We present 16 new manometric determinations of H2O solubility for a
range of natural silicate liquid compositions equilibrated up to 3 kbar
of H2O pressure, As the threshold temperature of dehydration of the
quenched glasses during measurements of the H2O content becomes lower as
a function both of bulk silicate composition and the dissolved H2O
contest, we measured the H2O released on heating over a range of
temperature intervals. For example, alkali-rich samples having a
dissolved H2O content greater than similar to 6 wt% start to evolve H2O
at temperatures less than 150 degrees C, whereas more mafic samples and
silicic samples with less than 6 wt% H2O begin to dehydrate at
temperatures greater than 200 degrees C, This behavior is consistent
with the concept that alkali-rich liquids can have their glass
transition temperatures lowered substantially by dissolved H2O and that
H2O is released only significantly on heating in the supercooled liquid
region, rather than in the glass region, Using these new data, in
conjunction with previous data from the literature, we refined and
extended the empirical H2O solubility model of Moore et al. (1995b). The
new model works well (2 sigma = +/- 0.5 wt%) between 700-1200 degrees C
and 1-3000 bar and can be applied to any natural silicate liquid in that
range. The model may also be used for systems where X-H2O < 1 in the
vapor phase.