HR: 1340h
AN: V43D-1637    [Abstracts]
TI: Coupled Petrological and Geodynamic Models of Mantle Flow in Subduction Zones; the Importance of Chlorite in the Emergence of a Low-Viscosity Channel
AU: * Smith, P M
EM: psmith@gps.caltech.edu
AF: GPS Caltech, 1200 E. California Blvd., Pasadena, CA 91125, United States
AU: Baker, L J
EM: labaker@gps.caltech.edu
AF: GPS Caltech, 1200 E. California Blvd., Pasadena, CA 91125, United States
AU: Asimow, P D
EM: asimow@gps.caltech.edu
AF: GPS Caltech, 1200 E. California Blvd., Pasadena, CA 91125, United States
AU: Gurnis, M C
EM: gurnis@gps.caltech.edu
AF: GPS Caltech, 1200 E. California Blvd., Pasadena, CA 91125, United States
AB: Seismic velocity and attenuation studies have shown that 5-20 km thick low velocity layers exist above seismically fast slabs and are associated with broad zones of high attenuation in many subduction zones. These observations are generally interpreted as formation of hydrous phases by dehydration of the slab, although the impact of water in nominally anhydrous minerals (NAM) on seismic wave propagation is largely unknown. Recent petrological experiments on hydrous peridotite at subduction zone conditions suggest that chlorite will be stable adjacent to the subducting slab in sufficient quantities to be a significant water sink. We use a scheme that couples a petrological model (pHMELTS) with a 2-D thermal and variable viscosity flow model (ConMan) to model energy and mass transfer within a subduction zone. By varying input parameters including the convergence rate and slab dip we have developed models for cases in the Costa-Rica and Izu- Bonin-Marianas arc systems and are able to predict major and trace element compositions of primary melts, as well as geophysical observables, such as the topography and geoid. We find that the emergence of a slab- adjacent low-viscosity channel (LVC) is a natural consequence of the thermal and chemical controls on mantle dynamics and feedback between them. In our earlier models, as the LVC is dragged downwards by the subducting slab, hornblende breaks down at about 2.5 GPa and other hydrous phases such as serpentine are secondary in importance to the NAM water reservoir. The spatial limit of the LVC is the water-saturated solidus of the hydrated peridotite; the LVC thickens as the peridotite is progressively depleted by melting and the solidus migrates into the warmer wedge, despite water replenishment at depth. pHMELTS is a hybrid of the pMELTS model of Ghiorso and co-workers and includes amphiboles, serpentines and micas. Chlorite was lacking but we have recently rectified this omission. Following De Capitani and co- workers, we have chosen the tri-octahedral FeMg-chlorite model of Hunziker [2003, PhD thesis, University of Basel] for its relative simplicity, suitable P-T-X experimental calibration for modeling igneous systems and internal consistency with the Berman thermodynamic dataset. For incorporation into the MELTS family of algorithms the four end-member speciation model must be reformulated as a three end-member solution with an ordering parameter, appropriate interaction parameters and analytical partial derivatives up to third order. When chlorite is included as a potential phase in the coupled scheme a persistent layer of the mineral forms extending from the location of the shallowest water source to at least 5 GPa. Preliminary results suggest that the LVC no longer thickens with depth as described above and melting is restricted to shallower depths. These effects have implications for the chemistry of extracted arc melts, predicted topography and geoid signal, migration of water towards the back-arc basin and the fate of hydrated material as subduction proceeds. Chlorite is the dominant hydrous phase, forming up to 25 wt% of the hydrous assemblage, but the NAM account for 10% or more of the water budget within the LVC. At pressures greater than 5 GPa, chlorite should become unstable and the NAM are likely to be important in transport of water towards the transition zone.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1042 Mineral and crystal chemistry (3620)
DE: 3611 Thermodynamics (0766, 1011, 8411)
DE: 8120 Dynamics of lithosphere and mantle: general (1213)
DE: 8178 Tectonics and magmatism
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting