A SUBDUCTION ZONE-SPREADING RIDGE TRANSITION SIGNATURE PRESERVED IN RECENT VOLCANIC ACTIVITY IN THE NE LAU BASIN

The NE Lau Basin suprasubduction zone hosts young, boninitic volcanic activity that has been the subject of several investigations into melt petrogenesis and mantle source contributions during early stages of subduction. In particular, the cluster of small, active volcanoes forming the Mata Volcanic Field (MVF) have provided valuable insight into backarc melt generation. Here we present the first Fe isotopic data for a suite of MVF boninitic glasses. As these melts are derived directly from underlying mantle, the coupling of stable isotope ratios to compositional data provides clues into the geographic distribution of source mantle heterogeneity, including possible transitions between dominant tectonic styles.

Using magmatic differentiation models to relate melt composition and Fe isotope fractionation, we find that estimates of parental melt compositions in the northwestern part of the MVF proximal to the NE Lau Spreading Center, particularly Tolu Volcano, have higher Mg#_modeled (~0.68 to 0.71) and higher δ⁵⁶Fe_modeled (~-0.22 to -0.19‰). To the southeast, parental melt estimates with lower Mg#_modeled (~0.64 to 0.67) and lower δ⁵⁶Fe_modeled (~-0.27 to -0.20‰) are dominant, particularly in West Mata Volcano. This suggests West Mata-type melts may contain an isotopically light, enriched (lower Mg#) subduction-derived fluid component, while Tolu-type compositions more closely resemble depleted upper mantle. Volcanoes lying between these two bear transitional δ⁵⁶Fe-Mg# compositions. The general relationship between parental melt Mg# and δ⁵⁶Fe estimates in the MVF may capture a transition in tectonic style from subduction (enriched, isotopically light compositions) in the southeast to spreading center (depleted, isotopically heavy compositions) in the northwest.