Water content and activity in model felsic liquids at 1 GPa

Abstract:

Water solubility and activity in granitic, trondhjemitic and syenitic magmas at their source is important for understanding crustal melting. We investigated model felsic systems in this study: Ab(NaAlSi₃O₈)-Or(KAlSi₃O₈)- Qz(SiO₂), Ab-Qz , and Ab. The experiments were all conducted at 1 GPa. For the first two systems, synthetic glass + H₂O was homogenized well above the liquidus T, then T was lowered by increments until quartz and alkali feldspar crystalized. To establish reversed equilibrium, we crystallized the homogenized melt at the lower T, then raised the T until we found that the crystalline phases were resorbed into the liquid. For the Ab system, crystalline albite was used and the liquidus was bracketed by varying water concentration. All curves were fit with cubic equations to the experimental data, giving

1. T(°C)= -0.063 w_H2O³ + 3.892 w_H2O² - 79.217 w_H2O + 1166 (Ab_36.25-Or_36.25-Qz_27.5)

2. T(°C)= -0.050 w_H2O³ + 2.872 w_H2O² - 64.864 w_H2O + 1196 (Ab_72.5-Qz_27.5)

3. T(°C)= -0.021 w_H2O³ + 2.224 w_H2O² - 63.188 w_H2O + 1239 (Ab₁₀₀)

where w_H2O and the dry starting mixtures are reported in weight percent. The new liquidi were combined with previous data on the melting of granite and albite in the presence of CO₂-H₂O fluids (Ebadi and Johannes, 1991; Eggler and Kadik,1979) and a CO₂-H₂O activity model (Aranovich and Newton, 1998) to derive activity-concentration relationships along the liquidi. The solubilities of CO₂ in liquids and of liquids in CO₂-H₂O fluid were neglected. We obtain the following relationships between water activity (a_H2O) and concentration along the liquidus

4. a_H2O = 0.0084 w_H2O² + 0.0117w_H2O for the Ab_36.25-Or_36.25-Qz_27.5system

5. a_H2O = 0.0028 w_H2O² + 0.0266 w_H2Ofor Ab system

The absence of data on Ab-Qz melting with CO₂-H₂O fluid prevents similar analysis for this system. The equations are accurate up to ~8 wt.% water and describe a rapid rise in a_H2O with increasing water content (decreasing liquidus T). At 8 wt% H₂O, a_H2O is 0.39 and 0.63 in albite and simple granite liquids, respectively. High water activity is consistent with abundant molecular H₂O in the liquids and is necessary for critical mixing in melt-H₂O systems. Our results provide a quantitative basis for understanding the chemical effects of water dissolution in felsic liquids during crustal melting.