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