HR: 0830h
AN: V51A-14    [Abstracts]
TI: U-series disequilibria in volcanic rocks from the Futuna spreading centre (North Fiji Basin)
AU: * le faouder, a
EM: antoine.lefaouder@sdt.univ-brest.fr
AF: IUEM, place nicolas copernic, plouzane, 29280 France
AU: hemond, c
EM: christophe.hemond@univ-brest.fr
AF: IUEM, place nicolas copernic, plouzane, 29280 France
AU: benoit, m
EM: mathieu.benoit@univ-brest.fr
AF: IUEM, place nicolas copernic, plouzane, 29280 France
AU: Lagabrielle, Y
EM: yves.lagabrielle@dstu.univ-montp2.fr
AF: Laboratoire Dynamique de la LithosphŠre ISTEEM, place bataillon, Montpellier, 34095 France
AU: Guivel, C
EM: Christelle.Guivel@chimie.univ-nantes.fr
AF: Laboratoire de Plan‚tologie et G‚odynamique, cedex 3, nantes, 44322
AU: Pelletier, B
EM: Bernard.Pelletier@noumea.ird.nc
AF: IRD/UMR G‚osciences Azur - Laboratoire de G‚ologie - G‚ophysique, B. P. A5 nouvelle cal‚donie, Noumea, France
AU: Bollinger, C
EM: claire.bassoullet@univ-brest.fr
AF: IUEM, place nicolas copernic, plouzane, 29280 France
AB: The North Fiji basin is the largest active back arc basin of the SW Pacific. It is 12 m.y. old and characterised by a regional upper mantle thermal anomaly (Garel, 2001). The 200Km long Futuna spreading centre (Pelletier et al., 2001) is located west of Futuna and Alofi islands at the border between Lau and North Fijian Basins. This ridge is composed of an axial valley spotted with numerous seamounts. It starts from the north of the Fiji platform (15° 40'S) and ends in the north Fiji transform zone, northwest of the Futuna and Alofi islands (13° 35'S). New U series measurements were performed by TIMS on fresh dredged glassy samples. As the topography of this spreading centre exhibits a peculiar structure, the geochemical signature is also unusual. U and Th concentrations range between 0,09<U<0,51 and 0,31<Th<2,16, with a corresponding large variation in Th/U ratio 3,4<Th/U<5,4. Such variations in Th/U ratios likely reflect the mixing between different long-lived sources. (230Th/238U) ratios vary from 1.15 to 1.42. Three samples have 230Th excesses >30%, six have a 230Th excess within 25 ñ 5% and three have 230Th excess of about 15%. These 230Th excesses are typical of MORB or OIB source melting and not a back arc volcanic processes. It is also symptomatic of a melting in presence of garnet. In the equiline diagram, samples fall on a linear correlation indicating a mixing between two end-members with different Th/Usources ratios. The trend observed in this diagram requires the involvement of an enriched component, likely an EMII component, as defined by the trace elements and Sr-Nd-Pb isotope compositions (Benoit et al., in prep). This trend falls within the OIB field and plots close to Samoan samples analysed by Newman et al. (1984). This trend demonstrates the influence of a melt may be even more enriched than the samples of Savai'i volcano. This EMII melt mixes, within the upper mantle, with the Indian ocean like component as it was already established that it is present below Lau and North Fiji Basins spreading centres (Loock, 1992; Turner et al., 1992; Hickey-Vargas et al, 1995; Zhang et al., 1999).
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2005 Joint Assembly