U-series isotope data on Lau Basin glasses: The role of subduction-related fluids during melt generation in back-arc basins

D W Peate, T F Kokfelt, Chris Hawkesworth, P W Van Calsteren, J M Hergt, J A Pearce

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    Abstract

    New thermal ionisation mass spectrometry U-series disequilibrium data are presented for 24 basaltic to dacitic glasses from active spreading centres in the back-arc Lau Basin (SW Pacific), together with additional inductively coupled plasma mass spectrometry trace element analyses and Sr-Nd-Pb isotope data. Valu Fa Ria ge samples, adjacent to the arc front, have high U/Th and (Th-230/U-238) < 1, implying a recent (much less than 350ka) addition of a U-rich slab-derived fluid. The Valu I, data can be combined with existing Th-230-U-238 data for the Central Tonga arc to infer a fluid addition event at similar to 50ka. The, similar sources and time scales for fluid transfer beneath the Valu Fa Ridge and beneath the an, itself suggest that the Valu Ea Ridge is propagating into the arc-front region. Central Lau Basin samples, further behind the me, have lower U/Th, and (Th-230/U-238) greater than or equal to 1, similar to typical mid-ocean ridge basalts (MORB). Within the Central Lau Basin, a water-rich subduction component is seen only in samples closest to the an,, and this fluid does not have the high-U/Th composition of the fluid at Valu Fa. Melt generation in the Central Lau Basin appears to be dominated by normal ridge-type processes, but the relatively low, (Th-230/U-238) for these shallow ridges compared with global MORB could be a consequence of increased melt productivity, as a result of the elevated water contents. The transition from U-238 to Th-230 excesses within the back-aw basin is not a smooth function of decreasing addition of a U-rich fluid moving away from the an,front, but also reflects the complex dynamics between two major mantle domains within the mantle wedge ('Pacific' beneath Valu Fa Ria ge, 'Indian' beneath the Central Lau Basin).

    Original languageEnglish
    Pages (from-to)1449-1470
    Number of pages22
    JournalJournal of Petrology
    Volume42
    Issue number8
    Publication statusPublished - Aug 2001

    Keywords

    • back-arc basins
    • U-series disequilibrium
    • fluids
    • subduction zones
    • melting
    • VALU-FA RIDGE
    • U-238 TH-230 DISEQUILIBRIUM
    • MANTLE WEDGE COMPOSITION
    • KERMADEC ISLAND-ARC
    • MID-ATLANTIC RIDGE
    • TRACE-ELEMENT
    • SPREADING CENTER
    • OCEANIC BASALTS
    • MIDOCEAN RIDGES
    • GEOCHEMICAL VARIATIONS

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