Deep Earth Water Cycle: Efficient Dehydration of Subducted Lithosphere

Details Deep Earth Water Cycle: Efficient Dehydration of Subducted Lithosphere Deep Earth Water Cycle: Efficient Dehydration of Subducted Lithosphere

Dec 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005agufm.v52a..04d&link_type=abstract

American Geophysical Union, Fall Meeting 2005, abstract #V52A-04

Other

1025 Composition Of The Mantle, 1030 Geochemical Cycles (0330), 1031 Subduction Zone Processes (3060, 3613, 8170, 8413), 1065 Major And Trace Element Geochemistry, 3037 Oceanic Hotspots And Intraplate Volcanism

Scientific paper

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.

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