Pre-Eruptive and Syn-Eruptive Conditions in the Black Butte, California Dacite: Insight Into Crystallization Kinetics in a Silicic Magma System
Journal of Volcanology and Geothermal Research 160 (2007) 263–284 www.elsevier.com/locate/jvolgeores Pre-eruptive and syn-eruptive conditions in the Black Butte, California dacite: Insight into crystallization kinetics in a silicic magma system Molly C. McCanta a,⁎, Malcolm J. Rutherford a, Julia E. Hammer b a Department of Geological Sciences, Brown University, Box 1846, Providence, RI 02912, USA b Department of Geology and Geophysics, University of Hawaii, 1680 East–West Road, Honolulu, HI 96822, USA Received 7 April 2006; received in revised form 26 August 2006; accepted 3 October 2006 Available online 28 November 2006 Abstract A series of experiments and petrographic analyses have been run to determine the pre-eruption phase equilibria and ascent dynamics of dacitic lavas composing Black Butte, a dome complex on the flank of Mount Shasta, California. Major and trace element analyses indicate that the Black Butte magma shared a common parent with contemporaneously erupted magmas at the Shasta summit. The Black Butte lava phenocryst phase assemblage (20 v.%) consists of amphibole, plagioclase (core An77.5), and Fe–Ti oxides in a fine-grained (b0.5 mm) groundmass of plagioclase, pyroxene, Fe–Ti oxides, amphibole, and cristobalite. The phenocryst assemblage and crystal compositions are reproduced experimentally between 890 °C and 910 °C, ≥300 MPa, X- H2O =1, and oxygen fugacity=NNO+1. This study has quantified the extent of three crystallization processes occurring in the Black Butte dacite that can be used to discern ascent processes. Magma ascent rate was quantified using the widths of amphibole breakdown rims in natural samples, using an experimental calibration of rim development in a similar magma at relevant conditions.
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