T136 Reading Igneous Textures

Monday, 23 September 2019
1:30 PM - 5:30 PM
Room 126ABC, North Building

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

Paper number: T136-9
Program: Technical Sessions
Time: 3:40 PM – 3:55 PM

Authors

Abstract

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 (G24h) were plotted against the width of the sample (W) and the data was best-fitted using a power function expressed as G24h=b·Wa. We extrapolated the G24h 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.

Geological Society of America Abstracts with Programs. Vol. 51, No. 5

doi: 10.1130/abs/2019AM-340199

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