Rapid crystallization of the Animikie Red Ace Pegmatite, Florence county, northeastern Wisconsin: inclusion microthermometry and conductive-cooling modeling

• Published in: Contributions to mineralogy and petrology Vol. 156; no. 3; pp. 289 - 305
• Main Authors: Sirbescu, Mona-Liza C., Hartwick, Emily E., Student, James J.
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer-Verlag 01.09.2008; Springer Nature B.V
• Subjects: Crystallization; Earth and Environmental Science; Earth Sciences; Geology; Geophysics; Magma; Mineral Resources; Mineralogy; Original Paper; Petrology; Thermodynamics; Florence County Wisconsin; Fluid inclusion assemblages; Florence county pegmatite field; Disequilibrium crystallization; Melt inclusion assemblages; Numerical simulation; Undercooling; LCT pegmatite; Microthermometry; Wisconsin
• ISSN: 0010-7999; 1432-0967
• DOI: 10.1007/s00410-008-0286-0

We evaluated the crystallization regime of a zoned pegmatite dike and the degree of magma undercooling at the onset of crystallization by analyzing coeval fluid and melt inclusion assemblages. The liquidus temperature of the pegmatite magma was ~720°C, based on re-melting of crystallized-melt inclusions in heating experiments. The magma crystallized sequentially starting with a thin border zone, which formed in less than one day at an average temperature of ~480°C based on primary fluid inclusions, meaning 240°C undercooling. The primary inclusions from the outer zones were postdated by secondary inclusions trapped between 580 and 720°C, representing fluid exsolved from hotter, still crystallizing inner pegmatite units. The huge temperature contrast between the pegmatite’s inner and outer zones was simulated by conductive-heat numerical modeling. A 2.5 m wide dike emplaced in 220°C rocks cools entirely to <400°C in less than 50 days. Unidirectional and skeletal textures also indicate rapid, disequilibrium crystallization.