<p>Hydrous equilibrium crystallization experiments were conducted on two boninitic melt compositions, low-Si boninite (LSB) and high-Si boninite (HSB), from the Izu-Bonin-Mariana (IBM) fore-arc to constrain the evolution of magma storage conditions and differentiation during subduction initiation. Experiments were performed at temperatures between 1020 and 1220 °C and at pressures of 200 and 400 MPa. Additionally, 14 natural boninite samples were analyzed for direct comparison. The experiments show that boninitic magmas crystallize orthopyroxene and/or olivine (± Cr-spinel) along the liquidus followed by clinopyroxene and finally plagioclase. Magmatic H₂O suppresses plagioclase formation and stabilizes olivine and clinopyroxene relative to orthopyroxene. Residual melts follow calc-alkaline trends, evolving to high-Mg andesitic and (high-Mg) dacitic compositions. Experimental results indicate that IBM boninitic magmas were strongly hydrous. Water contents of 1–3 wt% are estimated for LSB magmas and 3–5 wt% for HSB magmas. These different H₂O contents confirm the increasing importance of water fluxes during the evolution of boninitic magmas, from LSB in early stages to HSB in later stages. In contrast, systematic differences in magma storage conditions could not be detected. LSBs and HSBs both differentiated at temperatures between 1220 and 1040 °C and were stored at pressures between 100 and 300 MPa prior to eruption. Our study demonstrates that residual boninitic melt evolution was controlled by (near-)equilibrium crystallization processes. However, the contributions of additional magmatic processes including magma mixing and reworking of harzburgitic/dunitic cumulates during early stages of subduction initiation result in the large compositional variety observed for Izu-Bonin-Mariana fore-arc eruptive products.</p>