Understanding Pegmatite Texture: Kinetics of Crystallization in the Haplogranite-Li-B-H2O System

We investigated the crystallization behavior of haplogranite-Li-B-H2O melts in a series of dynamic experiments in order to constrain development of texture in Li-rich pegmatites. Current models propose that salient pegmatitic features such as sub-millimeter to meter scale crystal sizes and unidirectional, spherulitic, and skeletal intergrowths are a consequence of disequilibrium crystallization under conditions of large undercooling. Although pegmatite melts are inferred to be rich in water, the role of water in promoting large crystals during magmatic crystallization is not well understood. Hydrous haplogranite glasses containing 1% Li2O+2.3% B2O3 (composition C1, by weight) and 2% Li2O+4.6% B2O3 (C2) were synthesized at 1200°C and 300 MPa in sealed Pt capsules. The water ranged between 3 to 9% H2O. Time series of unseeded crystallization experiments lasting from 1 to 14 days were performed in hydrothermal autoclaves at temperatures ranging from 400 to 700°C at 200 and 300 MPa. The experimental conditions placed the hydrous melts under variable degrees of undercooling between their liquidus and glass transition. Reproducible phase assemblages including alkali-feldspars, muscovite, stuffed β-quartz (a silica-rich solid solution between quartz and petalite), petalite (LiAlSi4O10), and virgilite (a solid solution between petalite and spodumene - LiAlSi2O6) were documented using Raman spectroscopy, XRD, and EPMA. The incubation times varied with temperature and composition. At 500° C the incubation times were between 5 and 9 days for C1 and <5 days for C2. At 400°C C1 did not produce any crystals after 14 days whereas C2 contained trace amounts of skeletal crystals formed between 9 and 14 days. The largest crystals of up to 1.5 mm and maximum growth rates of 0.25 mm/day were produced in composition C2 at 550°C. Although crystallization did not exceed ~30%, pertinent textural characteristics of LCT pegmatites were simulated reproducibly. In all runs heterogeneous nucleation commenced at the charge margin and continued inwards. Muscovite was the only mineral able to nucleate homogeneously throughout the charge. Crystallization of C1 resembles feldspar-quartz±muscovite border zones and wall zones of LCT pegmatites including fine-grained “line rock” texture at 9 days and graphic alkali feldspar - quartz intergrowths at 14 days. C2 products of high growth rates coupled with low nucleation density resemble coarse fabric found in highly fractionated inner zones of LCT pegmatites. Kinetic boundary layers around crystals contain up to 11% B2O3, which suggests that crystal growth rates exceeded B diffusion rate. Water supersaturation appears to contribute to development of pegmatite texture by reducing nucleation and increasing crystal growth. For example, in 200 MPa experiments, gradual textural coarsening by a factor of 3 was observed in areas surrounding large fluid cavities. This resembles textures adjacent to “pockets” in relatively shallow miarolitic pegmatites.