex combination of infiltrative and diffusive fluid transport along
with mechan ical mixing that occurs progressively during prograde
metamorphism.
5. Sources of fluids at the subduction interface
The behavior of volatiles during subduction is of particular impor-
tance in governing the surface-to-mantle volatiles budget on Earth.
H
2
O is particularly significant since it serves as the major agent for dis-
tribution of elements along the subduction interface, traveling toward
the surface along the subduction thrust or vertically into the overlying
plate. Subduction of oceanic lithospheric plates initiates a continuum
of metamorphic reactions that release fluids, transitioning from shallow
realms in which fluids are mechanically expelled during compaction
and diagenetic reactions to deeper levels (N 15 km) at which low-
grade metamorphism can result in significant devolatilization.
Metamorphic reaction histories can strongly depend on the P–T paths
experienced by the rocks in subducting slab sections (see the examples
of peak P–T conditions recorded by subduction interface rocks and slab-
top P–T paths modeled for rocks subducting into selected modern mar-
gins in Fig. 4). Schmidt and Poli (2014) provide a review of the experi-
mental and field evidence for the devolatilization history of the wide
range of rock types present at the subduction interface.
Studies of de volatilization in metasedimentary suites comparing
rocks at various metamorphic stages have indicated that, in relatively
cool subdu ction margin s, relatively little devolatilization occurs to
depths approa ching those beneath volcanic fron ts (N 90 km; Bebout
et al., 1999, 2013). Bebout et al. (2013) suggested that up to only ~20%
of the H
2
O in HP/UHP metapelites in the W. Alps was lost during sub-
duction to depths of ~90 km, along a P–T gradient of ~7–8 °C/km (also
see Busigny et al., 2003). This implies that a large fraction of the volatiles
in these rocks remains to depths beneath arc volcanoes and that a signif-
icant fraction of the volatiles could be retained to beyond subarc regions
and into the deeper mantle. Based on thermodynamic calculations
(Collins et al., 2015; Kerrick and Connolly, 2001), it appears that far larg-
er fractions of structurally bound H
2
O are lost from metabasaltic rocks
subducted to these depths. On balance it appears that the combination
of deeply subducting sediment and basaltic rocks can release significant
amounts of H
2
O-rich fluid into the subduction interface.
There has been considerable debate recently regarding the flux of
H
2
O-rich fluid evolved via devolatili zation of upper man tle rocks in
subducting slab s. Bending of slabs just outboard of trenches (in the
outer rise region) is believed to result in infiltration of the slabs by sea-
water, resulting in the hydration of ultramafic rocks of the slab upper
mantle (Faccenda et al., 2009; Ranero et al., 2003, 2005). However, un-
certainties remain regarding the extent of hydra tion experienced by
these rocks during slab bending (see the recent discuss ion by Emry
and Wiens, 2015). Large amounts of fluids emanating from this ultra-
mafic section could have profound implications for the fluid and mass
transfer at the interface, into/within which fluids would be focused, par-
ticularly at greater depths (see Fig. 1). Dehydration of these sub-crustal
ultramafic rocks could occur at extremely variable depth s (~ 100–
200 km), depending on the thermal structure of the margin (van
Keken et al., 2011). The depth of this dehydration would also depend
on the bulk rock composition of the ultramaficrocks(Padrón-Navarta
et al., 2010).
Theoretical studies by Hacker (2008) and van Keken et al. (2011)
provided margin-by-margin assessments of H
2
O subduction to varying
depths in modern margins, which show a wide range of thermal states.
The calculations by Hacker (2008) suggested that approximately one-
third of the initially subducted H
2
O, including pore waters, is subducted
to beyond subarc regions, that is, entering the deeper mantle and avail-
able to contribute to its H
2
O inventory. Both studies stressed the very
large range of potential H
2
Olossprofiles and overall surfa ce-to-
mantle recycling efficiency resulting from the wide range in thermal
structure of modern subduction margins.
6. Field records of subduction interface processes
In this paper, we consider records of fluid and mass transfer at local-
ities representing various expressions of evolving paleo-interfaces,
ranging widely in the abundance and structural character of varying
rock types involved (ultramafic, mafic, sedimentary; see Whitney
et al., 2014). We have chosen to focus on suites for which there are
good field constraints and abundant observations related to fluid and
mass transfer in order to place these rocks in the context of our discus-
sion of deformation -enhanced fluid and mass transfer. Following are
brief descriptions of the records of fluid and mass transfer in these
high-P metamorphic suites (in order of increasing depths of interface
development; peak P–T and other relevant information regarding
these localities provided in Fig. 4 and Table 1).
6.1. Depths of b 40 km
Shallow to intermediate d epths in forearcs (b 40 km) incl ude
sediment-rich, paleo- accretionary se ttings involving accretion/
underplating and transient subduct ion interface s. Some of these
suites contain shear/mélange zones with tectonic blocks of mafic,
ultramafic, and sediment ary compositions. There has been great
interest in def ormation and fluid processes in this depth int erval
because of the cl ear connections with the generation of seismicity
(see Figs. 2 and 3; Angiboust et al., 2014a, 2015; Audet and
Bürgmann, 201 4; Bachmann et al., 2009; Fisher and Brantley, 201 4;
Yamaguchi et al., 2012).
Studies of fluid and mass transfer in these shallow complexes indi-
cate the mobility of dissolved material resulting in the precipitation of
quartz- and carbona te-bearing vein arrays (e.g., Meneghin i et al.,
2009). These certainly indicate the mobility of mass (and fluid) at
some scale, but assessments of scales of fluid mobility using stable iso-
topes have been complicated by the cryptic nature of infiltration by
fluids previously equilibrated with very similar roc ks in these very
thick sequences of largely sandstone-shale sequences (see Raimbourg
et al., 2015; Sadofsky and Bebout, 2001, 2004; Sakakibara et al., 2007).
235G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
Fisher and Brantley (1992) argued for local-scale derivation of abundant
quartz in veins in the Kodiak Formation, Alaska, involving diffusion of
the dissolved silica from the host-rocks into fractures. On the other
end of the spectrum, a record of km-scale fluid transport was docu-
mented for the Catalina Schist with clear evidence for fluid infiltration
in tectonometamorphic units of varying metamorphic grade, including
the mélange units from the lowest grade (lawsonite-albite and
lawsonite-blueschist facies) to the highest grade (amphibolite facies;
see Bebout and Barton, 1989; Bebout, 1991a).
Fluid flux at the very shallow levels of accretionary complexes in-
volves the mechanical expulsion of pore waters. Hacker (2008) estimat-
ed that, on a global basis, 46% of the H
2
O released in Earth's subduction
zones is released by closure of pores. This flux of H
2
O-rich fluid out of
the shal low levels has been observed in deep-sea drilling of fault
zones and as venting of fluids onto the seafloor. The fluids venting
into the oceans bear signatures of organ ic breakd own (to produce
CH
4
), diagenetic mineral-fluid reactions, and to varying degrees show
“freshening” attributed to contributions of low-salinity fluids produced
by low-grade metamorphic dehydration reactions (see Mottl et al.,
2004). These latter authors documented changes in pore fluid chemistry
emanating from serpentinite seamounts across the Marianas forearc
that seemingly reflect cross-forearc change in contributions due to dia-
genesis and low-grade metamorphism of the sedimentary sequence
subducting into this margin (also see the discussion by Bebout, 2014).
At deeper levels in accretionary complexes, and deeper within the sub-
duction channel, devolatilization reactions generate the volatiles being
expelled from the subducting slab.
6.1.1. Depths of b 30 km
6.1.1.1. A rosa Zone, Central Alps. Rocks of the Arosa Zone (Bachmann
et al., 2009 and references therein) display block in matrix (both shale
and serpentinite matrix) textures on a variety of scales (block sizes
range up to several km) with sedimentary, metabasaltic, metagabbroic
and serpentinite rocks derived from the lower plate and metamorphic
basem ent and dolomite derived from the upper plate (see Fig. 5).
Fig. 5. (A) Macroscopic schematic cross section through the subduction mélange and the Austroalpine upper plate at the Arosa locality, Switzerland. (B) Interpretative three-dimensional
cross-section of a forearc region illustrating interface deformation and veining and inferred stable and unstable regions (both figures are from Bachmann et al., 2009).
236 G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
Temperatures and pressures indicate relatively shallow (b 30 km)
depths within the subduction channel . The distribution of pseudo-
tachylyte is interpreted to represent a zone of unstable slip between
200 and 300 °C. The presence of veins within this same zone and con-
tinuing to greater depths is also related to rupture and failure possibly
associated with seismic events (Bachmann et al., 2009). Veins are
boudinaged and sheared, and the minerals filling veins are dominantly
large, well-developed crystals of quartz and calcite. In places, dilatation-
al jogs in veins display chaotic mineral growth, including fine-grained,
dynamically recrystallized calcite.
6.1.1.2. Shimanto and Chichibu Belts, Japan. The Tertiary Shimanto Belt
(Kimura et al., 2012) consists of a more coherent unit (Hiwasa) adjacent
to two mélange units (Mugi) on the islands of Kyushu, Shikoku, and
Honshu, Japan (see Yamaguchi et al., 2012). These mélange units are
b 200 m thick and contain basalts, pelagic cherts, red shales, and
hemipelagic shales in a scaly , black shale matrix. The mélange is
interpreted as tectonic, and is thought to represent fault rock al ong
the plate boundary. There are pseudotachylytes and cataclastic layers
found in the Shimanto Belt that are thought to record evidence for var-
ious types of earthquake events. Quartz veins are found, in some cases
along brittle faults, indicating late-stage fluid flow. Fluid inclusions in
veins from the southern Shimanto belt are mostly water-rich, but ap-
pear to trap both H
2
O and carbonic species, indicating temperatures be-
tween 215 and 255 °C and pressures of ~0.2 GPa (Lewis et al., 2000). The
flow within the accretionary prism was driven by deformation, and the
scale of that flow was relatively restricted (Lewis and Byrne, 2003).
Fluid inclusions in veins from the northern Shimanto belt record some-
what lower temperatures ranging from 150 to 170 °C at pressures of
~0.1 GPa (Yamaguchi et al., 2012). Carbon, oxygen and strontium iso-
topes indicate that while some vein s formed on the seafloor, others
formed during faulting due to fluids derived from several diffe rent
sources (shales , basalts, and seawate r; Yamaguchi et al., 2012;see
Fig. 6).
The Jurassic accretionary complex of the northern Chichibu belt in
western Shikoku, Japan (Sakakibara et al., 2007) consists of low-grade
metam orphosed or unmetamorphosed psammitic and peli tic rocks,
chert, greenstones and limestone. In the Hijikawa unit, there are low-
grade coherent schistos e and chert units separated by small-scale
thrusts and mél ange units comprised of pelitic schist with len ses of
chert and mafic schist that are sheared by faulting. Estimated tempera-
tures for the Hijikawa unit are ~230–290 °C and pressures are ~0.5–
0.6 GPa (Sakakibara et al., 2007). Ox ygen isotopic compositions of
veins and whole rocks were inferred to reflect up-dip flow of fluid de-
rived from higher-temperature pelitic rocks at greater depths within
the accretionary prism (Sakakibara et al., 2007).
6.1.1.3. Kodiak Islands, Alaska. The Kodiak Complex (Kodiak Island,
Alaska) has alternating formations of mélange and relatively coherent
low-grade (~200–300 °C) accretionary formation (Fisher and Byrne,
1990; Vrolijk et al., 1988). Adjacent to forearc rocks on the west side
of Kodiak Island there is the ~ 2 km thick Uyak Complex which is mé-
lange with greenstone, greywacke, gabbro and ultramafic blocks in a
chert-argillite matrix. This mélange was interpreted to have a tectonic
origin (Moore and Wheeler, 1978) based on lack of evidence for soft
sediment deformation and evidence for foliation formed under signifi-
cant confining pressure. Moving eastward, next is the Kodiak Formation
consisting of thin-bedded turbidites and massive sandstone that is more
coherent and only locally disrupted. Crack-sea l quartz veins in the
Kodiak Formation (see de scriptions by Fisher and Brantl ey, 1992,
2014; Fisher et al., 1995; Fisher, 1996) may have been produced in epi-
sodes that lasted b 10 days, a duration that is similar to the duration of
some slow earthquakes in nature, suggesting that silica redistribution
may play a ro le in determining the frequency of slip along the
Fig. 6. Schematic diagrams showing the tectonic setting of the Mugi mélange and the varied geological occurrences of syn-tectonic veins (from Yamaguchi et al., 2012, and references
therein). (A) Schematic profile of the Nankai Trough, as derived from seismic profiles, showing the position of the seismogenic zone. (B) Illustration of the structural and tectonic setting
of the Mugi mélange. (C) Schematic block diagram showing the geological occurrences of four types of veins that were identified by Yamaguchi et al. (2012), and their relationships with
the mélange and ramp thrust of the duplex structure. Veins are widely developed within the basaltic rocks. Boudin-neck veins are observed at the neck regions of pinch-and-swell and
boudinage structures in the mélange. Network veins are developed in damage zones along the ramp thrust, and fault-fill veins occur along dilational jogs in the thrust.
237G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
subduction interface (Fi sher a nd Brantley, 2014). Further east is the
~4 km thick Ghost Rocks Formation which is a disrupted mélange unit
with sandstone and shale and occurrences of pillow basalt. Occurrence
of pseudotachylyte and cataclasite in the Ghost Rocks Formation pro-
vides evidence for seismic activity on what is interpreted as a thrust sur-
face (Rowe et al., 2005, 2011). Methane inclusions in quartz veins from
both the Uyak and Ghost Rock Formations indicate decreasing pressures
from the edge of the vein to the center which was interpreted as evi-
dence for cycles of fluid buildup and release, which, over time, dropped
the overall fluid pressure (Vrolijk, 1987a,b; Vrolijk et al., 1988).
6.1.2. Depths of 30–40 km
6.1.2.1. Franciscan Complex, California. Together, the Coastal, Central, and
Eastern Belts of the Franciscan Complex provide a protracted history of
accretion, underplating and transient development of subduction inter-
faces. Each of these major units contains a rich history of fluid–rock in-
teraction, in the form of complex veining and, in more sheared mélange
domains, evidence of metasomatic exchange among disparate litholo-
gies. The O and C isotope compositio ns of carbon ate-bearing vein s
(calcite or ar agonite) and calculated fluid compositions using these
compositions indicate that, while in some cases fluids may be derived
from local host rocks, in other cases fluids were derived externa lly
(Fig. 7; Sadofsky and Bebout, 2001, 2004).
The Central Belt of the Franciscan Complex contains blueschists,
eclogites and amphibolites as blocks in shaly and serpentinite matrix
and also contains more intact, variably d eform ed blueschist terr ains
(several km thick and tens of km long; Cloos, 1986). Temperature and
pressure estimates from more intact Franciscan metasedimentary
rocks are ~200 °C and range from 2 to 8 kbar (Ernst, 2011, and refer-
ences therein). The ‘exotic’ mafic blocks in matrix and associated
Mg-rich reaction ‘rinds’ have been the focus of extensive st udy of
fluid–rock interactions during subduction metamorphism. Such reac-
tion rinds have been observed in many other localities described
below, and are inferred to represent fluid-assisted mixing of altered ma-
terial from mafic blocks and an ultramafi c matrix whic h is no longer
present in many examples in the Franciscan (Errico et al., 2013;
Penniston-Dorland et al., 2010; Sorensen and Grossman, 1993;
Sorensen et al., 1997; Ukar and Cloos, 2013). Recent constraints on the
maximum pressures and temperatures of metamorphism of high-
grade blocks in the Franciscan range from values for low-T blueschists
(~0.5–0.6 GPa, 300–350 °C; Ukar and Cloos, 2014) up to values for
high-P
eclogites (2–2
.5 GPa and 550–620 °C; Page et al., 2007;
Tsujimori et al., 2006). Evidence from th e LILE and Li concentrations
and isotopic compositions of mafic blocks and the O isotopic composi-
tions of garnets within those blocks attests to interaction of the mafic
block cores with flui ds during subduction (Errico et al., 2013; Page
et al., 2014; Penniston-Dorland et al., 2010; Sorensen et al., 1997). At
one Franciscan exposure, a profile across the contact between eclogite
and blueschist altered by fluids from eclogite demonstrates small-
scale variability in Li concentrations and δ
7
Li that are consistent with
diffusion within the eclogite into the altered blueschist. Diffusion
modeling suggests that the infiltration event that created the blueschist
was relatively brief (b 100 years; Penniston-Dorland et al., 2010).
6.1.2.2. Catalina Schist, California. The Catalina Schist exhibits a range
of metamorphic grades from lawsonite-albite up to amphibolite facies
(Grove and Bebout, 1995). Kilometer-scale regions of mélange
(Bebout and Barton, 2002) are observed throughout the Catalina Schist,
in which blocks of varying litho logies, including metasedimentary,
metabasalt, and metaultramafic rocks, are surrounded by finer-
grained matrix. The blocks exhibit a range of sizes up to hundreds of me-
ters in diameter grading into regions that exhibit more coherent behav-
ior. The most thorough work on mélange geochemistry to date has been
on the amphibolite-facies mélange unit in the Catalina Schist (Bebout,
1991a; Bebout and Barton, 1989, 1993, 2002; King et al., 2006, 2007;
Penniston-Dorland et al., 2012a, 2014; Sorensen, 1988; Sorensen and
Barton, 1987; Sorensen and Grossman, 1989). The mélange matrix dis-
plays chemical compositions consistent with mechanical mixing of ma-
terial derived from the blocks, but with some fluid-mediated removal
Fig. 7. Peak metamorphic temperature (˚C) vs. calculated (A) δ
18
Oand(B)δ
13
CforCH
4
-bear-
ing H
2
O-rich fluids equilibrated with carbonate-bearing veins in various units of the Francis-
can Complex and lawsonite albite (LA) and lawsonite blueschist (LB) units of the Catalina
Schist rocks (from Sadofsky and Bebout, 2001, 2004). Small black diamonds indicate the iso-
topic compositions of the veins, and shaded rectangles indicate ranges in calculated fluid δ
18
O
(A) and δ
13
C (B) based on the compositions of the vein carbonate. The black curve in
(A) indicates fluid δ
18
O calculated for quartz with δ
18
O
VSMOW
=+19‰ (curved line).
Match between this curve and fluid compositions calculated from vein δ
18
Ovalues(gray-
shaded boxes) suggests some buffering of fluid isotopic composition by the host rocks. How-
ever, host rock quartz δ
18
Orangesfrom+12to+19‰,thustheveinδ
18
O values are not uni-
formly equilibrated with the host rocks, perhaps consistent with some influence of externally
derived fluids and/or the homogenization of fluid isotopic compositions through mixing of
fluids equilibrated with sediments having a range of δ
18
O. The region “Decollement Fluids”
indicates the δ
18
OofH
2
O sampled from fault zones in active accretionary prisms
(e.g., Kastner et al., 1991). (B) Plot showing that δ
13
C
VPDB
values calculated for CH
4
in equi-
librium with the calcite veins are in isotopic equilibrium with reduced C (metamorphosed
organic matter) in the host rocks. (C) Field photograph of a typical exposure of
metasedimentary rock, in the lawsonite-albite-facies unit of the Catalina Schist, containing
multiple generations of calcite + quartz veins. Such veining is also observed in the Francis-
can Complex (see Sadofsky and Bebout, 2001, 2004; horizontal dimension of ~1 m).
238 G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
and addition of elements, most conspicuously an addition of Si (Bebout
and Barton, 2002,seeFig. 8). Evidence for extensive fluid–rock interac-
tions during subduction comes from O, C, H, N, and Li analyses of veins,
altered rocks and mélange matrix that suggest externally-derived fluids
sourced from metasedimentary rocks infiltrated the rocks and altered
their isotopic composition in some cases (e.g. O, see Fig. 9)homogeniz-
ing the isotopic composition on relatively large (km) scales (i.e., across
all metamorphic grades), in other cases (e.g. N, Li) homogenizing the
isotopic composition on intermediate scales (i.e. within a single meta-
morphic grade; Bebout, 1997; Penniston-Dorland et al., 2012a). Reac-
tion rinds surrounding mafic blocks in mélange matrix exhibit
evidence for a complex process of formation involving both mechanical
mixing and fluid infiltration (Sorensen, 1988; Bebout and Barton, 2002;
Penniston-Dorland et al., 2012b, 2014,seeFig. 10).
There are several reports of evidence for partial melting in subduc-
tion settings including for the Catalina Schist amphibolite unit
(Sorensen and Barton, 1987), the Sierra del Convento mélange in Cuba
(Garc ía-Casco et al., 2008; Lázaro and García-Casco, 2008), and the
Mt. Hochwart mélange unit in Italy (Marocchi et al., 2010). All of these
units experienced peak metamorphism at temperatures considerably
warmer for the estimated depths than predicted by thermal models
for subduction-related metamorphic rocks. However, these rocks pro-
vide some insight regarding the deeper high-T histories of rocks
subducting along cooler P–T paths, as many reactions in the more ultra-
mafic bulk compositions are fairly P-insensitive (see Bebout and Barton,
2002; Collins et a l., 2015; Kerrick and Connolly, 1998; Spandler and
Pirard, 2013; Spandler et al., 2008) and the chlorite-, talc-, and
amphibole-rich mineral assemblages are stable to far greater pressures.
6.1.2.3. Mt. Hochwart, Eastern Italian Alps. The Mount Hochwart locality
(Marocchi et al., 2010) contains a mélange unit with lens-shaped bodies
of peridotite found within garnet-kyanite-bearing gneisses. The perido-
tites are interpreted to have been fragments of mantle wedge incorpo-
rated into downgoing continental crust. Centimeter-thick metasomatic
contact bands of variable thickness are found at the contact between pe-
ridotite and gneiss, in which relatively fluid-mobile elements (such as
the LILE) are not observed to travel far into the peridotite. These results
suggest limited mobility of these elements in situations where transport
is not focused in fluid flow channels or veins. One exception is Li, which
appears across all the metasomatic zones.
Fig. 8. Major element (wt.%) and Cr (ppm) concentrations for mafic and ultramaficblocks,
reaction rinds, and samples of mélange matrix demonstrating that the mélange matrix
compositions can be explained by mixing of mafic and ultramafic rocks, but with the
necessity of significant addition of SiO
2
. Stable isotope compositions of the mélange matrix
are consistent with infilt ration of H
2
O-rich fluid previously equilibrated at somewha t
lowe r temperatures with metase dimentary rocks (Bebout and Barton, 1989;see
Figs. 9A,B). These fluids could also have delivered Si to the subduction interface along
which this mélange unit was developed (Bebout and Barton, 1993).
Fig. 9. (A) Calculated O isotope compositions of H
2
O in equilibrium with metasomatized
rocks from shear zones and fractures and some less altered rocks (from Bebout and Barton,
1989). In the highest-grade unit (amphibolite), large expanses of relatively intact
metabasaltic and metasedimentary rocks preserve δ
18
O out of isotopic equilibrium with
the fluids with δ
18
O=+13±1‰ (gray-shade d band) that preferentially infiltrated
shear zones and highly fractured domains on a regional scale (also see Bebout, 1991a).
(B) δ
18
O of calcite from relatively undeformed expanses of lawsonite-albite (darker-shad-
ed) and lawsonite-blueschist (lighter-shaded ) metasedimentary rock compared with
values for calcite in veins and in mélange matrix (from Bebout, 1991b). As in A, the vertical
gray-shaded band shows the uniform isotopic composition of the vein-forming fluids with
which the calcite in veins and mélange matrix are more extensively equilibrated (δ
18
O=
13 ± 1‰; Δ
18
O
calcite-water
for the relevant temperatures is in the range of +2.0 to +4.0‰;
see Bebout, 1991b). The field photographs are of lawsonite-blueschist mélange matrix (C;
horizontal dimension is ~30 cm) and amphibolite mélange matrix (D; hat for scale).
239G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
6.1.2.4. Dent Blanche-Sesia, Western Alps. The Dent Blanche Thrust Zone
(Angiboust et al., 2014a, 2015) consists of upper plate gneisses under-
lain by ophiolitic rocks (Tsaté Complex) on the lower plate. At the inter-
face of these tw o rock units, there are different types of regions
exhibiting mixing and cataclasis (Angiboust et al., 2014a, 2015). The
cataclasite zones occur in 1–100 m thick layers immediately below the
overlying gneisses. The cataclasite incorporates gneiss fragments and
also contains epidote-quartz veins in a matrix of fine-grained quartz,
albite, epidote, titanite, phengite, chlorite and actinolite. In one locality,
the Arolla Gneiss is underlain by a ~200 m wide serpentinite thrust zone
containing metagabbro pods (see Fig. 3). P–T–t constraints from these
rocks suggest progressive underplating and accretion of incoming mate-
rial at 30–40 km depth along the subduction interface over time with
the active subduction thrust migrating outboard over time (Angiboust
et al., 2014a).
6.2. Depths of 40–90 km
The suites representi ng 40–90 km contain intensely sheared do-
mains, with development of mélange zones in some localities that con-
tain greater degrees of disaggregation of slab and hanging-wall rocks
(and over a wide range of scales; e.g., Syros and Tinos, New Caledonia,
Sesia Zone, Sivrihisar, Voltri Massif, Tianshan). Monviso represents an
intraslab setting in which shear zones and fracture networks are devel-
oped in less deformed altered oceanic crust. Erro Tobbio and Cima di
Gagnone contain ultramafic rock, in sheare d/mélange zones, of
Fig. 10. Block-matrix relationships in the lawsonite-blueschist mélange unit of the Catalina Schist. (A) Field photo of mafic block core, rind and matrix. (B and C) Closeup (photograph and
sketch) of region in white box in A showing complex interface between block rind and matrix. (D) A comparison of the geochemistry of block rinds with block cores demonstrating rind
enrichment (higher concentrations) in elements associated with ultramafic rocks, variable concentrations of elements generally considered fluid mobile, and rind depletion (lower con-
centrations) in elements associated with mafic rocks relative to block cores. (E) Os concentrations and isotope compositions consistent with simple mixing (mixing curves shown) of mafic
and ultramafic components (from Penniston-Dorland et al., 2014).
240 G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
subduction-zone peridotites, presumed to be either seafloor-altered or
hanging-wall ultramafic rock. Hanging-wall rocks can also be previously
underplated rocks of either oceanic or continental crustal affinities or
they could conceivably be intact continental crust of the overlying plate.
6.2.1. Depths of 40–70 km
6.2.1.1. Sesia Zone. The Sesia Zone is distinct among the localities
discussed here because it represents one of the largest bodies of conti nen-
tal crust subducted to depths of ~60–65 km. The lithologies include
metapelitic rocks, metamorph osed calcareous pelites, and metagranitoids
intercalated with smaller slices of metabasites and lenses of carbonate-
and quartz-rich rocks (Compagnoni, 1977; Konrad-Schmolke et al.,
2006). Nonetheless, these rocks exhibit evidence for extensive fluid and
mass transport during their exhumation—while they were positioned
above the dehydrating subducting slab (see Halama et al., 2014;
Konrad-Sch molke and Halama, 2014).
Eclogite facies rocks contain subduction-related metamorphic min-
erals including garnet, omphacite, amphibole, and phengite that display
evidence for partial equilibration along grain boundaries, brittle frac-
tures and other fluid pathways (Konrad-Schmolke et al., 2011). Sharp
and diffuse mineral compositional zoning in some cases reflects up to
three different compositional variations, and coupled with evidence
from mineral replacement textures, suggests multiple influxes of fluid
in individual samples. Overall, open-system, pervasive reactive fluids
are thought to have infiltrated across large volumes of rock above the
subducting slab (see Halama et al., 2014).
6.2.1.2. New Caledonia. The northeast end of New Caledonia contains
subduction -related metamorphic rocks ranging from lawsonite-
blueschist to eclogite facies (Spandler et al., 2003, 2004, 2008). While
the lower grade rocks exposed on New Caledonia (the Diahot
Blueschists) ar e relatively coherent tracts of sedimentary rocks with
some intercalated basaltic and rhyolitic rocks, the higher-grade rocks
exposed in the northeast display mixing of materials characteristic of
mélange zones (see Fig. 11). This locality cont ains eclogite, garnet
blueschist, and garnet amphibolite along with metamorphosed ultra-
mafic rocks and metasedimentary rocks (quartz-garnet-mica schists).
Mélange matrix consists of talc, chlorite, and phengite along with quartz
clasts and coarse-grained amphibole clots, and is thought to have
formed prior to or during peak metamorphism. Hybrid talc and chlorite
schists formed due to metasomatism and mixing among ma fic,
metasedimentary, and ultramafic rocks in the mélange zone (Spandler
et al., 2008). Recent work on veins in eclogite in this suite demonstrated
a complex “drain age system” for fluids generated by dehydration
reaction which metasomatized host-rocks (Taetz et al., 2015).
6.2.1.3. Tianshan, China. The Tianshan HP-LT belt occurs in NW China and
contains blueschist, eclogite and greenschist facies metasedimentary
rock
s, mafic metavolcanic rocks, marbles and metamorphosed ultra-
maficrocks(Gao and Klemd, 2003; Klemd et al., 2011). Hosted within
greenschist and blueschist units are lenses, pods and slices of blueschist,
eclogite, marble, and ultramafic rocks, in classic mél ange style. The
Tianshan is the first locality where primary fluids at the blueschist to
eclogite transition were described (Gao and Klemd, 2001). These fluid
inclusions were low salinity, H
2
O-NaCl rich and were thought to repre-
sent fluids derived internally from the rocks during dehydration.
Alteration halos surrounding omphacite-quartz-calcite veins in
blueschist show that eclogite veins formed due to infiltration of an
externally-derived fluid (Beinlich et al., 2010; John et al., 2008, 2012).
These fluids interacted with the host rock and removed REE, HFSE,
some LILE (Cs, Rb, Ba, K) and Li from the host rock during the
eclogitization process but added Ca, Sr, and Pb (see Fig. 12). A profile
across the vein-host rock contact demonstrates small-scale variability
in Li concentrations and δ
7
Li that are consistent with diffusion into the
eclogite. Diffusion modeling suggests that the infiltration event that cre-
ated the vein was relatively brief (perhaps ~200 years; John et al., 2012).
6.2.1.4. Syros, Tinos. The Greek islands of Syros and Tinos contain
mélange formations metamorphosed through the Eocene that are
composed of eclogites, blueschists, metagabbros, serpentinites,
metaplagiograni tes, and metasedimentary rocks (Okrusch and
Bröcker, 1990; Bröcker and Enders, 2001; Ague, 2007; Ague and
Nicolescu, 2014; Miller et al., 2009; Pogge von Strandmann et al.,
2015). The mélange matrix on Syros is in some places serpentinite
and in others metasedimentary rock. Metasomatic exchange at litholog-
ic contacts between mélange blocks and serpentinite matrix resulted in
the development of asymmetric reaction rinds rich in omphacite,
glaucophane, and/or chlorite on the edges of some mafic blocks. Two-
dimensional numerical models of fluid flow within mél ange matrix
Fig. 11. (A) Field relations in shear zones in New Caledonia (from Spandler et al., 2008).
Highly deformed matrix in shear zones between larger volumes of intact maficrocks
and containing smaller blocks of mafic, sedimentary, and ultramaficrocks.Shearedmatrix
is either chlorite- or talc-rich. (B and C) Trace element compositions of mafic, ultramafic,
and hybridized rocks in the shear zone, normalized to chondrite. Matrix talc and chlorite
schists exhibit compositions ranging from mafictoultramafic, demonstrating derivation
of matrix from both lithologies. (B) Compositions of talc schist sample 2814, talc-amphi-
bole schist sample 1015, chlorite schist sample 2816, and serpentinites (various open
symbols). The field for associated mafic rocks is shown as the dark grey field (data from
Spandler et al., 2004). (C) Serpentinite (gray field) and talc schists.
241G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
suggest that, as permeability decreases within blocks, flow is diverted
around blocks into matrix leading to enhanced fluid–rock reaction at
the block margins (Ague, 2007).
Multiple studies have observed changes in chemical composition
in these rocks at interfaces between mafic, ultrama fic, and meta-
sedimentary ro cks, resulting in ‘hybrid’ chemical compositions of
rocks and also inferred changes in fluid composition, resulting ultimate-
ly in transferring the chemical signature of these fluids and rocks to arc
magmas or deep into the mantle (see Fig. 13; Breeding et al., 2004;
Miller et al., 2009). Formation of reaction zones adjacent to veins in
metacarbonate rocks produces chemical change including loss of CO
2
that suggests fluid infiltration from surroun ding rocks resulting in
dissolution of carbonate and precipitation of silicate minerals (Ague
and Nicolescu, 2014). Although the volumetric significance of carbonate
dissolution remains unknown, this fluid–rock interaction could provide
a mechanism, in a ddition to decarbonati on, f or the release of large
quantities of CO
2
from subducting slabs (see Fig. 14; Ag ue and
Nicolescu, 2014; Collins et al., 2015).
Fig. 12. Mineralogy and major and trace element concentrations across a Tianshan vein envelope developed in an intraslab fracture network (from Beinlich et al., 2010). (A) Variation in
mineral modes, (B) variation in major element concentrations, (C) loss of some elements adjacent to the vein (Li, Cs, Rb, Ba, K
2
O), and (D) metasomatic addition of some elements (CaO, Sr,
Pb). Shown in (E) is a field photograph of the vein, the eclogitic selvage, the transition zone, and the host-rock blueschist (from Beinlich et al., 2010; also see the study of the same traverse
by John et al., 2012).
242 G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
6.2.1.5. Erro Tobbio and Voltri Units, Ligurian Alps. At this locality, ultra-
mafic tectoni c blocks occur within a matrix of tectonized ultramafic
rocks (Scambelluri et al., 2001) and mafic and metasedimentary lenses
are found in a chlorite-rich matrix (Federico et al., 2007). Major and
trace-element metasomatism at the rims of the ultramafic blocks is
interpreted to represent the effects of seafloor serpentinization, where-
as the compositions of veins in the ultramafic rocks provide constraints
on the composition of fluids released by dehydration reactions
(Scambelluri et al., 2001). The compositions of the veins reflect enrich-
ment of elements derived locally due to dehydration of serpentinite,
suggesting lack of externally derived fluid infiltration, and the authors
suggest that REE, Cl, Sr, some alkalis, in addition to H
2
O, can be conveyed
to great depths in subduction zones in such lithologies.
6.2.2. Depths of 70–90 km
6.2.2.1. Siv rihisar, Turkey. Th e Sivrihisar massif is part of the Tavsa nli
zone of central Turkey, which is one of the largest high-pressure/low-
tempe rature belts (250 × 40 km) exposed on Ea rth (Whitney et al.,
2014). Interlayered, deformed marbles, quartzites, and metabasalts are
continuously exposed on scales up to kilometers within the Sivrihisar
massif. Elliptical lawsonite eclogite pods occur within blueschist,
quartzite, and calc-schist ranging in size from 1 cm to 5 m. Many of
these pods are folded and have millimeter-scale alternating layers of
blueschist and eclogite at the margins. Additionally, pods and lenses of
metatuff, metagabbro, and serpentinite occur surrounded by blueschist.
Serpentinite lenses have reaction zones of chlorite-lawsonite ± talc ±
oxides ± carbonate. Alt hough a detailed study of fluid processes in
these rocks has not yet been conducted, the presence of lawsonite-
bearing and other veins seemingly reflects diverse fluid–rock interac-
tions (Martin et al., 2014).
6.2.2.2. Monviso (Western Italian Alps). Extensive work on fluid–rock in-
teractions has been conducted on rocks from this locality, beginning
with Philippot and Selverstone (1991) and Nadeau et al. (1993),
which focused on veins in Fe–Ti metagabbros in the Lago Superiore
Unit at this locality. That work documen ted relati vely closed-system
fluid–rock behavior during the formation of these veins. Spandler
et al. (2011) suggested early closed-system behavior, leading to vein
formation, followed by open-system behavior, based on compositions
of garnet and other vein phases, and proposed that dehydrating ultra-
mafic rocks were the sources of this fluid.
More recently, Angiboust et al. (2011a,b; 2014b) placed the various
eclogitic units, and intervening deformed ultramafic rocks, into the con-
text of shearing at/near the subduction interface, suggesting that the
Lower Shear Zone at Monviso represents an intraslab shear zone (see
Fig. 13. Evidence for fluid-mediated metasomatic exchange between mélange matrix and metasedimentary rocks in the Syros mélange unit (from Breeding et al., 2004). (A) Sketch of
blocks in matrix and reaction zone adjacent to matrix, and (B) geochemical data demonstrating that in the metasedimentary rocks, the destabilization of phengite resulted in large deple-
tions in K, Rb, Cs and Ba.
243G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
Fig. 15;alsoseeRubatto and Angiboust, 2015). Mafic eclogites near and
within this shear zone are brecciated and variably metasomatized and
the authors used a combination of garnet chemistry and whole-rock
chemistry to document metasomatic exchange between the maficand
ultramafic lithologies, in a fluid-rich setting (Fig. 15). A more detailed,
larger-scale study of fluid–rock interactions, using stable isotope data,
has not yet been conducted at this locality.
6.2.2.3. Cima di Gagnon e, Adula Unit, Swiss Central Alps. At Cima di
Gagnone, pelitic schists and gneiss enclose chlorite harzburgite and
Fig. 14. Evidence for the dissolution of carbonate in high-P metamorphic rocks exposed on Syros and Tinos (from Ague and Nicolescu, 2014). (A) Photograph of unaltered carbonate on
right, green altered carbonate on left, (B) photograph of glaucophane (gl)–epidote (ep) rich alteration zone in carbonate rock adjacent to quartz (q) vein, (C) field photograph of epidote–
quartz vein surrounded by omphacite (om)-rich alteration zone, (D) photograph of omphacite and glaucophane rich alteration zone adjacent to vein. (E and F) Proposed tectonic settings
for carbonate dissolution in these exposures, also from Ague and Nicolescu (2014), on Syros (E) and Tinos (F). This dissolution in underplated carbonate-bearing rocks could be facilitated
by infiltration of fluids from evolving major shear zones (mélange zones) acting as fluid channelways (also see Breeding et al., 2004; Fig. 13).
244 G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
garnet peridotite lenses, and Cannaó et al. (2015; also see Scambelluri
et al., 2014) suggested that this unit represents a slab-top mélange
(see Fig. 16A). These authors documented multiple hydration/
dehydration events related to antigorite and chlorite breakdown. The
fluids released by these reactions facilitated metasomatic interactions
among the dispar ate lithologies in this mélange unit, as reflected in
the redistribution of many trace elements and B and Pb isotopes. The
scenario of mixing of mantle and crustal materials within the subduc-
tion interface that is invoked by these authors (see also Scambelluri
et al., 2015) is similar to one model proposed by Spandler and Pirard
(2013; see Fig. 16B) in volving the production of a hydrated mélange
zone at the top of a subducting slab, in this case largely containing ultra-
mafic lithologies with smaller components of mafic and sedimentary
rocks. These models both suggest mixing of mantle and crustal mate-
rials at the subduction interface and do not require the same large-
scale transport of fluids and mass as is required if the mantle-derived
component is sourced in the sub-oceanic lithospheric mantle. Fluids
with compositions reflecting sediment sources and seafloor hydrother-
mal alteration, can be released from metasomatized ultramafic rocks
along the subduction interface (Scambelluri et al., 2015).
Fig. 15. Field relations, geochemistry, and tectonic setting of an intraslab shear zone exposed at Monviso, NW Italy (from Angiboust et al., 2014b). Fluid–rock interaction resulted in ad-
ditions of the LILE Rb and Ba and elements sourced in the ultramafic shear zone matrix (Cr and Ni). (A) Field relations at/near the brecciated margin of a metabasaltic block in this
shear zone. (B) Zoning anomalies, including Cr zoning, in garnets growing in the progressively metasomatizing blocks indicating possible mobility of these elements in metamorphic fluids.
(C) Subduction interface setting for this shear zone and structural relationships at Monviso, where intraslab shear zones at/near the interface are developed between large zones/sheets of
intact mafic eclogite showing different peak P–T history (from Angiboust et al., 2014b). The LSZ (Lower Shear Zone) contains smaller, more spherical mafic blocks with metasomatic rinds
and brecciation at their rims (Angiboust et al., 2012). Fluid mobility, indicated by blue arrows, was concentrated in the shear zone and in fractures in mafic blocks showing brittle defor-
mation behavior.
245G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
7. Discussion
7.1. Common themes in mass transfer at the subduction interface
It is possible to draw some generalities from the metasomatic para-
geneses observed in rocks from the subduction interface and to infer
some relative mobilities of major and trace elements in HP/UHP meta-
morphic fluids (also see Bebout and Barton, 1993; Bebout, 2013;
Dolejs and Manning, 2010; Manning, 2014). Transfer of mass in H
2
O-
rich fluids is thought to vary dramatically depending on the P–T path
the fluids take, and the rocks encountered, once liberated from
subducting slabs.
7.1.1. Quartz ± calcite veining
As predicted from solubility relations (see Manning, 1994), quartz
solubility is sufficiently high that Si metasomatism can dominate the
metasomatic record (also see Dolejs and Manning, 2010). Veins contain-
ing quartz, with or without carbonate (generally calcite or aragonite), are
nearly ubiquitous in metasedimentary-dominated (ordinarily quartz-
saturated) subduction interface rocks representing depths of b 40 km,
Fig. 16. Views of the subduction interface from Spandler et al. (2008) and Cannaó et al. (2015) emphasizing the development of mélange and the importance of the dehydration of chlorite,
antigorite, and talc for the fluid and mass transfer in this region. (A) Schematic model of interaction between different geochemical reservoirs at the plate interface environmentandmain
geochemical implications (from Cannaó et al., 2015; redrawn after Trommsdorff, 1990). a) Release of H
2
O-rich fluids sourced from mélange and dehydration of mélange serpentinites and
of serpentinizedwedge, could trigger arc magmatism and explain the geochemical anomalies characteristic of manyarc lavas; b) geochemical anomalies introduced into arc magma source
by ascent of serpentinized wedge diapirs due to their low density compared to the surrounding materials and their “spongy behavior” for sedimentary-like elements and isotopes (from
Cannaó et al., 2015). (B) One model for the devolatilization and partial melting resulting in arc magmatism, invoking the presence of a hybridized zone at the interface that is partially
melted, yielding hybrid-slab chemical signatures (from Spandler and Pirard, 2013).
246 G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
suggesting Si addition, and in some cases can make up a large fraction of
the volume of an individual exposure (see Breeding and Ague, 2002,who
also suggested removal of large amounts of Si by dissolution in the
metasedimentary host rocks). In general, this deposition of quartz and
carbonate is consistent with up-dip flow of fluid, that is, down-T and
down-P and parallel to the subduction thrust (see path 1 in Fig. 17;see
Section 7.5.6). Examples of suites discussed in this review containing
such veins are Arosa, Franciscan, Shimanto, Kodiak Island, and Catalina
Schist. Rocks in these suites commonly contain multiple generations of
veins containing these minerals, showing cross-cutting relations and di-
verse textures relative to the deformation textures of the host-rocks (see
Bachmann et al., 2009; Meneghini et al., 2009; Sadofsky and Bebout,
2004; Yamaguchi et al., 2012). In a smaller number of cases, these
quartz-calcite veins also contain minerals that are part of the peak-
metamorphic mineral assemblage of the host-rocks or minerals that
are more clearly retrograde.
7.1.2. Other silica metasomatism
Fluid–rock interaction can result in either enrichment or depletion
of silica depending on the fluid P–T paths, and the diverse rock compo-
sitions encountered along fluid fl ow paths, (see Bebout and Barton,
1993; Bebout, 2013; Dolejs and Manning, 2010; Manning, 1997). Aque-
ous fluids containing appreciable dissolved Si through interaction with
quartz-bearing metasedimentary rocks (see Fig. 17) would be expected
to drop much of this silica when they interact with Si-poor lithologies
with large ultramafic components. Thus the possibility exists for a
fluid to change considerably in its dissolved Si (and other dissolved ele-
ment load) along a complex P–T path involving traverses across widely
disparate lithologies (see Bebout, 2013). For example, regional/km-
scale addition of Si to amphibolite-grade mélange ultramafic rocks by
fluids generated in lower-grade metasedimentary rocks in the Catalina
Schist was documented by Bebout and Barton (2002) (see the evidence
for Si addition to this mélange unit in Fig. 8B).
7.1.3. LILE metasomatism
The addition or removal of LILE, from rocks of varying bulk composi-
tions, has been documented in a number of studies, particularly in stud-
ies of the metasomatism of rocks initially poor in these elements
(e.g., ultramaficandmafic rocks). Bebout (2007) documented that
many HP and UHP metabasaltic rocks, commonly occurring as tectonic
blocks in sheared matrix, show whole-rock enrichments in LILE such
as Ba, Rb, Cs, K, and Pb. Whole-rock enrichments in Ba and Pb can be
most directly associated with subduction-zone metasomatism, as the
other elements are also commonly enriched on the seafloor during in-
teractions with seawater. This type of metasomatism can occur during
both prograde and retrograde metamorphism. For example, Sorensen
et al. (1997) documented addition of LILE to mafic rocks of the Catalina
Schist and Samana Complex (Dominican Republic) associated with the
addition of phengite and documented prograde and retrograde crystal-
liz
ation of the phengites (bas ed on textural relationships) indicating
multi-stage addition of LILE . Gabbroic and dioritic cobbles in
blueschist-facies-metamorphosed conglomerates in the Catalina Schist
similarly show large co-enrichments of Cs, Rb, and Ba, along with en-
richments in B and N (Bebout, 1997; Bebout, 2013).
7.1.4. Na-rich metasomatic parageneses
Jadeitites (rocks rich in jadeite, Na–Al pyroxene) are found in
subduction-related mélange in a range of localities around the world
(see the review by Harlow et al., 2015). Jadeitite mineral textures sug-
gest that in some cases these rocks crystallized from fluids in veins,
and thereby their presence attests to transport of Na, Al, and Si by meta-
morphic fluids (e.g., Cárdenas-Párraga et al., 2012; Harlow and
Sorensen, 2005). Evidence from jadeite that has been replaced domi-
nantly by albite (albitites) in the Sierra de las Minas, Guatemala, sug-
gests subsequent mobility of LILE (K, Rb, Cs, Ba, Sr, Pb) and also Hf, Zr,
Th and U, all thought to be introd uced by externally-derived fluids
(Sorensen et al., 2010). Further evidence for the mobility of Na, Al, and
Si is documented by the presence of sodic-amphibole-bearing veins
and their alteration envelopes, and albite-ri ch veins, both found in
blueschist facies metasedimentary rocks of the Catalina Schist (Bebout
and Barton, 1993). In both the amphibolite-grade unit of the Catalina
Schist and the Mt. Hochwart mélange unit, leucosomes and pegmatites
largely tonalitic/trondhjemitic in composition reflec t H
2
O-saturated
partial melting and mobilization of an “albitic component” (for the
Catalina Schist, Bebout and Barton, 1993; Bebout, 2013; Sorensen and
Barton, 1987; for Mt. Hochwart, Marocchi et al., 2010). Sorensen and
Grossman (1989) suggested that some eclogite blocks in the Catalina
Schist amphibolite -grade mélange experienced removal of an “albite
component” likely in the form of partial melt.
7.1.5. Other trace element metasomatism along the subduction interface
The hierarchy of trace element mobilities at depth along the subduc-
tion interface more or less mimics the mobilities predicted by experi-
mental studies (see experiments on aqueous fl uid-MORB by Kessel
et al., 2005). In these and other experiments, trace elements with higher
fluid–solid partition coefficients include Cs, Rb, Ba, Sr, B, Li, Pb, Th and U,
all of which are commonly enriched in metasomatic products at the in-
terface. Boron and Cs are sufficiently partitioned into fluids during
devolatilization of metasedimentary rocks to produce observable loss
(Bebout et al., 1999). The experiments by Kessel et al. (2005) indicate
relative immobility of HFSE such as Nb, Ta, Zr, and Hf, except at temper-
atures N 800 °C, and the REE are also relatively immobile but show in-
creasing partit ioning into fluids wi th de creasing atomic weight. The
increasing potential of higher-T aqueous fluids and supercritical/
transitional fluids to mobilize trace elements ( see Hermann et al.,
2006) is borne out by experiments of Kessel et al. (2005), which show
1–2 orders of magnitude increase in fluid-MORB partitioning in these
higher-T fluids. In accordance with this experimental finding, elements
such as the HFSE and REE typically do not show evidence of their mobil-
ity, in the form of significant depletions in devolatilized rocks, in HP and
Fig. 17. Quartz solubility as a function of P and T, plotted as contours of dissolved silica
(contours of log molality of aqueous SiO
2
in equ ilibrium with quartz, from Manning,
1994), and generalized “fluid” P–T flow trajectories that can be envisioned in a slab–man-
tle interface environment (see text for discussion; also see B ebout and Barton, 1993;
Bebout, 2013). The inset shows the possible geometry of fluids experiencing P–T Paths 1
(blue) and 2 (green) on this diagram, Path 1 up-dip along the interface and Path 2 initially
up-T within the slab toward the interface, then down-T during up-dip transport similar to
that experienced along Path 1. A critical end point on the hydrous melting curve of quartz
lies at 1080_C and 9.5–10 kbar (Newton and Manning, 2008).
247G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
UHP interface rocks all peak-metamorphosed at temperatures b 600 °C.
However, these elements certainly do occur in some veins and in meta-
somatic rinds on tectonic blocks in mélange units, indicating some
mobility.
Evidence from metamorphosed basaltic rocks from the Franciscan
Complex, Samana Metamorphic Complex (Dominica n Republic) and
the Catalina Schist suggests that Li is mobile in subducti on-related
metamorphic fluids, and provides a source of information about the his-
tory of multiple fluid sources during metamorphism. Mafic blocks found
in mélange generally have higher Li concentrations and lower δ
7
Li than
presumed MORB or altered MORB protoliths, suggesting infiltration of
fluids derived from Li-rich sedimentary sources. Retrograde reaction
zones surrounding blocks have higher δ
7
Li compared to mafic blocks
and in some cases have lower and in others higher Li concentrations
compared to mafic blocks, suggesting a later fluid, whose source cannot
be constrained by Li composi tions (Penniston-Dorland et al., 2010,
2012a,b).
In some settings, there is evidence for REE and/or HFSE mobility. The
metamorphosed mafic rocks of the Catali na Schist and the Shuksan
Metamorphic Suite (Washington State, USA) exhibit enrich ments in
HFSE and REE in some garnet amphibolite blocks compared to others
and in reaction rinds relative to associated maficblocks(Sorensen and
Grossman, 1993). These enrichments are associated with crystallization
of accessory minerals such as epidote, rutile, and titanite at relatively
high-T conditions (500–800 °C, 0.7–1.4 GPa). Lawsonitites in the Alpine
Corsica (France) are found as reaction rinds at the contacts between
metamafic, serpentinite or ophicarb onates and enclosing meta-
sedimentary rocks (Vitale Brovarone et al., 2014) at somewhat lower-
T conditions (lawsonite-blueschist unit T ~350–460 °C, P ~1.5–
1.8 GPa, lawsonite-eclogite unit T ~ 490–550 °C, P ~2.2–2.4 GPa).
These rocks exhibit extreme (N 1000% mass gain) enric hment in Ca
and Sr along with significant (50–500% mass gain) enrichment in REE
and loss of LILE such as K, Rb, Cs and Ba.
7.1.6. Likely importance of the transition from hydrous fluid to silicate melt
The element transport capabilities differ greatly for aqueous fluid
and silicate melt (see Hermann et al., 2006; Kessel et al., 2005). The sig-
nificance of supercritical fluid for transporting mass has remained un-
certain due to uncertainty in the positions of the relevant second
critical end points in various H
2
O–rock systems (see the discussions
by Manning, 2004; Zheng and Hermann, 2014). Nonetheless, the P–T
conditions at the subduction interface beneath arcs (i.e., at depths of
80–120 km) are sufficiently high (N 600 °C) that some H
2
O-saturated
partial melting can occur (particularly of metasedimentary rocks), pre-
sumably providing greater potential for mass transfer.
7.2. Contrasting metasomatic behaviors: closed vs. open system
There are two endmember metasomatic styles observed at/near the
subduction interfaces based on investigati ons of HP metamorphic
suites. In one endmember, more intact or coherent volumes of rocks
(sedimentary, mafic, ultramafic) behave as relatively closed systems.
This has been documented in a number of the Alpine eclogite localities
(e.g., Monviso: Nadeau et al., 1993; Philippot and Selverstone, 1991;
Rubatto and Hermann, 2003; Spandler et al., 2011; the Swiss Alp s:
Barnicoat and Cartwright, 1995; Cartwright and Barnicoat, 1999;
Getty a nd Selverstone, 1994; Widmer and Thompson, 2001). For
metasedimentary rocks, Bebout and Fogel (1 992), Bebout et al.
(1999), Bebout and Nakamura (2003),andBebout et al. (2013) invoked
relatively closed-system Rayleigh-like devolatilization behavior in larg-
er volumes of metapelites and metacarbonates. Collins et al. (2015) ar-
gued for relatively closed-system behavior in exposures of deeply
subducted W. Alps metabasalts and ophicarbon ate away from major
shear zones and containing few larger, through-going veins. Closed-
system behavior was indicated by a lack of evidence for appreciable de-
carbonation, consistent with little or no infiltration of the rocks by H
2
O-
rich fluids. Also, in many of these rocks, carbo nate δ
18
O is consistent
with closed-system exchange with more abundant silicate minerals.
In the other endmember, fluid flow is focused into shear zones
(including mélange zones) and fracture networks, as suggested by
greater abundances of metasomatic fea tures in shear zone matrices,
relative to more intact rocks, and isotopic homogenization observed
in shear zone matrices and veins (Barnicoat and Cartwright, 1995;
Bebout, 1991a,b, 2014; Selversto ne et al., 1991; Spandler and
Hermann, 2006; see the discussions of permeability in subduction chan-
nel settings by Ague, 2007, 2014; Angiboust et al., 2014b; Kawano et al.,
2011; Spandler et al., 2011; van der Straaten et al., 2012; Zack and John,
2007). This relationship was demonstrated for the Catalina Schist
paleoaccretionary complex, using the O, C, H, and N isotope composi-
tions of rocks from mélange zones and mineralized fractures in compar-
ison wi th compositions for m etasedimentary and metamaficrocks
away from these structural features (see Fig. 9). Carbonate dissolution
and carbonate reduction recently documented by Galvez et al. (2013)
and Ague and Nicolescu (2014; also see the discussion by Lazar et al.,
2014) indicate localized larger fluid flux in zones of structural weakness
(i.e., particularly high permeability), in these cases along veins and in a
fold hinge.
7.3. Contrasting deformation behaviors: intact vs. highly deformed
Field-based studies of HP metamorphic suites point to some com-
mon themes in the structural/lithologic mak eup of the subduction
plate interface, and the various field studies likely s ample differing
parts of evolving subduction interfaces (see Fig. 1). At one end of the
deformation spectrum, relatively intact or coherent volumes of
identifiably metasedimentary, metabasaltic/metagabbroic, and meta-
ultramafic rocks occur within or separated by shear zones. These larger,
more intact bodies in some cases appear to be sheets/tabular in mor-
phology. The scales of these features vary widely, from 10s of cms to
kms (Angiboust et al., 2011a,b; Bachmann et al., 2009; Whitney et al.,
2014), and can be seen fragmenting into smaller blocks that begin to ac-
quire more spherical geometries. Some aspect of all metamorphic expo-
sures can represent this endmember behavior: intact intraslab rocks
(with veins, less-developed shear zones; e.g., Sivrihisar; Whitney
et al., 2014), large intact bodies with intervening more highly developed
shear zones (e.g., Monviso; Angiboust et al., 2011a,b), smaller spherical-
geometry blocks within expanses of sheared matrix (Catalina Schist ,
New Caledonia; Bebout and Barton, 1993, 2002; Spandler et al., 2008),
and intact volumes of variably disaggregated meta-peridotite believed
to represent hanging- wall ( mantle wedge) rocks to varying degrees
metasomatized by fluids emanating from a subducting slab and sedi-
ments (e.g., Cannaó et al., 2015). These exposures lead to depictions of
the subduction interface as a zone developed between subducting lith-
osphere sections and hanging wall (peridotite or other lithologies, par-
ticularly at greater depths; see Fig. 1).
At the other end of the deformation spectrum are highly deformed
zones with schistose matrix. These zones are commonly referred to as
mélange zones and generally have a fine-grained matrix that is com-
posed either of sedimentary particles, such as clay minerals or have a
large ultramafic component (serpentinite or talc/chlorite schists). The
matrix rocks can include large tracts of chlorite-, talc-, and amphibole-
rich schists containing few other minerals (
Bebout and Barton, 1989,
19
93, 2002; Marsch all and Schumacher, 2012; Miller et al., 2009;
Spandler et al., 2008). Where studied, the isotopic compositions of
these matrix rocks are relatively homogenized, at up to km-scales,
seemingly reflecting extensive fluid–rock interaction at such scales.
These matrices contain bodies of less deformed mafic, sedimentary,
and ultramafic rocks. The blocks found within these matrices have vary-
ing geometries, are up to km-scale, and can preserve disparate P–T his-
tories reflecting dynamics of incorporation and entrainment. The blocks
can contain mineralized fracture networks (vein arrays), at least in
some cases with isotopic compositions indicating communication with
248 G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
the fluids mobilized within the matrices (see Bebout, 1991a,b, 1997;
Penniston-Dorland et al., 2012a).
7.4. It's a matter of scale
It is easy to imagine that the deformation, and related fluid and mass
transfer, at the subduction interface is distributed across multiple km
scales, consistent with geophysical observations regarding a low seismic
velocity zone at tops of many subducting slabs (see Section 1). Exhumed
mélange zones in some cases are kilometers wide, such as the
amphibolite-grade mélange unit in the Catalina Schist, within which
tectonic blocks of mafic and ultramafic rock range in scale from meters
to 100s of meters (Bebout and Barton, 2002). In other suites, it is likely
that the scale of the tectonic blocks is sufficiently large that the identifi-
cation of more deformed matrix is greatly complicated. In some locali-
ties, preferential weathering of the mélange matrices leads to
preferential exposure of the tectonic blocks, with or without attached
matrix (e.g., in the Franciscan Complex, Tiburon Peninsula exposure).
The subduction interface should be thought of as a km-scale zone
over which the distribution of strain is quite heterogeneous, with inter-
leaving of strongly deformed zones and domains experiencing little de-
formation — from this perspective, a mélange unit such as that mapped
by Bebout and Barton (2002) could be thought of as representing one of
these more highly deformed zones. In this spirit, we have incorporated
observations from subduction-zone metamorphic suites in which a
quite diverse set of lithologies has experienced juxtaposition. We em-
phasize the implications of the styles of deformation in these zones for
the release and mobility of fluids and the related mass transfer that is fo-
cused in the more deformed parts of the section. In summary, many of
the differences in conclusions regarding fl uid and mass transfer
reviewed here relate to the scales at which the key observations were
made and to which part of an evolving subduction interface is being
investigated (see Fig. 1).
7.5. Significance and impacts of interface fluid and mass transfer
At all depths along the subduction interface there is evidence for
fluids, whether in the form of veins, metasomatic reaction zones, or as
alteration of the isotopic composition of more highly deformed rocks.
In this section, we argue for the significance of fluid and mass transfer
along the subduction interface and describe its impacts on the structural
and geochemical evolution of subduction zones.
7.5.1. Role of fluids and mass transfer for seismicity
At shallow depths (b 40 km), the rocks display a variety of deforma-
tion features (e.g. veins, slickenfibers, pseudotachylytes, cataclastic
zones) that may have been associated with significant seismicity (see
Angiboust et al., 2015; Deseta et al., 2014; Fagereng et al., 2011; Fisher
and Brantley, 2014; John and Schenk, 2006; Rowe et al., 2005, 2011).
Evidence from crack-seal veins of the Kodiak Formation led to the
conclusion that silica distribution may play a role in modulating the
frequency of plate boundary instability (Fisher and Brantley, 2014). Re-
lationships between brittle deformation structures (cataclasites, veins)
and ductile deformation structures (mylonitic rocks) suggest switching
between brittle and ductile behavior in Arosa (Bachmann et al., 2009)
and the Dent Blanche Zone (
Angiboust et al., 2015),
and suggest varia-
tions in strain rates and fluid pressures which are thought to be related
to seismic activity. These observations suggest a strong relationship be-
tween build up of fluid pressure, seismic failure, and migration of fluids
(see Husen and Kissling, 2001; Saffer and Wallace, 2015).
The presence of weak, hydrous sheet silicate minerals and free fluids
(and higher pore fluid pressure) is generally believed to decrease the
strength of the interface (Abers et al., 2013; Wada et al., 2008, 2012),
and some believe that fluids play a key role in episodic tremor and slip
(ETS) events obse rved in some subduction zones (e.g., Cascadia, NW
Japan; Audet and Bürgmann, 2014; Audet et al., 2009; Gomberg et al.,
2010; Peacock, 2009; Peacock et al., 2011). Recently, some have sug-
gested that the redistribution of quartz by fluids could significantly af-
fect elastic properties and play a role in modulatin g periodic events
such as earthquakes or slow earthquakes (Audet and Bürgmann,
2014; Fisher and Brantley, 2014; see Fig. 2). For the Cascadia margin,
Hyndman et al. (2015) suggested a relationship between silica additions
to forearc crust and the positioning of the ETS zone.
7.5.2. Rheology and deformation-enhanced fluid infiltration
The highly deformed mélange units found in many subduction inter-
face exposures preserve a mixture of a variety of materials that have sig-
nificantly different rheology. These materials respond differently to
strain, in many cases resulting in the localization of strain within weaker
materials. Relatively weak materials that may act as lubricating layers
within subduction zones include sedimentary rocks (at P b 2GPa)and
serpentinites (at P b 3 GPa, Guillot et al., 2009). Experimental results
on the deformation behavior of antigorite suggest that it may have a rel-
atively low viscosity during deformation at low temperatures character-
istic of subduction zones (P =0.85to4GPa,T =200–625 °C; Chernak
and Hirth, 2010; Hilairet et al., 2007). Other sheet silicates such as talc
(Escartin et al., 2008) and muscovite (Mares and Kronenberg, 1993)
also behave in a relatively weak fashion and can lead to strain localiza-
tion. The finer-grained matrices of plate interface mélanges are com-
monly composed of these relatively weak materials. Strain localization
has been proposed to explain different records of strain in coexisting
rocks. For example, in the Erro Tobbio unit (Voltri massif, Western
Alps) antigorite mylonites and cross-cutting en-echelon olivine veins
(indicative of prograde metamorphism) both record top-to-the-NW ki-
nematics consistent with deformation during subduction. Pre-Alpine
peridotite bodies contain ed within the antigorite mylonites, on the
other hand, record only minor evidence of Alpine deformation
(Hermann et al., 2000). This difference between the peridotite bodies
and the antigorite mylonites is explained by localization of strain within
the weaker serpentinite (Hermann et al., 2000
).
I
n this way, strain can be partitioned in mélange zones into regions
that contain relatively weak material. At Monviso, strain localization oc-
curred in shear zones at lithologic interfaces regardless of the strength
of materials (Angiboust et al., 2011a,b). While ductile deformation dom-
inated within these shear zones local brittle deformation is documented
by eclogite breccias and fractured garnets thought to be linked with
subduction zone seismicity (Angiboust et al., 2011a,b, 2012). Evidence
from localities such as the Catalina Schist (e.g. Bebout, 1997;
Penniston-Dorland et al., 2012 a) and the Sesia-Lanzo Zone (Babist
et al., 2006; Konrad-Schmolke et al., 2011) suggests that enhanced per-
meability due to the highly foliated nature of the matrix focuses fluid
flow into these relatively weak regions of high strain (see the discussion
by Ague, 2014).
Numerical modeling of the behavior of materials in subduction
zones indicates that zones or layers of mixed material can develop at
the subduction interface. Flow of material within the subduction inter-
face has been modeled by Cloos (1982), Cloos and Shreve (1988a, b),
Gerya and Stöckhert (2002),andGerya et al. (2002) and is thought to
be a possible mechanism for the exhumation of HP rocks. The effective
rheology of the material within the subduction interface and the relative
densities of the materials control this movement and the rate of exhu-
mation (Cloos, 1982; Gerya and Stöckhert, 2002; Gerya et al., 2002).
Density contrasts contribute to relative buoyancy of some subduction-
related materials and have been postulated to lead to exhumation
(e.g., Hermann et al., 2000; discussion by Agard et al., 2009).
Serpentinite has a relatively low density (≤ 2.7 g/cm
3
), which suggests
that it can behave in a buoyant fashion, similar to continental crust
within subduction zones (Reynard, 2013). Gerya et al. (2002) demon-
strated that the lower the effective viscosity of the matrix material,
the greater the degree of chaotic distribution of materials within the
subduction interface. These studies have also concluded that mélange
likely has a Newtonian rheology (dissolution precipitation creep) and
249G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
not a power law rheology (dislocation creep) in order for material to be
exhumed efficiently (e.g., Stöckhert, 2002). The rheology of mixed mate-
rials was investigated by Grigull et al. (2012). This study found that blocks
compose a volume percent that typically ranges from 5 to 50% of the
block-in-matrix terrain. The study further modeled the subduction inter-
face as a suspension of spheres in a low viscosity (≤ 10
19
Pa s) matrix and
found that the effect of spheres within this matrix on the overall viscosity
of the matrix was relatively minor—it changed the overall viscosity by
less than one order of magnitude (Grigull et al., 2012).
Fluids are known to play key roles in deformation and mass transfer
in shear zones (e.g., Axen et al., 2001; Barnes et al., 2004; Dipple and
Ferry, 1992; Goncalves et al., 2012; Keller et al., 2004; Selverstone
et al., 1991). The interplay between fluid flow, the development of hy-
drous minerals, and enhanced deformation can lead over time to the
channelization of fluid flow which in turn results in significant mass
transfer within these zones of channelized fluid flow as is observed in
many of the mélange zones described in this review.
7.5.3. Duration of fluid infiltration events
Regular spatial variations in elemental abundances and isotopic
compositions present in reaction zones, and consistent with diffusive
transport through an intergranular fluid, have previously been well doc-
umented for some elements, such as Li (John et a l., 2012;
Penniston-Dorland et al., 2010)andMg(Pogge von Strandmann et al.,
2015). In these studies, the scale of diffusive transfer yields information
regarding the timescales of diffusion. Petrographic evidence for fluid–
rock interaction is coupled with evidence for variation in whole-rock
isotopic compositions along profiles at the scale of cm to m. Numerical
models are fit to the profile data and use existing information about el-
emental and isotopic diffusivities to extract information about the dura-
tion of the diffusive event which in turn relates to the duration of the
fluid–rock interaction. The results yiel d timescales on the order of
100 years or less, suggesting that fluid is traveling through the rocks
in relatively short pulses or bursts.
7.5.4. Connection with arc lava compositions (and the deeper mantle)
The heating related to the transit of subducting slabs and overly-
ing sediment and sheared rocks as they come in contact with hot,
convecting mantle, and the resulting great er devolatili zation of
some of these rocks, should be a recipe for the mobility of fluid and
dissolved material. To add to this, the fluids are expected at these
conditions to be transitioning from hydrous fluids to silicate melts,
greatly increasing their capacity to dissolve and transport mass
(see Hermann et al., 2006; see Fig. 1 in Kessel et al., 2005). It is pos-
sible that the enhanced me tasomatism possible at subar c depths
and temperatures influences the compositions of more deeply
subducted rocks contributing to mantle geochemical reservoirs sam-
pled by ocean-island basalts (see discussion by Bebout, 2007; Ryan
and Chauvel, 2014; Hofmann, 2014).
The fl
uids and/or silicate melts released in subducting sections be-
nea
th arcs may disproportionately bear chemical/isotopic signatures
of rocks in higher-permeability zones that channelize and interact
with these fluids to a greater degree. Thus, if km-wide shear zones
(akin to the mélange zones examined in field studies of HP rocks)
exist along the subduction interface, the fluid flow focused along these
zones should exert a strong influence on the composition of the slab sig-
nature being delivered to the subarc mantle wedge. Likewise, if these
fluids are mobilized along fracture networks developed in more coher-
ent parts of the subducting section (i.e., the zones showing more brittle
deformation), they would tend to inherit chemical compositions related
to exchange with host-rocks along fracture surfaces.
In general, the evidence for element mobility along the deep-forearc
subduction interface is compatible with the compositions of fluids en-
tering the mantle wedge and contributing to arc magma generation
(e.g., the “IRS fluids” of Gill, 1981). As has been discussed previously
by Bebout and Barton (2002), King et al. (2006, 2007), Spandler et al.
(2008),andMarschall and Schumacher (2012), fluids emanating from
slab sections and interacting with mélange-like zones would be expect-
ed to acquire “hybrid” signatures related to this mixing in these zones
(see Figs. 18, 19). It is possible that fluids bearing these signatures
could simultaneously satisfy the various mixing endmember criteria de-
rived from the study of arc volcanic rocks. King et al. (2006) noted such
hybrid sign atures in individual hand samples of the Catalina Schist
amphibolite-grade mélange matrix. There, rocks that appear to repre-
sent endmember ultramafic compositions (e.g., an ultramaficchlorite-
anthophyllite schist with low SiO
2
(41.3 wt.%) and high MgO
(28.6 wt.%) and low alkalis) preserve Sr and Nd isotope ratios indicative
of a more evolved rock (e.g.,
87
Sr/
86
Sr = 0.70590, εNd = − 3.30). King
et al. (2006) noted that it could be difficult to discern whether contribu-
tions from various rock types calculated from arc volcanic data repre-
sent proportions of distinctly separate parts of the pre-subduction
section, or if they reflect the signature of a hybridized rock. Mélange ma-
trix samples from the various units of the Catalina Schist, many of them
now ultramafic or chlorite-rich schists, likely bear Pb isotope composi-
tions of the sediments subducted into this paleomargin, mixed to vary-
ing degrees with maficcompositions(Fig. 18; King et al., 2007). Those
authors argued that arc volcanic rocks could bear signatures of mixture
between a hybridized mélange component and an upper mantle source.
Marschall and Schumacher (2012) similarly argued that mélange rocks
carry much of the geochemical signature of arc volcanic rocks and that
devolatilization and melting of such rocks, as they rise in diapirs in the
mantle wedge, could deliver this signature to arc source regions (see
Fig. 19).
The fluid and mass transfer along structurally complex subduction
interfaces can result in significant metasomatic alteration of rocks con-
veyed to depths beyond those beneath magmatic arcs. The metasoma-
tism and transit of these rocks to great depths in the mantle could
lead to the production of some of the chemically/isotopically distinct
reservoirs invoked in studies of ocean island basalts (Hofmann, 2014;
Ryan and Chauvel, 2014; also see Brenan et al., 1995). Bebout (2007)
discussed the general lack of evidence in exhumed subduction-related
metamorphic rocks for the fairly massive element losses (e.g., for Pb
and U relative to Th) invoked to generate the geochemical characteris-
tics of mantl e reservoirs such as HIMU, EM1, and EM2 (Hofmann,
Fig. 18. Compositions of “hybrid” rocks developed in mélange zones for Pb isotopes com-
pared with representative arc volcanic compositions (from King et al., 2007). The data for
the Izu arc volcanic rocks, and subducting sediment and altered oceanic crust (AOC) are
from Hauff et al. (200 3). The average composition of the Izu volcanic rocks (star) has
been explained as representing a mixture of an AOC component (90%) and a sediment
component (10%; see the black, nearly vertical line indicating this mixing), whereas this
composition could alternatively be produced by a mixture of a hybridized mafic–sedimen-
tary–ultramafic mélange component with the composition of the mantle source (see the
red line). NHRL = Northern Hemisphere Reference Line (after Hart, 1984).
250 G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
2014; Zindler and Hart, 1986) and suggested that losses of this magni-
tude could be achieved at greater depths beneath arcs that are not rep-
resented in the HP/UHP metamorphic suites (i.e., at subarc depths and
beyond). We suggest that deformation-enhanced fluid infiltration and
related mass transfer at depths N 80 km, where the interface experiences
temperatures N 600 °C (see the P–T paths in Fig. 4), could contribute to
greater element loss and possibly lead to residues with compositions
corresponding to these reservoirs.
7.5.5. Stabilization of hydrous mineral assemblages and importance for vol-
atiles cycling
Fluid–rock interactions at the interface in many cases result in the
production of hydrous, nearly mono-mineralic rocks (e.g., chlorite-talc
schists) that can stabilize a hydrous component to great depths in sub-
duction zones (Bebout and Barton, 2002; Marschall and Schumacher,
2012; Miller et al., 2009; Spandler et al. , 2008). I n general, hydrous
phases suc h as chlorite and talc have far greater P– T stability when
they are essentially th e only ph ase present in a rock than they have
when in rocks containing multiple phases (e.g., subducting sediments
and basalts). As noted by Bebout (1991a), chlorite in subducting basaltic
and sedimentary rocks would be expected to break down at shallower
depths than chlorite in hybrid rocks such as mélange matrix, with
breakdown conditions also depending on the P–T paths they experi-
ence, which in turn are related to the dynamics of the subduction mar-
gin. The depth of breakdown of these hydrous minerals will determine
the cycling of H
2
O during subduction (see the study of chlorite stability
by Lakey and Hermann, 2015), thus the presence of hybrid rocks will
stabilize H
2
O to greater depth s and lead to greater amounts of H
2
O
subducted deep into the mantle.
7.5.6. Deformation-enhanced decarbonation and carbonate dissolution
Recent work by Cook-Kollars et al. (2014) and Collins et al. (2015) is
a reminder that carbonate-bearing rocks often require infiltration
by low-X
CO2
fluids in order to drive extensive decarbon ation
reactions. Cook-Kollars et al. (2014) demonstrated that, without such
infiltration, much or all of the carbonate in the Schistes Lustres/Cignana
metasedimentary suite would have been retained to depths of up to
90 km approaching those beneath many volcanic fronts. This relation-
ship is demonstrated in Fig. 20A, which shows contours of concen-
trations in volatiles CO
2
and H
2
O calculated for closed-system
devolatilization behavior for a hydrated and carbonated basaltic compo-
sition (from ODP Site 417/418). For this closed-system scenario, the ba-
salts largely retain their CO
2
to depths of 90 km. However, as is shown
for the same basaltic composition in Fig. 20B, infiltration by extremely
H
2
O-rich fluids, perhaps in part from dehydrating sub-crustal ultramafic
rocks in slabs, can drive large amounts of decarbonation at relatively
low temperatures (b 500 °C). Theoretical calculations of carbonate solu-
bility suggest that fluids released from dehydration of serpentinites at
the base of the subducting slab could dissolve additional C in transit up-
ward toward the plate interface (Kelemen and Manning, 2015; Fig. 21).
Fig. 19. Trace element compositions of “hybrid” rocks developed in mélange zones (open circles in A and C, black line in B), compared with arc volcanic rock compositions (from Marschall
and Schumacher, 2012). These authors argued that there is great overlap in the overall geochemical characteristics of mélange rocks with the compositions of arc volcanic rocks. Thus, as
was also suggested by Bebout and Barton (2002) and King et al. (2006, 2007), partial melting of a previously hybridized lithology (mélange) could deliver the apparent mixture of sed-
iment, AOC, and upper mantle proposed for arc magmas.
251G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
Fluids could then enter the mantle wedge, contributing to forearc and
subarc C storage in ultramafic rocks or addition of C to arc magmas, or
they could be transported up-dip along the subduction interface, in
some cases (depending on the P–T path they experience, see for exam-
ple path 1 in Fig. 17) leading to precipitation of C in minerals. The abun-
dant carbonate veins in many forearc metamorphic suites could reflect
this up-dip fluid transport and related precipitation (see the contours
of [C] in ppm in Fig. 21, from Kelemen and Manning, 2015).
Another important factor is the tendency of mixed carbonate –
silicate compositions to undergo more dramatic decarbonation than
more pure carbonate rocks, related to the need for silicates in order
for carbonates to react and decarbonate. In one extreme, a 100% carbon-
ate rock could be subducted to extreme depths in the upper mantle be-
fore experiencing decarbonation to produce CaO and CO
2
. Presumably,
access of fluids to subduct ing rocks with evolving permeability (Ord
and Oliver, 1997), is also necessary to facilitate the carbonate dissolu-
tion and re dox-related C loss recently documented by Ague and
Nicolescu (2014) and Galvez et al. (201 3). The mixing that occurs in
shear/méla nge zones can juxtapose carbonate-rich and carbonate-
poor rock types, in a fluid-rich environment, leading to metasomatic ex-
change and resulting reactions in some cases involving decarbonation.
Thus, processes of the type demonstrated for these zones of mixing
could greatly enhance loss of CO
2
, particularly beneath deep forearcs
and subarcs, conceivably releasing suffi cient CO
2
to balance with the
arc volcanic CO
2
output (see the discussion by Collins et al., 2015). The
extent to which this occurs could depend on the unknown amounts of
H
2
O-rich fluid released from sub-crustal ultramafic rocks in subducting
slabs. This H
2
O-rich fluid could rise from hydrated oceanic lithospheric
mantle into the subduction inte rface and be focused in shear zones
and more highly fractured rock volumes to react with the hybrid rocks
within the interface.
8. Conclusions and future directions
One commonality in virtually all studies of HP metamorphic rocks is
the abundant evidence for the presence of a free fluid phase and hetero-
geneity in the availability and transport of fluid as related to structural
setting. At the shallowe st levels, pore fluids contri bute significantly,
mixing with fluids generated by diagenetic and metamorphic reactions.
At greater depths, fluids are generated by devolatilization reactions. At
all depths studied and considered in this review, fluid flow is focused
along fractures and in shear zones, likely with a far smaller degree of
pervasive fl uid transfer. The heterogeneous deformation that occu rs
A
B
P (GPa)
T (˚C)
T (˚C)
X
CO
2
X
CO
2
Weight %
Fig. 20. Demonstration of the enhanced decarbonation that can occur at the subduction in-
terface due to infiltration of carbonate-bearing rocks by extremely H
2
O-rich fluids (from
Collins et al., 2015). (A) P–T diagram showing calculated volatiles concentrations for a
ODP Site 417/418 altered oceanic crustal composition from Staudigel et al. (1996),fora
closed system scenario (i.e., no infiltration by H
2
O-rich fluids), and recently published pro-
grade P–T paths for sediment cover on subducting slabs (T = Tonga, NV = N. Vanuatu,
N = Nankai, C = Cascadia) from Syracuse et al. (2010; also van Keken et al., 2011, van
Keken, pers. comm., 2013). Contours are the H
2
O and CO
2
concentrations of this rock com-
position and the shad ed rectangles ar e peak P– T of the metabasaltic units studied by
Collins et al. (2015). The large brown-shaded arrow indicates the expected P–T trajectory
taken at ~80–120 km depths by rocks subducting into most modern margins, involving
heating to N 600 °C related to exposure of the slab to the convecting mantle wedge (see
Syracuse et al., 2010). (B) T–X
CO2
diagram for the same basaltic composition for ODP Sites
417/418 (from Collins et al., 2015), showing mineral stabilities at 2.5 GPa as a function of
T–X
CO2
. Reactions releasing CO
2
occur at lower T when fluid composition is H
2
O-rich, lead-
ing to significant progress of decarbonation reactions when rocks are infiltrated by H
2
O-
rich fluids. Panels 1–2 in the lower part of (B) detail the modal abundance changes that
would occur, according to relationships in the T–X
CO2
diagram above, during the infiltra-
tion of this rock by an extremely H
2
O-rich fluid, at two different temperatures at 2.5 GPa
(the tw o dashed arrows in the middle diagram; see the discussion by Collins et al.,
2015). Note the complete disappearance of carbonate in both scenarios when rocks are in-
filtrated by extremely low-X
CO2
fluids. Without infiltration by these H
2
O-rich fluids, this
rock would require heating to N 600 °C for significant loss of CO
2
by decarbonation, as is
shown in (A).
252 G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
along subduction interfaces in ge neral leads to focusing of fluid flow
along the more highly deformed regions.
The exact fluid loss profiles, for subducting lithologies along subduc-
tion interfaces, likely vary as functions of the rock types and thermal
structure of the margin, the latter dictating the prograde P–T paths the
rocks experience. These differences in thermal structure will also affect
the element/mineral solubilities along the subduction interface, and
thus the mass transfer that occurs. A number of recent studies of forearc
devolatilization appear to document the retention of large fractions of
the init ially subducted volatiles content of several important rock
types. However, these studies have in general been focused on consider-
ation of devolatilization in more intact volumes of these rock types, not
in more highly deformed zones (shear zones and more highly fractured
volumes) where disparate rock types can be structurally juxtaposed and
through which flow of externa lly-derive d fluid can be focused.
Devolatilization, and related mass transfer, can in general be enhanced
along subduction interfaces, particularly if loss of large amounts of
H
2
O-rich fluid from sub-crustal ultramafic rocks in slabs results in flush-
ing of the overlying interface.
Studies of the subduction interface have been restricted to exposures
representing forearc depths, due to the general lack of suitable
exposures represe nting the greater depths beneath magmatic arcs
(i.e., N 100 km). We suggest that the fluid processes beneath arcs could
lead to far more extensive mass transfer and related metasomatism of
the subducting section and overlying mantle wedge, enhanced by the
higher te mperatures and the transition of the dominant fluid type
from hydrous fluid to silicate melt. Beneath most arcs, subduction inter-
faces experience temperatures of N 600 °C, approaching the tempera-
tures of the wet solidi for relevant rock compositions, and in general
resulting in far higher solubilities of elements of interest in studies of
surface-to-mantle chemical cycling.
We suggest some research directions that will likely be fruitful in
shedding light on the nature of the subduction interface and attendant
fluid and mass transfer.
— Some extremely important questions remain regarding large-scale
chemical cycling of elements and compounds, particularly volatiles,
and the roles of interface deformation and fluid processes in this cy-
cling. Significant effort has been directed toward the understanding
of the cycling of H
2
O, and future work should focus on understand-
ing the other elements forming major volatile species (C, N, and S).
Other geochemical indicators such as those that have the potential
to shed light on fluid flow within subduction zones include fluid-
mobile elements such as B and Li and the halogens.
— The roles played by deformation and rheology in fluid flow and mass
transfer are still relatively poorly understood. Future work should in-
clude studies aimed at shedding light on the relationship between
deformation and mass transfer.
— The work to date has demonstrated a number of ways in which
fluids are related to patterns of seismicity. Evidence for relatively
short durations of fluid flow events suggests a more episodic nature
of fluid flow during subduction metamorphism. Further attention
should be paid to understand the periodicity of such flow and its re-
lationship to seismicity.
— Quantitative estimation of fluid fluxes based on evidence from
subduction-related metamorphic rocks remains a somewhat elusive
goal. In many cases, the spatial constraints necessary for this evalua-
tion are missing. Further evaluation of fluid fluxes during subduction
metamorphism, using the most recent thermodynamic modeling re-
sults (e.g., Dolejs and Manning, 2010; Sverjensky et al., 2015), would
shed further light on the quantities of fluids traveling through meta-
morphic rocks during subduction zone metamorphism.
— For a comprehensive understanding of subduction zones, observa-
tions and analyses of samples collected in the field, such as those
presented here, need to be integrated with experimental work
and thermodynamic and geodynamic modeling. Sample collection
and the analytical strategies chosen need to be informed by questions
raised by geodynamic models. The results of geodynamic models
in turn need to be evaluated by comparisons with evidence from
natural samples (e.g., Penniston-Dorland et al., 2015). Interpretations
of fluid fluxes and fluid sources have a more quantitative basis
when thermodynamic modeling and experimental constraints are
considered.
— Geophysical observations should also be used to directly test petro-
logic models. Intriguing suggestions that mélange lithologies along
the interface become gravitationally unstable and rise diapirically
into the mantle wedge, conveying volatiles and transferring geo-
chemical signatures of the interface into arc magma source regions
Fig. 21. Demonstration of the CaCO
3
solubility in aqueous fluids and the possible dissolution that could occur at/near the subduction interface (from Kelemen and Manning, 2015).
(A) Pressure–temperature diagram showing contours of [C] (C concentration) in parts per million for aqueous fluids saturated in CaCO
3
.OverlainaretheP–T paths modeled for several
modern subduction margins (from Syracuse et al., 2010). Dashed lines represent the paths for the base of the subducting oceanic crust and the solid lines are the paths for the top of
the volcanics and sediments (i.e., at the subduction interface). The solid grey curve is the stability limit of serpentine (from Ulmer and Trommsdorff, 1995). (B) Figure similar to that in
(A) but showing only the Aleutian P–T paths. The upper arrow represents the P–T trajectory that would be taken by fluids generated by dehydration of serpentine in the upper mantle
of the subducting slab. C solubility increases along this path. The lower arrow shows a trajectory that could be taken by fluids exsolved from melts rising into the mantle lithosphere
and crust of the overlying plate. The degree to which flushing of fluids derived from dehydrating sub-crustal ultramafic sections occurs within the subduction interface is a key open ques-
tion in considering the fluid–rock evolution of the subduction interface.
253G.E. Bebout, S.C. Penniston-Dorland / Lithos 240–243 (2016) 228–258
(see Behn et al., 2011; Gerya and Yuen, 2003; Marschall and
Schumacher, 2012) could potentially be tested through geophysical
observations.
— Further investigation of rocks from the subduction interface should
be focused on the amounts of fluid contributed to subduction inter-
faces from dehydrating ultramafic rocks in the sub-crustal mantle of
subducting slabs. Massive flushing of the interface by this fluid
could have profound implications for fluid and mass transfer, includ-
ing C release, at depth along the interface.
Acknowledgments
We tha nk the journal editors for inviting this review article.
Constructive comments by Brad Hacker and an anonymous reviewer
greatly imp roved the quality of this contribution. We thank Wenlu
Zhu for discussion of rock deformation and Samuel Angiboust for dis-
cussion of W. Alps subduction interface exposures. We thank our col-
leagues who provided high-resolution versions of their previously
published figures. Much of the research by the au thors presented in
this article, on these topics, was facilitated by funding from the National
Science Foundation, most recently grant EAR-1119264 to GEB and
grants EAR-1119111 and EAR-1419871 to SPD.
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