HR: 08:00h
AN: V11C-01    [Abstracts]
TI: Lateral Offset of the Volcanic Front: Implications for Fluid Pathways in the Mantle Wedge
AU: * Feineman, M D
EM: feineman@uclink.berkeley.edu
AF: University of California, Dept. of Earth & Planetary Sci. MC 4767, Berkeley, CA 94720 United States
AU: * Feineman, M D
EM: feineman@uclink.berkeley.edu
AF: Lawrence Livermore Natl. Lab., Earth Sciences Division PO Box 808, L--206, Livermore, CA 94551 United States
AU: Ryerson, F J
EM: ryerson1@llnl.gov
AF: Lawrence Livermore Natl. Lab., Earth Sciences Division PO Box 808, L--206, Livermore, CA 94551 United States
AU: DePaolo, D J
EM: depaolo@eps.berkeley.edu
AF: University of California, Dept. of Earth & Planetary Sci. MC 4767, Berkeley, CA 94720 United States
AU: DePaolo, D J
EM: depaolo@eps.berkeley.edu
AF: Lawrence Berkeley Natl. Lab., Earth Sciences Division MS 90R-1116, Berkeley, CA 94720 United States
AB: One of the most striking common features of subduction zones worldwide is the appearance of the volcanic front at a height of approximately 120 km above the subducting slab. The water-rich compositions of the lavas erupted at the volcanic front suggest that melting is initiated by dehydration of hydrous phases in the slab, primarily amphibole. However, the location of the front is offset considerably from the predicted origin of fluids due to amphibole dehydration at $\sim$80km slab depth. The lateral offset at the surface varies with subduction angle, but generally the predicted site of fluid release is $\sim$20-70 km trench-ward of the actual volcanic front. Many of the proposed mechanisms for generating this offset involve stalling the fluid in the mantle such that it is drawn down and/or back into the mantle wedge due to viscous flow in the solid mantle. For example, the fluid may re-crystallize as phlogopite and pargasitic amphibole in the portion of the mantle that is viscously coupled to the subducting slab. These newly formed hydrous minerals have higher breakdown pressures than glaucophane, the dominant hydrous mineral in the slab, and could explain the offset to deeper apparent depths of dehydration. However, processes that rely upon solid mantle flow are very slow. For a slab descending at 100mm/yr, 5x10$^{5}$ years are required to descend 40 km vertically ($\sim$50 km along-slab). Such long periods of time spent in transport in the mantle are seemingly contradicted by strong U-series isotopic disequilibria in arc lavas. Although special circumstances may be evoked in order to allow U-series disequilibria to be extended in time, it is also possible that reaction rates in the cold descending slab are sluggish to the point that fluids are not released at the expected depth of 80 km, but instead are retained to greater depths where increasing temperatures allow reactions to proceed. In this scenario, the fluids would then be able to proceed relatively quickly to the region of melting, preserving their isotopic disequilibria.
DE: 1025 Composition of the mantle
DE: 1065 Trace elements (3670)
DE: 1212 Earth's interior--composition and state (8105)
DE: 1213 Earth's interior--dynamics (8115, 8120)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting