Carbon dioxide in silica-undersaturated melt. Part I: The effect of mixed alkalis (K and Na) on CO2 solubility and speciation

Abstract

Silica undersaturated melts such as nephelinites are very peculiar magmatic materials. Their occurrence on Earth is often associated with carbonatite melts. These low-silica melts can dissolve a large quantity of CO2 and are rich in alkalis. However, the way CO2 dissolves into these melts and the effect of different alkali elements are poorly constrained.We present experimental results on the CO2 solubility and speciation in synthetic nephelinite in the NKCMAS system, equilibrated at high-pressure (50–300MPa), high-temperature (1250°C) with an excess C–O–H fluid phase. The nephelinitic glasses with 20mol.% total alkali oxides were synthesized with varying K2O/(K2O+Na2O) ratio (denoted as #K) in order to investigate the differential effect of those two alkali cations on CO2 solubility and speciation. All experiments were conducted under oxidizing conditions (>NNO+3) resulting in binary fluid phase compositions with CO2 and H2O species. The CO2 content and speciation were investigated using micro-Raman and Solid State NMR spectroscopies for 13C nucleus.We observe an increase in CO2 content as a function of pressure, consistent with previous studies but CO2 solubility is much higher than in alkali-poorer melts. The CO2 content is above 1wt.% at 50MPa and increases up to 4.5wt.% at 300MPa. The progressive replacement of Na by K (#K between 0 and 1) induces an increase in CO2 content. At 50MPa, the CO2 solubility is ∼1.75wt.% in the K-free glass (#K=0) and increases up to ∼3.0wt.% CO2 in Na-free glass (#K=1). The change in CO2 solubility as a function of #K is discussed in terms of carbonatite genesis.The 13C NMR spectra show that carbonate (CO32−) environments can be attributed to carbonate species associated to non-bridging oxygen in agreement with the depolymerized nature of the investigated compositions. Two singular additional carbonated species were also identified with 13C NMR signatures at 161 and 165ppm. Those species are assigned to isolated K+…CO32−…H+ and Na+…CO32−…H+ carbonate species. The presence of such isolated carbonate species is interpreted as possible precursors to carbonatitic melt genesis.