Three-dimensional observation of the boundary region between massive feldspar and graphic granite by X-ray computed tomography

Abstract

The three-dimensional (3-D) structure of crystals around the interface between massive potassium (K)-feldspar and quartz-feldspar intergrowth (graphic granite) of two samples collected from a pegmatite body in Ishikawa town, Fukushima Prefecture, Japan, was determined by X-ray computed tomography (CT). Based on the results and additional information obtained by serial thin section and polarizing observations, the formation processes of graphic granite and pegmatite bodies are discussed. The most important finding of the 3-D observation using X-ray CT is the very flat spatial distribution of the tips of the quartz crystals. The sharp interface between the graphic and single-phase parts suggests that the change from graphic granite to massive K-feldspar crystallization corresponds to the transition of the thermodynamic conditions at the growth front from non-equilibrium to equilibrium. This indication leads to a new model for the pegmatite genesis, that is, graphic granite forms at the marginal zone under non-equilibrium conditions shortly after the intrusion of magma due to rapid cooling by the cold host rock body and, subsequently, the crystallization of massive K-feldspar starts when the conditions at the crystal growth front reenter the 'feldspar + melt' field in the phase diagram due to the relaxation of the conditions. Other results related to the conditions of the growth environment with coexisting aqueous fluid, such as the connectivity of the quartz crystals, are also discussed, for example, the influence of the supercritical state on the nucleation of quartz crystals.