Experimental insights into liquid lines of descent of evolved high- and low-Si boninites from the Izu-Bonin-Mariana fore-arc

The Izu-Bonin-Mariana (IBM) fore-arc is a rare example where flux melting of the refractory mantle and the generation and differentiation 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). However, given the strongly 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 and magma storage conditions of IBM boninites, 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 200-400 MPa and 1020-1220 °C with varying H₂O contents.

For the high-Si boninite, orthopyroxene is the liquidus phase. For the low-Si boninite, orthopyroxene is the liquidus phase at low water concentrations while olivine is the liquidus phase at high H₂O contents. Differentiation for both compositions proceeds with clinopyroxene and plagioclase crystallization. Residual melts 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 as well as mineral compositions can be reproduced at 200 MPa and bulk H₂O concentrations >2 wt%. We, therefore, conclude that natural boninitic magmas differentiated at higher pressures than previously estimated.

(1) Whattam, S. A. et al. (2020): Am. Mineral. 105 (10)

(2) Coulthard, D. A. et al. (2021): Geochem. Geophys. Geosyst. 22 (1)