Other
Scientific paper
Dec 2002
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002agufm.t22c..12d&link_type=abstract
American Geophysical Union, Fall Meeting 2002, abstract #T22C-12
Other
1010 Chemical Evolution, 1020 Composition Of The Crust, 1025 Composition Of The Mantle, 1065 Trace Elements (3670), 3670 Minor And Trace Element Composition
Scientific paper
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% [1]. 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 [2 and 3]) 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 [e.g., 4]. I propose that the same is true for water. Water 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 250) [5]. 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. A model for mantle water evolution will be presented in which FOZO is composed dominantly of slightly-depleted, primitive, peridotitic mantle metasomatized by a small amount (1-3%) of a small extent melt (2-6%) of a HIMU component. 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 lithosphere and accompanying continental-derived materials and, ultimately, the mantle. 1. Abe Y, et al.., Origin of the Earth and Moon., Univ. of Arizona Press, Tucson (2000). 2. Dixon J E and Clague D A,. J. Petrol. 42, 627-654 (2001). 3. Asimow P, et al., G-cubed in prep. (2002). 4. Chauvel C, et al., Chem. Geol. 126, 65-75 (1995). 5. Dixon J E, et al., Nature, submitted .
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