Melt Boundary Layers  
  The central question posed by London (2004) regarding the origins of pegmatitic rocks is this: the fluxing components of H2O, B, F, and P all promote the formation of pegmatitic features, but most pegmatites contain negligibly small quantities of these components. They lack evidence that they once carried much more flux that was lost to surrounding rocks. How, then, can a flux-based model for the development of pegmatite be valid (e.g., London et al., 1989)? The answer proposed by me in London (1992, 1996), and elaborated in London (2004), is that the rapid growth of crystals from a viscous granitic melt can generate layers of flux-enriched melt in advance of the crystal front (Fig. 5). The flux-enriched melt occurs as a discrete boundary layer, and its composition can be far different from that of the main melt. In Fig. 5, the dark boundary layer of melt (now glass) that developed along the crystallization front of quartz and feldspar contains, at point 1, nearly 50 mole % B2O3 and H2O, a 2-fold increase over the bulk melt, especially in boron (3-fold).  

Fig. 5. A boundary layer liquid frozen as glass in an experiment that contained 3 wt% B2O3 at the start. From London (2004).

 

    

The properties of boundary layers in pegmatite-forming melts are just beginning to be examined, and right now major disparities exist in estimates of their physical properties. At the conditions at which pegmatites would form, are they viscous gels, or are they runny fluids? How, also, do they affect the partitioning of trace elements? What we know of that partitioning is based on crystal growth from rather ordinary granitic melts, which are not likely to apply in this case.