Examination of the Role of Boron in the Petrogenesis of Natural Granite Systems

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This proposal is for a petrologic and geochemical study of the role of boron in the differentiation of granite magmas. Recent experiments (Chorlton and Martin, 1978; Pichaant, 19981, 1987; Maning, 1981) have demonstrated that the addition of boron and fluorine causes the minimum in the Q-Ab-Or system to move toward albite, that the solidus temperature is lowered and that the solubility of water in the melt is decreased (Bernard et al., 1985). In addition, the fluid/melt partition coefficient for boron of about 3 (Pichavant, 1981), suggests that much boron can be mobilized by a fluid coexisting with a granitic melt. The aim of the proposed study is to apply the available experimental results toward understanding the differentiation of boron-rich magmas, using the Harney Peak Granite, South Dakota, as an example. Using the combination of major and minor element, including boron, stable isotope, microprobe, fluid inclusion and Sm-Nd isotope analysis we will be able to describe the role of boron in large and small-scale chemical and mineralogic fractionations in granites. These include a wide range in Na/K ratios commonly observed in boron-rich granites, the generation of pegmatitic melts, and tourmalinization.