THE EFFECTS OF MICRO-INCLUSIONS ON INTERNAL STRUCTURE OF ZONED GARNET: AN EXAMPLE FROM THE TOCANTINS RHODOLITE GARNETS, TOCANTINS STATE, BRAZIL

Mineral inclusions are a key tool to understanding the complex history of geologic environments. Investigating the composition of inclusion and tying that information to growth zone compositions can provide detailed insight into changing thermal and chemical conditions in both igneous and metamorphic systems. Here we present an examination of micro-inclusions and garnet mineral chemistry on the internal structure of almandine-spessartine-pyrope garnets. We examine the effect of geochemical variation in the crystal structure as a function of deformation on the physical properties and diffusion rates of Mn (II) and Fe (III). Substitution for Mn by Mg or Fe along the pyrope-almandine-spessartine boundary involves change in cation size producing changes in the thermoelastic properties.

The Tocantins area of North-East Brazil produces both gem quality and highly included rhodolite garnets from a working mine on the Rodolita Farm. The garnets occur in mylonitic schist lenses of kyanite-staurolite grade in a migmatitic gneiss. These garnets contain high Mn cores and low Mn rims and increase in Mg from core to rim creating a color change from red in the core to purple in the rims. Fe content is within error from core to rim for all garnets analyzed. Preliminary analysis of the inclusions in the garnets range from aligned quartz, and Fe-Ti oxides. The substitution of Mg for Mn in the rims of the garnets not only produces a color change but cause fracturing perpendicular to the chemical zoning.