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UBC Theses and Dissertations

Field, kinetic, and thermodynamic studies of magmatic assimilation in the Northern cordilleran volcanic province, northwestern British Columbia Edwards, Benjamin Ralph


This thesis investigates the process of magmatic assimilation. Field observations, kinetic considerations, and thermodynamic calculations are used together to form a calculational model for assimilation processes in mafic magmas. Field and petrographic constraints derive from lavas within the newly defined Northern Cordilleran Volcanic Province (NCVP) of northwestern British Columbia and the Yukon Territories (Chapter One). Petrographic and geochemical evidence for assimilation is common throughout the NCVP and is especially important at the Hoodoo Mountain volcanic complex (HMVC), a pair of previously unstudied alkaline to peralkaline volcanoes within the Stikine subprovince of the NCVP, in northwestern British Columbia. Chapter Two presents the geology, stratigraphy, geochronology, petrography, and geochemistry of the HMVC and discusses its chemical and physical evolution. Subglacial volcanism and magmatic assimilation are shown to be important processes in the physical and chemical evolution of the HMVC. Chapter Three reviews kinetic constraints on magmatic assimilation and the available experimental data for rates of mineral dissolution in mafic silicate melts. Published mineral dissolution experiments in basaltic melts are consistent with a time-independent dissolution mechanism. The most extensive experimental dataset (Donaldson 1985) is used to develop a model for predicting rates of mineral dissolution in basaltic silicate melts. It is shown that rates of dissolution can be simply predicted by calculating the thermodynamic driving force for the dissolution reactions (the Affinity), using the thermodynamic database of Ghiorso and Sack (1995). An existing model based soley on equilibrium thermodynamics (MELTS; Ghiorso and Sack 1995) demonstrates that crystal growth and compositional zoning are sensitive recorders of specific assimilation paths (Chapter Four). The thermodynamic database for the MELTS model is combined with the kinetic model for predicting mineral dissolution rates to produce a new model for magmatic assimilation (Chapter Five). The new model is different from preexisting models because it predicts cooling and crystallization rates implied by kinetically-controlled magmatic assimilation. Results from the new model demonstrate that magmatic assimilation processes that are controlled by kinetics operate on the time-scale of days to weeks. Given the time-scale of magma transport and volcanism, magmatic assimilation is shown to be a temporally, as well as geochemically, viable process during transport of mantle derived magmas and in the evolution of Hoodoo Mountain volcanic complex (-240 Ka).

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