Sample preparation

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environments of the framework cations) and speciation of carbon in silicate glasses quenched from melts at pressures up to 8 GPa in the sodium

trisilicate binary system (Na2O-3SiO2, NS3) and the sodium aluminosilicate ternary system (NaAlSi3O8, albite), using high-resolution multi-nuclear (²⁹Si,

27Al, ¹⁷O, and ¹³C) solid-state NMR. We seek to reveal the pressure-induced structural changes of carbon species in silicate melts in the upper mantle (up to 240 km depth), such as the formation of bridging carbonates up to 8 GPa.

We also explore the effect of pressure on the spin-lattice relation time (T1) of CO2 in albite melts and derive quantitative information about the speciation and connectivity of carbon in silicate melts. We report the first experimental results of detailed coordination environments of framework cations (Si and Al) at high pressure above 4 GPa up to 8 GPa. We then discuss the effect of the structural changes in thermodynamic properties (i.e., solubility, isotope fractionation) of carbon-bearing silicate melts.

3.2. Experimental Methods

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prepared by adding powdered 99.7% enriched Na213CO3 (2.71 wt% and 16.63 wt%, respectively), balanced with Al2O3 and SiO2 powders, into the pre-synthesized albite glass. The amount of Na213CO3 needed for saturation of ¹³CO2 in albite glass was estimated from the linear extrapolation of previous results (Brooker et al., 1999; Stolper et al., 1987). Because the carbon solubility in silicate melts has only been available up to 4 GPa, the amount of Na213CO3 in the starting oxide-carbonate mixture above 4 GPa needs to be assumed. Here, a factor of 2 was multiplied to the predicted solubility of ¹³CO2 in albite glasses at 6 GPa from the previous study to ensure that the system was saturated with the CO2. Although the presence of void space and/or fluid inclusion is not observed in the carbon-bearing albite glasses at 6 GPa, the calculated carbon contents in albite melts at 6 GPa are similar to the predicted solubility of carbon in albite melts at 6 GPa (see section 3.3.5 for the estimation of carbon contents). After each of the starting materials was loaded into a Pt capsule and sealed, the samples were loaded into a piston cylinder and a multi-anvil apparatus at the Geophysical Laboratory with a 1/2-inch assembly and an 18/11 (octahedron edge

length/truncated edge length of the anvils) assembly, respectively. The samples were fused at 1.5 GPa and 1400 °C in the piston-cylinder apparatus for 25 min and at 6 GPa and 1700 °C in the multi-anvil device for 30 min, and then quenched to glasses by turning off the power. The initial quenching rate was approximately 500 °C/s. The carbon-free albite glass sample at 8 GPa was previously synthesized (Lee et al., 2004).

NS3 glasses were synthesized in the same manner as the synthesis of albite glasses using Na2CO3 and SiO2. The amount of Na2CO3 needed for saturation of ¹³CO2 in NS3 glass for 4, 6, and 8 GPa was estimated from the

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Table 3.1. Experimental condition on carbon-bearing silicate glasses quenched from melts at high pressure.

Albite glass NS3 glass

Pressure 1.5 GPa 6 GPa 4 GPa 6 GPa 8 GPa

wt% of

13CO2

(Na2 13CO3)

0.76 wt%

(2.71 wt%)

4.66 wt%

(16.63 wt%)

0.09 wt%

(0.39 wt%)

0.18 wt%

(0.75 wt%)

0.27 wt%

(1.13 wt%) T, time 1350 °C,

25 min

1700 °C, 30 min

1400 °C, 30 min

1400 °C, 30 min

1400 °C, 30 min Apparatus Piston

cylinder

Multi- anvil press

Multi- anvil press

Multi- anvil press

Multi- anvil press

* The amount of Na2CO3 needed for saturation of ¹³CO2 in albite and NS3 melts was estimated from previous study (Brooker et al., 1999; Mysen et al., 2009; Stolper et al., 1987), and the factor of 2 was multiplied.

linear extrapolation of previous solubility data from melts formed in reduced conditions with an iron-wüstite buffer (IW, fO2 value of ~ -12) (Mysen et al., 2009). Note that carbon solubility into melts also depends on oxygen fugacity (fO2). For instance, the solubility of carbon in Na2O-4SiO2

(NS4) melts in magnesite-hematite buffer (MH with estimated fO2 value of ~- 4) is reported to be twice larger than that in iron-wüstite buffer (Mysen et al., 2011). The estimated fO2 in the current experiment was similar to that of C-CO (CCO) buffer (with estimated fO2 value of ~-7.5) (Zhang and Duan, 2010). Taking these into consideration, the amount of Na213CO3 in the starting NS3 glasses was calculated as a factor of 2 multiplied to the

predicted solubility of carbon in NS3 glasses in reduced conditions (Mysen et al., 2009). 0.39, 0.75, and 1.13 wt% of Na213CO3 (corresponding to 0.09,

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0.18, and 0.27 wt% of ¹³CO2, respectively) were added to a pre-synthesized glass for experiments at 4, 6, and 8 GPa, respectively. The presence of void space was observed in carbon-bearing NS3 glasses quenched from melts at 6 GPa, suggesting that the NS3 glasses at 6 GPa is saturated with carbon under the current oxygen fugacity conditions. The samples were loaded into a multi-anvil device at the Geophysical Laboratory with the 18/11

assembly. All the samples were fused at approximately 1400 °C for 30 min and then quenched to glasses. Table 3.1 shows experimental conditions on carbon-bearing silicate glasses quenched from melts at high pressure.

17O-enriched NS3 glasses used for experiments at 6 GPa were synthesized in the same manner as the synthesis of NS3 glasses, using Na213CO3 and ¹⁷O-enriched SiO2 obtained from hydrolysis of 40% ¹⁷O water with SiCl4. The recovered samples were used for NMR experiments without grinding them into powder in order to avoid hydration and minimize the structural changes that might be associated with crushing and grinding. A previous study for ²⁷Al NMR of albite glasses at high pressure showed a potential drop in melt pressure during rapid quenching of the melts

(Gaudio et al., 2015). The pressure conditions of the current study could also be slightly lower, leading to less fraction of highly-coordinated network- forming cations (e.g., ^[5,6]Al) and lowering the solubility of total carbon in the silicate melts at high pressure. The glasses quenched from melts at high pressure preserve pressure-induced structural transitions in super-cooled melts (e.g., Lee, 2010; Xue et al., 1991). Therefore, the solubility and

speciation of carbon in the glasses could be somewhat different from those in the melts. See supplementary materials 3.S1 for the effect of quench on the solubility of CO2 in silicate melts and glasses.

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Due to the difficulty in the sample synthesis of fluid-bearing silicate glasses at high pressure using multi-anvil press and the relatively long NMR data acquisition time for the ~ mg of sample (see, e.g., Lee, 2010 for the detailed review), it is challenging to get the sufficient spectral data. We have limited our study to 4, 6, and 8 GPa at which sufficient amount of sample can be synthesized in our largest sample assembly. Further study with more structural information at varying pressure conditions is

desirable, but additional technical development and/or application of high- field NMR are required for sufficient number of NMR data acquisition.