, Volume 66, Issue 2, pp 113-117

Experimental calibration of the partitioning of Fe and Mg between biotite and garnet

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The cation exchange reaction Fe3Al2Si3O12 +KMg3AlSi3O10(OH)2 = Mg3Al2Si3O12+KFe3-AlSi3 O10(OH)2 has been investigated by determining the partitioning of Fe and Mg between synthetic garnet, (Fe, Mg)3Al2Si3O12, and synthetic biotite, K(Fe, Mg)3AlSi3O10(OH)2. Experimental results at 2.07 kbar and 550 °–800 ° C are consistent with In [(Mg/Fe) garnet/(Mg/Fe) biotite] = -2109/T(°K) +0.782. The preferred estimates for Δ¯H and Δ¯S of the exchange reaction are 12,454 cal and 4.662 e.u., respectively. Mixtures of garnet and biotite in which the ratio garnet/biotite=49/1 were used in the cation exchange experiments. Consequently the composition of garnet-biotite pairs could approach equilibrium values in the experiments with minimal change in garnet composition (few tenths of a mole percent). Equilibrium was demonstrated at each temperature by reversal of the exchange reaction. Numerical analysis of the experimental data yields a geothermometer for rocks containing biotite and garnet that are close to binary Fe-Mg compounds.