Solubility of Water in Carbonatites

Water solubility in carbonatite magmas at 1 bar total pressure has been explored. Three compositions of carbonatite magma have been studied: natrocarbonatite from Oldoinyo Lengai, Tanzania, and two synthetic compositions CaCO3–Na2CO3–MgCO3 and CaCO3–Na2CO3–CaF2–NaF. All three were characterized with...

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Bibliographic Details
Published in:ACS earth and space chemistry 2020-11, Vol.4 (11), p.2144-2152
Main Authors: Jacobson, Nathan S, Fegley, Bruce, Setlock, John A, Costa, Gustavo
Format: Article
Language:English
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Summary:Water solubility in carbonatite magmas at 1 bar total pressure has been explored. Three compositions of carbonatite magma have been studied: natrocarbonatite from Oldoinyo Lengai, Tanzania, and two synthetic compositions CaCO3–Na2CO3–MgCO3 and CaCO3–Na2CO3–CaF2–NaF. All three were characterized with thermal analysis (DSC-TG). Solubility measurements were conducted with a novel microbalance based method adapted from chemical metallurgy. The mixtures were melted under 1 bar of CO2, and then a mixture of H2O/CO2 was admitted. Thus, a direct measurement was made with no effects of cooling such as crystallization and phase changes. The natrocarbonate showed no measurable solubility at 0.57 bar H2O and 800 °C. This is attributed to its altered form from atmosphere exposure, likely compositional changes on weathering. CaCO3–Na2CO3–MgCO3 showed a small solubility (0.47 ± 0.07 wt % H2O at 900 °C and 0.57 bar H2O). This was consistent with an extrapolation from work by other investigators and also additional oxide anions reacting with water molecules in this study. The CaCO3–Na2CO3–CaF2–NaF synthetic magma showed a larger solubility 6.0 ± 1.4 wt % H2O at 0.57 bar H2O and 675 °C. The resultant CaCO3–Na2CO3–CaF2–NaF–H2O melt is described with a Temkin/Regular solution model to understand the dissolution process. The mechanism of dissolution is discussed with particular attention to the effect of fluoride on enhancing water solubility.
ISSN:2472-3452
2472-3452
DOI:10.1021/acsearthspacechem.0c00222