The effect of garnet cell volume on the distribution of Mg and Fe in garnet exchange equilibria

The distribution of Mg in garnet exchange equilibria has been examined in three regional metamorphic and four eclogitic parageneses. Garnet cell volume has been found to have a significant effect on the distribution of Mg between garnet and coexisting ferromagnesian minerals. When the activity of Mg is corrected for the variation in cell volume, the range in variation of the distribution coefficients is strongly reduced within each group. Also the relative positions of the different groups in the distribution diagrams are in better accordance with geological evidence of crystallization temperatures. It is proposed that the effect of the garnet cell volume on the Mg distribution may be used as a geobarometer. The distribution of Ca between garnet and plagioclase is also discussed and it is found that the Ca partitioning is not suitable for geobarometry, as this partioning is dependent on the Mg/Fe ratio of the garnet.