serpentinization of the forearc mantle - phil skemer

Serpentinization of the forearc mantle

Roy D. Hyndman a;�, Simon M. Peacock b

a Paci¢c Geoscience Centre, Geological Survey of Canada, 9860 W. Saanich Rd., Sidney, BC, Canada V8L 4B2b Department of Geological Sciences, Arizona State University, Tempe, AZ 85287-1404, USA

Received 15 July 2002; received in revised form 5 May 2003; accepted 9 May 2003

Abstract

A wide range of geophysical and geological data indicate that extensive serpentinization in the forearc mantle isboth expected and observed. Large volumes of aqueous fluids must be released upwards by dehydration reactions insubducting oceanic crust and sediments. Subduction of oceanic lithosphere cools the overlying forearc such that lowtemperature hydrous serpentine minerals are stable in the forearc mantle. Over several tens of millions of yearsestimated fluid fluxes from the subducting plate are sufficient to serpentinize the entire forearc mantle wedge.However, fluid infiltration is probably fracture controlled such that mantle serpentinization is heterogeneous.Geological evidence for hydration of the forearc mantle includes serpentine mud volcanoes in the Mariana forearcand serpentinites present in exposed paleo-forearcs. The serpentinization process dramatically reduces the seismicvelocity and density of the mantle while increasing Poisson’s ratio. Serpentinization may generate seismic reflectivity,an increase in magnetization, an increase in electrical conductivity, and a reduction in mechanical strength.Geophysical evidence for serpentinized forearc mantle has been reported for a number of subduction zones includingAlaska, Aleutians, central Andes, Cascadia, Izu-Bonin^Mariana, and central Japan. Serpentinization may explainwhy the forearc mantle is commonly aseismic and in cool subduction zones may control the downdip limit of greatsubduction thrust earthquakes. Flow in the mantle wedge, induced by the subducting plate, may be modified by thelow density, weak serpentinized forearc mantle. Large volumes of H2O may be released from serpentinized forearcmantle by heating during ridge subduction or continent collision.Crown Copyright 6 2003 Elsevier Science B.V. All rights reserved.

Keywords: forearc; mantle; serpentine; subduction

1. Introduction

Large volumes of aqueous £uids are releasedfrom subducting plates (e.g., [1]) and at depths

of about 100 km such £uids may trigger partialmelting in the overlying mantle wedge, the sourceof arc volcanism (e.g., [2]). Subducting sedimentsand altered oceanic crust contain free water inpore spaces and bound water in hydrous minerals.At shallow depths, free water is expelled by com-paction of subducted sediments and collapse ofporosity in the upper oceanic crust. At greaterdepths, extending to at least 200 km, aqueous£uids are produced by progressive metamorphic

0012-821X / 03 / $ ^ see front matter Crown Copyright 6 2003 Elsevier Science B.V. All rights reserved.doi:10.1016/S0012-821X(03)00263-2

* Corresponding author. Tel. : +1-250-363-6428;Fax: +1-250-363-6565.E-mail addresses: [email protected] (R.D. Hyndman),

[email protected] (S.M. Peacock).

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Earth and Planetary Science Letters 212 (2003) 417^432

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dehydration reactions involving numerous hy-drous minerals (e.g., [3,4]). Fluid production gen-erally decreases with depth; most £uid is releasedbeneath the forearc, whereas smaller amounts areexpected to be released at depths of arc magmageneration and beneath the backarc. Note that inthis text we use ‘water’ for H20, although theconditions may be such that it is a supercritical£uid.

In this article we document the wide range ofgeological and geophysical evidence and modelingthat give a consistent picture of extensive serpen-tinization of the forearc mantle between thetrench and volcanic arc (Fig. 1). To our knowl-edge, serpentinization of the forearc mantle was¢rst proposed by Fyfe and McBirney [5] to ex-plain the uplift of coast ranges that commonlyparallel subduction trenches. In subduction zones,the forearc mantle is exceptionally cool such thatserpentine and related hydrous minerals are sta-ble. In contrast, the backarc mantle is usually toohot for hydrous minerals to be stable. The forearcis underlain by a subducting plate that releasesH2O-rich £uids that may migrate into and hy-drate the overlying mantle. The presence of ser-pentine and other hydrous minerals has signi¢cante¡ects on the physical and mechanical propertiesof the forearc mantle, including a decrease in seis-mic velocity, increase in Poisson’s ratio, genera-tion of seismic re£ectivity, increase in magnetiza-tion, reduction in density, increase in electrical

conductivity, and reduction in mechanicalstrength.

2. Serpentinization

Dry forearc mantle is inferred to be composedof depleted ultrama¢c rocks consisting primarilyof olivine and orthopyroxene with lesser amountsof clinopyroxene and spinel. Harzburgites (olivine+orthopyroxene rocks) and dunites (olivine-richrocks) are the most abundant ultrama¢c rocks insupra-subduction zone ophiolites (e.g., [6]). Proto-liths of serpentinized ultrama¢c rocks recoveredfrom Mariana forearc serpentine seamounts onOcean Drilling Project Leg 125 consist mostly ofharzburgite with lesser amounts of dunite [7].

At temperatures less than 700‡C, a number ofhydrous minerals are stable in ultrama¢c bulkcompositions. The hydration of depleted mantlecan be described using the simple MgO^SiO2^H2O system (e.g., [8,9]). The addition of H2O toa mantle wedge composed of olivine and ortho-pyroxene may generate a variety of hydrous min-erals including serpentine (antigorite, chrysotile,lizardite), talc, and brucite, with the speci¢c min-eral assemblage depending on temperature (T),pressure (P), and bulk composition [10,11] (Fig.2).

Serpentine minerals, VMg3Si2O5(OH)4, are themost abundant hydrous minerals in altered ultra-ma¢c rocks because the Mg:Si ratio of serpentine(1.5) lies between that of olivine (2) and orthopy-roxene (1). Antigorite is the stable serpentine min-eral in ultrama¢c rocks metamorphosed under themoderate temperatures of blueschist and green-schist facies conditions [10] and is stable to 620‡Cat 1 GPa [12] (Fig. 2). At lower temperatures(6V350‡C), chrysotile and lizardite are stablein the lower grade zeolite and pumpellyite facies[10].

Brucite, Mg(OH)2, coexists with serpentine inolivine-rich compositions at T6 500‡C (Fig. 2).Brucite is a common mineral in serpentinites[13], but its ¢ne-grained nature makes it di⁄cultto estimate modal abundances. The maximumamount of brucite in hydrated mantle will occurfor a dunite protolith consisting of 100% olivine.

Fig. 1. Schematic cross section illustrating £uid expulsionfrom subducting oceanic crust and sediments, and serpentini-zation of the overlying forearc mantle.

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Complete hydration of pure forsterite (olivine) at400‡C and 1 GPa will yield a rock consisting of 78vol% antigorite and 22 vol% brucite ; however,only smaller amounts of brucite are usually ob-served (e.g., [17]). Talc, Mg3Si4O10(OH)2, coexistswith serpentine in pyroxene-rich compositions atT6 700‡C. Talc also may form in the forearcmantle along the plate interface (Fig. 1) [14]where in¢ltrating £uids derived from the under-lying subducting crust are silica-saturated [15] andmechanical mixing of mantle with siliceous sedi-ments may occur [16].

Additional minerals, which lie outside the sim-ple MgO^SiO2^H2O system, may form in hy-drated forearc mantle depending on bulk compo-sition. Hydration of Al-bearing mantle rocks will

form chlorite. At T6V500‡C, diopside is thestable Ca-bearing mineral ; tremolite is the stableCa-bearing mineral at higher temperatures [10].Fe partitions into serpentine, brucite, and talc toa limited extent, but the serpentinization processinvariably produces magnetite (e.g., [17]). Mostserpentinite is strongly magnetic, to the degreethat commonly there are strong deviations inmagnetic compass directions near surface out-crops. Thus, cold serpentinized mantle might bedetected by magnetic anomalies caused by mag-netite as discussed below.

Except for unusual metasomatic rocks, serpen-tine is the most abundant hydrous mineral in hy-drated ultrama¢c rocks at T6 500‡C. Hacker etal. [18] present normative calculations, assumingfull hydration, for three di¡erent ultrama¢c com-positions: lherzolite (enriched upper mantle), de-pleted lherzolite, and harzburgite (depleted uppermantle). In their calculation, at P=1 GPa andT6 500‡C, antigorite makes up s 50 vol% ofthe rock in each of the three bulk compositions.

3. Thermal structure of the forearc mantle

The thermal structure of a subduction zone isthe primary control on the location of slab dehy-dration reactions that produce aqueous £uids andthe region where hydrous minerals are stable inthe forearc mantle. In addition, temperature isimportant because of the thermal dependence ofthe physical properties of the forearc mantlerocks. Interpretation of geophysical data in termsof forearc mantle hydration requires separation ofthe e¡ects of temperature from the e¡ects of hy-dration.

Numerous general subduction zone thermalmodels have been presented in the past (see reviewby [19]). More recently, two-dimensional ¢nite el-ement models have been constructed for speci¢csubduction zones that more accurately predictslab and forearc temperatures down to V100km or more (e.g., [20,21]). In these models thethermal structure of the forearc mantle is foundto be most sensitive to the age of the incomingoceanic plate, the convergence rate, and the ge-ometry of the plate interface (slab dip). Also im-

Fig. 2. P^T diagram showing important reactions in theMgO^SiO2^H2O system and calculated P^T conditions ofcool forearc mantle in oceanic (shallow dark shaded area)and continental (deeper light shaded area) subduction zones.Antigorite serpentine is stable over a wide range of forearcconditions. Ternary diagrams show stable three-phase miner-al assemblages in hydrated ultrama¢c bulk compositions;note brucite is not stable in hydrated mantle at Ts 500‡C(shaded triangles). Chrysotile reactions from [10]; talc reac-tions from [12]; antigorite+brucite breakdown reaction calcu-lated using Holland and Powell’s (1998) thermodynamic da-tabase. Stability ¢eld for anthophyllite at 6006T6 800‡Cand P6 1.2 GPa omitted for clarity. Mineral abbreviations:antig, antigorite; bru, brucite; en, enstatite (orthopyroxene);fo, forsterite (olivine); per, periclase; qtz, quartz.

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portant are the thickness of insulating sedimenton the incoming oceanic crust, and the radioactiveheat generation of the overlying accretionaryprism and forearc crust [22]. Most studies haveconcluded that the thermal e¡ect of frictionalshear heating and metamorphic reactions aresmall (e.g., [19,23,24]), but these processes remaina source of uncertainty in the model temperatures.Advective heat transport by the expelled £uidsalso should not be important except perhaps atthe very start of underthrust sediment consolida-tion. At least 1 mm/yr is needed (e.g., [26]).

Heat £ow pro¢les across the forearc provide animportant check on the thermal models. Mostsubduction zone forearcs are characterized by ob-servations of very low heat £ow (30^40 mW m32)as predicted by the models. Forearcs are very coolas a consequence of the heat removed by theunderthrusting of the cool near-surface rocks ofthe oceanic lithosphere. Extensive marine andland heat £ow data are available for comparisonwith model predictions, such as Cascadia [22,25](Fig. 3). On this margin, a correction is needed forthe heat £ow reduction e¡ect of rapidly thicken-ing large accretionary prisms (e.g., [26]). After thiscorrection, good agreement has been found be-tween the subduction thermal model and observedheat £ow, within the uncertainties in the heat £owand other thermal data. Heat £ow decreases land-ward for young hot subduction zones, whereasheat £ow is nearly constant across the forearc

for margins subducting old and cold oceanic lith-osphere [27].

Calculated temperatures in the forearc mantleare signi¢cantly di¡erent for subduction zoneswhere old cold lithosphere is being underthrustcompared to young hot oceanic lithosphere (e.g.,[21]) (Fig. 4). Calculated forearc temperatures are400^600‡C for warm continental subductionzones with young incoming oceanic lithospheresuch as Cascadia (Fig. 3), SW Japan (Fig. 4),Mexico [28], and S. Chile near the subductingChile ridge [27]. In cool continental subductionzone forearcs such as NE Japan (Fig. 4), Alaska,and N. Chile, where the subducting slab intersectsthe forearc Moho at 30^50 km depth, the calcu-lated temperatures in the uppermost forearc man-tle are 150^250‡C. Uppermost forearc mantletemperatures are especially low for island arcswith thin forearc crust (10^15 km) such as theIzu-Bonin subduction zone where the tempera-

Fig. 3. Calculated thermal structure of the warm N. Casca-dia continental subduction zone and associated surface heat£ow data (after [20]) (BSR, bottom simulating re£ector;ODP, Ocean Drilling Program). Numerical two-dimensionalmodel results extend to 200 km seaward from the trench.Deep temperatures near the arc and abrupt rise are fromsimple one-dimensional models (dashed isotherms).

Fig. 4. Calculated thermal structure of (A) warm continentalsubduction zone (SW Japan; Cascadia is similar), (B) coolcontinental subduction zone (NE Japan), and (C) cool ocean-ic subduction zone (Izu-Bonin) [21,99]. Forearc mantle whereserpentine is stable is shaded.

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tures in the mantle wedge corner are as low asV100‡C (Fig. 4C).

The region of the forearc upper mantle inwhich hydrous minerals such as serpentine arestable is commonly 50^100 km wide for warmcontinental subduction zones (Fig. 4), and 100^150 km wide for cool continental subductionzones (Fig. 4). The hydrated mantle region maybe as much as 200 km wide for cool oceanic sub-duction zones, where the forearc crust is thin andthe forearc mantle is reached by the underthrust-ing oceanic crust at shallow depth within about 50km of the trench. However, the steeper slab anglesfor some West Paci¢c subduction zones such asthe Mariana, reduce the trench^arc distance.

4. Dehydration of the downgoing plate and upward£uid expulsion rates

Large amounts of aqueous £uid are expelledfrom the downgoing oceanic crust and overlyingsediments with increasing pressure and tempera-ture. Estimated £uid production rates of V0.1mm/yr or 100 m/Myr (Fig. 5) suggest that overseveral tens of Myr enough water is released fromthe subducting oceanic crust and sediments to hy-drate the entire forearc mantle [4,29]. The mainfactors controlling £uid production beneath theforearc mantle are the convergence rate, the thick-ness of the forearc crust (i.e., forearc Mohodepth), and the amount of water in the incomingcrust and sediments. The rate at which free andbound water enters a subduction zone is approx-imately proportional to the convergence rate. Inoceanic subduction zones where the forearc crustis thin, the subduction thrust intersects the forearcmantle at a shallow depth where larger amountsof £uid are driven o¡ the subducting sedimentsand oceanic crust. For the thicker continentalcrust, more of the water goes into the forearccrust, rather than the forearc mantle. Thus,more £uid may be available for hydration of theforearc mantle of oceanic compared to continen-tal subduction zones. The degree of serpentiniza-tion will be controlled by the amount of H2O thatactually chemically interacts with the forearcmantle. Field observations indicated that £uid

£ow during serpentinization tends to be fracturecontrolled rather than pervasive [30]. Serpentini-zation of the forearc mantle is probably very het-erogeneous and some £uid may escape to the sur-face. In higher temperature forearcs, fasterreaction rates and more rapid di¡usion shouldpermit greater £uid penetration from fracturesand channels. However, as noted above, in highertemperature subduction zones there may be less£uid available because a larger fraction of the slabdehydration will have occurred before the sub-ducting slab reaches the forearc mantle.

Fig. 5. Estimated amounts of £uid released upward from po-rosity collapse and dehydration reactions of subducted sedi-ments and oceanic crust for (1) a cool subduction zone (N.Japan), and (2) a warm subduction zone (SW Japan). Loca-tions of the main progressive dehydration reactions aremarked; £uid expulsion is probably more smoothed withdepth. Note (a) the scales are a factor of two di¡erent to ac-commodate the approximately factor of two di¡erence insubduction rate, (b) the amount of £uid for 1, 2, 3 is ap-proximately proportional to the amount of sediment sub-ducted. See text for discussion.

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Fig. 5 shows estimates of the amounts of £uidsthat may be expelled from the subducting oceaniccrust and overlying sediments as a function ofdistance from the trench for examples of cool(N. Japan) and warm (SW Japan) continentalsubduction zones. In this simple ¢rst order ap-proximation, an extreme example of 1000 m of50% porosity sediment is assumed to be under-thrust. The H2O expelled at shallow depths comesprimarily from this source and the amount of £u-id is nearly proportional to the thickness of sub-ducted sediments (components 1, 2, 3, in Fig. 5).Therefore, the £uid expelled from subducted sedi-ments may be roughly scaled by the thickness ofsubducted sediment section. The upper 1000 m ofthe oceanic crust is assumed to contain an averageof 7% porosity based upon Ocean Drilling Pro-gram core and downhole measurements (e.g.,[31]). The porosity deeper in the oceanic crust issmaller and is ignored in this ¢rst order discus-sion. Also, the porosity may decrease with agingof the crust. In the forearc, we assume the 500 mof fully compacted, zero-porosity sediments con-sists of 400 m of terrigenous sediments containing7.5 wt% bound H2O, 50 m of siliceous sedimentscontaining 11 wt% H2O, and 50 m of anhydrouscalcareous sediments [32,33]. The basaltic andgabbroic sections of the oceanic crust are assumedto contain 2 and 1 wt% bound H2O, respectively[3]. The bound H2O estimates have an uncertaintyof about a factor of two. The dehydration of ser-pentinized peridotite in the uppermost mantle ofthe incoming plate or incorporated into the oce-anic crust have not been included, but they couldbe a signi¢cant additional source of £uids (e.g.,[34,35]). At shallow depths in subducting plates,there are two competing processes in the under-thrusting sediments and oceanic crust: £uid lossfrom the free water released through porosity col-lapse and £uid incorporated into low temperaturehydrous minerals, such as zeolites. Thus, theamount of £uid expelled from the slab is quiteuncertain. However, for the underthrusting sedi-ments and porous uppermost oceanic crust, thereis more than enough water for complete hydra-tion, and in our calculations we assume the re-maining water is expelled. We assume vertical up-ward £uid £ow. Some £uid may be expelled

seaward along the subduction thrust, but we arenot aware of any direct evidence of such £ow orof an associated heat £ow anomaly.

In both the cool and warm continental subduc-tion zones, most H2O released beneath the forearcmantle comes from the subducted oceanic crust(elements 4^6 in Fig. 5). H2O released from com-paction and dehydration of subducted sedimentsoccurs mostly beneath the forearc crust (Fig. 5).In an oceanic subduction zone (not shown), wherethe forearc mantle is shallower, the release of H2Ofrom subducted sediments should extend downdipto beneath the forearc mantle. However, in manyoceanic subduction zones there is very little in-coming sediment.

5. Geological evidence for serpentinization of theforearc mantle

5.1. Serpentine mud volcanoes

Active and inactive serpentinite mud volcanoesobserved in the Mariana and Izu-Bonin forearcsprovide direct evidence for hydration of the fore-arc mantle by slab derived £uids (e.g., [36]). Geo-chemical arguments that £uids emanating fromthe mud volcanoes rise from the subducting plateare summarized by [37^39]. These serpentinitemud volcanoes may occur just arcward of theintersection of the subduction thrust with theforearc mantle. The Mariana forearc crust isV10 km thick [40] and Fryer et al. [39] estimatedepths of 15^25 km to the subducting slab be-neath the Mariana mud volcanoes. In the Izu-Bo-nin forearc, the seismic structure model of Suye-hiro et al. [41] suggests that the serpentine mudvolcanoes lie above the intersection of the plateinterface and forearc Moho.

Serpentine mud volcanoes may be particularlywell exposed in the Mariana and Izu-Bonin oce-anic subduction zones because there the forearccrust is especially thin and sediment cover issparse. In oceanic subduction zones, the forearcmantle is reached by the subducting plate at shal-low depths of V10 km where the largest amountsof £uid are being driven o¡ the subducting sedi-ments and upper oceanic crust. Although still sig-

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ni¢cant, smaller amounts of £uid are being driveno¡ at the V40 km depths of continental forearcs.An example of possible serpentine masses reach-ing the surface in a continental forearc is de-scribed by Kido et al. [42] for Shikoku in south-west Japan.

5.2. Forearcs of paleo-subduction zones

Exposed ultrama¢c rocks, interpreted to haveformed in paleo-subduction zones, are extensivelyhydrated. In many cases, it is concluded that thehydration predates exposure, i.e., serpentinizationis not the result of meteoric water in¢ltration sub-sequent to exposure at the surface (e.g., [17]). Thebase of the ultrama¢c section of Cordilleran-typeophiolites is commonly serpentinized by £uids de-rived from underthrust rocks (e.g., [43]). In sev-eral well studied regions the ultrama¢c hangingwall of a paleo-subduction zone is extensively hy-drated, including Santa Catalina Island, Califor-nia [16], the Trinity peridotite and Josephineophiolite in the Klamath Mountains of California[30,44], and the Shuksan suite of Washington [45].

6. Seismic properties of serpentinite

Hydrated mineral assemblages generally havesubstantially lower seismic velocities than theirparent rocks; this e¡ect is well known for serpen-tinite. Fig. 6 shows compressional wave velocity(Vp), shear wave velocity (Vs), and Poisson’s ratiofor hydrated mantle peridotite samples as a func-tion of degree of serpentinization from [46] (seealso [47^49]). The data were obtained at P=1GPa appropriate for the base of V35 km thickcontinental crust. Seismic velocities are about0.3 km s31 slower for the 200 MPa con¢ningpressure appropriate for the base of thinner oce-anic crust. However, at 200 MPa there may besome residual cracks generated by sampling re-maining in the laboratory samples, so the lattervelocities should be taken as minimum values.The average zero-serpentine Vp for this suite oflaboratory data is V8.4 km s31, similar to thatfrom larger compilations of unaltered ultrama¢crocks by Rudnick and Fountain [50] and Chris-

tensen and Mooney [51]. The temperature correc-tion from room temperature to 400^500‡C ofabout 30.2 km s31 gives good agreement withthat commonly observed for stable continentaluppermost mantle (average 8.13 km s31 [51]). AVp of 7.2^7.6 km s31 in the forearc mantle thusrepresents 15^30% serpentinization (Fig. 6). TheVp for 100% serpentinization is about 5.1 km s31.The temperature coe⁄cient of velocity for serpen-tinite is small up to temperatures between 550 and800‡C for di¡erent hydrous minerals [52].

The Poisson’s ratio (related to Vp/Vs) also isdiagnostic of the degree of serpentinization. Inthe compilation of laboratory data of Fig. 6[46,47], the Poisson’s ratio for peridotite of0.26^0.28 (Vp/Vs = 1.76^1.81) increases to about0.30 (1.87) at 15% serpentinization, and to 0.38

Fig. 6. A compilation of laboratory mantle peridotite Vp, Vs,Vp/Vs, and Poisson’s ratio as a function of degree of serpen-tinization [46], with best ¢t linear relations. Vp also is showncorrected to the temperature of normal stable uppermostmantle.

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(2.27) at 100% serpentinization. The best ¢t Pois-son’s ratio versus degree of serpentinization linefor these data gives an unaltered peridotite valueof 0.28 (1.81). This is slightly higher than that forlarger compilations of unaltered ultrama¢c rocksof 0.26 (Vp/Vs = 1.76) [48], but within the datascatter. Also, Poisson’s ratio is sensitive to theolivine composition [48]. An o¡set of about30.02 for Poisson’s ratio (30.05 for Vp/Vs) maybe needed for the application of the laboratorydata best ¢t line to averages from seismic data.Christensen [48] and Rudnick and Fountain [50]conclude that the temperature e¡ect on Poisson’sratio is usually very small. We note that the Pois-son’s ratio for ma¢c deep crustal rocks, 0.29^0.30(Vp/Vs = 1.84^1.87) (e.g., [48]), is higher than thatfor unhydrated ultrama¢c mantle rocks of 0.26(1.76). Therefore, both the Vp and Poisson’s ratioof moderately hydrated mantle compositions(V30% serpentinization) are not much di¡erentfrom unaltered ma¢c crustal rocks. However, athigher degrees of serpentinization (over 40%,Vp 6 6.5 km s31), the Poisson’s ratio for serpenti-nized mantle rocks (s 0.32) (Vp/Vs s 1.94) ismuch higher than that for ma¢c rocks with asimilar Vp (e.g., [53]).

7. Geophysical evidence for forearc mantleserpentinite

7.1. Upper mantle reference velocities with noserpentinite

The uppermost mantle refracted or head wave(Pn) velocity is usually one of the best determinedparameters in wide-angle seismic refraction stud-ies. The relation between Pn velocity and Mohotemperature has been estimated from heat £owdata by Black and Braile [54] (Fig. 7). Thesedata show the large temperature e¡ect on Pn ve-locity. For cratons and other stable continentalareas, the estimated Moho temperature is gener-ally 350^450‡C and the Pn velocity is 8.15^8.25km s31. For continental backarc areas the Pn ve-locity is commonly 7.8^7.9 km s31, in agreementwith that predicted for ultrama¢c composition athigh temperatures with no serpentinization. An

excellent example is the detailed series of Lithop-robe seismic refraction lines in the southern Cana-dian Cordillera backarc summarized by Clowes etal. [55]. The Moho temperature is estimated to be800^1000‡C [56], and the Pn velocity range of 7.8^7.9 km s31 expected for that temperature is ob-served (Fig. 7). Fig. 7 also shows the correlationof Pn velocity with heat £ow for continental crus-tal areas. Except for uncommon areas of veryhigh crustal heat generation, uppermost mantletemperatures usually correlate closely with surfaceheat £ow. Low heat £ow areas (V40 mW m32)

Fig. 7. Uppermost mantle Pn velocity as a function of (a)surface heat £ow, and (b) inferred Moho temperature (low-er). Range of velocity^temperature relations from laboratorydata also is given for comparison. Pn velocity for cold fore-arcs is unusually low, deviating substantially from the predic-tions of both relations.

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exhibit Pn velocities of about 8.2 km s31 and mosthigh heat £ow areas (80^100 mW m32) exhibitPn velocities of 7.8^7.9 km s31.

Other important controls of uppermost mantleseismic velocities where there is no hydration arecomposition variations and anisotropy, but webelieve that temperature is the most importantcontrol. This conclusion is in contrast to thatfor deep crustal velocities where composition ap-pears to be the dominant control of seismic veloc-ity. The other two e¡ects remain important un-certainties for upper mantle velocity. Fliedner andKlemperer [57] discuss the e¡ect of upper mantlecomposition on velocity; unusual compositionsare required for the Vp to be as low as 7.8 kms31 at low temperatures. In extreme cases the con-tinental mantle velocity anisotropy may be aslarge as 0.2 km s31 (e.g., [58]), but other wellstudied continental areas exhibit almost no Pn

velocity anisotropy in the upper mantle (e.g.,[55]).

7.2. Low seismic velocities and high Poisson’sratios in forearc mantle

Low seismic velocities and high Poisson’s ratiosobserved in subduction zone forearcs stronglysuggest that forearc mantle is partially serpenti-nized. Several types of data provide informationon forearc mantle velocities : (1) Both Vp and Vs

in the uppermost mantle can be obtained by seis-mic tomography studies using a suite of earth-quake sources, (2) Vs contrasts across the forearcMoho can be determined by receiver functionstudies, (3) forearc Pn arrivals provide Vp infor-mation for the uppermost forearc mantle,although they are often weak, (4) that forearcPn arrivals are weak or absent is also diagnosticof low mantle velocities such that the Moho con-trast is lowered (e.g., [59]), and may provide animportant mapping tool for hydration. We notethe compilation of Zhao [60] who summarizedlow upper mantle velocities in forearc of the Ma-riana, Izu-Bonin, northeast Japan, Alaska, Chile,and northern New Zealand.

Where observed, forearc P-wave velocities arecommonly 7.8 km s31 or less, similar or lowerthan the adjacent backarc. Surface heat £ows in

forearcs are 30^40 mW m32, indicating forearcmantle T6 400‡C, so the expected Pn velocityfor anhydrous mantle is greater than 8.2 kms31. At these low temperatures, P-wave velocitiesof less than 7.8 km s31 can be explained by morethan 15% average serpentinization of the upper-most mantle.

7.2.1. CascadiaAn excellent compilation of the evidence for

low P-wave and S-wave velocities in the Cascadiaforearc has been presented recently by Brocher etal. [71], and we provide only a short summary.Four types of evidence are available : P- andS-wave velocities from seismic tomography, Pnvelocities from wide-angle active source experi-ments, receiver function S-wave contrast at theMoho (e.g. [69]), and lack of or weak uppermostmantle Pn velocities. The most striking evidencefor serpentinization in the Cascadia forearc man-tle is the receiver function data recorded by adense broadband array. Scattered teleseismicwaves reveal a normal continental Moho at36 km depth in the Cascadia volcanic arc. Sea-ward, as the trench is approached, the velocitycontrast at the Moho ¢rst disappears and thenreappears with reverse polarity [70]. The reversepolarity indicates a Vs for the uppermost forearcmantle that is less than for the overlying crust.Near the wedge corner, the maximum Vs down-ward perturbation is V10% suggesting mantleserpentinization levels could be as high as 50^60% in this region [70]. In the northern Cascadiaforearc, Pn velocities are usually less than 7.8 kms31 [67,68]. Brocher et al. [71] mapped low mantlevelocities from controlled source and earthquaketomography and weak wide-angle re£ections fromthe top of the forearc upper mantle in a narrowregion along the margin. The forearc upper man-tle velocities range from 7.2 to 7.7 km s31 repre-senting up to 30% serpentinization.

7.2.2. Central AndesThe shallow mantle beneath the western Cor-

dillera of Bolivia (18^21‡S) exhibits slightly re-duced average Vp (6 8 km s31), slightly reducedaverage Vs (6 4.4 km s31), and high Vp/Vs

(V1.83) [61]. Locally strong seismic velocity

EPSL 6685 2-7-03


anomalies are observed with VpV7.8 km s31,VsV4.3 km s31, and Vp/VsV1.86 [61]. The lowseismic velocity, high Vp/Vs region lies above thesubducted Nazca plate beneath the forearc andarc region and is interpreted to re£ect extensivehydration [61]. These authors also suggest possi-ble local partial melting in the mantle, but this ismore likely beneath the arc, rather than in thevery cool forearc. Beneath northern Chile(V33‡S), the Andean forearc mantle is character-ized by anomalously low Vp and high Vp/Vs (1.79to s 1.87), based on three-dimensional tomo-graphic images derived from simultaneous inver-sions of local earthquake data [62]. The zone ofhigh Vp/Vs occurs between 70 and 120 km depth,overlies the subducted Nazca plate, and is inter-preted to re£ect hydration of the mantle wedgecaused by £uids released from the subductingplate [62].

7.2.3. Central JapanKamiya and Kobayashi [63] detected serpenti-

nized forearc mantle beneath the Kanto district ofcentral Japan. They observed low seismic veloc-ities (Vp = 6.9 km s31, Vs = 3.4 km s31) 20^70 kmdeep in the forearc mantle adjacent to the sub-ducted Philippine Sea plate. Between 20 and45 km depth, the Poisson’s ratio is greater than0.3 (up to 0.34), as compared to surroundingrocks with normal Poisson’s ratios of V0.25. Ka-miya and Kobayashi [63] interpreted the forearcmantle to be 50% serpentinized in this region. Thehigh Poisson’s ratio region also corresponds ap-proximately to the downdip limit of thrust earth-quakes, to the rupture limit of the Kanto 1923great earthquake, and to the base of the sub-duction thrust locked zone estimated from GPSdata.

7.2.4. Izu-Bonin^MarianaA marine seismic re£ection^refraction survey of

the Izu-Bonin subduction zone (32‡N) by Suye-hiro and colleagues [41,64] found that the wellde¢ned Moho beneath the arc disappears beneaththe forearc because of low seismic velocities in themantle (VpV7.2 km s31) inferred to result fromhydration. Very low Vp (V5 km s31) are ob-served beneath a forearc serpentinite diapir [41].

An extreme case may be the Mariana forearcwhere Hussong and Uyeda [40] concluded thatthe uppermost mantle velocities are 6.1^7.2 kms31, although the data for this region are limited.These velocities represent 40^60% serpentinization(Fig. 6).

7.2.5. Alaska^AleutiansA tomographic inversion beneath southern

Alaska revealed a pronounced low Vp anomaly(at least 10% reduction) in the forearc mantle[65]. The anomaly beneath the Cook Inlet extendsupward from the subducting Paci¢c plate into theoverlying mantle wedge from 20 to 120 km depth[65]. In the same area, a tomographic study byZhao et al. [66] revealed a V50 km thick zoneof low Vp located V50 km above the subductedslab that may correspond, in part, to the anomalyseen in Kissling and Lahr’s [65] study. Fliednerand Klemperer [57] found average Vp of V7.7km s31, and as low as 7.4 km s31, for the easternAleutian forearc mantle. Vp increases downwardand away from the mantle wedge corner. Fliednerand Klemperer [57] proposed that the low seismicvelocities could be explained by pyroxene-rich cu-mulates, but also suggested that serpentinizationcould account for the low seismic velocities in theforearc mantle, and might also explain the ob-served mantle re£ectivity. These velocities corre-spond to 10^20% serpentinization (Fig. 3). Thelimited Vs data of Fliedner and Klemperer [57]also suggest an unusually high Poisson’s ratio inthe Aleutian forearc mantle consistent with ser-pentinization.

7.3. Other geophysical expressions of forearcmantle serpentinite

Magnetic susceptibilities increase up to 30 timeswith increasing serpentinization, from about 1033

to about 3U1032 S.I. [72]. Thus, cold serpenti-nized mantle where the temperature is below themagnetite Curie temperature may be detected bymagnetic anomalies caused by magnetite. A mag-netic source located in the uppermost mantlewould yield a long spatial wavelength anomaly,although tectonic processes often emplace serpen-tinite into the crust. Strong trench-parallel mag-

EPSL 6685 2-7-03


netic anomalies are ubiquitous just seaward of thevolcanic arcs in present and former subductionforearcs, including NE Japan [73], Alaska [74],and along much of western North America fromthe southwestern British Columbia coast rangeplutonic complex, through Washington and Ore-gon to the Great Valley of California [75,76].Coles and Currie [75] and others show that theshort spatial wavelengths of the Cascadia mag-netic anomalies require that at least part of thesource must lie in the upper to middle crust. Fur-ther study is required to determine whether a sig-ni¢cant part of the magnetic signal is comingfrom the forearc mantle.

Some serpentinites are characterized by highelectrical conductivity interpreted to be due tointerconnected magnetite [77], the same sourcematerial suggested for the high magnetizationsof serpentinite. We have not examined this e¡ectin detail, but one example of high conductivitythat probably lies in the forearc mantle is fromthe MSLAB Cascadia magnetotelluric and mag-netovariation study [78,79]. High conductivitiesare found at a depth below about 25 km forabout 100 km seaward of the volcanic arc. Incontrast, only moderate conductivity was esti-mated for the Andes forearc by Echternacht etal. [80].

Density also decreases substantially with in-creasing degree of serpentinization, from about3200 kg m33 for unaltered ultrama¢c rocks toabout 2500 kg m33 for 100% serpentinization[47^49]. Such a large density reduction may resultin substantial gravity anomalies, as inferred forthe Aleutian arc [81]. However, a large-scale 15^30% serpentinization as suggested by the P-wavevelocity data in several subduction zones, onlyreduces the upper mantle density from 3200 toabout 3000 kg m33, which is still higher thannormal crustal densities. Where there is a veryhigh degree of serpentinization, the reduction ofdensity and seismic velocity are such that forearcmantle serpentinite will not be easily distinguishedfrom greater crustal thickness in gravity modeling.The e¡ect of dynamic topography in subductionzones also complicates gravity modeling whichmakes detection of reduced density forearc mantledi⁄cult.

8. Consequences of forearc serpentinization

8.1. Aseismic forearc mantle

In subduction zones, the forearc mantle appearsto be aseismic. The maximum temperature forearthquake behavior in continental crustal com-positions is about 350‡C, based on laboratoryand ¢eld data (e.g., summary by Hyndman andWang [22]) which encompasses most forearccrusts. The brittle^ductile transition for dry man-tle rocks is 600^800‡C (e.g., [82]) which exceedsthe maximum temperature of most forearc uppermantles. Dry mantle rocks in cool forearcs shouldbe much stronger than crustal rocks (e.g., [82]).Thus, earthquakes might be expected to occur toconsiderable depth in forearc mantle. However,few if any earthquakes are reliably located withinthe forearc mantle (e.g., for Cascadia, [83] ; forSouth America [84]). Serpentinite is dramaticallyweaker than dry mantle rocks and may deformaseismically under forearc P^T conditions.

8.2. Downdip limit of subduction thrustearthquakes

For cool subduction zones, the downdip limitfor subduction great thrust earthquakes often cor-responds to the intersection of the thrust with theforearc mantle (Fig. 1) (e.g., [24,85]). This limitmay be explained by aseismic hydrous mineralssuch as serpentinite present in the forearc mantlewedge that exhibit stable sliding [14,27,86]. Justabove the slab interface itself, talc-rich rocksmay form in the mantle by the addition of silicatransported by rising £uids and by mechanicalmixing of underthrust siliceous rocks with theoverlying mantle [14]. In the laboratory, the be-havior of serpentine minerals is complex, but theygenerally exhibit stable-sliding aseismic behavior(e.g., [87,88]). Recent experiments demonstratethat brucite, a layered hydroxide, has a very lowcoe⁄cient of friction (WV0.30 at room tempera-ture decreasing to 0.20 at 300‡C and 0.23 at450‡C) [89]. Serpentinites containing disseminatedbrucite will have lower coe⁄cients of friction (10^15% lower) than pure serpentinites [89]. Furtherdata are required to con¢rm this behavior under

EPSL 6685 2-7-03


forearc P^T conditions. We have not found dataon the sliding behavior of talc, but its layeredsilicate structure also suggests very weak stable-sliding behavior. Slow earthquakes (silent slipevents) that rupture the plate interface downdipof the subduction thrust have recently been rec-ognized in Cascadia [90]. This phenomenon maybe controlled by the rheologic behavior of forearcserpentinite (e.g., [71]). The depth to the forearcmantle (i.e., forearc Moho) and inferred maxi-mum depth for subduction thrust earthquakesvaries from nearly 50 km in some continental sub-duction zones with a thick forearc crust to as littleas 10^15 km in oceanic subduction zones such asthe Mariana, that have thin forearc crust,although the depth of the forearc Moho intersec-tion is usually poorly known.

8.3. Flow in the mantle wedge

In subduction zones, mantle wedge convectionmay be driven primarily by viscous coupling be-tween the subducting plate and mantle wedge(forced convection) [91] and by horizontal ther-mal gradients (free convection) (e.g., [92]). Forthe thermal models presented in Fig. 4, mantlewedge convection was simulated using an analyt-ical corner £ow solution [93] in which a no-slipvelocity boundary condition is employed alongthe plate interface and the wedge material has aconstant viscosity. More realistic models of man-tle wedge convection incorporate the strong tem-perature dependence of mantle viscosity andbuoyancy forces (e.g., [94,95]).

Serpentinized forearc mantle, because of itsweak rheology and low density, will likely alterthe pattern of convection in the mantle wedge.In almost all mantle wedge convection models,the subducting slab and mantle wedge are as-sumed to be fully coupled such that the base ofthe mantle wedge moves downward at the samevelocity as the subducting plate. Where the tem-perature is low, serpentinite, present at the base ofthe mantle wedge seaward of the arc, should de-crease the coupling between the subducting plateand mantle wedge. Rather than deformationbeing distributed throughout the mantle wedge,much of the shear deformation will be focused

in the weak, serpentinized mantle adjacent tothe subducting plate. In addition, the positivebuoyancy resulting from serpentinization will actto counter the negative thermal buoyancy thatdrives the free convection component of mantlewedge £ow. We propose that the weak rheologyand positive buoyancy of serpentinite will act toisolate hydrated forearc mantle from the overallmantle wedge £ow system. Models of mantlewedge convection linking arc magmatism todowndragged hydrous mantle wedge material(e.g., [94,96]) need to consider the possible rheo-logical e¡ects of forearc serpentinization.

8.4. Heating and dehydration of previouslyhydrated forearc mantle

The dehydration of forearc mantle serpentinitecan occur by a number of processes. Simple stop-ping of subduction will allow temperatures toslowly rise for some tens of Myr resulting inslow dehydration of at least the deeper portionsof the forearc mantle wedge. More rapid dehydra-tion and large upward £uid £uxes may occur fromhigh temperatures associated with other changesin margin tectonics. Forearc temperatures arehigh where spreading ridges are being subducted[97] and there are slab windows (e.g., [98]), incontrast to normal low forearc temperatures. Insuch special areas the forearc upper mantle maybe largely dehydrated. Over tens of millions ofyears, many active continental margins havebeen subjected to a short period of high temper-atures caused by the subduction of a spreadingridge that swept along the margin. The conse-quence may be a period of forearc mantle dehy-dration and strong upward £uid expulsion thatmigrates along the margin. Rehydration may fol-low ridge subduction as older oceanic lithosphereis once again subducted and the forearc cools.Although not as dramatic as ridge subduction, asigni¢cant decrease in convergence rate can leadto forearc warming and dehydration. A trench-ward shift in the axis of arc volcanism, perhapsdue to slab steepening, might also trigger dehy-dration of the landward portion of previously hy-drated forearc mantle.

The petrologic and thermal structure of the two

EPSL 6685 2-7-03


converging continental margins prior to continen-tal collision is an important control for the natureof orogeny. We have not examined the role ofhydrous minerals in the forearc mantle duringcollision, but we point out several important ex-pected consequences. First, although forearcshave very low temperatures and might be takento be strong (in contrast of hot weak backarcs),they may be weaker than expected due to thepresence of weak hydrous minerals in the forearcupper mantle. A weak upper mantle may allowdetachment just below the base of the forearccrust. The hydrated forearc mantle also may pro-vide substantial aqueous £uid for orogenic mag-matism during continental collision if there isforearc heating, such as may occur due to under-thrusting of high radioactive heat generation con-tinental crust beneath the forearc.

9. Conclusions

We conclude that serpentinization of the fore-arc mantle is a global phenomenon linked to thelarge volumes of free and bound water releasedfrom subducted oceanic crust and sediments. Sub-duction chills the forearc mantle such that serpen-tine and related hydrous minerals are stable. Hy-drous £uids derived from the subducting plate willreact strongly with ultrama¢c rocks in the over-lying forearc mantle to produce serpentine miner-als, brucite, and talc. There is direct geologicalevidence of forearc mantle hydration in serpentinemud volcanoes of the Mariana and Izu-Boninsubduction zones and indirect evidence in exposedpaleo-subduction zones. Geophysical observa-tions, including reduced seismic velocities andhigh Poisson’s ratio, suggest forearc mantle maycommonly be V20% serpentinized; locally, ser-pentinization may reach 50%. Serpentinized fore-arc mantle may provide an explanation for theaseismic nature of the forearc mantle and thedowndip limit of subduction thrust earthquakes.The weak rheology and positive buoyancy of ser-pentinite will act to isolate hydrated forearc man-tle from the mantle wedge £ow system. Dehydra-tion of forearc serpentinite may provide a £uid

source to £ux magmatism during ridge subductionand continental collision.

Acknowledgements

Geol. Survey of Canada Publication Number:2001017. R.D.H. acknowledges support from theGeological Survey of Canada, Natural Sciencesand Engineering Research Council of Canadaand the U.S. Geol. Survey National EarthquakeHazards Reduction Program. S.M.P. acknowl-edges NSF support through grants EAR97-25406 and 98-09602.[SK]

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serpentinization of the forearc mantle - phil skemer

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