HR: 0800h
AN: V41C-1469    [Abstracts]
TI: Mantle petrogenesis of the Eastern Lau Spreading Center basalts and andesites and the role of subduction-related fluids
AU: Bezos, A
EM: bezos@eps.harvard.edu
AF: Harvard University, Earth & Planetary Sciences 20 Oxford Street, Cambridge, MA 02138 United States
AU: * Langmuir, C
EM: langmuir@eps.harvard.edu
AF: Harvard University, Earth & Planetary Sciences 20 Oxford Street, Cambridge, MA 02138 United States
AU: Escrig, S
EM: escrig@eps.harvard.edu
AF: Harvard University, Earth & Planetary Sciences 20 Oxford Street, Cambridge, MA 02138 United States
AU: Michael, P
EM: pjm@utulsa.edu
AF: University of Tulsa, Department of Geosciences 600 S.College Ave, Tulsa, OK 74104 United States
AU: Asimow, P
EM: asimow@gps.caltech.edu
AF: Caltech, Geological & Planetary Sciences 1200 E. California Blvd., Pasadena, CA 91125 United States
AU: Woods, S
EM: woods@eps.harvard.edu
AF: Harvard University, Earth & Planetary Sciences 20 Oxford Street, Cambridge, MA 02138 United States
AB: The Eastern Lau Spreading Center (ELSC) varies progressively in distance from the volcanic front of the Tonga Arc. The southern end (Valu Fa Ridge) is 40 km from the arc front while the northern end is 100km. Cruise KM0417 carried out detailed sampling of the ELSC and the northern end of the Valu Fa Spreading Center (less than 3km sample spacing), as well as sampling of seamounts between the spreading centers and the arc (see Michael et al, this meeting). The samples provide an unparalleled coverage of magmatism at varying distances from a volcanic front. This unique sample set, with its high spatial density, can be used to assess the influence of subduction-related fluids on mantle composition and melting processes, and the regularity of variations with increasing distance behind the arc. Previous studies of this region showed a marked contrast between "arc-like" lavas in the Valu Fa region in the south, and "MORB-like" lavas to the north. The new closely spaced samples show that the transition from basaltic to andesitic magmatism along the ELSC occurs almost entirely along a single 15 km long segment located 70 km behind the Tonga arc. The petrological boundary is accompanied by an abrupt change in ridge morphology as the axis shifts from an axial rift valley in the MORB-like domain to an axial high in the arc-like domain. Along this transition, samples are progressively enriched in U Th, Ba, K and Rb, with progressively greater Ti depletion. To complicate the picture, discrete spikes of arc-like volcanism (apparently associated with hydrothermal venting) occur also in the northern ELSC domain. Great variations of the subduction signal can be observed for samples collected within hundreds of meters of each other. The ELSC andesitic arc like magmatism displays, at both regional and local scale, a tight relation between enrichment of fluid mobile elements, high water content and fluid immobile element depletion (Nb, Ta, Zr, Hf, Ti and Y). Modeling this petrological and geochemical signature requires a two-step melting event: first, a small extent of melting to deplete the mantle wedge, followed by a second hydrous melting event associated with addition of fluid-mobile elements. The southern ELSC domain displays gentle geochemical gradients well organized with respect to the arc-backarc distance. However, detailed trace element systematics show that there are discrete enriched components. Modeling of this well organized geochemical boundary and gradient allow characterization of the nature of different subduction components and relation to the tectonic setting.
DE: 1000 GEOCHEMISTRY
DE: 1037 Magma genesis and partial melting (3619)
DE: 1065 Major and trace element geochemistry
DE: 3600 MINERALOGY AND PETROLOGY
DE: 3619 Magma genesis and partial melting (1037)
SC: Tectonophysics [T]
MN: Fall Meeting 2005