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 -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 curves can 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 2817.2,, 4351 _ 0.26789" _l) (3) t t.,--- T where andalusite is the ALSiO5 polymorph. Since the minerals are not pure phases we add terms for activity of anorthite solid solution in plagioclase and activity of grossular solid solution garnet: _1)1) ,_L3_11qq__r) o:-z:+8.3969 TT * loe "i"i,'"'.ii, "' "". " 3 log "t',Til"J'""",j" 7t0 (l c)
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A POTENTIAL GEOBAROMETER-GEOTHERMOM ETER 7tl For a discussion of the derivation
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AM61_710 GASP original - American Mineralogist, Volume 6l ,...

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