Characterizing the effect of mantle source, subduction input and melting in the Fonualei Spreading Center, Lau Basin: Constraints on the origin of the boninitic signature of the back‐arc lavas

Abstract

New major element, trace element and Pb‐Sr‐Nd isotope data for glasses from the Fonualei Spreading Center (FSC) constrain the genesis of back‐arc basin basalts and the origins of boninites. The FSC is an end‐member for global back‐arc lavas in terms of low Ti8.0 and Na8.0, and contains lavas with a boninitic signature. Latitudinal variations reveal a correspondence in location between back‐arc and adjacent arc volcanism. The locations of spikes in subduction input and positive bathymetric anomalies along the FSC correspond to the projected location of the arc volcanoes, likely reflecting 3‐D convective structure of the mantle wedge. Non‐mobile trace elements in arc and back‐arc lavas reveal an increasing proportion northward of a re‐enriched refractory mantle source, which is supported by isotope data. Quantitative modeling constrains the extents of melting, fraction of enriched mantle and subduction input. For the FSC, extents of melting are exceptionally large. We show a general relationship between extent of melting, subduction input and distance from the arc that applies to both the Eastern Lau Spreading Center and the FSC segment closer to the arc. In the center of the FSC where the arc volcanism is captured by the back‐arc, exceptionally high subduction input and even greater extents of melting are observed, producing melts with boninitic signature. Boninitic samples require the juxtaposition of high subduction input and refractory mantle, leading to large integrated extents of melting, an occurrence that can be produced by multiple causes.