Intellectual Merit: New evidence suggests that thin sheets of crystal-free granitic melts intruded in the upper crust can cool to subliquidus temperatures within weeks. Because hydrous silicic melts are sluggish to nucleate, such strong undercooling may govern their crystallization and geochemical evolution paths. It is proposed that dissolved water and other fluxing components (e.g., Li, B) lower melt viscosity and the glass-transition temperature, thus allowing the silicate liquids to persist at unusually low temperatures. Geothermometric, textural and compositional data have revealed important aspects of the crystallization history of such melts, but more experimental constraints are needed to decipher the textural and geochemical consequences of undercooling. The goal of this three-year proposal is to identify links between texture, composition, and crystallization history of silicic intrusive rocks by using field-based and experimental observations. Reconnaissance experimental evidence has demonstrated the existence of hydrous Li- and B-bearing haplogranitic melts at temperatures as low as 400Â°C at a pressure of 300 MPa. Glass transition temperatures (ranging between 250-300Â°C) measured for these compositions support the experimental observations. Nucleation and crystallization of these undercooled liquids was metastably delayed for at least 30 days. When crystallization finally took place, distinctive textures resulted depending on degree of undercooling and amount of dissolved water, that are specific to pegmatites (spherulitic, graphic, skeletal, unidirectional, etc.), aplites (fine grained, equigranular), and rhyolites (porphyritic, euhedral). High crystal growth rates can lead to chemical and isotopic boundary layers ahead of the solidification front. Nucleation and growth rates are consistent with cooling timelines of small intrusions or sheets within larger incrementally built plutons. A field-based component of the study will constrain emplacement temperatures, temperature gradients, cooling rates, undercooling, and fluid saturation during inward crystallization of magma sheets. Proposed research includes fluid and melt inclusion studies, conductive-cooling simulations, and mineral and isotopic analyses on samples from two study areas. Focus will be on chilled margins and layered, unidirectional, and spherulitic textures interpreted as consequences of rapid crystallization. Attempts will be made to identify and quantify the existence of chemical and isotopic boundary layers. An experimental component will address crystallization of synthetic melt compositions seeking to quantify the kinetics of crystallization of fluxed granitic melt at 300 MPa and various degrees of undercooling and cooling rates. We will expand our study on nucleation and crystal growth rates in the B-Li-H2O-haplogranite system with emphasis on the role of water on controlling the crystallization kinetics and, ultimately, the magmatic texture. For a complete characterization of the synthetic melts, the viscosities and glass transition temperatures will be measured at the Univ. of Missouri, in collaboration with Dr. Whittington. Chemical diffusion ahead of the solidification front during rapid, disequilibrium crystallization will also be investigated in experimental charges. Diffusion-driven boundary layers and their potential impact on mineral and textural zoning, melt inclusion compositions, and geochemical fractionation will be evaluated. The impact within the broader petrologic community will be significant, because the interpretation of texture based on integrated field-experiment observations, identification of low temperature melts in nature, and diffusion-related isotopic fractionation produced by rapid crystallization will improve our understanding of undercooled melts, regardless of their composition.
Broader Impact: To disseminate our results within the University and larger community, an educational exhibit on the origin and significance of magmatic texture will be developed as part of the Museum of Cultural and Natural History at Central Michigan University (CMU) and public presentations will be given with hands on crystallization activities. This RUI project will benefit the CMU undergraduate program in that it will allow the PI to train and mentor several undergraduates involved at all stages of research. Resources and a very strong commitment to undergraduate research exist at CMU. Undergraduate CMU students will have the opportunity to travel and work at prestigious research laboratories at GFZ-Potsdam, Germany, Univ. of Michigan, and Univ. of Missouri.