The Transition from Granite to Pegmatite

For half a century, petrologists have taken pegmatites as de facto evidence for aqueous vapor saturation of granitic melt, because Jahns and Burnham (1969) ascribed the textural transition from granite to pegmatite to the exsolution of buoyant aqueous vapor and its inferred effects upon the crystallization of melt near the thermal minimum of the hydrous haplogranite liquidus (Tuttle and Bowen, 1958).

There is no known causal relation between aqueous vapor saturation of melt and the formation of giant crystals. Growth of giant crystals in pegmatites requires more rapid transport (mass, if not rate) of Si and Al than an aqueous vapor can provide in the timeframe of cooling. In analogue experiments, a flux-rich melt moves 10⁷ times the mass of Si as aqueous vapor at 10⁷ times the rate of diffusion in hydrous haplogranitic melt at 800^oC and 200 MPa. A hydrous flux-rich melt created via constitutional zone refining at crystal-melt interfaces provides the necessary mass transport in pegmatite-forming environments.

There is no known causal relation between aqueous vapor saturation of melt and the mineralogical zonation that is prevalent in many granitic pegmatites. Crystallization is sequential (or oscillatory) in highly undercooled granitic melts, even those of invariant composition, because of (1) differences in the curvature of the liquidus surfaces for quartz and feldspars, and (2) interactions among fluxing components and major melt components that suppress the nucleation of rock-forming minerals.

Empirical evidence indicates that granitic pegmatites become saturated in H₂O only toward the end of their consolidation. Fractionation models for pegmatites imply that their source magmas should be H₂O-saturated prior to the derivation of pegmatite-forming melts. Resolution of this question, when granitic melts become saturated in H₂O and the evidence for this transition (if not pegmatites), remains as one of the most significant problems in igneous petrology today.