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

Abstract

Research Article| January 01, 1975 Crystallization of Alkali Feldspar and Quartz in the Haplogranite System NaAlSi3O8-KAlSi3O8-SiO2-H2O at 4 Kb JEFFREY C. STEINER; JEFFREY C. STEINER 1Department of Geology, Stanford University, Stanford, California 943052Present address: Department of Earth and Planetary Sciences, City College of New York, New York, New York 10031. Search for other works by this author on: GSW Google Scholar RICHARD H. JAHNS; RICHARD H. JAHNS 1Department of Geology, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar WILLIAM C. LUTH WILLIAM C. LUTH 1Department of Geology, Stanford University, Stanford, California 94305 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1975) 86 (1): 83–98. https://doi.org/10.1130/0016-7606(1975)86<83:COAFAQ>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation JEFFREY C. STEINER, RICHARD H. JAHNS, WILLIAM C. LUTH; Crystallization of Alkali Feldspar and Quartz in the Haplogranite System NaAlSi3O8-KAlSi3O8-SiO2-H2O at 4 Kb. GSA Bulletin 1975;; 86 (1): 83–98. doi: https://doi.org/10.1130/0016-7606(1975)86<83:COAFAQ>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Results of experimental studies in the system NaAlSi3O8-KAlSi3O8-SiO2-H2O, 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 H2O content in the system, toward SiO2 for NaAlSi3O8-rich compositions and away from SiO2 for KAlSi3O8-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 H2O 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 NaAlSi3O8, 45 KAlSi3O8, 35 SiO2.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 (PH2O ≈ Pfluid = Ptot) are highly restricted in their pertinence to the crystallization histories of most granitic plutonites. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.