Effect of pressure on the short-range structure and speciation of carbon in alkali silicate and aluminosilicate glasses and melts at high pressure up to 8 GPa: 13C, 27Al, 17O and 29Si solid-state NMR study

Despite the pioneering efforts to explore the nature of carbon in carbon-bearing silicate melts under compression, experimental data for the speciation and the solubility of carbon in silicate melts above 4 GPa have not been reported. Here, we explore the speciation of carbon and pressure-induced changes in network structures of carbon-bearing silicate (Na2O-3SiO(2), NS3) and sodium aluminosilicate (NaAlSi3O8, albite) glasses quenched from melts at high pressure up to 8 GPa using multinuclear solid-state NMR. The Al-27 triple quantum (3Q) MAS NMR spectra for carbon-bearing albite melts revealed the pressure-induced increase in the topological disorder around 4 coordinated Al (Al-[4]) without forming Al-[5,Al-6]. These structural changes are similar to those in volatile-free albite melts at high pressure, indicating that the addition of CO2 in silicate melts may not induce any additional increase in the topological disorder around Al at high pressure. C-13 MAS NMR spectra for carbon-bearing albite melts show multiple carbonate species, including Si-[4](CO3)Si-[4], Si-[4](CO3)Al-[4], Al-[4](CO3)Al-[4], and free CO32-. The fraction of Si-[4](CO3)Al-[4] increases with increasing pressure, while those of other bridging carbonate species decrease, indicating that the addition of CO2 may enhance mixing of Si and Al at high pressure. A noticeable change is not observed for Si-29 NMR spectra for the carbon-bearing albite glasses with varying pressure at 1.5-6 GPa. These NMR results confirm that the densification mechanisms established for fluid-free, polymerized aluminosilicate melts can be applied to the carbon-bearing albite melts at high pressure. In contrast, the Si-29 MAS NMR spectra for partially depolymerized, carbon-bearing NS3 glasses show that the fraction of Si-[5,Si-6] increases with increasing pressure at the expense of Q(3) species ([4] Si species with one non-bridging oxygen as the nearest neighbor). The pressure-induced increase in topological disorder around Si is evident from an increase in peak width of Si-[4] with pressure. O-17 NMR spectrum shows that the fraction of Na center dot center dot center dot O-([5]) Si in carbon-bearing NS3 glasses is less than that of carbon-free NS3 glasses at 6 GPa potentially due to the formation of bridging carbonate species. While its presence is not evident from the O-17 NMR spectrum primarily due to low carbon concentration, C-13 MAS NMR results imply the formation of bridging carbonates, Si-[4](CO3)Si-[4], above 6 GPa. The spin-lattice relaxation time (T-1) of CO2 in albite melts increases with increasing pressure from 42 s (at 1.5 GPa) to 149 s (at 6 GPa). Taking the pressure-induced change in T-1 of carbon species into consideration, total carbon content in carbon-bearing albite melts increases with pressure from similar to 1 wt% at 1.5 GPa to similar to 4.1 wt % at 6 GPa. The results also reveal a noticeable drop in the peak intensity of free carbonates in carbon-bearing NS3 melts at 6 GPa, implying a potential non-linear change in the carbon solubility with pressure. The current results of carbon speciation in the silicate melts above 4 GPa provide an improved link among the atomic configurations around carbon species, their carbon contents, and isotope composition of carbon-bearing melts in the upper mantle. (C) 2018 Elsevier Ltd. All rights reserved.