Do carbonate liquids become denser than silicate liquids at pressure? Constraints from the fusion curve of K2CO3 to 3.2 GPa

Abstract

Brackets on the melting temperature of K2CO3 were experimentally determined at 1.86 ± 0.02 GPa (1,163–1,167°C), 2.79 ± 0.03 GPa (1,187–1,195°C), and 3.16 ± 0.04 GPa (1,183–1,189°C) in a piston-cylinder apparatus. These new data, in combination with published experiments at low pressure (<0.5 GPa), establish the K2CO3 fusion curve to 3.2 GPa. On the basis of these experiments and published thermodynamic data for crystalline and liquid K2CO3, the high-pressure density and compressibility of K2CO3 liquid were derived from the fusion curve. The pressure dependence of the liquid compressibility (K′0 = dK 0/dP, where K 0 = 1/β0) is between 16.2 and 11.6, with a best estimate of 13.7, in a third-order Birch–Murnaghan equation of state (EOS). This liquid K′0 leads to a density of 2,175 ± 36 kg/m3 at 4 GPa and 1,500°C, which is ∼30% lower than that reported in the literature on the basis of the falling-sphere method at the same conditions. The uncertainty in the liquid K′0 leads to an error in melt density of ± 2% at 4 GPa; the error decreases with decreasing pressure. With a K′0 of 13.7, the compressibility of K2CO3 at 1,500°C and 1 bar (K 0 = 3.8 GPa) drops rapidly with increasing pressure ( $${K}_{4\,{\rm GPa}}=33\,\hbox{GPa}$$ ), which prevents a density crossover with silicate melts, such as CaAlSi2O8 and CaMgSi2O6, at upper mantle depths.