Chemical and isotopic data for volcanic glasses from the Valu Fa Ridge (VFR) in the southern Lau Basin closely resemble the composition of volcanics from the nearby Tonga arc. The most primitive basalts from the VFR were used to reconstruct the primary magma composition which is believed to reflect the composition of the bulk crust. The magma production rate at the Valu Fa Ridge was estimated (0.7-1.5¿106 kg/a/m), and chemical fluxes of various elements have been calculated based on these values and the primary magma composition. The chemical and isotopic composition of VFR lavas is totally consistent with the release of fluid-soluble elements from the subducting altered oceanic crust and sediments. Mass balance calculations indicate the important role of the altered basaltic basement as a source of H2O, K, Rb, Cs, Sr, U, and Pb in supra-subduction zone rocks. ¿ 1998 American Geophysical Union