HR: 13:55h
AN: V53C-02    [Abstracts]
TI: The Metasomatic Alternative for the Origin of OIB: a Model which Reconciles Experimental Petrology and Geochemistry
AU: * Pilet, S
EM: Sebastien.Pilet@img.unil.ch
AF: Institute of Mineralogy and Geochemistry University of Lausanne, BFSH 2, Lausanne, 1015 Switzerland
AU: Hernandez, J
EM: jean.hernandez@img.unil.ch
AF: Institute of Mineralogy and Geochemistry University of Lausanne, BFSH 2, Lausanne, 1015 Switzerland
AU: Sylvester, P J
EM: pauls@esd.mun.ca
AF: Dept. of Earth Science Memorial University of Newfoundland, Alexander Murray Building, St John's, NL A1B 3X5 Canada
AB: Variation of trace element and isotopic ratios in OIB is commonly ascribed to the recycling of ancient oceanic crust associated with crustal or pelagic sediment assimilation. However this model based on geochemical arguments is in opposition with experimental petrology data. Partial melts of oceanic-crust lithologies produce silica saturated liquids whereas many oceanic island rocks are characterized by silica undersaturated compositions. Experimental data indicate that only partial melting of peridotite in presence of carbonate [1] or of pyroxenite [2] produce liquids which are close to the nephelinite or basanite major element compositions observed in oceanic islands. In this way, recycling of subducted oceanic basal lithosphere enriched by metasomatic veins seems to represent a convincing alternative for the source of OIB [3]. However, this hypothesis does not explain the formation of isotopic heterogeneity observed in OIB. Chemical variations observed in Cantal basalt (France), interpreted as the result of a lithospheric metasomatic mechanism [4], allow us to constrain the chemical evolution of a metasomatic agent within basal lithosphere. These data demonstrate that - 1) fractionation of trace element ratios (U/Pb, Th/Pb, Rb/Sr, Sm/Nd, Nb/La .) necessary to generate, after subduction and isolation, the EM and HIMU components can be explained by metasomatic process and - 2) partial melting of veins-plus-enclosing lithospheric mantle produce basalt with composition perfectly similar to major and trace elements composition observe in OIB. This suggest that isotopic and trace element variations observed in basalts from individual oceanic islands may more likely be the result of melting heterogeneous, metasomatised, subducted oceanic lithosphere rather than a mixture of chemically distinct mantle reservoirs. End-member isotopic compositions would correspond to the extreme trace element fractionation generated by metasomatic process within the lithosphere. The new interpretation of OIB sources proposed here requires a re-evaluation of the processes that control chemical evolution of Earth's mantle reservoirs and plumes geochemical tracers. Recycled metasomatised lherzolite may be the major mantle component sampled by OIBs; recycled oceanic crust and sediment may be less common in OIB sources than is commonly assumed. The metasomatic hypothesis is entirely consistent with isotopic heterogeneity observed in the source of oceanic basalts and experimental data which indicate that the most plausible source of OIB material is pyroxenites. [1] K. Hirose (1997) Geophys. Res. Lett. 24, 2837 [2] M.M. Hirschmann et al. (2003) Geology, 31, 481. [3] Y. Nui, M. O'Hara. (2003) J. Geophys. Res.,108, B4, 2209. [4] S. Pilet et al. (2004) Geology, 32, 2, 113.
DE: 8450 Planetary volcanism (5480)
DE: 3640 Igneous petrology
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
DE: 1010 Chemical evolution
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting