Influence of subducted components on back‐arc melting dynamics in the Manus Basin

Abstract

Lavas erupted in back‐arc basins afford the opportunity to explore the extent to which decompression and subduction‐related components influence partial melting in this setting. We present U‐Th‐Ra disequilibria data from 24 well‐characterized lavas from the Manus Basin behind the New Britain volcanic arc, supplemented by some additional trace element and Sr‐Nd‐Pb isotope data. The lavas range in composition from 49.6 to 57.7 wt % SiO2 and can be subdivided into those that are broadly like mid‐ocean ridge basalts (MORB) with Ba/Nb < 16 and back‐arc basin basalts (BABB) that are variably influenced by subduction components and have Ba/Nb > 16. Rifts closest to the arc are dominated by BABB, whereas both lavas types erupt further away at the Manus Spreading Center. The MORB have small 230Th excesses (up to 5%) and are displaced below the global correlation of (230Th/238U) with ridge depth. In most respects the BABB closely resemble lavas erupted along the New Britain arc front, including 238U excesses that reach 26%. The Pb isotope data can be explained by mixing of a subduction component into an Indian MORB mantle source. The Pb in the subduction component is derived from both the subducted sediment (5%) and fluids from the subducting altered Solomon Sea oceanic crust (95%), and these were mixed prior to addition to the mantle wedge. U/Th ratios, Fe3+/ΣFe, and H2O contents all increase with increasing 206Pb/204Pb. A model in which addition of the subduction component to the mantle wedge is followed by 230Th in‐growth during decompression and dynamic melting all less than 140 kyr prior to eruption can simulate the data. However, our preferred model is one of dynamic decompression melting in which subduction‐modified, more oxidized mantle had DU ≪ DTh leading to 238U excesses in contrast to unmodified mantle that yields 230Th excess. Large 226Ra excesses in some southern rift samples require addition of a fluid <8 kyr ago but elsewhere reflect melting under low‐porosity conditions.