Over-Arching Themes:

(See Science Plans, page 67)

• Elucidate the effect of “forcing functions” on magma & fluid production from trench to back-arc
• Determine how and where volatiles affect and cycle through subduction zones
• Determine the chemical and mass balance across subduction zones and its relationship to the growth of the continents

Methods of Studying SubFac Processes:

(See detailed discussion in Science Plans, page 78)

Focus Areas and Rationale:

• General Site Criteria
  • Active volcanic arc
  • Tectonic steady-state
  • Variable and well-characterized forcing functions
  • Abundant mantle-derived magmas
  • Logistically reasonable for geochemical and geophysical sampling of inputs and outputs of the system including seismic and drilling at a variety of scales
• Central America
  • Dramatic along-strike variations in magma chemistry
  • Well-characterized variations of input, which often mimic variation in output
    • Moderately young incoming plate
    • Carbonate subduction and evidence for high volatile output
• Subaerial exposures of basement and igneous rocks Miocene-modern
• Direct link to SEIZE site
• Strong local support and international collaboration
• Izu-Bonin-Marianas (IBM)
  • Back-arc basins present
  • Along-strike and across-strike variations in magma chemistry
  • Entire magmatic history of arc and back-arc volcanism is accessible
  • Old, cold incoming plate
  • Documented fluid egress from trench to back-arc
  • Vp = 6kms -1 mid crustal layer evident in region
  • Carbonate-absent input
  • Along-strike and temporal changes in sediment thickness and composition
  • Exposed forearc serpentinite mud volcanoes sample the lower-plate and hydrated shallow mantle
  • Strong local support and long-term Japanese collaborations

Workshops and Theoretical Earth Institutes (TEIs):

• Subduction Factory Workshop, La Jolla, 1998
• Inside the Subduction Factory Theoretical Earth Institute, Eugene, 2000. Publication: Inside the Subduction Factory , AGU Geophysical Monograph 138, 2003
• Central America Workshop, Heredia, Costa Rica, 2001
• Izu-Bonin Marianas Workshop, Honolulu, 2002
• Geodynamical Modeling Workshop, Ann Arbor, 2002
• Field workshop on volatiles, Costa Rica and Nicaragua, 2003

Major Research Activities (MARGINS funding 1999-present):

Input at the Trench:

• TicoFlux dense heat flow survey of Cocos Plate (CA) (Stein et al.)
• ODP drilling of subducting crust (IBM and CA)
• Chemical and mineralogical characterizations of inputs (IBM and CA) (Plank et al.; Brown)

Output at the Arc:

• Chemical and mineralogical compositions of modern mantle-derived magmas and their derivatives (IBM and CA) (Chan; Kent; Plank et al.; Stern; Eiler)
• Volatile fluxes and slab dehydration (IBM and CA) (Fehn et al.; Fehn et al.; Fischer; Fischer; Fischer et al.; Plank et al.; Rudnick & McDonough)
• History of volcanic outputs (IBM and CA) (Carr et al.; Hanan et al.; Reagan et al.)
• Volcanological, geochemical, and geophysical studies of the 2003 eruption of Anatahan (Fischer et al.)

Geophysical Studies:

• U.S.-Japan integrated hydroacoustic, active and passive seismic imaging experiments (IBM) (Taylor et al.)
• Seismic and ROV imaging of serpentine volcanoes (IBM)
• Broadband passive seismic imaging in CA (starting 2004) (Abers et al.)
• SEIZE-related passive seismic and GPS imaging in Costa Rica (Schwartz et al.; Dixon et al.)
• Reflection imaging of forearc and incoming plate (CA, IBM) (McIntosh)
• Seismic anisotropy from global seismic data (IBM) (Fouch)

Experimental and Theoretical:

• Experimental investigations of basalt generation in subduction zones (Grove et al.; Plank et al.)
• Phase equilibria in carbonate-bearing and carbonate-lacking sediments
• Modeling the role of upwelling for magma production in the mantle wedge (van Keken)
• Modeling how more sophisticated/realistic rheologies (e.g. temperature and stress-dependent viscosity) influence thermal structure and melting (Conder & Wiens)
• Modeling and experimental studies on slab geometry and feedbacks between slab kinematics and mantle flow/melting (Hirth et al.; Kincaid; Fouch)

Intellectual Progress During MARGINS:

• Input at the Trench:
• Volcanic chemistry correlates with regional and temporal variations in geochemistry of subducting sediment and basalt (IBM, CA)
• Subducting plate could be significantly serpentinized, and contribute large fraction of H₂O delivered to depth
• Vigorous low-T fluid flow in oceanic section of the incoming plate (CA)
• Output at the Arc:
• Volatile output sampled directly from glass inclusions shows extent of flux melting in arc and backarc (IBM, CA)
• History of magmatism (IBM, CA)
• Mantle sources for arc and back-arc volcanism are linked isotopically but do not follow obviously linked flow paths (IBM)
• Boron and oxygen isotopes may indicate that serpentine-derived fluids contribute to arc magmas (CA)
• Identified N in arc magmas as a sensitive tracer of hemipelagic sediments and showed that N is almost entirely recycled by arcs
• Anatahan eruption event response—first results (IBM)
• Discovery of subarc mantle xenoliths (CA)
• Along-strike variations in lava chemistry have persisted since the Miocene, and are moderated by input changes (CA)
• Geophysical Imaging:
• Outer-rise faulting penetrates the mantle, and is preserved beyond the trench (CA)
• Anomalous heat-flow provinces delineated in the Cocos Plate are evidence of a hydrothermal origin of low heat flow (CA)
• Major active and passive-source imaging datasets presently being collected (IBM, active and passive; CA, passive)
• Experimental and Theoretical Advances:
• High-H₂O contents in arc sources are implied by H₂O-saturated phase equilibria for Mariana basalts
• Constraints on trace element partitioning during hydrous mantle melting
• Significant advances in thermal models for subduction zones, showing importance of temperature-dependent viscosity
• Dynamic models can now predict realistic slab geometry

Map (PDF) prepared by the MARGINS Office, showing the available information on where work has been funded to date in the Costa Rica-Nicaragua focus area. (Click map for a larger version with explanatory caption.)

Map (PDF) prepared by the MARGINS Office, showing the available information on where work has been funded to date in the Izu-Bonin-Mariana focus area. (Click map for a larger version with explanatory caption.)

Major Research Gaps:

• General:
  • Samples of dikes and gabbros in incoming plate seaward of trench, including complete volatile and elemental inventory
• In situ tests of the hypothesis of massive serpentinization at the outer rise
• Chemical compositions (especially volatile concentrations) in glass inclusions through the entire evolution of the arc
• Reliable determination of magma and volatile fluxes along the length of the arc front
• Achieve an adequate isotopic database for subducted plate materials ( i.e., one that more closely matches the detail of trace element studies)
• Detailed geology and geochemistry of individual volcanoes to determine patterns of mafic, intermediate, and silicic volcanism along the length of each arc and a chemical mass balance for quantitative comparison with the continental crust
• Izu-Bonin-Marianas:
  • Geochemistry of rock-fragments from the subducted plate found in serpentine volcanoes
  • Paleogeographic and modeling studies of the location of the arc and behavior of backarc spreading with time
  • Analyses of samples collected from basement of arc crust (e.g. sheeted dikes and gabbros of “arc ophiolite”; tonalitic middle crust)  
• Central America:
  • Enhanced database for the chemical compositions of subducted materials
  • Hafnium isotopic compositions of arc inputs and outputs
• Geophysical Imaging:
  • Active-source seismic imaging of arc crustal structure to establish a baseline for crustal growth, composition, and mantle flux (CA)
  • Magnetotelluric imaging of slab and wedge resistivity structure to complement seismic constraints on fluid distribution and connectivity
  • Compilation and integration of gravity and magnetic data
  • Complete heat flow transects of forearc and arc, with which to constrain thermal models
• Modeling and Experimental studies:
  • Quantitative models of the dynamics of fluid flux from the slab to mantle wedge in mantle flow models
  • Constraints on variations in trace element partition coefficients under varying conditions of magma generation
  • Experimental constraints on the conditions responsible for boninitic magma formation
  • Establishing guidelines for differentiating effects on seismic properties of melt, fluid, and composition on seismic properties
  • Geochemical and geophysical constraints on mantle flow through the wedge

Ongoing Scientific Problems Motivated by MARGINS Research:

Work to date highlights the longevity of the scientific themes developed in the Science Plan, and sharpens the focus of many questions.

• Input at the Trench:
  • What is the total (sediment + crust + mantle) input budget of H₂O and CO₂?
  • Does massive serpentinization at the outer rise take place, and if so what is the mechanical process that allows hydration?
  • How significant is tectonic erosion for overall mass balance at the arc, and what controls it?
• Output at Arc and Forearc:
  • What is the rate of arc growth? What is the total output chemical and volatile flux?
  • What controls variations in concentrations of H₂O and tracers for subducted material in arcs? 
    • CA issue: Nicaraguan O isotope signatures that suggest a subduction component derived from deep crustal or serpentinized mantle sources, are seen in the same regions where trace element and ¹⁰Be data suggest a strong sediment component. Why are these two signals closely affiliated and what are the implications for the sources and pathways of fluids?
  • What is the CO₂ output flux, and how does it relate to input?
  • At what temperatures does mantle melting occur, and how much melting takes place?
  • What controls the interaction of hydrous and decompression melting?
  • What process controls long-lived variations in output that are not obviously related to input variations?
• Inner Workings of the Subduction Factory:
  • What is the pathway of fluid from the slab to the melting region? Of magma to the surface?  
  • What is the pattern of solid flow within the wedge, and how can corner flow models be reconciled with seismic anisotropy constraints? What do variations in the locations and spreading directions in back-arc basins tell us about mantle circulation patterns in the back-arc?
  • What controls the locus of the volcanic front and its variations? Volcanic spacing?
    • CA issue: Strong shifts in arc lava composition coincide with the large offset in the volcanic front, but not the “Quesada Sharp Contortion” in the subducting plate. Is the offset in the volcanic front controlled by the upper plate? If so, how does the upper plate exert control on arc magma composition?
  • What is the transition from thrust faulting to wedge flow, and how does that impact the temperature structure at the top of the slab?
  • How consistently depleted is the mantle source for arc magmas?
  • Does melting in the back-arc affect the degree of depletion of the arc mantle source?   What is the spatial and temporal history of paired arc and back-arc mantle evolution and what are the implications for mantle circulation above the subducting slab?

International Collaborations:

• Central America:
  • GEOMAR - active-source imaging offshore, petrology, and igneous geochemistry
  • ETH - collaboration in tomographic imaging
  • OVSICORI/UNA - seismology, volcanology, petrology, volatile geochemistry, and geodesy in Costa Rica
  • INETER - most geophysical exercises in Nicaragua
• IBM:
  • IFREE/JAMSTEC - collaborations in active and passive imaging, submersible and onshore sampling, petrology, igneous geochemistry, volatile geochemistry, magnetotelluric imaging, geodesy, marine data acquisition
  • CNMI Emergency Management Office - response to Anatahan eruption

SF Presentations and Publications:

(See “Research” section)

Nuggets:

(Proposal start dates in parentheses)

• Abers et al. (April, 2002): “Volcanic Origins ILluminated in Costa Rica And Nicaragua (VOLCAN): Imaging the mantle in the Central American Subduction Factory”
• Carr et al. (July, 2002): “Determination of Volcanic Flux Rates and Application to Understanding Regional Geochemical Trends and Element Mass Balances in Central America”
• Chan (August, 2002): “Lithium Isotope Compositions of Volcanic Arc Lavas: a Study of Processes and Fluxes in Subduction Zones”
• Conder & Wiens (July, 2003): “A Numerical Investigation of the Relative Importance of Different Melting Mechanisms at Volcanic Arc”
• Eiler (August, 2001): “MARGINS: Oxygen Isotope Studies of the Central American Arc”
• Fehn et al. (June, 99): “Volatiles in the Central American Volcanic arc: Constraints on Subduction Processes from I-129, C, N, and Noble Gas Determinations”
• Fehn et al. (February, 2003): “Tracing Volatiles Across Volcanic Arcs: 129I, He, N2, CH4 and CO2 in Fluids From New Zealand and Japan”
• Fischer (August, 2000): “The Flux of Volatiles from Active MARGINS: the Central American Volcanic Arc”
• Fischer (January, 2003): “Field Workshop On The Chemistry And Fluxes Of Volatiles From The Central American Volcanic Arc: Nicaragua And Costa Rica”
• Fischer et al. (June, 2003): “Collaborative Research: Determining The Volatile And Slab Flux From The Izu-Bonin-Marianas Active Margin Using Geothermal Fluids, Phenocrysts And Melt Inclusions”
• Fouch (July, 2003): “Mantle Dynamics of the Izu-Bonin Subduction System”
• Hanan et al. (January, 2001): “Collaborative Research: Magma Generation in the Early Mariana Arc System Revisited”
• Hirth et al. (May, 2002): “Convection in the Mantle Wedge Above Subduction Zones”
• van Keken (August, 2001): “Margins: Workshop on Modeling of Subduction Zone Dynamics and Thermal Structure”
• Kent (July, 2003): “Melt Inclusions in Izu Lavas”
• Kincaid (August, 2001): “MARGINS: Laboratory Experiments on 3-D Circulation and Temperature Distribution in Subduction Zones”
• McIntosh (December, 2002): “Structure of the Nicaragua/Costa Rica Subduction Zone: A Framework for the Subduction Factory and Seismogenic Zone Initiatives. U.S./German/Nicaraguan Collaborative Project”
• Grove et al. (September, 2000): “Collaborative Research: Volatiles (H₂0 and CO₂) in Mariana and Izu Arc Magmas”
• Plank et al. (September, 2000):  "Collaborative Research: Volatiles (H₂0 and CO₂) in Mariana and Izu Arc Magmas”
• Plank et al. (March, 2002): “Collaborative Research: Hf-Nd Isotopic and Trace-Element Geochemistry of Globally Subducting Sediments”
• Plank et al. (August, 2000): “Collaborative Research: Volatiles (H₂0 and CO₂) in Mariana and Izu Arc Magmas”
• Reagan et al. (January, 2001): “Collaborative Research: Magma Generation and Tectonics in the Early Mariana Arc System Revisited”
• Rudnick et al. (June, 2002): “Li Isotopic Investigations of the Crust And Mantle”
• Stern (September, 2001): “Mantle Inputs to the Subduction Factory: Assessing Scales of Spatial Variability along and Across the IBM Convergent Margin”
• Taylor et al. (January, 2001): “U.S.-Japan Collaborative Research: Multi-Scale Seismic Imaging of the Mariana Subduction Factory”

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