Effect of grossular-content in garnet on the partitioning of Fe and Mg between garnet and biotite |
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| Authors: | Georg Hoinkes |
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| Institution: | (1) Institut für Mineralogie und Petrographie der Universität Innsbruck, Innrain 52, A-6020 Innsbruck, Austria |
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| Abstract: | In contrast to Ferry (1980) (X
Ca)-values in garnet even lower than 0.1 have a significant effect on the calculated equilibrium temperature using the experimental calibration of the Fe and Mg paritioning between garnet and biotite. Garnet compositions and Mg/Fe — distribution coefficients from samples of the Eoalpine staurolite — in zone in the southern Ötztal are related by the quadratic regression equation: InK
D= -1.7500 (±0.0226) + 2.978 (±0.5317)X
Ca
Gt
-5.906(±2.359)(X
Ca
Gt
)2 Temperatures derived by the Ferry and Spear (1978) calibration using  chemistry — corrected K
D values are petrologically realistic.Analysis of our data supports non ideal mixing of grossular with almandine — pyrope solid solution. The derived excess mixing energies are quite small for the almandine — pyrope solution (W
FeMg= –133 cal/mole) and about +2775 cal/mole for the difference between pyrope-grossular and almandine-grossular solutions (W
MgCa —W
FeCa) at metamorphic conditions of  570° C and 5,000 bar. The mixing parameters proposed by Ganguly and Saxena (1984) are not confirmed by our data as they would result in significantly lower temperatures. |
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