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Metastable melting in the granite system Qz-Or-Ab-An-H2O

Authors:	W Johannes

Institution:	(1) Mineralogisches Institut der Universität Hannover, Weifengarten 1, 3000 Hannover, Federal Republic of Germany

Abstract:	Time studies were performed in the quinary system Qz-Or-Ab-An-H₂O at $\operatorname{P} \operatorname{H} _2 O = 5$ kbars and T=665 ° and 660 ° C. Starting material was a mixture of quartz, alkali feldspar Or₈₀ and plagioclase An₃₁. The compositions of plagioclases of run products were determined and compared with the plagioclase of stable solidus conditions.The solidus of the granite system was fixed at P _HäO=5 kbars using various plagioclase — and appropriate alkali feldspar — compositions besides quartz in the starting mixture (Fig. 1).The results of time studies (Table 3 and Fig. 3) reveal metastable melting in the granite system Qz-Or-Ab-An-H₂O. Plagioclase melts almost stoichiometrically. The new plagioclase compositions formed during melting of cotectic compositions approach the theoretically expected stable plagioclase compositions only extremely slowly. An extrapolation of the data achieved in run times of 5–1,500 h indicates attainment of equilibrium after 10¹⁴ years. Metastable melting of granitic compositions is not only considered as an experimental problem but also as a rock forming process in nature.

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