HR: 11:50h
AN: V52A-07    [Abstracts]
TI: Trace element and isotopic constraints on mantle source heterogeneity and subduction- related fluids along the Eastern Lau Spreading Center
AU: * Escrig, S
EM: escrig@eps.harvard.edu
AF: Harvard University, 20 Oxford Street, Cambridge, MA 02138, United States
AU: Goldstein, S L
EM: steveg@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Rt. 9W, Palisades, NY 10964, United States
AU: Bezos, A
EM: antoine.bezos@univ-nantes.fr
AF: Harvard University, 20 Oxford Street, Cambridge, MA 02138, United States
AU: Bezos, A
EM: antoine.bezos@univ-nantes.fr
AF: Laboratoire de Planetologie et Geodynamique, Universite de Nantes, Nantes, 44322, France
AU: Langmuir, C H
EM: langmuir@eps.harvard.edu
AF: Harvard University, 20 Oxford Street, Cambridge, MA 02138, United States
AU: Michael, P J
EM: pjm@utulsa.edu
AF: University of Tulsa, 600 S. Ave., Tulsa, OK 74104, United States
AB: The Eastern Lau Spreading Center (ELSC) is located in the Lau Basin behind the Tonga arc. Its distance from the arc front changes progressively from 40 km at the Valu Fa Ridge in the south to 100km in the north. We report >65 new Pb-Sr-Nd isotope analyses and over 130 trace element analyses of axial lavas. The excellent correlations between the trace element and isotope compositions allow the determination of the subduction component and the background (unaffected by subduction) mantle compositions, as well the changes occurring along the arc (N-S), and with distance from the arc (E-W). We see clear N-S changes in "background" mantle, with stronger Indian (Dupal) signatures toward the north. In the southern Valu Fa the isotope ratios intercept the Tonga arc array. ²⁰⁶Pb/²⁰⁴Pb decrease northward (from 18.66 to 18.13), with an increasing offset to high ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb compared to N. Atlantic-Pacific data. With a few exceptions, ⁸⁷Sr/⁸⁶ are high but show little variation (0.70323±8), and values are similar to Indian MORB and some Tonga islands. ¹⁴³Nd/¹⁴⁴Nd are even more uniform (0.51306±3). Despite the small variability, the data show that the "background" mantle is less Indian-like in the south. For example, Valu Fa lavas define a trend, distinct from the other segments, toward lower ⁸⁷Sr/⁸⁶ for a given ²⁰⁶Pb/²⁰⁴Pb, indicating a more "Pacific-like" background mantle in the southernmost segment. The ODP sites located in the Lau Basin show that the "background" mantle was Pacific- like in earlier stages of volcanism Our on-axis data gives the current compositions of the Lau Basin upper mantle and combined with the ODP data indicate that Pacific mantle has been progressively replaced with Indian mantle from north to south. Latitudinal variations in the compositions of the subduction components can be clearly seen in back-arc lavas. Relationships between Pb isotopes and Nb/U or Ba/Nb reveal that the compositions of subduction components affecting the back arc mantle are the same as those affecting adjacent volcanoes in the main arc (for example Valu Fa is like Ata, and ELSC2 and ELSC3 are Kao and Tofua). Overall, the contributions from subduction components to back-arc basalts decrease with increasing distance from the arc. The southernmost segments display smooth changes of Pb and fluid mobile/immobile element ratios with latitude, whereas further north and farther from the arc, some segments show local subduction signature "spikes". These results show that, close to the arc, the mantle beneath the back-arc is pervasively affected by subduction-related fluids or melts, while far from the arc, the subduction signature is localized into geographically discrete portions of the ridge.
DE: 1000 GEOCHEMISTRY
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1038 Mantle processes (3621)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting