Magmatic Response to Subduction Initiation, Part II: Boninites and Related Rocks of the Izu‐Bonin Arc From IOPD Expedition 352

International Ocean Discovery Program Expedition 352 to the Izu-Bonin forearc cored over 800 m of basement comprising boninite and boninite-series lavas. This is the most extensive, well-constrained suite of boninite series lavas ever obtained from in situ oceanic crust. The boninites are characterized as high-silica boninite (HSB), low-silica boninite (LSB), or basaltic boninite based on their SiO₂-MgO-TiO₂ relations. The principal fractionation products of all three series are high-Mg andesites (HMA). Lavas recovered >250 meters below seafloor (mbsf) erupted at a forearc spreading axis and are dominated by LSB and HMA. Lavas recovered from <250 mbsf erupted off-axis and are dominated by HSB. The axial and off-axis lavas are characterized by distinct chemostratigraphic trends in their major, trace, and isotopic compositions. The off-axis lavas are chemically similar to boninite from the type locality at Chichijima, with concave-upward rare earth elements patterns. In contrast, the more abundant axial lavas have distinctly light rare earth element-depleted patterns and represent a new, previously unsampled precursor to the Chichijima-type boninite lavas. Petrogenetic modeling suggests that the axial lavas formed by fluxing of refractory mantle (likely the residue from forearc basalt extraction), with amphibolite-facies melt derived from subducting altered oceanic crust. The upper, off-axis lavas require an additional component of sediment-derived melt in addition. Both models are consistent with previously published isotopic data.