Water-driven evolution of hydrous boron-bearing silicate melts at high pressures up to 8 GPa: Implications for dynamic properties and boron isotope content in hydrous mantle melts

Abstract

A prominent seismic weakening has suggested the potential presence of H2O-bearing complex silicate melts in the Earth’s interiors. As boron is typically accompanied by the hydrous fluids and melts and may be transported into deep mantle, boron isotopic evolution enables us to trace the deep-water cycle. The impact of H2O on the structures of boron-bearing magmas is essential to grasp the melting processes and the seismic weakening in Earth’s mantle. In this study, we report experimental results on the structural changes in volatile-bearing, partially-depolymerized aluminoborosilicate glasses (NaAl0.5B0.5Si3O8) - an analog for boron-rich rhyolitic melts - with varying H2O content and at pressure conditions from 6 to 8 GPa (∼180–240 km depth in the mantle) via multi-nuclear (1H, 17O, 27Al, 11B, 29Si, and 23Na) magnetic resonance (NMR) spectroscopy. At isobaric conditions both at 6 GPa and 8 GPa, compared to the coordination environments of anhydrous quenched melts, adding water up to 4 wt% generally promotes the formation of [4]B and systematically suppresses the [5,6]Al fractions; however, in the glasses with 8 wt% water (at 8 GPa), a dramatic increase in both [6]Al and [4]B is observed, while Si coordination number remains unchanged. In contrast, at a constant H2O content of 4 wt%, an increase in the pressure from 6 to 8 GPa results in an increase in the [5,6]Al fractions and a decrease in the [4]B fraction. The results highlight complex water-pressure dependences on the cation coordination transformation. Upon adding water content at isobaric condition, boron is more prone to be highly-coordinated over aluminum. In contrast, with increasing pressure at a constant water content, aluminum prefers to be highly-coordinated over boron. These NMR results allow us to refine water dissolution schemes, highlighted by selective network depolymerization of B and Al-bearing bridging oxygens and subsequent annihilation of oxygen triclusters consisting primarily of Al, consistent with a water-driven evolution of B-Al coordination numbers in response to pressurization. These changes induce non-linear decrease in melt viscosity and seismic weakening in the mantle at depths of ∼180–240 km. Observed boron coordination evolutions indicate that deep mantle melts could exhibit complex pressure-water-driven changes in the tendency to enrich 10B. In particular, the effect of 8 wt% water contributes to ∼41 % of the overall reduction of δ11B in deep melts. Finally, as the deep magmatic melts could also be more mafic than those studied here, the water-driven structural changes in more mafic melts with much less boron contents under compression remain to be explored. While the details may vary with melt composition, the current probing of the structures of boron-bearing compressed melts with varying degree of hydration enables better prediction of the deep boron and water cycles.