Magmatic Evolution of Early Subduction Zones: Geochemical Modeling and Chemical Stratigraphy of Boninite and Fore Arc Basalt from the Bonin Fore Arc

The Izu-Bonin-Mariana arc stretches south from Japan to Guam in the Western Pacific. International Ocean Discovery Project Expedition 352 drilled four core in the fore arc of the Izu-Bonin arc east of the Bonin Islands: U1439C, U1440B, U1441A, and U1442A. From the four core, 124 samples were retrieved and analyzed for major and trace elements. Two main rock types were identified: FAB and boninite. FAB is a Mid-Ocean Ridge Basalt (MORB)-like tholeiite with variable fluid mobile element enrichment such as Rb, Ba, and Sr, and low Ti/V ratios more similar to an island arc volcanic than a mid-ocean ridge volcanic. Boninite is a hydrous high-Mg andesite with low TiO2 and distinctive subduction zone characteristics in the form of elevated fluid mobile elements and melt mobile elements. FAB was assumed to be formed from a Depleted MORB-Mantle (DMM) source and the boninite was formed from a depleted mantle source, presumably the mantle after FAB melt was extracted. Here, we used the Rare Earth Elements (REE) of the samples to model melt scenarios for the FAB and boninite in order to better understand the initial volcanic product of subduction zones. This research was funded by in joint by the National Science Foundation, Consortium for Ocean Leadership, and International Ocean Discovery Program. 124 samples were analyzed using an X-Ray Fluorescence (XRF) and Inductively Coupled Plasma - Mass Spectrometer (ICP-MS) to determine the major and trace elements. These analyses were then used to recreate the chemostratigraphy defined by the shipboard crew and determine variations within the core. We found that there was variability as the magma evolved over time and mixed with other melts, seen in magma mixing horizons. Boninite samples were separated based on their SiO2 and MgO concentrations into Basaltic Boninite (BB), Low-Silica Boninite (LSB), and High-Silica Boninite (HSB) with BB being more primitive and HSB being more evolved. These volcanics are the first known products of the subduction zone and were used to model the early evolution of the subduction zone. FAB was the first product due to its proximity to the trench and greater age than the boninite. Assumed to be generated from DMM, FAB was modeled with a total melt extraction of ~20% spinel lherzolite and 1% garnet lherzolite. Boninite was assumed to be generated from the FAB residue because it requires a depleted source and because the FAB residue was within the hydrous flux melt zone of the subduction factory. Boninite was modeled at high degree of melt from the FAB residue, however an additional melt must be added to the model to match the observed samples. We proposed a small fraction of FAB melt mixed with the models because it is still present in the subduction factory, observed in core U1439C with a FAB sample in the HSB regime