Petrological and geophysical conclusions include the following: (1) Free CO/sub 2/ cannot exist in the mantle it is stored as carbonate. Experimental data and theoretical analysis permit construction of a series of partly schematic phase diagrams. The points are connected by a series of liquidus reactions involving the carbonates and mantle silicates, at temperatures generally lower than the silicate-CO/sub 2/ more » melting reactions. Fusion curves for mantle mineral assemblages involving forsterite, orthopyroxene, and clinopyroxene in the presence of CO/sub 2/, extending from higher temperature regions, terminate at these same invariant points. Each decarbonation reaction terminates at an invariant point involving a liquid, CO/sub 2/ vapor, carbonate minerals, and one or more of the silicate minerals. « lessĭecarbonation reactions in the system CaO-MgO-SiO/sub 2/-CO/sub 2/ involve calcite, dolomite, magnesite, and quartz, and the products enstatite, forsterite, diopside, and wollastonite, among others. The authors' model stresses the importance of the contribution that recycled crustal materials can have on the composition of the upper mantle, in particular the recognition of a crustal signature in a mantle regime. Best results were obtained from mixing a maximum of 3 to 5% seawater-altered basalt and 2 to 3% pelagic sediment, with a LREE-enriched mantle precursor. In their model, it was assumed that negative Ce anomalies cannot be produced by magmatic or metasomatic processes. In order to define the petrogenesis of more » this peridotite within the constraints of the present experimental data, the authors have attempted various mixing models with the end-members: mantle peridotite, Pacific sediment and seawater-altered basalt. A phlogopite component appears to have been important in metasomatic fluids related to the Chokai basalts which were more » generated at relatively great depths ( conditions, such a process also seems plausible for the generation of negative Ce anomalies. The Chokai basalt sources are distinctly more enriched in K and Rb relative to Sr, Ce and Zr and lower in K/Rb than the Nasu tholeiite sources. The source peridotites of these basalts above the subducted Pacific plate have large LIL (large ion lithophile)/HFS (high field strength) element ratios but are differentially enriched in individual LIL elements. Geochemical data are compiled for basalts from the Nasu zone at the trench side and the Chokai zone at the back-arc side, Northeast Japan arc.
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