Direct observation of immiscibility in pyrope-almandine-grossular garnet

Although recent thermodynamic models predict a low-temperature miscibility gap in the pyrope-almandine-grossular garnet solid-solution series, coexist ence of two immiscible garnets consistent with the calculated miscibility g ap has not been demonstrated previously. Here we report the discovery of su ch a coexistence in nature. which provides direct evidence for immiscibilit y in the pyrope-almandine-grossular system. Garnet inclusions were found ne ar solidified fluid/melt inclusions in a single pyrope crystal collected fr om the ultramafic diatreme at Garnet Ridge, Arizona. Microprobe analyses cl ose to the contact between the pyrope host and one garnet inclusion yield f ormulae of (Mg1.98Ca0.52Fe0.50Mn0.02)(Al1.89Cr0.09)Si3O12 for the host and (Mg1.28Ca1.31Fe0.41Mn0.02)(Al1.93Cr0.07)Si3O12 for the inclusion. High-reso lution analytical traverses across the two-garnet boundary reveal that the compositional difference between the inclusion and the host increases as th e contact is approached. implying that the two garnet grains represent an i mmiscible pair rather than a disequilibrium coexistence. The compositions o f the coexisting garnets are in good agreement with those predicted from re cent thermodynamic mixing models for pyrope-almandine-grossular garnet.