We report major and trace element data and water contents for 25 volcanic glasses and Sr, Nd, and Pb isotopic compositions for 19 representative samples from the Valu Fa Ridge (VFR, 21°50'S-22°30'S), the Central Lau Spreading Center (CLSC, 18°35'S-18°45'S), and a ridge segment in the North Fiji Basin (NFB) at 16°18'S-16°25'S and 177°20'E-177°30'E.
The Pb, Sr, and Nd isotopic compositions of CLSC and VFR samples agree with previously reported data (Hergt and Hawkesworth, 1994; Loock et al., 1990) and support the concept of an Indian Ocean-type upper mantle with high 87Sr/86Sr, low 143Nd/144Nd, and high 207Pb/204Pb - 208Pb/204Pb for a given 206Pb/204Pb below the CLSC. This situation is contrasted by the chemical evidence in samples from the VFR for melting of Pacific mantle. VFR samples are chemically and isotopically very similar to the nearby Tofua island arc. Trace element ratios (e.g., Ce/Pb, Ba/Nb, Nb/U) in conjunction with high water contents in the VFR basalts-dacite suite provide evidence for a sediment-modified mantle. Our mixing calculations suggest that less than 0.5 wt.% of pelagic sediment can account for the Sr and Pb isotopic compositions, which is in agreement with earlier estimates Volpe et al., 1988). Mass balance calculations of chemical fluxes in the Tonga subduction show that the elemental fluxes at the VFR can be balanced by subducted sediment and altered oceanic crust which is in agreement with the isotopic evidence. Using a method similar to that of Stolper and Newman (1994), the chemical composition of the fluxing agent has been calculated. The fluxing agent is water-rich and contains a minimum of 15 to 20 wt.% of dissolved components. The composition of the migrating fluid indicates disequilibrium with the mantle wedge. As little as 0.35 wt.% of such a fluid is sufficient to account for the trace element and water enrichment in the VFR basalts, corresponding to a water content of ~0.3 wt.% in the mantle source. The elevated water content results in high degrees of melting (~25 wt.%), consistent with recent experimental results (Hirose and Kawamoto, 1995). The CLSC basalts show only slight enrichment of water, in accordance with chromatographic buffering of the subducting lithosphere signal as the distance between subducted plate and back-arc ridge increases. This conclusion is corroborated by decreasing Ba/Nb and increasing Ce/Pb at the VFR from south to north. Our results support the model of Pearce et al. (1994) who suggested that differential diffusion of elements within the mantle wedge accounts for the along axis trends north of the VFR.
Sr and Nd isotope compositions of the NFB basalts form a trend between Samoa post-erosional basalts, basalts from the northeastern Lau Basin, Fiji alkali basalts, and a mantle component with radiogenic Nd and Sr. Our data include the highest 143Nd/144Nd ratios (up to 0.51322) reported to date for samples from the NFB. Such radiogenic Nd isotopic ratios are unusual in Indian MORB and we interpret them as evidence for presence of a Pacific MORB source, although Sr isotope ratios are somewhat higher than average Pacific MORB. Basalts from the NFB have higher 206Pb/204Pb ratios than samples from the CLSC with Indian-MORB characteristics. For a given 206Pb/204Pb ratio, 207Pb/204Pb ratios are typical of Pacific MORB and plot along the NHRL, whereas higher 208Pb/204Pb ratios suggest a long-term source evolution with Th/U up to 4.5. We suggest that the Nd and Pb isotopic compositions of the NFB basalts reflect melting of a significant proportion of a Pacific MORB source. We propose a model of a westward convecting Pacific asthenosphere (including Samoa components) below the NFB and the northern Lau Basin, opposed by an easterly flow of Indian asthenosphere below the southern and central Lau Basin. This scenario agrees with the eastward subduction beneath the New Hebrides island arc and a west-directed subduction beneath the Tonga Ridge. The model implies that the slab-induced corner flow controls the regional flow in the asthenoshere above both subduction zones. The subducting Pacific Plate at the Tonga Trench appears to isolate the Samoa-type mantle components in the Lau Basin south of a line between the northernmost Tonga Ridge and the southern tip of the New Hebrides Arc. To the north of this line (e.g., in the NFB, northern Lau Basin, Fiji islands) Samoa-type mantle components obviously represent important back-arc basin basalt sources.
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