HR: 09:45h
AN: T31F-08    [Abstracts]
TI: Water in Mantle-Derived Melts: Constraints on the Mantle Water Cycle
AU: * Dixon, J
EM: jdixon@rsmas.miami.edu
AF: RSMAS/Univ. of Miami, 4600 Rickenbacker Cswy., Miami, FL 33149 United States
AB: I review mantle water concentrations derived from study of mantle-derived melts. Average MORB mantle contains about 120 ppm H$_{2}$O, with estimates ranging from about 50 (NMORB) to 450 (EMORB) ppm. Mantle sources of oceanic island basalts (OIB) typically have water concentrations (about 400 to 1000 ppm) greater than MORB. Deep mantle sources with water concentrations greater than 1000 ppm have not been found. The amount of residual water that survives the "subduction gauntlet" and is retained in the down-going slab to enter the deep mantle has until recently been quite uncertain. Measurements of volatile contents of OIB containing components of previously subducted lithosphere (EM-type) contain roughly half the water found in other OIB mantle sources (FOZO-type), indicating that water is efficiently extracted during the subduction process. Thus, water in the subducted slab must be expelled through fluid venting in the fore-arc, expelled during island arc magmatic processes, or sequestered in the hydrated, overlying mantle wedge. Effective partitioning of water into the exosphere results in depletion of water relative to other incompatible elements in the recycled lithosphere, accompanying continental-derived materials, and ultimately the mantle. Water concentrations in arc and back-arc lavas and in melt inclusion in phenocrysts confirm that the source regions for arc lavas are anomalously wet (from 250 ppm to 1 wt% H$_{2}$O) compared to MORB and OIB sources. The question to be answered is: If the transition zone contains from 0.5 to 1 wt% H$_{2}$O, as suggested by solubility experiments and seismic velocities, why don't we see any evidence for highly hydrous, deep mantle-derived melts?
DE: 3015 Heat flow (benthic) and hydrothermal processes
DE: 1025 Composition of the mantle
DE: 1030 Geochemical cycles (0330)
DE: 1213 Earth's interior--dynamics (8115, 8120)
DE: 1010 Chemical evolution
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting