U51C-01 INVITED
Metamorphic Perspectives of Subduction Zone Volatiles Cycling
Field study of HP/UHP metamorphic rocks provides "ground-truthing" for experimental and theoretical petrologic studies estimating extents of deep volatiles subduction, and provides information regarding devolatilization and deep subduction-zone fluid flow that can be used to reconcile estimates of subduction inputs and arc volcanic outputs for volatiles such as H2O, N, and C. Considerable attention has been paid to H2O subduction in various bulk compositions, and, based on calculated phase assemblages, it is thought that a large fraction of the initially structurally bound H2O is subducted to, and beyond, subarc regions in most modern subduction zones (Hacker, 2008, G-cubed). Field studies of HP/UHP mafic and sedimentary rocks demonstrate the impressive retention of volatiles (and fluid-mobile elements) to depths approaching those beneath arcs. At the slab-mantle interface, high-variance lithologies containing hydrous phases such as mica, amphibole, talc, and chlorite could further stabilize H2O to great depth. Trench hydration in sub-crustal parts of oceanic lithosphere could profoundly increase subduction inputs of particularly H2O, and massive flux of H2O-rich fluids from these regions into the slab-mantle interface could lead to extensive metasomatism. Consideration of sedimentary N concentrations and δ15N at ODP Site 1039 (Li and Bebout, 2005, JGR), together with estimates of the N concentration of subducting altered oceanic crust (AOC), indicates that ~42% of the N subducting beneath Nicaragua is returned in the corresponding volcanic arc (Elkins et al., 2006, GCA). Study of N in HP/UHP sedimentary and basaltic rocks indicates that much of the N initially subducted in these lithologies would be retained to depths approaching 100 km and thus available for addition to arcs. The more altered upper part of subducting oceanic crust most likely to contribute to arcs has sediment-like δ15NAir (0 to +10 per mil; Li et al., 2007, GCA), and study of HP/UHP eclogites indicates retention of seafloor N signatures and, in some cases, enrichments in sedimentary N due to forearc metamorphic fluid-rock interactions (Halama et al., this session). A global estimate of C cycling, using seafloor inputs (carbonate and organic matter) and estimates of volcanic CO2 outputs, indicates ~40% return (with large uncertainty) of the subducting C in volcanic gases. This imbalance appears plausible, given the evidence for deep carbonate subduction, in UHP marbles, and the preservation of graphite in UHP metasediments, together seemingly indicating that large fractions of subducting C survive forearc-to-subarc metamorphism. Estimates of return efficiency in the Central America arc, based on data for volcanic gases, are lower and variable along strike (12-29%), quite reasonably explained by de Leeuw et al. (2007, EPSL) as resulting from incomplete decarbonation of subducting sediment and AOC, fluid flow patterns expected given sediment section thickness, and varying degrees of forearc underplating. The attempts to mass-balance C and N across individual arc-trench systems demonstrate valuable integration of information from geophysical, field, petrologic, and geochemical observations. Studies of subduction-zone metamorphic suites can yield constraints on the evolution of deeply subducting rocks and the physicochemical characteristics of fluids released in forearcs and contributing to return flux in arc volcanic gases.
U51C-02
Fluid Venting Along the Central Chilean Continental Margin: A Geophysical Perspective
The Central Chilean subduction zone between S35 and S37 was recently investigated during a cruise with RV James Cook in order to identify, document and possibly understand fluid flow and fluid venting within the forearc region. Several areas were mapped using multibeam bathymetry, multibeam backscatter, high- resolution Sidescan sonar, Chirp subbottom profiling and reflection seismic data. First analysis of the dataset shows surprisingly little indications for fluid venting along the mid-slope region, which is different from other subduction zones such as Central America or New Zealand. In particular, accretionary ridges between S35.5 and S36 were suspected to show fluid venting, but this fact could not be confirmed with our dataset. Within the closely investigated area abundant indications for fluid venting are observed along the mid-slope between S36.5 and S36.8 along the seaward margin of an intra-slope basin. Here, backscatter anomalies indicate widespread authigenic carbonate precipitation, which probably resulted from the expulsion of methane-rich fluids. We speculate that these fluids are of biogenic origin and generated within the slope sediments, but this will likely be confirmed during a sampling cruise in the coming year. However, similar patterns are reported from the accretionary margin off New Zealand, while the erosive margin off Central America shows venting of fluids derived from the subducting plate. Subsequent analysis of the data might show that this is a common difference between accretionary and erosive active margins.
U51C-03
Decarbonation of the Subducting Pacific Plate Triggered by the Lawsonite-to-Epidote Transition Beneath the Mariana Forearc Serpentinite Mud Volcanoes
A band of serpentinite mud volcanoes in the outer half of the Mariana forearc provides a unique view into conditions, processes, and fluxes in the shallowest part of a subduction zone, to depths of ~25 km. These large mud volcanoes, up to 2 km high and 50 km across, are abundant along a 600-km swath from 13°47'N to 19°33'N and from 50 to 90 km behind the trench. They form when water generated by dehydration of the subducting Pacific Plate ascends into the overlying mantle of the Philippine Plate and converts it to serpentinite. This low-density rock then rises buoyantly along fractures and extrudes at the seafloor, usually as a point source, producing a mud volcano with a central conduit that is narrow relative to the diameter of the volcano. This conduit feeds flows of unconsolidated sedimentary serpentinite that comprise the bulk of the seamount and contain variably serpentinized clasts of harzburgite ranging in size from silt to boulders. The upwelling serpentinite brings up fragments of subducted ocean crust metamorphosed in the blueschist facies. Also rising up are the aqueous fluids generated during, and responsible for, this metamorphism and serpentinization, that exit the seafloor as springs on the summits of the mud volcanoes. Because depleted harzburgite is much simpler chemically and mineralogically than most igneous rocks, these upwelling pore waters retain a clear chemical signal of their deep metamorphic origin in spite of their long ascent. The ascending fluids are all fresher than seawater because of slab dehydration. Their chemistry varies abruptly with distance: near the trench, at 48-54 km, they have pH 10.7, much higher Ca and Sr than seawater and much lower alkalinity, sulfate, Na/Cl, K, Rb, and B. Farther from the trench, at 70 to 90 km, the waters have pH 12.5 and show the opposite trends relative to seawater for all of these species. Sulfate, Na/Cl, K, Rb, Cs, and B all increase regularly with distance from the trench, leached from the subducting sediment and altered basalt in response to increasing temperature at depth from ~100- 350°C. The pH, alkalinity, and methane content of the springs increase abruptly with distance because carbonate dissolution joins dehydration as a major process at the top of the subducting plate. Because serpentinization during ascent generates both high pH and H2, the resulting dissolved carbonate is reduced to methane such that carbonate alkalinity is replaced by hydroxyl alkalinity: 4H2 + CO3= = CH4 + H2O + 2OH-. This reaction accounts for the much higher pH of the distal springs and the fact that most of the ascending C is as methane rather than dissolved carbonate. The abrupt transition occurs ~70 km from the trench, where metabasites recovered from the serpentinite mud indicate the transition from lawsonite-blueschist to epidote-blueschist facies also occurs. Thermodynamic modeling indicates that this transition triggers decarbonation of the subducting slab, as replacement of lawsonite by epidote drastically depletes the solution in Ca and shifts the equilibrium toward massive dissolution of subducted carbonate. Fluxes of sulfate, C, Na, K, Rb, Cs, B, Ca, and Sr in the forearc springs represent only a few percent of the amounts subducted, consistent with continued supply at greater depth.
U51C-04
Fast Breakdown of Hydrous Minerals may Trigger Intermediate-depth Earthquakes in subduction Zones
Earthquakes are located along a double seismic zone (DSZ) in many subduction zones. Dehydration embrittlement is the mechanism usually proposed to explain these earthquakes, as the pressure-temperature (P,T) location of some hydrous minerals breakdown reactions coincide with intermediate-depth hypocenters. This model assumes that the release of water during dehydration reactions is fast enough to cause hydraulic rupture. Therefore, we have measured the kinetics of dehydration of antigorite, talc and 10Å phase at (P,T) conditions relevant to subduction zones. In situ real-time measurements were performed by X-Ray Diffraction (XRD) at high-P and high-T in the Paris- Edinburgh press at the European Synchrotron Radiation Facility (ESRF, ID27). Hydrous minerals were loaded in a transparent, pressure-sealed titanium capsule that ensured a closed thermochemical system. P and T were calculated from the unit-cell parameters of Au and NaCl, using the EoS cross calibration method. During dehydration, (P,T) conditions were kept constant and XRD patterns were acquired every 3 min. The extent of dehydration as a function of time was monitored by the variation in intensity of characteristic diffraction lines of both the hydrous phase and the reaction products. The fit of the isothermal kinetics data to the Avrami's equation and the texture of the reaction products after quench confirm that dehydration rates are limited by diffusion, as expected if a fluid is present. Minimum rates of fluid production during dehydration vary from 10-4 to 8.10-6 m3fluid.m-3rock.s-1. Such rates are definitely faster than the characteristic relaxation rate of the weakest mineral in subduction slabs, calculated as the inverse Maxwell relaxation time of antigorite. Hence, the sudden release of water by dehydration of antigorite, talc and 10Å phase is fast enough to provoke hydrofractures. We also show that rates of fluid release are comparable between antigorite, talc and 10Å phase. Consequently, we suggest that dehydration of all hydrous minerals may be fast enough to trigger earthquakes, and that the deepest among intermediate-depth earthquakes may actually locate the dehydration limit in the downgoing lithosphere.
U51C-05 INVITED
The distinct hydrogeological system of the forearc of the Middle America Trench: significance for long-term tectonics and updip limit of the interplate seismogenic zone.
The distribution and flow of fluid has been widely studied at accretionary prisms, but at convergent margins where tectonic erosion affects overriding plates fluid distribution and tectonics are far less understood. Observations along the erosional subduction zone of Middle America Trench indicates a hydrogeological system distinctly different from those that have been described at accretionary prisms. The hydrogeological system has been studied by: 1) compiling an inventory of focused seepage sites at the seafloor using a multiscale mapping approach, sequentially applying methods of increasing spatial resolution during successive ship cruises, 2) mapping the relative distribution of fluid at the plate boundary with seismic data, and 3) calculating the forearc fluid budget after estimating flow rates from thermal structure and pore fluid chemistry. Most fluid originally contained at the plate boundary migrates by focused flow across a fractured overriding plate, contrasting with conceptual models of accretionary margins where the decollement has been inferred to be the main fluid flow conduit. The distribution of fluid created by the hydrogeological system influences the locus of long-term tectonic erosion determining which areas of the margin are tectonically thinned. Where fluid is more abundant along the plate boundary, the overriding plate is being actively thinned, and fractures and subsides to form the continental slope. Also, the transition with depth from aseismic to the area of nucleation of earthquakes along the plate boundary appears related to fluid distribution. Earthquakes nucleate where fluid appears to be less abundant indicating a first order control on subduction zone thrust earthquakes.
U51C-06
The Thermal Structure of the Costa Rica Margin along the Middle America Trench
The thermal structure of convergent margins provides critical information related to the natural hazards and geodynamics associated with active plate boundaries. We are integrating in-situ thermal data, bottom simulating reflectors (BSRs) imaged with seismic data, data from ODP cores, and numerical models of subduction to estimate the shallow thermal structure along the Middle America Trench off shore Costa Rica. Seismic reflection data from two cruises, R/V Sonne SO-81, and BGR-99, show that BSRs are widespread between the lower slope and shelf edge. These BSRs reflect the temperature at the base of the hydrate stability zone and are used to estimate geothermal gradients along the margin. Thermal conductivity data comes from both in-situ probe measurements and needle probe measurements on ODP cores. Comparisons between in situ and BSR derived values of heat flow are good and improve landward with distance from the deformation front likely reflecting the influence of fluid flow at the trench. Heat flow values across the margin generally decrease landward consistent with subduction. Along the margin heat flow generally increases from north to south and likely reflects the advective fluid flow and/or active deformation. Additional heat flow data seaward of the trench is being used to initialize thermal models of subduction. Estimates of the thermal structure of the margin are coupled to a conduction-advection thermal model of subduction and temperature estimates of the subduction thrust are investigated.
U51C-07
Seismic velocity structure of subducting Pacific Ocean slab near Japan trench deduced by airgun-OBS surveys
Recent seismic studies around the Japan trench subduction zone have revealed that both the interplate and intraplate earthquakes occur under strong influence of fluid, in particular the water. Although it is believed that the subducted slab is the most significant carrier of the water into the earth interior, it is not fully understood how the water is transported into the slab before it starts to subduct. Extensive faulting forming the horst-graben structure along the outer slope of the trench can provide the sea water with pathways. If the water is transported enough deep into the slab, the mantle is expected to reduce its seismic velocity due to the serpentinization. The purpose of our study is to search for variations in the uppermost mantle of the Pacific plate near the Japan trench. In order to estimate the seismic velocity of the subducting slab, we conducted two seismic reflection/ refraction experiments along the trench: one is inner-trench (IT) line located at the bottom of the landward slope with the length of 348 km and the other is line OR with 115 km length set in the outer-rise area, where a normal faulting earthquake with M7.0 occurred in 2005. These profiles are almost parallel to the trench axis. The distance from the trench axis to IT and OR is 20 km and 80 km, respectively. We used a 2-D ray tracing method (Zelt and Smith, 1992) for traveltime analysis with a try and error approach. In our result, Pn velocity of the uppermost mantle in the outer-rise area is estimated to be 7.9 km/s. This value is slightly lower than that observed at the site WP2, in the middle of the NW Pacific basin (Shinohara et al, 2008), where no hydration of the uppermost mantle is expected. On the other hand, Pn velocity obtained from the survey made along the landward slope is 7.7 km/s. Although our result does not show velocity variations across these three sites in continuous manner, our result suggests that the Pn velocity is already reduced at the outer-rise area and that the further reduction of Pn velocity keeps going near the axial part of the Japan trench. Since the horst-graben structure is less developed to the east of the outer-rise site, we assume that such a velocity reduction reflects the hydration process of the uppermost mantle: the water infiltrates into the mantle by way of the normal faults, which are continuously developing toward the trench.
U51C-08
P-wave Velocity Modeling Using Onshore/Offshore Passive Seismic Networks Along the Middle America Subduction Zone, Costa Rica and Nicaragua
The Middle America subduction zone along Costa Rica and Nicaragua transitions from subduction of thicken oceanic crust associated with the Cocos Ridge in southern Costa Rica to subduction of thinner, but pervasively faulted and hydrothermally cooled, crust along Nicaragua. The overlying volcanic arc also exhibits a high degree of geochemical and spatial variability along strike. Here, we combine arrival time information from five onshore/offshore passive networks deployed along this margin from 1999-2006 to image along-strike changes in P-wave velocity and shallow subduction zone background seismicity. We combine data collected along southern and northern Costa Rica as part of the NSF sponsored Costa Rica Seismogenic Zone Experiment and along central Costa Rica and Nicaragua as part of the German SFB 574 program, yielding ~600 km of along-strike coverage of the subduction seismogenic zone. We test multiple earthquake tomography methods, including double-difference techniques, and find that the combined dataset improves velocity resolution across major tectonic transitions that fall along the individual network boundaries. For example, the Nicaragua network recorded many more events near the trench at depths consistent with oceanic plate or ocean mantle sources, while seismicity under northern Costa Rica occurs primarily along the plate interface above 40 km depth. Low VP and high VP/VS imaged in the forearc mantle wedge and oceanic slab along Nicaragua also transition to more normal values imaged by the northern Costa Rica network. Both transitions are better resolved using the combined dataset, while initial velocity results within each network are consistent with previously published local earthquake tomography studies using those networks. The revised velocity and hypocenter results will be compared with recent heat flow and pore fluid pressure estimates to better constrain how fluids influence seismogenesis along this margin.