HR: 12:05h
AN: T21F-08    [PDF]
TI: Some Geochemical Implications of the Mantle Transition-Zone Water-Filter Model
AU: * Karato, S
EM: shun-ichiro.karato@yale.edu
AF: Yale University, Department of Geology & Geophysics, New Haven, CT 06520 United States
AU: Bercovici, D
EM: david.bercovici@yale.edu
AF: Yale University, Department of Geology & Geophysics, New Haven, CT 06520 United States
AB: Geochemical observations are usually interpreted in terms of the Earth's initial composition and subsequent differentiation, including (1) core-mantle separation, (2) continental crust formation and (3) partial melting at mid-ocean ridges and/or subduction-zone upper mantle. In this traditional picture, significant chemical differentiation occurs only in the shallow upper mantle [process (3)], in which case it is difficult to reconcile geochemical evidence for isolated mantle reservoirs with geophysical evidence for mantle-wide convective circulation. However, we recently proposed [Bercovici and Karato, Nature, Sept 4, 2003] that dehydration-induced partial melting at 410km will act as a filter for incompatible elements which are extracted into a dense melt that is then returned to the deep mantle; the trace-element circulation is thus partially decoupled from the convective circulation of the bulk of the mantle which occurs at the whole-mantle scale. In this model, the OIB source materials are drawn directly from the undepleted and heterogeneous deep mantle (deeper than 410km), while the MORB source materials are processed through the putative 410km dehydration-melting "filter". The separation of MORB and OIB source materials is thus assumed to occur at 410km and therefore it is important to examine if our hypothesis is consistent with observations of isotopes with very slow radioactive decays (i.e., $^{87}Sr$/$^{86}Sr$, $^{143}Nd$/${144}Nd$). The $^{87}Sr$/$^{86}Sr$ and $^{143}Nd$/$^{144}Nd$ ratios are distinct between OIBs and MORBs ( i.e., OIBs have a wider distribution than MORBs, and MORB isotopic ratios correspond to "depleted" chemistry). These isotopic ratios are controlled by (i) the chemical composition (Rb/Sr and Sm/Nd, respectively) and (ii) the long-term radioactive decay (half-life of 48 and 106 Gyrs, respectively). A simple analysis was made to calculate the contrast in the $^{87}Sr$/$^{86}Sr$ and $^{143}Nd$/$^{144}Nd$ ratios between OIBs and MORBs corresponding to our model. Two factors control these isotopic ratios in our model: (i) chemical differentiation at 410km between Rb/Sr and Sm/Nd and (ii) the volume fraction of OIB sources with different isotopic ratios (and the degree of mixing of these materials in the 410km dense melt layer). We find that the observed isotopic ratios are consistent with our model if (i) the source region of OIBs is volumetrically dominated by materials with isotopic ratios similar to Hawaii or FOZO source regions (but not EM1 or HIMU), (ii) significant chemical fractionation occurs by partial melting at 410km for Rb/Sr but not for Sm/Nd and (iii) the mixing at 410km is extensive. Implications for other geochemical signatures including isotopic ratios of rare gas elements and their volumetric amounts and the trace element distribution pattern will also be discussed.
DE: 1010 Chemical evolution
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1065 Trace elements (3670)
DE: 3919 Equations of state
SC: Tectonophysics [T]
MN: 2003 Fall Meeting