GEOCHEMICAL MODELING OF BONINITE AND FAB FROM IODP EXPEDITION 352, IZU-BONIN FOREARC

IODP expedition 352 drilled four core in the Bonin forearc, two dominated by boninite and high-Mg andesite, and two with fore arc basalt (FAB). Three boninite suites were defined: high silica boninite (HSB), low silica boninite (LSB), and basaltic boninite (BB), representing at least two distinct mantle sources. In order to understand how these different magma suites formed, we evaluated trace element melting models using a depleted MORB mantle (DMM) source and a range of melting scenarios. We focus on the rare earth elements (REE) to evaluate melting, then use an expanded suite to study fluid and melt addition from the slab. The observed compositions were modeled as pooled fractional melts using a 0.1% retained melt.

FAB are depleted in LREE, Zr, Hf, Nb and Ta relative to N-MORB. Best fit models require extraction of ~1% melt in the garnet lherzolite field prior to 8.5% melt in the spinel lherzolite field. Although this matches the HREE, modeled fluid mobile element (FME) (e.g., Rb, Ba, Pb) concentrations are too low, suggesting fluid input from the subducting slab.

Boninites have lower REE and other incompatible trace element concentrations relative to FAB, but are flat to slightly enriched in LREE/HREE. Further, major element data show that distinct sources are required for HSB and LSB/BB magmas. Models using FAB residual mantle as a source produce melts far too depleted in the LREE compared to any of the observed boninites. Models using DMM as a source require significant melt extraction: after clinopyroxene depletion from spinel lherzolite at approximately 29% melting, another 20-30% melting of spinel harzburgite is required to match the heavy REE. Model concentrations of the LREE-MREE, fluid mobile elements, and some high field strength elements (Zr, Nb) are all too low to match the observed compositions. This requires a significant slab-derived hydrous melt component, or a source that is neither DMM nor FAB.

Our modeling results show that melt formation during subduction initiation is complex, with distinct sources being tapped serially, and a progressive increase in both fluid flux and the addition of slab-derived melts. Future efforts will focus on understanding how the boninite suites are related to each other, and the role of FAB (if any) in boninite formation.