HR: 16:45h
AN: V34A-04 INVITED     [Abstracts]
TI: The Contribution from Hot, Subducted Lithosphere to Mantle Wedge: Melt or Fluid?
AU: * Grove, T L
EM: tlgrove@mit.edu
AF: Massachusetts Institute of Technology, Dept. EAPS, 54-1220, 77 Mass. Ave., Cambridge, MA 02139 United States
AB: In the Mt. Shasta region, N. Calif., USA, primitive basaltic andesites and andesites (similar to adakites) preserve a remarkable record of subducted lithosphere and mantle wedge elemental contributions. Estimates of pre-eruptive water contents allow the development of models of magma generation. When combined with a mantle melting model, one can characterize the chemical composition of the subducted slab contribution. Mt. Shasta lies above the young Juan de Fuca plate, where a hot slab environment has been proposed for the origin of the lavas. Melting is modeled as a process where an initial melt is formed in the mantle wedge above the slab by vapor-saturated melting of peridotite, metasomatized and enriched by the slab-derived melt or fluid. Vapor-saturated melting leads to the production of a water-rich melt (25-30 wt. % H₂O) that ascends into the overlying mantle and continuously reacts as it encounters hotter, shallower mantle. The melt fraction increases, the water content decreases and the slab contribution is modified and diluted. The result is a flux melt whose major elements are dominantly derived by mantle melting and whose trace elements and isotopic characteristics reflect the subducted oceanic lithosphere. Two distinct sources are indicated by Sr, Nd and Pb isotopic evidence: a MORB and a sediment source. When the major element signature of the mantle wedge is removed, the slab contribution more closely resembles a low degree melt (2 to 5 wt. %) of a garnet + clinopyroxene source. The subducted component is less similar to experimental fluids equilibrated with eclogite. Although the LIL elements are a good match with a fluid, the model abundances of rare earth elements (Ce, Sm and Yb) are low by several orders of magnitude. Thus, a melt seems a better fit at Mt. Shasta based on our current state of understanding of melt vs. fluid equilibrium in the deep subduction environment.
DE: 3619 Magma genesis and partial melting (1037)
DE: 3690 Field relationships (1090, 8486)
DE: 8410 Geochemical modeling (1009, 3610)
DE: 8412 Reactions and phase equilibria (1012, 3612)
DE: 8413 Subduction zone processes (1031, 3060, 3613, 8170)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005