Heat capacity of hydrous basaltic glasses and liquids

Abstract

We determined the heat capacities of four series of glasses and liquids of basaltic and basaltic andesite compositions from remelted volcanic rock samples and Fe-free synthetic analogues. The samples are low-alkali, Ca- and Mg-rich aluminosilicates with non-bridging oxygen to tetrahedrally-coordinated cation ratios (NBO/T) ranging between 0.33 and 0.67. Differential scanning calorimetry measurements were performed at atmospheric pressure between room temperature and ~100K above the glass transition for hydrous samples and up to ~1800K for dry samples. The water contents investigated range up to 5.34wt.% (16.4mol%). Water does not measurably affect the heat capacity of glasses. We derived a new value of the partial molar heat capacity of water in silicate glasses of C¯P,H2Oglass=82.804+10−3T−48.274×10−5T−2 (J/molK) using our new data in combination with literature data on more and less polymerized compositions. The increase in heat capacity at the glass transition is of the order of ~30–40% and generally increases with increasing water content. The onset of the glass transition in hydrous samples occurs below the Dulong–Petit limit of 3R/g atom. The configurational heat capacity, i.e., the magnitude of the change in heat capacity observed at the glass transition, generally increases as polymerization decreases and as water content increases. We obtained a partial molar heat capacity of water in silicate liquids of basaltic composition of ~86J/molK. This value is comparable to the partial molar values for the major oxides which range from ~79 to 230J/molK. The partial molar heat capacity of water in silicate liquids appears to be compositionally-dependent, increasing as melt polymerization decreases. Such a dependence is certainly linked to the speciation and structural roles of water in complex silicate melts, however, a single value of ~93J/molK could reproduce the heat capacity of hydrous liquids of a wide range of NBO/T (0–1.51) at temperatures up to ~100K above the glass transition and water contents of 0–3.76wt.% with a root-mean square deviation of only 3.23J/molK.