HR: 08:00h
AN: T31F-01 INVITED     [Abstracts]
TI: The transition-zone water-filter model of mantle convection and chemistry
AU: * Bercovici, D
EM: david.bercovici@yale.edu
AF: Yale University Department of Geology and Geophysics, PO Box 208109, New Haven, CT 06520-8109 United States
AU: Karato, S
EM: shun-ichiro.karato@yale.edu
AF: Yale University Department of Geology and Geophysics, PO Box 208109, New Haven, CT 06520-8109 United States
AB: The transition-zone water-filter model of mantle convection was recently proposed (Bercovici and Karato, Nature v425, p39, 2003), to reconcile geochemical observations of isolated chemical mantle reservoirs (evident in distinct chemical signatures of ocean island and mid-ocean ridge basalts, OIB and MORB, respectively) with geophysical, especially seismological evidence for whole mantle circulation and mixing. The model proposes that as the background ambient mantle upwelling (rising in response to the downward flux of subducting slabs) moves out of the high-water-solubility transition zone into the low-solubility upper mantle above 410km, it undergoes water supersaturation and partial melting that filters out incompatible elements; the remaining solid phase continues to ascend and supplies relatively dry and depleted materials to the MORB source region. The residual melt is presumed denser than the surrounding solid and is thus trapped at the 410km boundary until slab entrainment recirculates the enriched material back into the deeper mantle. The filtering effect is suppressed for hotter mantle plumes, because of their greater ascent rates and lower water solubility in the transition zone, allowing plumes to generate more enriched OIBs. Simple calculations of elemental-vs-compatability spectra ("spidergrams") using the two-stage melting of ambient mantle (melting in the filter mechanism and at ridges) and single stage melting of plumes (at hotspots only) reproduce chemical observations for mid-ocean ridges and Hawaiian plume spectra. Differences in radiogenic isotope ratios between MORB and OIB are explained by different compatibilities of parent and daughter products (causing their different depletion at the filter zone) and a short separation in isotopic evolution; the necessary compatibilities and parent:daughter ratios to yield the observed isotopic composition is consistent with field and laboratory measurements. The model also predicts that radiogenic internal heating would also be non-uniform, i.e., largely sequestered in the sub-410km mantle and even in the transition zone itself. Numerical models of convection with such heat-source distributions show that no layering or implausible circulation is induced since all boundaries of the mantle are permeable. Various seismic studies over the last decade indicate the presence of a melt zone above the 410km discontinuity, which provides partial support for our hypothesis. Further geodynamical studies of the horizontal structure of the 410-km melt zone and mechanisms for recirculating water through the transition zone are underway and will be discussed.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8124 Earth's interior--composition and state (old 8105)
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting