ENHANCED NUMERICAL SIMULATION OF CRYSTALLIZATION USING NEW CONSTRAINTS FROM GARNET CHEMICAL ZONING

Various forms of evidence of kinetic controls on metamorphic processes are commonly hidden in mineral textures and chemical compositions, but if extracted, they can be used as high-order constraints on numerical models of metamorphism. Precise textural constraints from high-resolution X-ray computed tomography, including crystal-number density, crystal-size distribution (CSD), and mode (volume %), have been used to fit numerical models to natural systems. Despite the value of textural constraints in fitting numerical simulations, the addition of mineral chemistry provides a new, more nuanced control on numerical analysis of crystallization in metamorphic systems. We now use garnet chemical zoning, measured in tens to hundreds of centrally sectioned crystals quantitatively analyzed along rim-core-rim traverses, and previous textural measurements to fit numerical simulations of porphyroblastic nucleation and growth. The simulations permit new examination of (1) classical nucleation theory applied to natural (complex) systems; (2) the relative contributions of the drivers of nutrient transport; (3) garnet growth rates (dependent on heating rates used as model input); and (4) the degree of nutrient competition among porphyroblasts at any point during crystallization. Application of the technique requires sufficient textural and chemical characterization of individual samples and a demonstration of rock-wide equilibration of at least one chemical component of garnet (e.g., Mn). Preliminary results from four highly-characterized Picuris Mountains (New Mexico) lower-amphibolite facies metapelitic quartzites suggest that fits to rock textures alone may not sufficiently describe chemical zoning among all crystals. In some of these rocks, nucleation rates may diverge from those described by classical nucleation theory, and significant drivers of nutrient transport may include fluid flux affected by changing porosity, rather than thermally driven intergranular diffusion as the dominant control. In sum, the inclusion of chemical data across multiple porphyroblasts documenting relative nucleation and growth rates through time provides a uniquely rigorous and powerful test of our understanding of the processes controlling metamorphic crystallization.