Thermal Structure Of Subduction Zones Research Articles

Very-low-temperature record of the subduction process: A review of worldwide lawsonite eclogites

Lawsonite eclogites preserve a record of very-low-temperature conditions in subduction zones. All occur at active margin settings, typically characterized by accretionary complexes lithologies and as tectonic blocks within serpentinite-matrix mélange. Peak lawsonite-eclogite facies mineral assemblages (garnet + omphacite + lawsonite + rutile) typically occur in prograde-zoned garnet porphyroblasts. Their matrix is commonly overprinted by higher-temperature epidote-bearing assemblages; greenschist- or amphibolite-facies conditions erase former lawsonite-eclogite relics. Various pseudomorphs after lawsonite occur, particularly in some blueschist/eclogite transitional facies rocks. Coesite-bearing lawsonite-eclogite xenoliths in kimberlitic pipes and lawsonite pseudomorphs in some relatively low-temperature ultrahigh-pressure eclogites are known. Using inclusion assemblages in garnet, lawsonite eclogites can be classified into two types: L-type, such as those from Guatemala and British Columbia, contain garnet porphyroblasts that grew only within the lawsonite stability field and E-type, such as from the Dominican Republic, record maximum temperature in the epidote-stability field. Formation and preservation of lawsonite eclogites requires cold subduction to mantle depths and rapid exhumation. The earliest occurrences of lawsonite-eclogite facies mineral assemblages are Early Paleozoic in Spitsbergen and the New England fold belt of Australia; this suggests that since the Phanerozoic, secular cooling of Earth and subduction-zone thermal structures evolved the necessary high pressure/temperature conditions. Buoyancy of serpentinite and oblique convergence with a major strike-slip component may facilitate the exhumation of lawsonite eclogites from mantle depths.

The thermal structure of subduction zones constrained by seismic imaging: Implications for slab dehydration and wedge flow

Thermal models of subduction zones often base their slab–wedge geometry from seismicity at mantle depths and, consequently, cannot be used to evaluate the relationship between seismicity and structure. Here, high-resolution seismic observations from the recent Broadband Experiment Across the Alaska Range (BEAAR) constrain, in a rare instance, the subducting slab geometry and mantle wedge temperature independent of seismicity. Receiver functions reveal that the subducting crust descends less steeply than the Wadati-Benioff Zone. Attenuation tomography of the mantle wedge reveals a high Q and presumably cold region where the slab is less than 80 km deep. To understand these two observations, we generate thermal models that use the improved wedge geometry from receiver functions and that incorporate temperature- and strain-rate-dependent olivine rheology. These calculations show that seismicity within the subducting crust falls in a narrow belt of pressure–temperature conditions, illuminating an effective Clapeyron slope of 0.1 K/MPa at temperatures of 450–750 °C. These conditions typify the breakdown of high-pressure hydrous minerals such as lawsonite and suggest that a single set of dehydration reactions may trigger intermediate-depth seismicity. The models also require that the upper, cold nose of the mantle wedge be isolated from the main flow in the mantle wedge in order to sustain the cold temperatures inferred from the Q tomography. Possibly, sufficient mechanical decoupling occurs at the top of the downgoing slab along a localized shear zone to 80 km depth, considerably deeper than inferred from thrust zone seismicity.

A case for hot slab surface temperatures in numerical viscous flow models of subduction zones with an improved fault zone parameterization

Viscous flow models play a fundamental role in our understanding of the dynamics and thermal structure of subduction zones. For example, the relatively cool slabs produced in 2D viscous flow models have been central to the argument against slab and sediment melting at subduction zones except in special cases, such as near slab edges. Because flow models have such an important role in our insight to the subduction process, it is imperative to understand how various assumptions in the models affect their results. The fault zone is a key part of subduction models, but the effects of different parameterizations on the results have been largely ignored. The two main fault parameterizations used in the literature are weak (low viscosity) nodes imposed at the fault surface and an imposed rigid overlying plate. Each formulation has pros and cons, but their only physical justification is that they keep the overlying mechanical lithosphere from deforming in unrealistic ways. Because the assigned thickness of the imposed plate or the dimensions of the weak zone governs the amount of ablation that occurs in the corner and the temperature of the material advected to the subducting slab, different arbitrary implementations can result in significantly different flow patterns and thermal structures. A fault zone parameterization with a rigid plate defined by temperature and strain rate rather than by depth or other geometry could ameliorate the predicament. This formulation removes much of the arbitrariness from the modeling, as it is based on a physical process. This formulation has the advantages of restricting viscous deformation in the overriding plate without limiting processes such as ablation by imposed boundary conditions. Model results with this parameterization show hotter slab surface temperatures than previously shown in the literature. The results also agree as well as or better than other formulations with geophysical observations such as heat flow, seismic velocity, and seismic attenuation. The calculated higher slab surface temperatures could lead to a reappreciation of sediment melting at subduction zones without requiring an atypical tectonic environment.

A simple analytical approximation to the temperature structure in subduction zones

SUMMARY A simple model is employed to find scaling relations between key features of the temperature structure of subduction zones and subduction zone parameters. Flow in the wedge of mantle between the slab and the overriding plate is approximated by the corner flow of a Newtonian viscous fluid. The flow maintains an advective boundary layer on top of the slab that controls the temperature at the interface between the slab and the wedge. This temperature, the maximum temperature in the wedge above the slab, and the thickness of the advective boundary layer, all depend on a single dimensionless distance, Vrδ2/κ, where V is the speed of plate convergence, r is distance from the corner of the wedge, δ is the dip of the slab and κ is thermal diffusivity. The observation that volcanic fronts at island arcs lie above places where the slab reaches a depth that correlates negatively with convergence rate and slab dip suggests that the thermal structure of subduction zones may be described by the simple scaling developed here, and that the locations of the arcs are controlled by a strongly temperature-dependent process taking place in the wedge.

Phase relations of peridotites under H 2O-saturated conditions and ability of subducting plates for transportation of H 2O

Phase relations of peridotites under H 2O-saturated conditions up to 28 GPa and 2600 °C have been clarified based on the high-pressure experimental results in the MgO(–FeO)–SiO 2–H 2O, MgO–SiO 2–Al 2O 3–H 2O, CaO–MgO–Al 2O 3–SiO 2–H 2O and natural systems. Based on the phase assemblages deduced and the chemical compositions of the phases, the maximum H 2O contents for two bulk rock compositions (lherzolite and harzburgite) have been calculated using mass balance. Then the potential ability of subducting plates for transportation of H 2O is discussed by simple models combining the phase relations and the thermal structures of subduction zones as a function of the age of subducting plate a (15–130 Ma), the subduction velocity u 0 (2.25–18 cm/year) and angle α (30° and 60°), and the mantle potential temperature T p (1300 and 1350 °C). The results confirm the importance of “choke point” (a cusp around 6.2 GPa and 550 °C at which the stability field of major hydrous phases is minimized in terms of temperature), although the choke point is exceeded by the Mg-sursassite-bearing assemblage that has the maximum H 2O content of 0.4–0.7 wt.% around 5–7 GPa and 550–700 °C. If the geotherm of the coldest part across the subducting plate passes below the choke point, maximum 4.6 wt.% H 2O can subduct, whereas H 2O less than 0.4–0.7 wt.% can subduct above the choke point. The critical conditions are clarified as follows: if α=30° and T p=1300 °C, a∼15 Ma with u 0=9 cm/year, or a∼30 Ma with u 0=2.25 cm/year is the critical condition. If α=60° and T p=1300 °C, or α=30° and T p=1350 °C, the critical conditions slightly shift to an older age or a greater velocity (e.g., a∼30 Ma with u 0=3.5 cm/year). The slab thermal parameter ( au 0sin α) is useful to predict roughly the critical condition. However, within the younger plate, the spatial extent of a cold region with major hydrous phases is thinner, hence less H 2O is subducted, even with the same slab thermal parameter. This indicates that the age of the plate and the subduction velocity act differently on the H 2O-transportation ability of subducting plates.

The structure and dynamics of the mantle wedge

A large amount of water is brought into the Earth’s mantle at subduction zones. Upon subduction, water is released from the subducting slab in a series of metamorphic reactions. The resulting flux into the mantle wedge modifies its chemical and physical properties by mineral hydration with associated weakening, flux melting and changes in the dynamics and thermal structure of subduction zones. Water guides the formation of volcanoes, earthquakes, continent formation and the long-term chemical evolution of the Earth’s mantle. Recent observational advances include the better documentation of the role of water in causing melting from minor and trace elements in arc lavas, improved structure of the mantle wedge derived from seismic tomography, and documentation of hydration of the mantle wedge from converted phases. High-pressure experiments allow for a quantification of the role of water on seismic velocities and attenuation and rheological changes, which provide essential input into models of subduction zones. Computational models provide additional evidence for the importance of the mantle wedge in subduction zone dynamics.

High‐resolution models of subduction zones: Implications for mineral dehydration reactions and the transport of water into the deep mantle

Arc volcanism is intimately linked to mineral dehydration reactions in the subducting oceanic mantle, crust, and sediments. The location of slab dehydration reactions depends strongly on the temperature and pressure conditions at the top of the subducting plate and hence on the detailed thermal structure of subduction zones. A particularly important physical property of subduction zone thermal models is the viscosity of the mantle wedge. The introduction of an olivine rheology, with appropriate stress and temperature dependence, focuses flow into the tip of the mantle wedge. This leads to a temperature increase of a few hundred degrees in the wedge and the top of the slab as compared to the isoviscous case. Sensitivity tests show that this conclusion is robust under a variety of subduction zone parameters. The new high‐resolution finite element models are used to reevaluate the thermal structure of the Honshu and Cascadia subduction zones using the more realistic olivine rheology. For Honshu, the model predicts slab–mantle interface temperatures of ∼800°C beneath the volcanic front. Deeper parts of the subducting oceanic crust and upper mantle remain relative cool (e.g., the subducted Moho beneath the volcanic front is ∼400°C) because of the rapid subduction of old Pacific lithosphere. High interface temperatures are consistent with Th and Be data, indicating sediment melting, whereas cooler crustal temperatures are consistent with boron evidence, indicating lower temperature dehydration. The strong temperature gradient at the top of the subducting plate may thus reconcile conflicting estimates of slab temperature in subduction zones that are characterized by rapid subduction of mature oceanic lithosphere. Low temperatures persist to great depth in the shallow upper mantle of the subducting plate, which allows for water transport in the form of hydrous minerals to the deep mantle. The thermal structure of the Cascadia subduction zone is markedly different because of the age of the incoming lithosphere and low subduction speed. In this case, shallow dehydration and melting of the subducting crust and lithosphere is predicted.

Effects of active crustal movements on thermal structure in subduction zones

SUMMARY In young suduction zones we observe steady uplift of island arcs. The steady uplift of island arcs is always accompanied by surface erosion. The long duration of uplift and erosion eVectively transports heat at depth to shallower parts by advection. If the rates of uplift and erosion are suYciently large, such a process of heat transportation will strongly aVect thermal structure in subduction zones. First, we quantitatively examine the eVects of uplift and erosion on thermal structure by using a simple 1-D heat conduction model, based on the assumption that the initial thermal state is in equilibrium. The results show that temperature increase, DT , due to uplift and erosion can be approximately evaluated by DT =n e tb at depth, where n e is the rate of uplift (erosion), t is the duration of uplift (erosion), and b is the gradient of the geotherm in the initial state. Next, considering the eVects of vertical crustal movements such as uplift and erosion in island arcs and subsidence and sedimentation in ocean trenches, in addition to the eVects of radioactive heat generation in the crust, frictional heating at plate boundaries and accretion of oceanic sediments to overriding continental plates, we numerically simulate the evolution process of the thermal structure in subduction zones. The result shows that the temperature beneath the island arc gradually increases as a result of uplift and erosion as plate subduction progresses. Near the ocean trench, on the other hand, the low-temperature region gradually expands as a result of sedimentation and accretion in addition to direct cooling by the cold descending slab. The surface heat flow expected from this model is low in fore-arc basins, high in island arcs and moderately high in back-arc regions.

Simple analytic model for subduction zone thermal structure

A new analytic model is presented for the thermal structure of subduction zones. It applies to the deeper regions of a subduction zone, where the overriding mantle is no longer rigid but flows parallel to the slab surface. The model captures the development of one thermal boundary layer out into the mantle wedge, and another into the subducting slab. By combining this model with the analytic model of Royden (1993a,b), which applies to regions in which the overriding plate is rigid, a nearly complete analytic model for the thermal structure of a steady-state subduction zone can be achieved. A good agreement is demonstrated between the output of the combined analytic model and a numerical finite element calculation. The advantages of this analytic approach include (1) efficiency (only limited computing resources are needed); (2) flexibility (non-linear slab shape, and processes such as erosion, and shear heating are easily incorporated); and (3) transparency (the effect of changes in input variables can be seen directly).

On the generation of deep focus earthquakes in subduction zones

Following a quasi-dynamic scheme proposed by Minear and Toksoz (1970), thermal structures of subduction zones for different models by finite element method (FEM) were calculated. Density distribution and P-wave anomaly of subduction zones were calculated at the same time. Comparing with seismological evidences and results of laboratories, it is proposed that earthquakes occurred below 400 km depth are probably controlled by anti-crack mechanism.

The Finero phlogopite-peridotite massif: an example of subduction-related metasomatism

The Finero peridotite massif is a harzburgite that suffered a dramatic metasomatic enrichment resulting in the pervasive presence of amphibole and phlogopite and in the sporadic occurrence of apatite and carbonate (dolomite)-bearing domains. Pyroxenite (websterite) dykes also contain phlogopite and amphibole, but are rare. Peridotite bulk-rock composition retained highly depleted major element characteristics, but was enriched in K, Rb, Ba, Sr, LREE (light rare earth elements) (LaN/YbN = 8–17) and depleted in Nb. It has high radiogenic Sr (87Sr/86Sr(270) = 0.7055–0.7093), low radiogenic Nd (ɛNd(270) = −1 to −3) and EMII-like Pb isotopes. Two pyroxenite – peridotite sections examined in detail show the virtual absence of major and trace element gradients in the mineral phases. In both rock types, pyroxenes and olivines have the most unfertile major element composition observed in Ivrea peridotites, spinels are the richest in Cr, and amphibole is pargasite. Clinopyroxenes exhibit LREE-enriched patterns (LaN/YbN ∼16), negative Ti and Zr and generally positive Sr anomaly. Amphibole has similar characteristics, except a weak negative Sr anomaly, but incompatible element concentration ∼1.9 (Sr) to ∼7.9 (Ti) times higher than that of coexisting clinopyroxene. Marked geochemical gradients occur toward apatite and carbonate-bearing domains which are randomly distributed in both the sections examined. In these regions, pyroxenes and amphibole (edenite) are lower in mg## and higher in Na2O, and spinels and phlogopite are richer in Cr2O3. Both the mineral assemblage and the incompatible trace element characteristics of the mineral phases recall the typical signatures of “carbonatite” metasomatism (HFSE depletion, Sr, LILE and LREE enrichment). Clinopyroxene has higher REE and Sr concentrations than amphibole (amph/cpxDREE,Sr = 0.7–0.9) and lower Ti and Zr concentrations. It is proposed that the petrographic and geochemical features observed at Finero are consistent with a subduction environment. The lack of chemical gradients between pyroxenite and peridotite is explained by a model where melts derived from an eclogite-facies slab infiltrate the overhanging harzburgitic mantle wedge and, because of the special thermal structure of subduction zones, become heated to the temperature of the peridotite. If the resulting temperature is above that of the incipient melting of the hydrous peridotite system, the slab-derived melt equilibrates with the harzburgite and a crystal mush consisting of harzburgite and a silica saturated, hydrous melt is formed. During cooling, the crystal mush crystallizes producing the observed sequence of mineral phases and their observed chemical characteristics. In this context pyroxenites are regions of higher concentration of the melt in equilibrium with the harzburgite and not passage-ways through which exotic melts percolated. Only negligible chemical gradients can appear as an effect of the crystallization process, which also accounts for the high amphibole/clinopyroxene incompatible trace element ratios. The major element refractory composition is explained by an initially high peridotite/melt ratio. The apatite, carbonate-bearing domains are the result of the presence of some CO2 in the slab-derived melt. The CO2/H2O ratio in the peridotite mush increased by crystallization of hydrous phases (amphibole and phlogopite) locally resulting in the unmixing of a late carbonate fluid. The proposed scenario is consistent with subduction of probably Variscan age and with the occurrence of modal metasomatism before peridotite incorporation in the crust.

Fluids in Subduction Zone Magmatism: Implications of Boron Geochemistry

The roles of fluids, sediments, and slab melts in convergent margin magmatism are difficult to quantify and likely vary from one arc to another. Decoupling of trace elements with differential fluidand melt-mobility can illuminate specific subdnction processes; the fluid-mobile element (FME) boron is particularly useful in this regard. Variable B-enrichment (up to > 100 x ) in arc compared to within-plate lavas can be attributed to direct subduction contributions (as implied by correlations with l~ in arc suites). The fact that B enrichment is decoupled from that of fluid-immobile elements (REE, Zr, Ti, Nb, etc.) but parallels that of fluid-mobile elements (Pb, As, Sb) implies dominant transfer of the latter elements into arc magma sources via aqueous fluids rather than melts. B commonly is decoupled from or poorly correlated with Ba, Sr, and alkalies - suggesting that these elements are subject to more complex controls. In general, element mobility must be considered in light of element partitioning between all stable mineral, fluid, or melt phases. Enrichments of B (e.g. relative to Zr) vary widely from one arc to another, and generally correlate with such factors as 'slab length' or 'age of subducting crust', both of which are manifestations of slab thermal state. Locally, in cross-arc transects, Benrichment is strongest at the volcanic front (VF) and diminishes toward back-arc (BA) regions. Coupled with data for metamorphic suites that indicate progressive B-depletion with increasing grade, such trends suggest that B is mobilized from subducting slabs due to progressive slab dehydration. Thus, Benrichment in arc basalts appears to reflect the thermal evolution of subducted slabs. B-enrichment also appears to be correlated with degree of Uisotope disequilibria. Available B isotopic data for arc lavas tend to show both regional and local variations consistent with the above. Different ranges in boron isotopic composition of siliciclastic sediments (5lIB: -5--}-5%o), altered oceanic crust (811B: 4.3+_5.4%o), and peridotite (sllB: 8 to 13%o) an covariations between degree of B enrichment, 811B, and 878r/86Sr isotopic data may be used to discriminate the relative contributions of hydrous fluids vs. sediments to arc magma sources. In western Pacific arcs (old, cold slab), elevated 811B (> +3%0) in VF lavas is consistent with derivation of excess B from subducted seawateraltered oceanic crust rather than sediments, whereas low 8lIB and elevated 878r/86Sr in some arc lava suites (Lesser Antilles, Aeolian) point to sediment or crustal involvement as well. Moreover, inter-arc variations in 811B could in part reflect progressive decrease in 811B of the subduction component during devolatization of subducted slabs, with less intense metamorphism and lower liB-loss in cold subduction zones and vice versa. To highlight the relative importance of these processes in arc-magma genesis we have analysed volcanic rocks from arcs with different geological settings and subduction zone thermal structures. Tonarini & Leeman (this volume) discuss the behaviour of B in specific arcs. Overall, 8 lIB varies widely (between -10%o and +21%.) in arc lavas; the highest values apparently occur in lavas derived from highly depleted sources that are easily modified by the subduction component (Fig. 1). The B/Nb-B systematics similar to those of the Izu and Kuriles arcs (Ishikawa and Nakamura, 1994; Ishikawa and Tera, 1997) are sometimes observed in subsets of our data, but overall relations are more complex. Also, 87Sr/S6Sr is correlated with 811B inside each arc but not in general due to differing slab and sediment contributions worldwide. From plots of 'fluid-immobile element'/B ratios vs ~IlB, extrapolated to zero, ~llB values for the subduction component(s) can be estimated for each arc studied. Typical values are: +16%o for the South Sandwich arc (and perhaps locally in Tonga [Hunga Ha'apai]); +7%0 for Tonga, Mariana, Izu (Ishikawa and Nakamura, 1994), and Kurile (Ishikawa and Tera, 1997) arcs; +3.7%0 for the Aleutian arc; and

Subduction zone melting of pelagic sediments constrained by melting experiments

The fate of pelagic sediments in subduction zones has an important bearing on global geochemical cycles and the thermal structure of subduction zones. Recent mass-balance calculations have indicated that about 20% of subducted sediments are recycled to volcanic arcs. Trace-element studies of these volcanic arcs imply that the subducted sediment melts while the gabbroic crust that underlies it dehydrates. This requires a rather specific thermal structure in subduction zones. Here we report laboratory melting experiments that allow us to derive melting curves for pelagic red clay at pressures of up to 40 kbar, equivalent to depths of 120 km within the mantle. The melting behaviour of wet red clay is similar to that for wet gabbro, but melting occurs at a slightly lower temperature, showing that sediment melting and gabbro dehydration can occur at the same temperature. The combination of trace-element data and phase diagrams such as that derived here may thus be used to constrain the temperature of the slab–mantle boundary.

Fluid-melt-rock interaction in mafic eclogites and coesite-bearing metasediments: Constraints on volatile recycling during subduction

The question as to whether the metasomatizing-slab agent needed to form island-arc magmas is an aqueous fluid or a hydrous melt requires a knowledge of fluid-melt-rock interaction during high-pressure low-temperature metamorphism. In the Western Alps, typical “anhydrous” mafic eclogites (Monviso Massif) and coesite-bearing metasediments (Dora-Maira Massif) have experienced a prograde P- T path characteristic of a mature (cold) subduction zone. Microstructural, petrologic, geochemical and fluid inclusion studies show that fluid flow was limited during eclogite and high-grade blueschist facies metamorphism. Most of the fluids were driven off the rocks at pressures lower than 1 and 1.6 GPa, respectively. With increasing pressure and temperature, the remaining fluid phase was released during crystal plastic flow processes or devolatilisation reactions but retain the host rocks to form veins, partial melts or dense hydrous silicates. Tectonic erosion at modern convergent margins delivers large amounts of terrigenous sediments (KMASH system) in subduction zones. In contrast to the basalt and peridotite systems, the KMASH system is characterized by a wide range of pressure-sensitive reactions which allow the reconstruction of dehydration/hydration depths in the subducted slab. Independently of the thermal structure of subduction zones, terrigenous sediments will begin to melt at pressures < 2 GPa. The melt phase can absorb tremendeous quantities of water and represent major sinks for devolatilisation fluids. Dense hydrous silicates will form in any subducted sediments at all depths. These silicates can store significant amount of water in their structure. In the subducted oceanic crust and upper mantle, heterogeneous distribution of fluids inherited from hydrothermal alteration at mid-ocean ridges will cause contemporaneous dehydration and hydration reactions on a local scale rather than large-scale fluid infiltration in the overlying mantle wedge. The situation where a cold, competent and fluid-rich subducted slab is bounded above by a hot, relatively weak and anhydrous material is likely to act against buoyant rise of devolatilisation fluids by forcing the fluids to flow downwards. At sub-arc depths (50 ± 10 to 80 150 km ) and for any geothermal gradient, continuous internal buffering of volatile activities will retain the fluid phase in the slab. This is in contrast with shallow, sub-forearc depths (20 to 50 ± 10 km), where tectonic vein arrays, localized thrust faults, serpentine diapirs, mud volcanoes and boninite magmas attest to the release of large volume of fluids into the overriding mantle wedge.

Upper-mantle seismic discontinuities and the thermal structure of subduction zones

The precise depths at which seismic velocities change abruptly in the upper mantle are revealed by the analysis of data from hundreds of seismometers across the western United States. The boundary near 410km depth is locally elevated, that near 660 km depressed. The depths of these boundaries, which mark phase transitions, provide an in situ thermometer in subduction zones: the observed temperature contrasts require at least moderate thickening of the subducting slab near 660 km depth. In addition, a reflector near 210 km depth may mark the bottom of the aesthenosphere.

Subduction zone thermal structure and mineralogy and their relationship to seismic wave reflections and conversions at the slab/mantle interface

Seismological studies of the interface between downgoing slabs of subducted lithosphere and the overriding mantle provide constraints against which models of mantle and slab temperature and bulk composition can be tested. These studies, employing waves reflected and converted at the slab/mantle interface, suggest that a velocity contrast of 5–10% occurs over a distance of 10–20 km to depths of at least 350 km. We have investigated whether such sharp contrasts can result from thermal structure, compositional differences, phase changes, or other effects such as preferred mineral orientation. Using a mantle and slab temperature model, a petrologic model of mantle and slab mineralogy, and a data base of elastic properties of mantle and slab phases, theoretical seismic velocities were calculated and compared with the observed velocity contrast and boundary thickness constraints. At 9.6 GPa and 1000°C (conditions modeled at the slab/mantle interface at ∼280‐km depth), cooler temperatures in the slab interior produce, at constant composition, only approximately a 1.75% Vp increase into the slab in any of the peridotitic and eclogitic mineralogies investigated. Variation in Vp among the different bulk compositions is only approximately 0.5%. These contrasts are substantially lower than those obtained from the seismological studies and indicate that temperature and bulk compositional differences between mantle and slab are inadequate to cause the observed velocity contrasts. We propose that phase transformations in mantle and slab mineralogies, notably elevation of the olivine‐spinel phase change normally occurring at 400‐km depth, are important factors in these sharp contrasts. Other contributing effects may include preferential mineral orientation near the slab surface and, at shallower depths, the presence of partial melt or hydrous fluid evolved from dehydration reactions along the interface.

Warm fluid migration along tectonic melanges in the Kodiak Accretionary Complex, Alaska

Analyses of methane + water fluid inclusions from syntectonic quartz veins in melange boudins of the Kodiak accretionary complex, Alaska, indicate fluid temperatures ranging from 215° to 290°C at depths between 10 and 14 km. These fluid temperatures are significantly higher than those predicted by conductive heat flow models of subduction zones. Given the available geological constraints, this discrepancy is interpreted as the result of migration of warm fluids from deeper structural levels along active fault zones during subduction‐related underthrusting of these units. This interpretation requires more careful consideration of how migrating fluids affect the general thermal structure of subduction zones.