Pressure-induced structural evolution in boron-bearing model rhyolitic glasses under compression: Implications for boron isotope compositions and properties of deep melts in Earth’s interior

Abstract

The pressure-induced structural evolutions of boron-bearing model rhyolitic melts under high pressures enable to infer the detailed geochemical processes (melting and fluid-rock-melt interactions) occurring in Earth interiors and to control the melt properties (viscosity and the boron isotope composition, δ11B) of complex magmatic melts, providing insights into the boron cycle toward the deeper part of the upper mantle (∼10 GPa). Despite the importance, the structures of multicomponent boron-bearing silicate melts above 3 GPa are currently unavailable. Here, we explore the structures, particularly, coordination transformation of constituent elements in boron-bearing nepheline and albite glasses – a model rhyolitic melts - upon compression to a depth of ∼270 km (∼9.2 GPa) in the mantle using multi-nuclear solid-state nuclear magnetic resonance (NMR) spectroscopy. The results showed that the conversion of [3]B into [4]B is prominent upon compression up to 6 GPa. In contrast, the formation of [5,6]Al is accompanied by the formation of oxygen tricluster above 6 GPa, where all the nonbridging oxygens are consumed. We quantify how the melt composition affects tendency to form highly coordinated B, Al, and Si upon compression. Particularly, the increase in the [4]B population tends to be larger for the glasses with low Si content as pressure increases to 9.2 GPa. We reveal the relationship between such structural adaptations of the compressed melts at high pressure and the melt properties, including viscosity and element partition coefficient in boron-bearing melts. The current NMR results also unravel the structural origins of 11B/10B ratios in rhyolitic melts at high pressure. Considering a preferential partitioning of 10B to [4]B, an increase in [4]B population in the melts leads to an pressure-induced enrichment of 10B. As the increase in Si/B ratio in the melts tends to decrease the pressure-induced increase in [4]B fraction, the contribution of boron coordination transformation on the 11B/10B ratios in silicate melt would be somewhat minor in deep mantle melts with increasing Si content. The detailed boron environments in rhyolitic melts at high pressure yield useful constraints for the isotope composition (11B/10B) of dense mantle melts, thereby enabling quantification of deep boron cycle.