HR: 08:55h
AN: V31A-04    [PDF]
TI: Metasomatic Control of Trace Element Ratios in Lithospheric Mantle: New Interpretation of OIB Isotopic Variations
AU: * Pilet, S
EM: Sebastien.Pilet@img.unil.ch
AF: Institut of Mineralogy and Geochemistry University of Lausanne, BFSH 2, Lausanne, VD 1005 Switzerland
AU: * Pilet, S
EM: Sebastien.Pilet@img.unil.ch
AF: Department of Earth Science Memorial University of Newfoundland, Alexander Murray Building, St John's, NL A1B 3X5 Canada
AU: Hernandez, J
EM: Jean.Hernandez@img.unil.ch
AF: Institut of Mineralogy and Geochemistry University of Lausanne, BFSH 2, Lausanne, VD 1005 Switzerland
AU: Sylvester, P
EM: pauls@sparky2.esd.mun.ca
AF: Department of Earth Science Memorial University of Newfoundland, Alexander Murray Building, St John's, NL A1B 3X5 Canada
AB: Metasomatized lherzolites from the deep portions of recycled oceanic lithosphere, rather than subducted oceanic crust, may be the principal component of OIB sources [1]. However, the exact chemical and physical nature of this metasomatic process is unclear. In particular, it is uncertain how metasomatism produces chemical variations in recycled lherzolite that reflect the different OIB isotopic "signatures". Chemical variations observed in Cantal basalt (France), interpreted as the result of a lithospheric metasomatic mechanism, provide important new constraints on the nature of this metasomatic process. The basalts, erupted between 13 and 3 My ago in the Cantal alkali massif, show an unusual increase in Nb/Th and Ce/Pb ratios from 10.5 and 23 respectively in the first emitted basalt to 18.7 and 44 in the last. The range in Nb/Th (10.5 to 18.7) is practically as large as that found in picrites and tholeiites from all major oceanic islands (8 to 20). These basalts are homogeneous with respect to their Sr, Nd, Pb isotopic composition, ruling out variable sediment contamination of their mantle sources. We postulate that the Nb/Th and Ce/Pb variations are the result of a metasomatic process, called "percolative fractional crystallization" (PFC) [2] which produced veins within the lithospheric mantle. A product of the PFC mechanism is observed in Cantal basalts erupted after 9.5 My ago as the appearance of green-core pyroxene xenocrysts (GCPX). The formation of this GCPX, observed worldwide, is related to the crystallization and fractionation of apatite, ilmenite, amphibole and rutile, during the percolation and differentiation within upper-mantle of an initial basaltic liquid. In this model, compositional variations between the first and the last erupted basalt are explained by an evolution of vein composition present at the depth of melting in the vein-plus enclosing mantle source. These new data indicate that there is a "short-term" mineralogic control on Nb/Th and Ce/Pb ratios through a vein-plus-enclosing mantle source model. This differs from previous models for variations in these ratios being related to the "long term" extraction of continental crust from the mantle and the extent of sediment recycling and mixing in the mantle. Reconstruction of lithospheric sources from the different basalt compositions indicates significant heterogeneities in Th/Pb, U/Pb and Sm/Nd resulting from the metasomatic process. If this type of metasomatised lithosphere was subducted and isolated during a long period of time (1-2 Ga), the chemical variations would create Nd and Pb isotopic variations in the different part of this lithosphere similar to isotopic range observed in some oceanic islands. Moreover, these isotopic variations would be correlated with trace element ratio variations commonly interpreted as the product of mixing between different end-member mantle components (HIMU and EM). This suggest that isotopic and trace element variations observed in basalts from individual oceanic islands may more likely be the result of melting metasomatized subducted oceanic lithosphere rather than from a mixture of chemically distinct mantle sources that formed and evolved independently from one another. [1] Y. Nui, M. O'Hara. (2003) J. Geophys. Res.,108, B4, 2209. [2] B. Harte, R. H. Hunter, P. D. Kinny. (1993) Philos. Trans. R. Soc. London, 342, 1-21.
DE: 1010 Chemical evolution
DE: 1025 Composition of the mantle
DE: 3600 MINERALOGY AND PETROLOGY (replaces
DE: 7218 Lithosphere and upper mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting