CONTRASTING ELEMENT TRANSPORT MECHANISMS DURING PORPHYROBLAST GROWTH IN HETEROGENEOUS MATRICES

Studies on porphyroblast growth typically describe homogenous matrices due to the lack of initial compositional gradients and fewer variables to constrain when modeling. The Castner Marble, TX, a layered calc-silicate, provides a natural setting to examine porphyroblast growth and elemental transport in heterogeneous matrices. Garnets in the Castner Marble show multiple of morphologies that include: 1) elongate crystals in hornfels at the hornfels-marble boundary, 2) idioblastic to hypidioblastic oscillatory zoned garnet growing from the marble-hornfels boundary in the marble, 3) hypidioblastic, grains in marble layers, and 4) elongate grains in marble layers with oscillatory zoning parallel to compositional layering. All morphologies of garnet in the marble are surrounded by recrystallized calcite that is coarser than calcite in the matrix.

Porphyroblasts grow by two mechanisms: interface- or diffusion-controlled nucleation and growth and the mechanisms may be distinguished between by EBSD. Polycrystals and agglomerates suggest interface-controlled nucleation and growth, while crystallographic single crystals represent possible diffusion-controlled nucleation and growth. EBSD maps of Castner Marble garnets show the presence of polycrystals, crystal agglomerates, and elongate single crystals. The presence of crystal agglomerates and polycrystals in all the textural settings indicate that interface-controlled processes contributed to porphyroblast growth in all settings.

REE compositions of calcite along profiles perpendicular to garnet were analyzed and show that recrystallized calcite is depleted in LREEs relative to calcite in the matrix, while both types of calcite show similar MREE + HREE abundances. This suggests the LREEs were partitioned into garnet, while the MREEs + HREEs were immobile. When REE abundances in calcite are plotted as a function of distance from garnet, LREE moved to the garnet interface by intergranular diffusion. This study shows divergence between the transport mechanisms of major- and trace-element nutrients towards the growing garnet, with fluid-aided (grain-boundary) transport of major element constituents, but diffusion-controlled transport of LREEs towards the garnet perhaps in response to intra-granular diffusion during recrystallization.