Solution of H2O in diopside melts: a thermodynamic model

Structural similarities between dry diopside melt and superhydrous albite melt (X_w>0.5) — both lack three-dimensional silicate units — suggest that thermodynamic relations may be similar. A model based on that assumption successfully predicts diopside melting relations and H₂O solubilities. For the model, the three partial differential equations describing solution of H₂O in albite melt for X_w>0.5 have been integrated for diopside melt from X_w=0 to X_w at least as large as 0.76, with two exceptions: an alternative partial differential equation for Henrian solution of H₂O in dilute melts was applied for X_w<0.20, and an alternative differential equation for the pressure dependence of a_w at pressures below 2 kbar was developed. The latter alternative equation yields relatively small ¯V_w's at low pressures rather than the large ¯V_w's calculated from the equation from the albite system. Available experimental solubility data are not precise enough to offer a choice between the small-¯V_w and large-¯V_w equations. Integration of all the partial differential equations was constrained solely by the P and T of a single experimentally-determined point on the H₂O-saturated solidus.Solubilities calculated by a Henrian-analogue solution model (a_di=X_di²) from the experimental H₂O saturated solidus lie outside experimental solubility constraints for dilute melts. On the other hand, a Henrian model (a_di=X_di) successfully predicts solubilities in dilute melts. The formulation of the Henrian model and magnitudes of model molar entropies of solution are consistent with the hypothesis that H₂O dissolves in diopside melt as an essentially undissociated species with little ordering on melt structural sites. That species could in turn be consistently, if not uniquely, interpreted to be molecular H₂O or a hydroxylation (OH^–) complex formed from nonbridging oxygens.