Composition of back-arc basin volcanics, Valu Fa Ridge, Lau Basin: Evidence for a slab-derived component in their mantle source

Samples dredged from 2 localities near the crest of the Valu Fa ridge, an active back-arc basin spreading centre in the Lau Basin, consist of highly vesicular lava fragments of andesitic composition. The samples are characterized by rare, euhedral An₈₅ plagioclase phenocrysts in a hypocrystalline groundmass of An₆₀ plagioclase laths, brown glass and rare subhedral clinopyroxene. Samples from within and, to a lesser extent, between the dredge hauls show remarkable isotopic and chemical homogeneity, with: ⁸⁷Sr/⁸⁶Sr - 0.70330 ± 2; ¹⁴³Nd/¹⁴⁴Nd - 0.51303 ± 2; ²⁰⁶Pb/²⁰⁴Pb - 18.65 ± 2; ²⁰⁷Pb/²⁰⁴Pb - 15.55 ± 1; ²⁰⁸Pb/²⁰⁴Pb - 38.34 ± 4; Sr - 165 ppm; Rb - 7 ppm; Cs - 0.17 ppm; K - 3300 to 4200 ppm; Ba - 96 ppm; and REE - LREE depleted with 12-18 × chondritic abundances. On Sr-Nd, Pb-Pb and Sr-Pb plots the volcanics lie just within or on the edge of the MORB fields, overlapping with island-arc volcanics from the Marianas and Tonga. Compared with MORB and ocean-island basalts, the samples show alkali-element enrichment relative to REE and higher Cs relative to Rb. The isotopic and geochemical characteristics of the Valu Fa Ridge volcanics clearly indicate a minor, but significant, slab-derived component in the back-arc basin mantle source.