The Izu-Bonin-Mariana (IBM) fore-arc is a rare example where flux melting of the refractory mantle and the generation of boninitic melts can be investigated. A previous study proposed that IBM boninitic magmas were stored at very shallow depths (20–40 MPa) and temperatures > 1100 °C, based on orthopyroxene-liquid and olivine-liquid thermobarometry [1]. Given the strong hydrous nature of these boninitic magmas [2] (up to 3.2 wt% H2O in quenched natural glasses), higher pressures seem likely.

To constrain the liquid lines of descent, magma storage conditions of IBM boninites and the effect of water on orthopyroxene liquidus temperatures, we conducted equilibrium crystallization experiments on evolved high- and low-Si boninites using an internally heated pressure vessel (IHPV). Experiments were performed over a P-T range of 200-500 MPa and 1040-1260 °C with varying melt H2O contents.

For the high-Si boninite, orthopyroxene is the liquidus phase and the effect of water on the orthopyroxene liquidus depression was calibrated to be ΔT = 26.02 • CH2O^(0.93) at 200 MPa, where ΔT represents the difference between the anhydrous liquidus temperature (°C) and the liquidus at a specific melt water concentration CH2O (wt%). For the low-Si boninite, orthopyroxene is the liquidus phase only at low H2O concentrations while olivine is the liquidus phase at high H2O contents. Differentiation for both compositions proceeds with clinopyroxene and plagioclase crystallization. Residual melt compositions follow calc-alkaline differentiation trends, evolving to high-Mg andesitic and dacitic compositions. Experimental results demonstrate that the liquid lines of descent of natural IBM boninitic glasses can be reproduced at 200 MPa and high melt H2O concentrations (>3 wt%). We, therefore, conclude that natural boninitic magmas differentiated at higher pressures than previously estimated by thermobarometry.

[1] Whattam, S. A. et al. (2020): Am. Mineral. 105 (10)
[2] Coulthard, D.A. et al. (2021): Geochem. Geophys. Geosyst. 22 (1)