RAPID, UNDERCOOLED CRYSTALLIZATION OF A THIN PEGMATITE DIKE. INSIGHTS FROM MICROTHERMOMETRY AND CONDUCTIVE-COOLING NUMERICAL MODELING

We used microthermometry of coeval fluid and melt inclusions combined with finite differencing computer modeling (Kware HEAT3D by K. Wohletz) to elucidate the cooling and crystallization history of the Animikie Red Ace granitic pegmatite (ARA), Florence County, northeastern Wisconsin. The ARA is a 0.5 to 2.5 m wide and ≥600 m long, internally zoned dike emplaced post-tectonically in Early Proterozoic schists. The emplacement pressure of ~3 kbars was constrained based on phase stability of aluminosilicates in the country rocks and of lithium-aluminosilicates in the ARA.

The liquidus temperature of the pegmatite magma ranged between 700 – 720 °C, based on homogenization of crystallized-melt inclusions from the ARA ≤ 3 mm wide quenched margin. For inclusion re-melting we used stepwise, 24-hour heating in internally heated pressure vessels. The 700 – 720 °C temperature is thus the temperature at which crystallization would have started in hypothetical, equilibrium conditions. However, the cooling rates as high as 370 °C/day in the ARA outer zones exceeded the crystal nucleation rate, resulting in disequilibrium crystallization at strong degrees of magma undercooling. Based on pressure-corrected trapping temperatures of primary H2O + NaCleq ± CO2 fluid inclusions, which record the actual crystallization conditions, the pegmatite magma crystallized sequentially, starting with the outer zones at an average temperature of ~460°C, equivalent to magma undercooling of ≥240°C. The primary inclusions from the already solidified outer zones were postdated by H2O + NaCleq secondary inclusions trapped between 580 and 700°C, representing fluid exsolved from hotter, slower cooling, inner pegmatite units.

The temperature difference between the inner and outer zones during the ARA crystallization was simulated through conductive-cooling modeling. Solidification took ≤10 days, assuming instantaneous emplacement of a 2 m wide magma dike in 220°C rocks. Rapid, disequilibrium crystallization is also indicated by unidirectional crystal growth and skeletal textures. This study documents secondary inclusions trapped at higher temperatures than primary inclusions. The rapid cooling of the thin dike emplaced in colder rocks promoted this telescopic overlap, and led to a minimum metasomatic transfer in the country rocks.