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AM61_710 GASP original

AM61_710 GASP original - American Mineralogist Volume 6l...

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American Mineralogist, Volume 6l , pages 7I0-714, 1976 Plagioclase-garnet-Al2SiOr-quartz:. s potential geobarometer-geothermometer Epwenr D. GHENI Department of Geolo.gy, Uniuersity of Calgary Calgary, Alberta T2N I N4 Canada Abstract For the reaction 3CaAlrSirO. : CarAlrSisO', + 2AlrSiOu + SiO2, the logarithm of the apparent distribution coefficient (Kp) is log Ko : 3X"%X'Jf!',o,, - 3XEl"^5',".;b?9 Log Ko varies from -0.5 to -1.4 for kyanite-bearing assemblages and from -2.1 to -3.4 for sillimanite and/or andalusite-bearing assemblages. Using equilibrium constant equations derived from experimental data on pure phases and an ideal solution model for garnet and plagioclase solid solutions, individual P-I curvescan be calculated for different values of log KD. If P or T can be independently estimated, these curves can be used as either geothermometers or geoba- rometers.The degree of non-ideality of grbssular solid solution in garnet can be estimated from calculations using the activity coefficients of anorthite in plagioclase and the kyanite- sillimanite P-Z equilibrium curve as a limiting case for kyanite and sillimanite-bearing assemblages. The equilibrium can be described by Introduction Plagioclase, garnet, qtJarIz, and an AlrSiOu poly- morph are a common assemblage in regionally meta- morphosed pelitic rocks. Several workers, e.g.,Miya- shiro (1953), Sturt (1962),have suggested that garnet compositions can be used as indicators of metamor- phic grade. Other workers have noted a relationship between the CaO content of garnet, the anorthite content of plagioclase, and metamorphic grade (Ke- pezhinskas, 1973). Unless an appropriate mineral as- semblage of low variance is chosen, however, the effects of bulk composition far overshadow the P-7 significance of garnet and plagioclase compositional variability. In this note I will attempt to demonstrate that the reaction 3CaAlzSi2O8 : Ca3Al2 Si.O', anorthite grossular + zAt,SiO, + SiO, (la) quartz provides data on the mineral assemblage plagioclase- garnet-AlrSiOu-quartz which is potentially useful in estimating pressures and temperatures of meta- morphism (Ghent, 1975). Reaction (la) with kyanite as the AlrSiOu poly- morph has been studied experimentally at elevated pressure and temperatures by Hays (1967) and by Hariya and Kennedy (1968). o : -+ + 8.3e6e - where Z is the temperature in degrees Kelvin and P is pressure in bars. The equation can be modified for the presence of sillimanite or andalusite as the AlrSiO5 polymorph by subtraction of equations of the form of (lb) for kyanite-sillimanite and kyanite- andalusite equilibria. Using the aluminum silicate diagram of Holdaway (1971) we obtain in place of equation (lb): 0.3448(P - t) o:-u!]'4+7.r7rr 0.2842(P - r) (2) T (1b) where sillimanite is the AlzSiO' polymorph and 2 8 1 7 . 2 , , 4 3 5 1 _ 0 . 2 6 7 8 9 " _ l ) ( 3 ) t t . , - - - T where andalusite is the ALSiO5 polymorph.
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