Water and magmas: Thermal effects of exsolution

The temperature changes caused by water exsolution from magma have been determined through calorimetric measurements performed on phonolitic and albitic compositions. The enthalpies of mixing of water with these melts have been derived from HF solution calorimetry, made at 323 K on glass samples containing up to 5 wt.% water, together with heat capacity data for the same series of samples. Mixing between aluminosilicate melts and water appears nearly ideal at magmatic temperatures, with small enthalpies of mixing that are negative for both melts at low pressures but can become positive for albite at high pressure. Regardless of the endothermic or exothermic nature of the process, water exsolution is associated with negligible temperature changes of only a few degrees even when 5 wt.% H₂O is degassed. However, thermal effects might be greater for more depolymerized melts such as basalts and related compositions.