Crystallization of Alkali Feldspar and Quartz in the Haplogranite System NaAlSi3O8-KAlSi3O8-SiO2-H2O at 4 Kb

Results of experimental studies in the system NaAlSi 3 O 8 -KAlSi 3 O 8 -SiO 2 -H 2 O, at 4 kb and from 650° to 1000°C, have been used to generate composition paths for liquid and crystal fractions as functions of temperature and bulk composition. In both the experimental work and the analyses of crystallization, attention has been devoted mainly to silicate liquid that is saturated with quartz and an alkali feldspar but unsaturated with respect to an aqueous vapor phase. Such liquid can be represented on a compound T-X surface that slopes, with decreasing H 2 O content in the system, toward SiO 2 for NaAlSi 3 O 8 -rich compositions and away from SiO 2 for KAlSi 3 O 8 -rich compositions. Compositions of the crystal fraction that separates from the liquid under equilibrium conditions in the temperature range 700° to 655°C have been calculated for an array of bulk compositions. Equilibrium crystallization history is characterized by numerous shifts and some reversals along paths that describe the composition of the crystal fraction. The various paths demonstrate that intensive parameters such as temperature, pressure, and H 2 O fugacity cannot be uniquely determined from the composition of a specific fraction. Paths also have been outlined, for a variety of bulk compositions, from the maximum temperature for separation of quartz ± alkali feldspar to the solidus temperature of 655° C under conditions of perfect fractional crystallization in which successive precipitates are immediately isolated from the silicate liquid. These crystal fraction paths are marked by discontinuities that indicate abrupt, discrete changes in composition, and with falling temperature, they tend to focus toward a compositional area centering about 20 NaAlSi 3 O 8 , 45 KAlSi 3 O 8 , 35 SiO 2 . The melting of crystalline assemblages that represent the haplogranite system can be considered as the inverse of equilibrium crystallization, fractional crystallization, or some combination of these idealized processes. Thus the composition path for an equilibrium crystal fraction can also serve for the tracing of compositional changes in a residue formed during equilibrium melting. A residue of given composition can be produced at constant elevated pressure under conditions representing many different combinations of temperature and bulk composition. The experimentally determined complexities of crystallization are reasonable indications of what can occur under natural plutonic conditions. They are potentially most useful, however, for the testing of genetic models based upon detailed studies of rocks in a geologic and petrographic context. They also lead to the conclusions that (1) the bulk composition of a granitic plutonite is not by itself sufficient for estimating conditions under which the rock was formed and (2) experimental data obtained for the haplogranite system under conditions that include the presence of an aqueous fluid phase ( P H 2 O ≈ P fluid = P tot ) are highly restricted in their pertinence to the crystallization histories of most granitic plutonites.