Retention of water in subducted slabs under core–mantle boundary conditions

Abstract

The hydrated SiO2 phase is a main carrier of water in subducting slabs in the lower mantle. Assuming its dehydration at high temperatures above the core–mantle boundary, it has been speculated that seismic anomalies observed in this enigmatic region and the uppermost core might be attributable to water released from slabs. Here we report melting experiments on a hydrous basalt up to conditions of the core–mantle boundary region at 25–144 GPa and 2,900–4,100 K. Secondary-ion mass spectrometry measurements with high-resolution imaging techniques reveal that the SiO2 phase and SiO2–AlOOH solid solution contain 0.5–3.6 wt% and ~3.5 wt% H2O, respectively, coexisting with melts holding 0.9–2.6 wt% H2O. The high solubility into SiO2 and high SiO2/melt partition coefficient of water at the high temperatures of the core–mantle boundary region suggest that practically water does not escape from subducted slabs at the base of the mantle. Even if the core–mantle boundary temperature were high enough to melt subducted crustal materials, most of the H2O would remain in the solid residue rather than entering a partial melt. Previously proposed consequences of slab dehydration are therefore unlikely to be responsible for chemical heterogeneities in the lowermost mantle and the topmost core. Water-bearing subducted slabs may not dehydrate and contribute to chemical heterogeneities at the core–mantle boundary, according to high-pressure and high-temperature melting experiments.