Mechanisms and Kinetics of Porphyroblast Nucleation

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Metamorphic rocks respond to changes in pressure, temperature, and their chemical environment by transforming existing minerals into new minerals. These changes in mineral assemblage create a record of the rocks' history that geologists can use to reconstruct ancient episodes of mountain building and erosion, and to understand the causes of these events and how they occur. The vital first step required to initiate the transformation of one mineral assemblage to another is creation of submicroscopic crystals (nuclei) of the new minerals, so detailed knowledge of this process of nucleation is essential in order for petrologists to read accurately the record of Earth history preserved in metamorphic rocks. In particular, if new minerals nucleate and grow significantly more slowly than changes occur in pressure, temperature, and chemical environment, the mineral transformations cannot keep pace with those changes in physical conditions. The petrologic record may then contain gaps or may be misread. The goal of this project is to understand the atomic-scale mechanisms involved in nucleation of metamorphic minerals, and to quantify the rates at which nucleation takes place in nature. This knowledge will improve the ability of geologists to reconstruct events in Earth history that have shaped the present-day distribution of mountain ranges and sedimentary basins, information that is vital to locating, exploiting, and conserving a variety of natural resources, including water, soil, industrial minerals, and multiple energy sources.In this project, the predictions of classical theories of nucleation will be compared to observations and measurements made of garnet porphyroblasts in diverse suites of rocks. The research will encompass three conceptually new applications of recently emergent technologies and simulation techniques. First, high-resolution X-ray computed tomography and electron backscattered diffraction measurements will be combined to determine the prevalence and common crystallographic characteristics of epitaxial orientational relationships between garnet crystals and substrates that may have controlled their nucleation. Second, new numerical models of nucleation and growth processes, constrained by quantitative microstructural data from X-ray computed tomography, will be used to determine the time variation of nucleation rates in a diverse suite of rocks, and to estimate relevant reaction affinities, interfacial energy effects, and steady-state limits on nucleation rates. Third, interfacial energies for a variety of garnet-garnet grain boundaries will be computed by means of molecular-statics calculations, as a first step toward similar calculation of polyphase interfacial energies directly relevant to garnet nucleation.