Subduction-related lavas have higher Fe3+/∑Fe than midocean ridge basalts (MORB). Hypotheses for this offset include imprint from subducting slabs and differentiation in thickened crust. These ideas are readily tested through examination of the time-dependent evolution of slab-derived signatures, thickening crust of the overriding plate, and evolving redox during subduction initiation. Here, we present Fe3+/ΣFe and volatile element abundances of volcanic glasses recovered from International Ocean Discovery Program (IODP) Expedition 352 to the Izu-Bonin-Mariana (IBM) forearc. The samples include forearc basalts (FAB) that are stratigraphically overlain by low- and high-silica boninite lavas. The FAB glasses have 0.18–0.85 wt% H2O, 75–233 ppm CO2, S contents controlled by saturation with a sulfide phase (602–1,386 ppm), Ba/La from 3.9-10, and Fe3+/ΣFe ratios from 0.136 to 0.177. These compositions are similar to MORB and suggest that decompression melting of dry and reduced mantle dominates the earliest stages of subduction initiation. Low- and high-silica boninite glasses have 1.51–3.19 wt% H2O, CO2 below detection, S contents below those required for sulfide saturation (5–235 ppm), Ba/La from 11 to 29, and Fe3+/∑Fe from 0.181 to 0.225. The compositions are broadly similar to modern arc lavas in the IBM arc. These data demonstrate that the establishment of fluid-fluxed melting of the mantle, which occurs in just 0.6–1.2 my after subduction initiation, is synchronous with the production of oxidized, mantle-derived magmas. The coherence of high Fe3+/∑Fe and Ba/La ratios with high H2O contents in Expedition 352 glasses and the modern IBM arc rocks strongly links the production of oxidized arc magmas to signatures of slab dehydration.

Brounce, M.Reagan, M. K.Kelley, K. A.Cottrell, E.Shimizu, K., & Almeev, R. (2021). Co-variation of slab tracers, volatiles and oxidation during subduction initiationGeochemistry, Geophysics, Geosystems22, e2021GC009823. abstract

(a) Map of the Izu-Bonin-Mariana subduction system in the western Pacific. Key geographic features are labeled. The location of International Ocean Discovery Program (IODP) Expedition 352 is marked with a black star. White stars mark the locations of samples studied by Brounce et al. (2015). Base map generated using GeoMapApp (Ryan et al., 2009; http://www.geomapapp.org). (b) Inset of (a) showing the four drill sites of IODP Expedition 352. (c) A schematic cross section of A-A′ from (b), modified after Reagan et al. (2015). Approximate locations and depths of the four drill sites of IODP Expedition 352 are marked in blue. Water depths are marked on the left in meters below sea level.