Episodes of fast crystal growth in pegmatites

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.

Widespread phosphorous excess in olivine, rapid crystal growth, and implications for magma dynamics

“…For the swallow-tailed dendritic olivines, which are ~5 mm in length and <50 µm in width, this corresponds to a growth rate of ~1-3 mm/h. These growth rates approach those in pegmatitic systems [Phelps et al 2020] and are consistent with the presence of dendritic olivines [Faure 2006].…”

“…Higher growth rates would be possible if the conditions were turbulent (Reynold's number >10 3 ) and boundary layer thicknesses were much thinner. Turbulent scenarios may occur during pegmatite formation [Phelps et al 2020], but for basaltic to felsic magmas at the scale of a phenocryst, laminar flow (i.e. low Reynold's number) dominates.…”

The physical and chemical evolution of magmatic fluids in near-solidus silicic magma reservoirs: Implications for the formation of pegmatites

“…to kinetic crystallization at turbulent, high-Reynolds number conditions (Phelps et al, 2020), characteristics for low-viscosity fluids.…”

Lithium

“…These mineral-rich fluids are incorporated into pegmatite crystals, which have been called 'scientific wonders', 4 due to their enormous grain-sizes. In extreme cases, such as the pegmatites found in the Black Hills of South Dakota, individual crystals can reach more than ten meters in length, forming at a rate of up to one to ten meters per day 5 -stunningly fast relative to most geological processes. These enormous crystals are not only a rich source of critical minerals, but also of quartz, feldspars and micas for industrial uses, as well as many of the world's finest gem and mineral specimens, including varieties of beryl, topaz and tourmaline.…”