Abstract
Pegmatites are shallow, coarse-grained magmatic intrusions with crystals occasionally approaching meters in length. Compared to their plutonic hosts, pegmatites are thought to have cooled rapidly, suggesting that these large crystals must have grown fast. Growth rates and conditions, however, remain poorly constrained. Here we investigate quartz crystals and their trace element compositions from miarolitic cavities in the Stewart pegmatite in southern California, USA, to quantify crystal growth rates. Trace element concentrations deviate considerably from equilibrium and are best explained by kinetic effects associated with rapid crystal growth. Kinetic crystal growth theory is used to show that crystals accelerated from an initial growth rate of 10−6–10−7 m s−1 to 10−5–10−4 m s−1 (10-100 mm day−1 to 1–10 m day−1), indicating meter sized crystals could have formed within days, if these rates are sustained throughout pegmatite formation. The rapid growth rates require that quartz crystals grew from thin (micron scale) chemical boundary layers at the fluid-crystal interfaces. A strong advective component is required to sustain such thin boundary layers. Turbulent conditions (high Reynolds number) in these miarolitic cavities are shown to exist during crystallization, suggesting that volatile exsolution, crystallization, and cavity generation occur together.
Highlights
Pegmatites are shallow, coarse-grained magmatic intrusions with crystals occasionally approaching meters in length
Based on conductive cooling models, Webber et al.[6] suggest that the pegmatitic body cooled from 650 ° C to below 550 °C within ~9 years after emplacement. This is an upper bound on cooling timescales because any convective cooling, which was not considered by Webber et al.[6], would decrease estimated cooling timescales
We explore the possibility that the trace element compositions reflect kinetic processes, the effects of rapid crystal growth
Summary
Pegmatites are shallow, coarse-grained magmatic intrusions with crystals occasionally approaching meters in length Compared to their plutonic hosts, pegmatites are thought to have cooled rapidly, suggesting that these large crystals must have grown fast. In rocks that cool more slowly (10–100 ky), such as in magmas that intrude or stall deep in the crust to form plutons instead of erupting to the surface, grains are coarser (up to cm size). We show that late-stage centimeter-scale crystals grew within hours If such growth rates can be sustained, decimeter to meter-sized crystals in pegmatitic systems could have grown in days, with growth occurring in highly dynamic rather than static conditions. We suggest that the crystals grew from a free fluid phase within a turbulent convective system
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
