Sources of constructional cross‐chain volcanism in the southern Havre Trough: New insights from HFSE and REE concentration and isotope systematics

Abstract

We report new compositional and isotopic data for submarine lavas erupted on the Rumble V Ridge cross chain behind the Kermadec Arc at ∼36°S and for locally subducting sediment. In order to constrain cross‐arc changes in the melt source, Havre Trough ambient mantle wedge isotope and trace element characteristics are interpreted from regional back‐arc basalts that are relatively free of slab‐derived components. They have MORB‐like trace element concentrations and are isotopically “Pacific” but define greater heterogeneity in 206Pb/204Pb and 176Hf/177Hf than previously known within the Havre Trough. In contrast with the ambient mantle, all Rumble V Ridge lavas have trace element and isotopic characteristics consistent with subduction zone contributions despite their rear‐arc setting but are less fluid‐enriched than at the Kermadec volcanic front. A broad trend in Nd‐Hf isotopic space and the correlation between 176Hf/177Hf and Hf concentration anomaly for Rumble V Ridge lavas is explained by across‐arc changes in (1) the mantle wedge component, (2) the nature of the subduction component, and (3) the mass fraction of subduction components added. Samples from the distal cross chain tend to have lower 176Hf/177Hf at similar 143Nd/144Nd compared with samples closer to the arc, suggesting that a low‐176Hf/177Hf component is preferentially removed from the mantle wedge during trenchward advection. Isotope trends suggest that locally subducting sediment is the primary slab component for Rumble V Ridge magmas, but bulk mixing of ambient mantle with sediment or slab‐derived fluids cannot account for cross‐arc trace element ratios. Instead, cross‐chain isotope and trace element characteristics are explained by the addition of 0.05%–2.0% sediment melts where trace zircon, monazite, and rutile are residual. The cross chain tracks an east to west increase in the mass fraction of a common subduction component. A particularly enriched subset of eastern basalts is interpreted as being derived from the addition of an even higher mass fraction of a different subduction component with greater apparent stability of refractory trace phases during sediment melting. An implication of this study is that both Nd and Hf can be mobile in sediment‐rich subduction zones, but the relative mobility depends on the sediment composition and depth of melting, and the absolute mobility is small.