HR: 12:05h
AN: V12A-08    [Abstracts]
TI: Subduction Dynamics and Mass Transfer: A Synthesis Model
AU: * Baker, L J
EM: labaker@gps.caltech.edu
AF: California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125 United States
AU: Hall, C E
EM: chall@gps.caltech.edu
AF: Seismological Laboratory, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125 United States
AU: Asimow, P D
EM: asimow@gps.caltech.edu
AF: California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125 United States
AU: Smith, P
EM: psmith@gps.caltech.edu
AF: California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125 United States
AU: Gurnis, M
EM: gurnis@gps.caltech.edu
AF: Seismological Laboratory, California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125 United States
AB: A full understanding of arc volcanism requires a synthesis of several key aspects, including slab and wedge mineralogy, dehydration reactions, water addition to the wedge, and melt generation within the wedge. Separate models exist to explain parts of the problem that independently address, for example, the thermal regime, the phase relations as a function of pressure and temperature, or the solid viscosity and melt properties as a function of major and trace element abundances. However, it is necessary to fully combine these separate models into a self-consistent, adaptable system that allows conclusions to be drawn simultaneously about both dynamics and mass transfer. We present a model that combines the petrological model pHMELTS with a 2-D thermal and variable-viscosity flow model of a subduction zone. We are able to define the thermal state and phase equilibria of the subducting oceanic slab and constrain a fluid flux into the wedge. This allows us to describe the process of hydration of the mantle wedge adjacent to the slab, including hydrous mineral stability, water content in olivine, melt generation, and physical properties of the wedge such as viscosity and density. This is the first step leading to predictions of major element chemistry and evolution of arc volcanics, thus providing a critical connection between model and observation. We have examined two end-member examples: slab thermal ages of 25 and 100 Myr, with a convergence rate of 3 cm/yr and a slab dip of 45\deg. We adopt an iterative approach to find compositional fields consistent with a given thermal model, updating the viscosity field to reflect its dependence on water content (1), and obtaining an updated solution of the velocity and temperature fields. We assume that vertical advection of fluid is instantaneous relative to the time-scale of solid advection (3). Allowing the viscosity to be both compositionally and thermally dependent permits a consistent linkage between the effect of water-addition on the mantle wedge and the wedge velocity field, possibly leading to large-scale changes in the flow field of the hydrated wedge relative to the anhydrous estimate. The importance of combining the influence of water with iterative viscosity calculations within a single model is obvious when considering the implications of the low-viscosity wedge scenario proposed by (4). We observe similar results to previous investigations (2), including the effect of the variable viscosity model on the thinning of the isotherms immediately above the slab, producing a hotter regime immediately adjacent to the slab as compared to the isoviscous case. 1) Hirth, G. & Kohlstedt, D. L. (1996) Earth Planet. Sci. Lett., 144 (1-2), 93-108. 2) Van Keken, P.E. et al. (2002) Geochem, Geophys, Geosyst., 3 (10), 1056, doi:10.1029/2001GC000256. 3) Scott, D.R. & Stevenson, D.J. (1989) J. Geophys. Res., 94, 2973-2988. 4) Billen, M.I. & Gurnis, M. (2001) Earth Planet. Sci. Lett., 193, 227-236.
DE: 1030 Geochemical cycles (0330)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting