CO 2 -bearing basaltic glasses

6.2.1. Influence of CO₂ on the viscosity of basaltic melts

The effect of CO2 on the viscosity of halogen-free basaltic melts is represented in figure 137.

The measured viscosity data is described by open symbols fitted with an Arrhenian equation (dashed line). One important aspect to mention is that the synthesised samples in IHPV generate low water contents. The presence of 0.10 wt% H₂O in silicate melts results in a decrease in viscosity by 0.2 log units (see 6.1.1. Influence of H₂O on the viscosity of basaltic melts). Therefore, this decreasing viscosity effect has to be calculated out of the viscosity of CO₂-bearing silicate melts. This calculation is represented as close symbols with Arrhenius plot (line). Besides the CO₂-bearing basaltic melts, the CO₂-free basaltic melts show 0.10 wt% H₂O content as well and thus all CO₂-bearing samples have a higher viscosity by 0.2 log units. Therefore, the relationship between these three melts remains constant.

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Figure 137: Effect of CO2 on viscosity of halogen-free basaltic melts with 0.10 wt% H2O. The original viscosity data is represented as open symbols with the Arrhenius plot (dashed line). The solid symbols demonstrate the water-corrected viscosity data (line). Rectangles shows the CO2-free melt, circles represents the 1479 ppm CO2

melt and triangles is the 2199 ppm CO2 melt. The error bars are smaller than the circles and rectangles.

In figure 138, the viscosity data of the present basaltic melt is plotted as a function of CO₂ content (wt%) and compared with the viscosity data of Bourgue and Richet (2001). The addition of 0.23 wt% CO₂ to the present basaltic melts results in a decrease in viscosity by 0.6 log units (green circles). This basaltic melts have 0.10 wt% H2O content. Previous studies have focused on the combined effect of H₂O and CO₂ on the viscosity of silicate melts.

That is the reason why a qualitative comparison with literature data does not work well.

Bourgue and Richet (2001) synthesised diverse H₂O- and CO₂-bearing silicate melts with the focus on SiO2/K2O ratio. Figure 138 shows the associated viscosity data (brown circles). The addition of 1.99 wt% CO₂ to the silicate melt results in a decrease in viscosity by 2.0 log units. It is important to note that the H₂O content varies between 0.20 and 0.39 wt%. The complex melt structure of the present basalt results in the strong decrease in viscosity.

Precise statement about the CO₂ effect on silicate melts requires more measurements with glasses of varying CO2-content without H2O.

The effect of CO₂ on the viscosity of halogen-free and –bearing basaltic melts is shown in figure 139. The presence of 2199 ppm CO2 in the present basalt shows the above mentioned decreasing effect on viscosity of 0.7 log units. The addition of 1853 ppm CO₂ to the fluorine-bearing basaltic melts results in the strongest decrease in viscosity by 2.0 log units, whereas

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the presence of 3658 ppm CO₂ in chlorine-bearing basalt shows a slighter decrease in viscosity by 0.5 log units. The addition of 2975 ppm CO₂ to (Cl^- + F^-)-bearing basaltic melts results in a mixed effect. The viscosity increases due to the addition of 1234 ppm CO2 and further addition of 2975 ppm CO₂ results in a decrease in viscosity by 0.3 log units. Thus, the presence of CO2 to halogen-free and –bearing basaltic melts results in a decrease in viscosity.

Figure 138: The change in viscosity due to the addition of CO2 at the same temperature as the original melt at 10¹² Pa s for present basalt (~ 0.10 wt% H2O). Literature data: Bourgue and Richet (2001) – K2Si2O5-K2Si3 + 0.22 wt% H2O. The numbers represent the Fe²⁺/Fetotal. The error bars are smaller than the symbols.

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Figure 139: The change in viscosity due to the addition of CO2 at the same temperature as the original glass at 10¹² Pa s for halogen-free and –bearing basaltic glasses. The numbers represent the Fe²⁺/Fetotal. The error bars of log10 viscosity are smaller than the symbols.

6.2.2. Effect of CO2 on the iron ratio of basaltic melts

The presence of 1579 ppm CO₂ in halogen-free basaltic glasses increases the Fe²⁺/Fe_total from 0.60 to 0.66, but the further addition of 2199 ppm CO2 shows a decrease in Fe²⁺/Fetotal

from 0.66 to 0.64. There is no clear dependence between Fe²⁺/Fe_total and CO₂ content.

Furthermore, the addition of CO2 to chlorine-bearing basaltic glasses results in a decrease in Fe²⁺/Fe_total, whereas the presence of CO₂ in fluorine-bearing basaltic glasses increases the Fe²⁺/Fe_total. This variation of Fe²⁺/Fe_total in halogen-bearing basaltic glasses is not only effected by the addition of CO2 but also halogens.

Figure 140 illustrates the effect of iron speciation on the normalised LF/HF (Raman spectra), whereas figure 141 shows the CO₂ content as a function of normalised LF/HF. The normalised LF/HF intensity ratios of halogen-free basaltic glasses increase with increasing Fe²⁺/Fe_total as well as with increasing CO₂ content. The addition of CO₂ to chlorine-bearing basaltic glasses results in a strong decrease in LF/HF intensity ratio due to increasing Fe²⁺/Fe_total and increasing CO₂ content. The increasing Fe²⁺/Fe_total in fluorine-bearing basaltic glasses shows a slight increase in LF/HF intensity ratio accompanied by a decreasing CO₂ content.

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Thus, the addition of 1234 ppm CO₂ to (Cl^- + F^-)-bearing basaltic melts increases the LF/HF intensity ratio as a function of increasing Fe²⁺/Fe_total, whereas the further addition of 2975 ppm CO2 decreases the LF/HF intensity ratio due to increasing Fe²⁺/Fetotal. The increasing CO₂ content results in an increase in LF/HF intensity ratio regardless of the iron speciation.

As a consequence, the Raman spectroscopy does not allow an observation of the structural change by the addition of CO₂ to basaltic melts due to varied iron speciation.

Figure 140: Effect of iron speciation on the intensity ratio LF/HF of halogen-free and halogen-bearing basaltic glasses with CO2.

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Figure 141: Effect of iron speciation on the intensity ratio LF/HF of halogen-free and halogen-bearing basaltic glasses with CO2.

6.2.3. Effect of CO2 on the structure of peralkaline melts

The solubility of CO₂ to peralkaline melts depends on the melting temperature and pressure (Brey 1976). Fine and Stolper (1985) suggested that the molecular CO2 to carbonate ratio changes by the silicate composition in the glass. In basaltic glasses, all CO₂ is dissolved in form of carbonate (Brey 1976; Fine and Stolper 1985), whereas high silica content results in the formation of molecular CO₂. The carbonate groups prefer the bonding to non-bridging oxygen atoms. The splitting of carbonate peak in infrared spectroscopy describes the asymmetric stretching vibrations, which are characterised for most natural glass (Ni and Keppler 2013).