Do carbonate liquids become denser than silicate liquids at pressure? Constraints from the fusion curve of K_2CO_3 to 3.2 GPa

摘要

Brackets on the melting temperature of K_2CO_3 were experimentally determined at 1.86 +- 0.02 GPa (1,163-1,167 deg C), 2.79 +- 0.03 GPa (1,187-1,195 deg C), and 3.16 +- 0.04 GPa (1,183-1,189 deg C) in a piston-cylinder apparatus. These new data, in combination with published experiments at low pressure (<0.5 GPa), establish the K_2CO_3 fusion curve to 3.2 GPa. On the basis of these experiments and published thermodynamic data for crystalline and liquid K_2CO_3, the high-pressure density and compressibility of K_2CO_3 liquid were derived from the fusion curve. The pressure dependence of the liquid compressibility (K'_0 = dK_0/dP, where K_0 = l/beta_0) 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/m~3 at 4 GPa and l,500 deg C, which is approx30 percent 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 percent at 4 GPa; the error decreases with decreasing pressure. With a K'_0 of 13.7, the compressibility of K_2CO_3 at l,500 deg C and 1 bar (K_0 = 3.8 GPa) drops rapidly with increasing pressure ( K_(4 GPa) = 33 GPa), which prevents a density crossover with silicate melts, such as CaAlSi_2O_8 and CaMgSi_2O_6, at upper mantle depths.