The effect of CO2 and H2O on Etna and Fondo Riccio (Phlegrean Fields) liquid viscosity, glass transition temperature and heat capacity

Viscosity (η), glass transition (Tg) and heat capacity (Cp) of Etna trachybasalt and Fondo Riccio latite (Phlegrean
Fields, Italy)were determined at lowand high temperatures for dry, hydrous and CO2 bearing samples.High temperature
experiments have been performed in the range of 1499 to 1700 K by concentric cylindermeasurements,
while low temperature experiments were carried out in the interval between 633 and 1093 K using the
micropenetration technique. Glass transition temperature and glassy and liquid heat capacitieswere investigated
using differential scanning calorimetry (DSC) up to 955 K.
The H2O content in themelts ranged from nominally dry to 6.32 wt.%, while CO2 ranged from 229 to 1907 ppm.
We combined low- and high-temperature viscosities and parameterized them by the use of a modified Vogel–
Fulcher–Tamman equation,which accommodates the non-Arrhenian temperature dependence ofmelt viscosity.
Experimental measurements showthat melt viscosity decreaseswith increasing temperature and water and CO2
contents. For latitic samples at 893 K, the introduction of CO2 (up to 732 ppm) decreases the liquid viscosity up to
one order of magnitude with respect to the measured viscosity for H2O-bearing liquid. Moreover, the results of
calorimetric measurements indicate that the glass transition temperature decreaseswith increasing volatile content
(H2O+CO2). The glass transition temperature decreases by about 25 K by adding up to 1907 ppmof CO2 in
the trachybasaltic samples. No appreciable effect on glassy [Cpg (Tg)] and liquid (Cpliq) heat capacities was observed
with the addition of water and CO2. Structural and volcanological implications (i.e. volatile speciation
and melt fragility) for water and CO2 dissolution in silicate melts are discussed in light of the presented results.