THERMAL MODELING AND NUCLEATION LAG TIMES IN PEGMATITIC MELTS

The London model of granitic pegmatite formation from supercooled melts requires tens to hundreds of degrees of supercooling before nucleation and growth of the giant crystals found in these rocks. However, the question arises as to whether cooling magmas can avoid crystal nucleation and growth before reaching deeply supercooled conditions. The answer to this question involves competition between nucleation lag times and cooling times: melts must reach deeply supercooled conditions before nucleation begins.

Nucleation lag times for crystallization of feldspars and quartz in granitic melts are poorly known. Classic studies suggest nucleation lag times varying from 4 to 23 h for supercoolings of 45 to 70 °C (Fenn, Can.Min. 1977), to ~100 h for supercoolings of ~150 °C (Swanson, Am. Mineral 1977). More recent studies of nucleation lag times are consistent with these previous ones and indicate that nucleation lag times of 100 to 200 h are expected for hydrous granitic melts supercooled by 100 to 150 °C (e.g., London & Morgan, Elements, 2012; Maneta & Baker, Am. Mineral. 2014).

Conductive cooling times of meter-scale pegmatites intruded at 650 °C into country rocks at 150 °C range between ~ 20 days for a 1 m-wide dike to fall below 500 °C, and more than 30 days for a 2 m-wide dike (Webber et al., 1999, Am. Mineral.). Our thermal modeling of dike intrusion as a laminar pipe flow (Kay & Nedderman, 1985, Chpt 18), where the initial melt is at 700 °C and the external temperature of the pipe is maintained at 525 °C (or a country rock ambient temperature of 350 °C), demonstrates that a 1-m diameter magma body cools to an average temperature of 525 °C, or a supercooling of 175 °C, in ~ 5 days, or 120 h, and that a 2-m diameter body evolves to the same conditions in less than 20 days, or 480 h. These times are much more rapid than simple conductive cooling times of a pegmatitic melt that require ~ 400 h for most of a 1-m dike to cool from 700 °C to 525 °C and greater than 800 h for a 2-m dike.

Comparison of the cooling times calculated by the pipe flow model and experimental nucleation lag times indicate that pegmatitic melts intruded into cool country rocks as meter-scale dikes can cool rapidly enough to produce nucleation lag times of 100 h and reach their final intrusive location with at most a few crystal nuclei that may form the seeds of giant pegmatite crystals.