HR: 16:15h
AN: V44B-02    [Abstracts]
TI: Are Subducting Slabs Wet or Dry in the Transition Zone? Comparisons of Seismicity and Model Predictions
AU: * Green, H W
EM: harry.green@ucr.edu
AF: Department of Earth Sciences and IGPP, University of California, Riverside, CA 92521, United States
AU: Brudzinski, M R
EM: brudzimr@muohio.edu
AF: Miami University, E. Miami University St., Miami, OH 45056, United States
AU: Chen, W
EM: wpchen@uiuc.edu
AF: Univ. of Illinois, Urbana-Champagne, Dept. Nat'l History and Geology 1301 W. Green St., Bldg. 245, Urbana-Champagne, IL 61801, United States
AB: We compare distribution of seismicity with what might be expected for dry and wet models of subducting slabs interacting with the mantle transition zone. For depths between about 100 to 250 km, earthquake distribution matches closely with where breakdowns of major hydrous minerals in the oceanic crust and mantle are expected -- a result consistent with dehydration embrittlement as the nucleation mechanism for intermediate-focus earthquakes. Below about 250 km, however, such mechanisms would have been exhausted; and correspondingly, at depths near 300 km, the frequency of earthquakes drops rapidly to a minimum of only one per year per 10 km interval in depth globally. In the "dry" model, disappearance of most H2O from the slab leads the way for transformation-induced faulting during the olivine-wadsleyite and olivine-ringwoodite reactions whose kinetics are temporally retarded by low temperatures in slabs. This scenario naturally explains abrupt termination of all deep earthquakes near the base of the transition zone, and is consistent with petrologic anomalies recently found in the transition zone beneath Tonga-Fiji and the Mariannas. In the "wet" model, earthquakes are essentially absent between depths of 250-350 km because H2O is bound into phase A and/or dissolved in olivine and pyroxene. Subsequently H2O is carried passively into the transition zone where it fluxes the olivine-wadsleyite reaction with little overstep of the reaction boundary and/or is passed from phase A to E and then to D. At greater depths, however, predictions from this model cannot reproduce observed patterns of deep earthquakes. As olivine reacts to form wadsleyite, the solubility of H2O becomes much higher, allowing for no free fluid and hence no earthquakes from dehydration. If transient release of H2O should occur, a flurry of earthquakes would be expected just below the "410 km" discontinuity (which occurs at depths of perhaps 300-380 km in slabs), at greater depths H2O would either remain dissolved in nominally anhydrous phases (hence not involved in triggering earthquakes at all), or it might conceivably generate some earthquakes but only at depths of ~500km and ~650 km as free H2O is transiently released during reactions to increasingly hydrous phases such as E and D. In places where some slabs may pass into the lower mantle (as in the Mariannas), breakdown of hydrous ringwoodite would be expected to release H2O, inducing a flurry of earthquakes in the uppermost lower mantle just below the 660-km discontinuity; or hydrous ringwoodite could be consumed in part by Phase D which would then be expected to release H2O and generate earthquakes at around depths of 900 km in the lower mantle. Either way, such predictions call for earthquakes to occur beneath the 660-km discontinuity where not a single earthquake has been observed.
DE: 3613 Subduction zone processes (1031, 3060, 8170, 8413)
DE: 3660 Metamorphic petrology
DE: 5120 Plasticity, diffusion, and creep
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting