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 (Na 2 O-3SiO 2 , NS3) and sodium aluminosilicate (NaAlSi 3 O 8 , albite) glasses quenched from melts at high pressure up to 8 GPa using multi-nuclear solid-state NMR. The 27 Al triple quantum (3Q) MAS NMR spectra for carbon-bearing albite melts revealed the pressure-induced increase in the topological disorder around 4 coordinated Al ( [4] Al) without forming [5,6] Al. These structural changes are similar to those in volatile-free albite melts at high pressure, indicating that the addition of CO 2 in silicate melts may not induce any additional increase in the topological disorder around Al at high pressure. 13 C MAS NMR spectra for carbon-bearing albite melts show multiple carbonate species, including [4] Si(CO 3 ) [4] Si, [4] Si(CO 3 ) [4] Al, [4] Al(CO 3 ) [4] Al, and free CO 3 2– . The fraction of [4] Si(CO 3 ) [4] Al increases with increasing pressure, while those of other bridging carbonate species decrease, indicating that the addition of CO 2 may enhance mixing of Si and Al at high pressure. A noticeable change is not observed for 29 Si 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 29 Si MAS NMR spectra for partially depolymerized, carbon-bearing NS3 glasses show that the fraction of [5,6] Si 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 [4] Si with pressure. 17 O NMR spectrum shows that the fraction of Na⋯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 17 O NMR spectrum primarily due to low carbon concentration, 13 C MAS NMR results imply the formation of bridging carbonates, [4] Si(CO 3 ) [4] Si, above 6 GPa. The spin-lattice relaxation time ( T 1 ) of CO 2 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 ∼1 wt% at 1.5 GPa 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. [ABSTRACT FROM AUTHOR]