Compositional re-equilibration of garnet: the importance of sub-grain boundaries

Author： Konrad-Schmolke Matthias O'Brien Patrick J. Heidelbach Florian

Publisher： E. Schweizerbart'sche Verlagsbuchhandlung

ISSN： 0935-1221

Source： European Journal of Mineralogy, Vol.19, Iss.4, 2007-08, pp. : 431-438

Disclaimer: Any content in publications that violate the sovereignty, the constitution or regulations of the PRC is not accepted or approved by CNPIEC.

Previous Menu Next

Abstract

Garnets from meta-granitoid high pressure rocks (Sesia Zone, Western Alps) show complex internal sub-grain textures in electron forescatter images. All investigated garnets consist of a large number of sub-grains with different shapes and sizes. Some garnets exhibit a sub-texture with very fine-grained (< 20 μm) sub-grains in their cores overgrown by palisade-like sub-grains in the rims. Sub-grain boundaries in these garnets have enabled diffusive element exchange between the garnet core and the surrounding matrix. Compositional mapping reveals zonation patterns of Mg that indicate modification of the garnet composition during prograde metamorphism. The extent of diffusional re-equilibration is dependent on sub-grain size and element diffusivities. Our samples show that X_Mg is strongly influenced by diffusion along the sub-grain boundaries, whereas apparently slow diffusing elements, such as Ca, Ti and Y preserve their original concentric zonation pattern. This differential re-equilibration leads to very complex chemical zonation that cannot be easily interpreted in terms of simple prograde growth zonation or of normally-applied spherical diffusion models. The observation that almost all garnets in the investigated samples exhibit a sub-grain pattern suggests this might be a common feature in high pressure low temperature rocks.