USING SIZE OF GIANT CRYSTALS TO CONSTRAIN CRYSTALLIZATION DURATION OF GRANITIC PEGMATITES

The crystal length (L) of the largest single crystals of feldspar, spodumene, or tourmaline in internally-zoned, granitic pegmatites may correlate with the width (W) of the magma body available in the direction of crystal growth at the time of nucleation. The unitless ratio X = L/W between the final crystal length and the initial width of the liquid can be used to determine the duration of crystallization, if the variation of the crystal growth-rate with time is known. We have tested this hypothesis in experiments of magmatic crystallization within samples of variable diameter conducted in diamond-anvil cell (DAC) and cold-seal pressure vessels (CSPV). Prior to each crystallization experiment, the starting H₂O-undersaturated, Li-B haplogranitic glass was remelted at ~720°C, to simulate superheated, nuclei-free pegmatite liquids. Typical pegmatite-like fabrics including “giant”-size alkali feldspar and graphic quartz-feldspar intergrowths were generated during magmatic crystallization at 200-400 MPa and 100-200 °C of undercooling below the liquidus temperature of ~660°C.

We used time-lapse photography during the DAC runs (71 to 185 h) and crystal sizes measured after the CSPV time-dependent runs (168 to 1440 h) to quantify the nucleation delays and growth rates of the largest, unobstructed crystals. Those crystals reaching a fraction X ≥ 0.1 of the sample diameter were classified as “giant”. Dominantly, the giant crystals followed a proportionate growth model, with a constant 3D growth rate and a nonlinear (cubic-root) 1D growth rate function of time. The 1D growth rates measured or estimated at 24h after nucleation (G_24h) were plotted against the width of the sample (W) and the data was best-fitted using a power function expressed as G_24h=b·W^a. We extrapolated the G_24h function from the 0.170 – 3 mm scale of experiments to the 10^-2 – 10² m scale of pegmatite dikes, assuming that similar undercooling conditions generated the texture similarity in experiments and nature. We then solved the time equation as a function of W and X. The crystallization durations of giant crystals in pegmatites resulted in this study are slightly larger than melt-longevity estimates from conductive-cooling computations but 1-2 orders of magnitude shorter and more realistic than growth times calculated based on experimental growth rates.