Dynamic Crystallization of a Haplogranitic Melt: Application to Pegmatites

Abstract

Abstract Both equilibrium and dynamic crystallization experiments have been performed on a hydrous haplogranitic melt at 200 MPa to model nucleation and growth mechanisms and simulate pegmatite textures. The equilibrium results provide a reference frame (phase assemblages and compositions, liquidus and solidus temperatures and dependence with the melt H2O concentration) to parametrize the kinetic experiments. The seven H2O-saturated dynamic crystallization experiments followed a specific time-temperature path. After a pre-conditioning step at 800°C, charges were cooled between 3·5 and 7°C/min to 700, 660 and 600°C corresponding to ΔT of 20, 60 and 120°C. Dwell times ranged from 42 up to 1440h. Variable mineral assemblages and textures, and two types of polymineralic assemblages were obtained depending on ΔT and t. For ΔT = 120°C, crystallization is sequential and includes graphic quartz–alkali-feldspar intergrowths characteristic of pegmatite textures. The crystallization sequence reflects nucleation and growth of kinetically-favoured metastable phases and solid solution compositions from the supercooled melt. Early alkali-feldspars are more K-rich than expected at equilibrium and late albites more Na-rich. The K-rich graphic texture progressively evolves to a Na-rich intergrowth texture. Melts also follow a progressive though limited sodic evolution with time. At the interface of growing alkali-feldspars, melts are enriched in SiO2 and depleted in Al2O3, Na2O and, to a lesser extent, K2O. H2O accumulates at the interface reaching concentration levels higher (by 1–2 wt %) than the saturation. Rejection of SiO2 and H2O at the interface controls the effective undercooling in the local melt and promotes rapid textural changes toward larger grain sizes at the front of graphic zones. Textural ripening takes place contemporaneously to sequential crystallization. Growth rates for quartz and alkali-feldspar are tightly grouped, between 7·3 x 10-11 and 1·6 x 10-12 m s-1. Textures from the dynamic crystallization experiments closely resemble natural pegmatites, but layered aplite units have not been reproduced. Our results confirm and strengthen the importance of liquidus undercooling to generate pegmatite textures.