Heat capacity of hydrous trachybasalt from Mt Etna: comparison with CaAl2Si2O8 (An)–CaMgSi2O6 (Di) as basaltic proxy compositions

The specific heat capacity (Cp) of six variably hydrated (~3.5 wt% H2O) iron-bearing Etna trachybasaltic glasses and liquids has been measured using differential scanning calorimetry from room temperature across the glass transition region. These data are compared to heat capacity measurements on thirteen melt compositions in the iron-free anorthite (An)–diopside (Di) system over a similar range of H2O contents. These data extend considerably the published Cp measurements for hydrous melts and glasses. The results for the Etna trachybasalts show nonlinear variations in, both, the heat capacity of the glass at the onset of the glass transition (i.e., Cpg) and the fully relaxed liquid (i.e., Cpl) with increasing H2O content. Similarly, the “configurational heat capacity” (i.e., Cpc = Cpl − Cpg) varies nonlinearly with H2O content. The An–Di hydrous compositions investigated show similar trends, with Cp values varying as a function of melt composition and H2O content. The results show that values in hydrous Cpg, Cpl and Cpc in the depolymerized glasses and liquids are substantially different from those observed for more polymerized hydrous albitic, leucogranitic, trachytic and phonolitic multicomponent compositions previously investigated. Polymerized melts have lower Cpl and Cpc and higher Cpg with respect to more depolymerized compositions. The covariation between Cp values and the degree of polymerization in glasses and melts is well described in terms of SMhydrous and NBO/Thydrous. Values of Cpc increase sharply with increasing depolymerization up to SMhydrous ~ 30–35 mol% (NBO/Thydrous ~ 0.5) and then stabilize to an almost constant value. The partial molar heat capacity of H2O for both glasses (Cgp H2O) and liquids (C1p H2O) appears to be independent of composition and, assuming ideal mixing, we obtain a value for C1p H2O of 79 J mol−1 K−1. However, we note that a range of values for C1p H2O (i.e., ~78–87 J mol−1 K−1) proposed by previous workers will reproduce the extended data to within experimental uncertainty. Our analysis supgests that more data are required in order to ascribe a compositional dependence (i.e., nonideal mixing) to C1p H2Ol.