Quantitative measurements of the speciation of carbon in albite

results and analyses

The initial ¹³CO2 contents in albite glasses are 0.76 and 4.66 wt% at 1.5 and 6 GPa, respectively. Whereas the initial input of carbon in albite glasses quenched from melts at 6 GPa is approximately six times larger than that at 1.5 GPa, the total peak intensity of carbon species in carbon-bearing albite melts (with a 5 s delay time) increases ~ 1.6 times from the sample at 1.5 GPa to the sample at 6 GPa (Figure 3.4). This result may be due to a relatively small increase in the carbon solubility into the silicate melts at high pressure or primarily due to the pressure-induced changes in the spin- lattice relaxation time. As for the latter, ¹³C MAS NMR spectroscopy is a quantitative analysis tool that is used when the relaxation delay time of the experimental conditions is sufficiently long (e.g., longer than three times the T1 relaxation time). As the delay time of 5 s was chosen from the

experimental conditions for CO2 in fluid inclusion in enstatite at 1.5 GPa (Kim et al., 2016), calibration of peak intensity with the spin-lattice relation time (T1) is necessary. It has been observed that the T1 relaxation time of carbon species increases with increasing pressure partly due to an increase

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in network rigidity with pressure, reducing the peak intensity with increasing pressure (Kim et al., 2016). Therefore, ¹³C saturation-recovery experiments were performed to determine whether the origin of the reduction of peak intensity is from the differences in the T1 relaxation time or from the lower solubility of carbon in albite glasses.

Figure 3.6 shows the peak intensity of the ¹³C NMR signal (M) normalized with respect to the peak area of a fully relaxed spectrum (M0) with varying delay times (τ, time between p/2 pulse and p pulse in T1

relaxation time measurement). The spin-lattice relaxation time can be described using the following equation (Abragam, 1961):

M / M0 = 1 - exp[-(τ/T1)] (4)

As shown in Figure 3.6, the ¹³C NMR peak intensities of CO2 for carbon-bearing albite glasses at 1.5 and 6 GPa were well fitted with 42 s and 149 s of T1 relaxation time, respectively. From the calculated T1 relaxation time of CO2 for carbon-bearing albite glasses, the estimated normalized peak intensity of CO2 with respect to the fully relaxed maximum peak intensity is ~ 0.16 at 1.5 GPa. Because of the difference in T1, the estimated normalized peak intensity of CO2 at 6 GPa is estimated to be ~ 0.06 at 6 GPa, indicating that the peak intensity is largely underestimated. The difference in the intensity ratios of CO2 at 1.5 and 6 GPa is ~ 2.8 from the normalized peak intensity of CO2 with respect to the fully relaxed maximum peak intensity, and ~ 3.5 from the differences in the T1 relaxation time. The T1

relaxation time for CO2 in albite glasses at 6 GPa is approximately three times longer than that at 1.5 GPa, so its peak intensity at 6 GPa is underestimated (~ 1/3 of the intensity at 1.5 GPa) when identical NMR acquisition condition (i.e., relaxation delay of 5 s) is used. Therefore, it is

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Figure 3.6. Saturation-recovery of CO2 in the carbon-bearing albite melts at 1.5 GPa and 6 GPa. Diamonds and circles refer to the normalized peak intensity of carbon-bearing albite glasses at 1.5 and 6 GPa, respectively, with varying delay time. Solid lines and dashed lines refer to calculated peak intensity, following the spin-lattice relaxation time equation. Error bars represent a 10% error.

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essential to calibrate the intensity at different pressures by determining the effect of pressure on the estimated T1 relaxation time.

Figure 3.7 illustrates the total carbon content and the relative amount of each carbon species (calibrated with difference in T1 relaxation time with pressure) in carbon-bearing albite melts with increasing pressure. Solubility data for carbon-bearing albite melts is available up to 3.5 GPa (Brooker et al., 1999; Stolper et al., 1987). We calculated the proportion of each carbon species in carbon-bearing albite melts from the peak area of each carbon species from the ¹³C MAS NMR spectra. The T1 for CO and CO32- species has not been measured because of their low S/N ratio and intensities.

Nevertheless, we assumed similar changes in pressure-induced T1

relaxation times for CO and CO32- (~ 3 times longer with increasing pressure from 1.5 to 6 GPa).

Taking into consideration of the T1 relaxation time of CO2 and similar increases in the relaxation times for other carbon species, the estimated amount of CO2, CO32-, and CO at 1.5 GPa are 0.52, 0.34, and 0.14 wt%, respectively. The calibrated amount of CO2, CO32-, and CO at 6 GPa are 2.69, 1.32, and 0.08 wt%, respectively. The total amount of carbon species in carbon-bearing albite glasses at 6 GPa is ~ 4.10 wt%. The calculated carbon contents in the current study are generally consistent with the previous studies (Brooker et al., 1999; Stolper et al., 1987). The current results may present a minimum solubility and thus the actual solubility of total carbon in albite melts up to 6 GPa is higher than 4 wt%. Note that without taking the T1 relaxation time of CO2 into consideration, the CO2 contents in albite melts at 6 GPa (marked as raw data in Figure 3.7) would be 0.95 wt%, and the total amount of carbon would be ~ 1.44 wt%, which is far below the

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Figure 3.7. Carbon contents in the albite melts with increasing pressure.

Black, blue, red, and violet closed circles refer to total carbon content and the amount of CO2, CO32-, and CO species in albite glasses calculated from ¹³C MAS NMR spectra, respectively. Open triangles and rectangles refer to data from Stolper et al. (1987) and Brooker et al.

(1999), respectively.

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input amount of carbon in the albite glasses at 6 GPa. Recent ¹³C NMR study presented a calibration curve between ¹³C abundance and ¹³C MAS NMR peak intensity of ADM-SiO2 mixture (Kim et al., 2016). Using this calibration curve, the calculated carbon contents in the carbon-bearing albite glasses is similar to the amount of ¹³C initially added in the starting silicates.

Note again that the focus of the study is to explore the effect of

pressure on carbon species in silicate melts and thus, quantitative estimation of the solubility may not be fully achievable based on current data alone.

Nevertheless, the speciation of carbon and the abundance of each carbon species in silicate melts at high pressure has been successfully calculated by using ¹³C NMR. These structural data are critical for understanding the solubility and the dissolution mechanism of carbon in silicate melts above 4 GPa. As the consideration the spinning sidebands does not affect the simulation results significantly (e.g., Lee and Ahn, 2014), the proportion of each carbon species in carbon-bearing albite glasses was calculated from the peak area of each carbon species without the consideration of the spinning sidebands. The estimated fractions of CO2, CO32-, and CO from the modified

13C MAS NMR spectrum are 52, 34, and 14% at 1.5 GPa, and 66, 32, and 2%

at 6 GPa, respectively, with a ± 10% error bar. Because the exact T1 times for CO and CO32- species are not available, the estimated fractions of carbon species may be subject to a larger uncertainty despite smaller uncertainty in the fitting results. Therefore, the proportions reported here are mainly for the semi-quantitative discussion. Despite the uncertainty, the trend of changes of carbonate species with increasing pressure is evident from the

13C MAS NMR spectra. The fraction of CO2 in albite melts, taking T1 time into consideration, matches well with the theoretical results of the fraction

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of CO2 (~ 60% among the carbon species) in rhyolite melts at 5 GPa that were calculated using the MD simulation (Guillot and Sator, 2011). Previous study showed that the fraction of CO32- to total carbon contents in albite glasses increased from 23–27% at 1.5 GPa to 27–34% at 3 GPa using IR spectroscopy (Stolper et al., 1987). However, the current results showed that the calibrated fraction of carbonates slightly decreases from ~34 % at 1.5 GPa to ~32 % at 6 GPa. This is consistent with the result for the carbon- bearing albite glasses using ¹³C NMR spectroscopy (Brooker et al., 1999).

The observed difference between the IR result and the study may reside in the difference in analytical methods.