Water and melt/melt immiscibility, the essential components in the formation of pegmatites; evidence from melt inclusions

In this paper we provide evidence for, and a statement of a general model of granititic pegmatite formation bases on melt and fluid inclusions in pegmatite minerals. We demonstrate that extremely H2O-rich residual granitic melt may undergo melt-melt immiscibility to form two mutually immiscible melts. One of these is relatively H2O- and alkali-poor (type-A melts), the other is extremely H2O- and alkali-rich (type-B melts). These melts have radically different compositions, mobility, and reactivity, and their separation, and interactions with wall-rocks, as well as potentially with each other, and their cooling and crystallization, can produce the many and varied types of pegmatites. Melt and fluid inclusions provide essential evidence in this model, since they are often the only remnants of the initial and intermediate stages of this process which survive. Thus, their compositions provide critical constraints on pegmatite crystallization. We will show that melt and fluid inclusion compositions are representative of these fugitive early stages, rather than boundary layer artefacts. From melt inclusions we can show that water is a major component in the formation of pegmatites, and we can demonstrate that complete miscibility between pegmatite-forming melts and water exist at or below 700°C and 1 kbar. Hydrothermal anvil cell experiments provide additional evidence to support our deductions by demonstrating the contrasting high and low pressure behavior of peralkaline melt fractions.