HR: 11:05h
AN: V52A-04 INVITED     [Abstracts]
TI: Deep Earth Water Cycle: Efficient Dehydration of Subducted Lithosphere
AU: * Dixon, J
EM: jdixon@rsmas.miami.edu
AF: Univ. of Miami/RSMAS, 4600 Rickenbacker Cswy., Miami, FL 33149 United States
AB: This talk will review estimates of water partitioning during subduction as determined by studies of mantle-derived melts. A major uncertainty in the earth's water cycle is the effect of subduction and recycling of hydrated lithosphere on deep mantle water concentrations. There is general agreement that the earth accreted "wet" and that post-accretion influx of water to Earth from comets is limited to <~20%. A minimum value of ~1000 ppm H₂O in the primitive mantle can be estimated by adding exosphere water (ocean, atmosphere, and crust) to the depleted mantle, assuming that this water comes from degassing of half the mantle and that the depleted mantle has ~100 ppm H₂O. Lower values of primitive mantle water (330 ppm for the Pacific and 430 ppm for the north Atlantic) are obtained by calculating the value required to produce a smooth depletion trend for highly incompatible elements in MORB. A similar phenomenon noted for Pb has been interpreted as preferential partitioning of Pb into the exosphere during subduction. I suggest that the same is true for water. Water concentrations in various mantle end-member components support this hypothesis. Mantle plumes enriched in recycled lithosphere (EM1, EM2, LOMU, and HIMU; H₂O/Ce<100) have lower ratios of water to similarly incompatible elements than plumes dominated by the common plume component (FOZO; H₂O/Ce=210 to 300). High H₂O/Ce in FOZO plumes cannot be derived from recycled lithosphere; therefore, a significant amount of water must be juvenile, left over from planetary accretion. Thus, dehydration during subduction effectively partitions water into the exosphere (mantle wedge, crust, ocean, atmosphere) resulting in time-integrated depletion of water relative to other incompatible elements in recycled (deeply subducted) lithosphere and sediments and, ultimately, the majority of the mantle. Entrainment of hydrated suprasubduction zone mantle (0.025 to ~2 wt% H₂O) may lead to water enrichments in the upper ~400 km, as evidenced by seismic tomography. Mantle below this depth is likely to be quite dry (<0.08 wt%), regardless of the existence of high pressure hydrous phases observed in laboratory studies.
DE: 1025 Composition of the mantle
DE: 1030 Geochemical cycles (0330)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1065 Major and trace element geochemistry
DE: 3037 Oceanic hotspots and intraplate volcanism
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005