Probing the atomic structure of volcanic glasses with synchrotron radiation

The Carbon (C) geochemical cycle, i.e. the transfer of C from the Earth's interior to the Earth's atmosphere, is subject to various constraints, related in particular to the pressure, temperature and chemical composition of the environments in which C is found. CO₂ behaviour in deep magmatic systems contributes to these constraints. While this behaviour is relatively well understood in kimberlite magmatic systems from a macroscopic point of view (i.e. solubility), on a microscopic scale however, there are several unexplored areas, such as:

1) the determination of the dissolution mechanisms of CO₂ (present as CO₃^2- ions), as well as:

2) the near atomic environment (i.e. alkaline and/or alkaline-earth) of the CO₃^2- groups for these kimberlite compositions, which are very low in silica and rich in CO₂.

In a series of kimberlitic glasses synthesized under high pressure conditions (up to 4 GPa, i.e. 120 km depth) and exhibiting CO₂ contents ranging up to 17 wt%, the researchers studied the Mg and Ca atomic environments using the XANES and EXAFS techniques on the LUCIA beamline, in order to determine if the CO₂ had a more pronounced affinity for one or other of these elements.

Figure 1: sample of kimberlitic glass

This study (spectra acquired at the Mg and Ca K-edges, Fig. 2), conducted with LUCIA scientists, determined that the Mg environment is not affected by the presence of CO₂ whereas the Ca environment is significantly modified in the presence of CO₂ (Fig 3A). Specifically, the EXAFS study of Ca demonstrates that Ca coordinates increase with the increase in CO₂ (Fig. 3B).

Figure 2: X-ray absorption spectra obtained on the LUCIA beamline at the Ca K-edge (~4030 eV) in kimberlitic composite glasses synthesized under high pressure conditions (1.5 GPa) and with different CO₂ contents ranging from 7 to 17 wt%. The spectra acquired for a CaSiO₃ glass serves as a standard and contains no CO₂.

Figure 3: A) Variation in the energy position of the white line (see Fig. 1) in the Ca and Mg spectra based on the glasses CO₂ content. While the position of the line for Mg remains relatively constant, the Ca line is significantly affected by the CO₂ presence. B) Change in the Ca coordination number in the vitreous structure according to the CO₂ content. The addition of CO₂ results in an increase in the Ca coordination number.

Conclusion

The main conclusion of this study is that CO₂ dissolves preferentially in the vicinity of the Ca environment within the atomic structure of kimberlite glasses. The CO₂ differential affinity for Ca and Mg is mainly explained for geometrical reasons; the Ca atom environment is less dense than the Mg (bond with nearest neighbour >2.5 Å for Ca and 2Å for Mg). This lack of space in Mg is due to the very nature of the element itself, which has a much higher cationic charge per volume leading to bonds with the nearest shorter neighbours that repulse CO₃^2- groups.

Globally, these results have an impact on the understanding of CO₂ transport mechanisms in the geochemical carbon cycle on a terrestrial scale:

1) primitive terrestrial Mg-rich magma (komatiites) would have been very poor CO₂ transporters in the Earth's primitive atmosphere;

2) conversely, the Ca-rich magmas of the East African rift transport a large quantity of CO₂;

3) kimberlitic magmas that undergo progressive enrichment in Mg during their ascent, have their CO₂ transport capacity reduced, thereby generating a fluid phase that is responsible for the explosive nature of the kimberlitic magmas.