Internal Evolution of Pegmatites
Understanding the causes of anisotropic fabrics, diverse and complex crystal habits, giant crystals and abrupt changes in crystal size, and mineralogical zonation in pegmatites (Fig. 1) has become a measure of how well we comprehend the basic processes of crystal growth from silicate melt or aqueous vapor. It is, therefore, an intellectual challenge for geoscientists, which is why the study of internal differentiation of pegmatites has attracted some of the world's most prominent mineralogists, petrologists, and geochemists since the mid-19th century.

At the University of Oklahoma, we have utilized experimental geochemistry to simulate all major features of natural pegmatites (see London, 2004), including graphic intergrowths (Detail 1A), layered aplites (Detail 1B), and highly oriented crystal growth fabrics perpendicular to contacts (Detail 1C). Important as they are, these simulations, however, merely serve as a test for hypotheses derived from the study of natural pegmatites in several field areas and through mineral and fluid inclusion analysis.


  

 

Fig. 1. Footwall section through the Little Three pegmatite, Ramona, California

  

 
     

The experimental basis for this model, which began with experimentation using a chemically evolved rhyolite obsidian from Macusani, Peru (London et al., 1989) has been detailed recently in London (2004). Field data from fluid inclusions (London, 1986) and chemical zoning profiles in pegmatites (Morgan and London, 1999) provide much of the ground truth to direct the experimental investigations.

Figure 1 is taken from one of 15 complete polished sections through 5 different pegmatite bodies up to 1 m thick. No one has ever had such complete cross sections to work from in the past, nor multiple sequential slices through the same rock to document changes in fabric. Working as a group, 2-3 graduate students at the M.S. or Ph.D. level will be able to fully characterize these pegmatite slices using all methods of analysis, including electron and ion microbeam analyzers (for majors, traces, and isotopes), fluid inclusion microthermometry, and image analysis programs.