THE EFFECT OF WATER ON THE PHYSICAL AND THERMODYNAMIC PROPERTIES OF CALC-ALKALINE BASALT AND BASALTIC ANDESITE

We present corresponding viscosity and heat capacity data obtained on a series of hydrated glasses and supercooled melts for Fe-free calc-alkaline basaltic (sb, NBO/T = 0.67) and basaltic andesite analogues (sba, NBO/T = 0.40). Water contents range from nominally anhydrous to 3.76 wt.% H₂O.

Viscosity data were obtained at temperatures in the neighbourhood of the glass transition using the parallel-plate method, and at superliquidus temperatures using the concentric-cylinder and falling-sphere methods. The effect of water on viscosity is most dramatic at low temperatures, with the addition of ~2 wt.% H₂O resulting in a reduction of the temperature at which the viscosity of the melt is 10¹² Pa s (T₁₂) of 170-180°C. The viscosity of a calc-alkaline basaltic andesite magma with 2 wt.% H₂O at depth would increase by a factor of ~100 upon complete degassing during ascent.

Calorimetric measurements were made up to 750°C, depending on the sample water content. The increase in heat capacity associated with the transition from a glass to a supercooled liquid is on the order of ~25-30% and is higher in magnitude the more depolymerized the liquid. Our preliminary results suggest that the heat capacity of the hydrous sba and sb liquids decreases with increasing temperature immediately above the glass transition, similar to borosilicate and titanosilicate melts. In the anhydrous titano- and boro-silicate melts, this anomalous behaviour was linked to T-dependent mixing of B or Ti with Si. In sb and sba, this behaviour may be related to the interplay of viscosity-increasing vs. viscosity-decreasing solution mechanisms of water in multi-component depolymerized melts. Measurements to higher temperatures are underway to obtain the heat capacity of the nominally anhydrous sb and sba liquids and rule out other possibilities.

The large changes in heat capacity measured at the glass transition for depolymerized melts compared to polymerized ones (25-30 vs. 10-15%) suggest that glasses and liquids in depolymerized systems are structurally significantly different. Large ΔC_P values correspond to higher fragility, i.e., sharp changes in viscosity at the glass transition. Water in sb and sba behaves calorimetrically like in depolymerized foidite and tephrite, and differently from more polymerized trachyte or phonolite.