Melting of plagioclase in the system Ab − An − $${\text{H}}_{\text{2}} {\text{O}}$$ and Qz − Ab − An − $${\text{H}}_{\text{2}} {\text{O}}$$ at P H 2 O =5 kbars, an equilibrium problem
Tóm tắt
Melting experiments were performed in the systems at P
H
2
O =5 kbars. The compositions of the plagioclases coexisting with melt or with melt and quartz were determined. The data obtained in the plagioclase-water systems show that the reaction rate decreases exponentially with decreasing temperature. Plagioclase compositions do not change any further for run times ≧ 1 h at 1000° C and are considered to have reached equilibrium. At 900° C run times of approximately 200 h are necessary to approach equilibrium. At 800° C the reaction is too slow to attain constant compositions within reasonable times. The solidus of the system Qz-Ab-An-H2O was reversed using mixtures of 95% plagioclase and 5% quartz. This solidus gives the equilibrium compositions of plagioclase for any combination plagioclase-quartz-melt. A series of comparative experiments was also made with An20 and An40, mixed with quartz in the ratio plagioclase/quartz: 1/1, because theoretically the equilibrium solidus composition of plagioclase should be independent of the plagioclase/quartz ratio. However, at various run times and temperatures, the plagioclase compositions thus obtained approached the equilibrium solidus compositions only very slowly. It is extrapolated that at 730° C about 100,000 years are needed to reach equilibrium composition. The experimental data show that it is not possible to reach equilibrium in the investigated tonalite- and anorthosite-system around and below 800° C in run times available in the laboratory. It is also concluded that many investigations performed in the granite system Qz-Or-Ab-An-H2O did not reach equilibrium.
Tài liệu tham khảo
Bambauer, H.U., Corlett, M., Eberhard, E., Viswanathan, K.: Diagrams for the determination of plagioclases using X-ray. Schweiz. Mineral. Petrog. Mitt. 47, 333–349 (1967)
Bowen, N.L.: The melting phenomena of the plagioclase feldspars. Am. J. Sci. 35, 577–599 (1913)
Brown, G.C., Fyfe, W.S.: The production of granitic melts during ultrametamorphism. Contrib. Mineral. Petrol. 28, 310–318 (1970)
Eberhard, E.: Zur Synthese der Plagioklase. Schweiz. Mineral. Petrog. Mitt. 47, 385–397 (1967)
Gibb, F.G.F.: Supercooling and the crystallization of plagioclase from a basaltic magma. Mineral. Mag. 39, 642–653 (1974)
Henkes, L., Johannes, W.: Solidustemperaturen und Plagioklaszu-sammensetzungen in Migmatiten. Fortschr. Mineral. 55, Beiheft 1, 42–43 (1977)
Karl, F.: Über die Zusammensetzung, Entstehung und gesteinssystematische Stellung tonalitisch-granitischer Gesteine. Tschermaks Mineral. Petrog. Mitt. 11, 413–438 (1966)
Lofgren, G.E., Donaldson, C.H.: Curved branching crystals and differentiation in comb-layered rocks. Contrib. Mineral. Petrol. 49, 309–319 (1975)
Luth, W.C., Jahns, R.H., Tuttle, O.F.: The granite system at pressures of 4 to 10 kb. J. Geophys. Res. 69, 759–773 (1964)
Piwinskii, A.J.: Experimental studies of igneous rock series: felsic bodies from the Wallowa batholith, Oregon. Geol. Soc. Am. Spec. Papers 101, 166–167 (1967)
Piwinskii, A.J., Martin, R.F.: An experimental study of equilibrium with granitic rocks at 10 kb. Contrib. Mineral. Petrol. 29, 1–10 (1970)
Piwinskii, A.J., Wyllie, P.J.: Experimental studies of igneous rock series: a zoned pluton in the Wallowa batholith, Oregon. J. Geol. 76, 205–234 (1968)
Platen, H. v.: Kristallisation granitischer Schmelzen. Beitr. Mineral. Petrog. 11, 334–381 (1965)
Raase, P., Morteani, G.: Über Zonarbau und basische Kerne in Plagioklasen I. Neues Jahrb. Mineral. Abhandl. 10, 81–105 (1968)
Stewart, D.B.: Four phase curve in the system CaAl2Si2O8-SiO2-H2O between 1 and 10 kilobars. Schweiz. Mineral. Petrog. Mitt. 47, 35–39 (1967)
Viswanathan, K.: Kationenaustausch an Plagioklasen. Contrib. Mineral. Petrol. 37, 277–290 (1972)
Winkler, H.G.F.: Petrogenesis of Metamorphic Rocks, 334 p. Berlin, Heidelberg, New York: Springer 1976
Winkler, H.G.F., Boese, M., Marcopoulos, T.: Low temperature granitic melts. Neues Jahrb. Mineral. Monatsh. 1975, 245–267 (1975)
Winkler, H.G.F., Das, B.K., Breitbarth, R.: Further data of low temperature melts existing on the quartz+ plagioclase+liquid.+vapour isobaric cotectic surface within the system Qz-Ab-Or-An-H2O Neues Jahrb. Mineral. Monatsh. 1977, 241–247 (1977)
Winkler, H.G.F., Ghose, N.C.: Further data on the eutectics in thesystem Qz-Or-An-H2O. Neues Jahrb. Mineral. Monatsh. 1973, 481–484 (1973)
Winkler, H.G.F., Lindemann, W.: The system Qz-Or-An-H2O within the granitic system Qz-Or-Ab-An-H2O. Application to granitic magma formation. Neues Jahrb. Mineral Monatsh. 1972, 49–61 (1972)
Winkler, H.G.F., Platen, H. v.: Experimentelle Gesteinsmetamorphose III. Anatektische Ultrametamorphose kalhaltiger Tone. Geochim. Cosmochim. Acta 18, 294–316 (1960)
Winkler, H.G.F., Platen, H. v.: Experimentelle Gesteinsmetamorphose IV. Bildung anatektischer Schmelzen aus metamorphisierten Grauwacken. Geochim. Cosmochim. Acta 24, 48–69 (1961)
Yoder, H.S., Jr.: Albite-anorthite-quartz-water at kb. Carnegie Inst. Wash Year Book 66, 477–478 (1968)
Yoder, H.S., Jr., Stewart, D.B., Smith, J.R.: Feldspars. Carnegie Inst. Wash. Year Book 56, 206–214 (1957)