HR: 09:00h
AN: T41F-05    [Abstracts]
TI: Seismic evidence for massive silica addition in some subduction zones
AU: * Abers, G A
EM: abers@bu.edu
AF: Boston University Department of Earth Sciences, 685 Commonwealth Av, Boston, MA 02215 United States
AU: Rossi, G
EM: giovanni@bu.edu
AF: Boston University Department of Earth Sciences, 685 Commonwealth Av, Boston, MA 02215 United States
AU: Rondenay, S
EM: rondenay@mit.edu
AF: Department of EAPS, Massachusetts Institute of Technology, Cambridge, MA 02139 United States
AU: Peacock, S
EM: simon.peacock@asu.edu
AF: Department of Geological Sciences, Arizona State Univ., Tempe, AZ 85287 United States
AU: Chrsistensen, D H
EM: doug@giseis.alaska.edu
AF: Geophysical Institute University of Alaska Fairbanks, 903 Koyukuk Dr., Fairbanks, AK 99775 United States
AB: Subduction zones alter the shallow mantle composition in a variety of ways, including melt removal, addition of metasomatic fluids, and tectonic mixing. These processes should produce distinctive signatures in seismic structure. The mantle wedge beneath many volcanic arcs is characterized by high attenuation (1/Q), low Vp, and high Vp/Vs, all consistent with high temperatures, the presence of small degrees of melt, and perhaps the presence of aqueous fluid. However, at least two subduction zones are characterized by high 1/Q, low Vp, but low Vp/Vs, in Central Alaska and the central Andes flat-slab segment (Wagner et al., 2004 JGR). The Alaska results, presented here, are derived from inversion of receiver functions for the Broadband Experiment Across the Alaska Range (BEAAR). New stacking techniques solve for interface depths and Vp/Vs, while accounting for dipping layers. Where the slab is deeper than 80 km, the overlying mantle wedge exhibits Vp/Vs of 1.62 - 1.68. Travel-time tomography confirms the low Vp/Vs. This is lower than predicted for any common mantle mineral (1.73-1.85), and lower than commonly measured in the mantle. Calculations show that isolated pores filled with a highly-compressible fluid can lower Vp/Vs slightly below that of the surrounding matrix, but can reach these low Vp/Vs only with physically absurd parameters (e.g. 10-20% porosity containing vacuum). Of the major rock-forming minerals, only quartz has a low Vp/Vs (Poisson's ratio). Thus, we suggest that some parts of these mantle wedges contain significant quantities of quartz. High quartz, and high overall silica, is a possible consequence of extreme metasomatism, or of mechanical addition of quartz-rich crustal rocks into the mantle during previous subduction events. Estimates of the fluid flux out of subducting slabs and silica solubilities rule out the possibility of large regions of the mantle wedge being silica oversaturated by metasomatism. In contrast, mechanical mixing of crust and mantle has been observed on the meter to kilometer scale in melange zones. During Alaskan terrane accretion events, large crustal slices may have been incorporated into the mantle wedge and may represent a significant mass-flux of silica into the mantle. Past sources of silica are less clear in the Andes, but tectonic erosion may be important. Perhaps, such silica addition is a common consequence of tectonic events at subduction zones.
DE: 3621 Mantle processes (1038)
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005