The competing models for the origin and internal evolution of granitic pegmatites in the light of melt and fluid inclusion research
Thomas, R and Davidson, Paul and Beurlen, H, The competing models for the origin and internal evolution of granitic pegmatites in the light of melt and fluid inclusion research, Mineralogy and Petrology, 106, (1-2) pp. 55-73. ISSN 0930-0708 (2012) [Refereed Article]
In this paper we discuss the main petrographic models for granitic pegmatites and how these models have evolved over time. We suggest that the present state of knowledge requires that some aspects of these models to be modified, or absorbed into newer ones. Pegmatite formation and internal evolution have long supposed the need for highly water- and flux-enriched magmas to explain the differences between pegmatites and other intrusives of similar major element composition. Using compositions and textural characteristics of fluid and melt inclusions in pegmatite minerals can provide strong evidence for such magmas. Furthermore, we show that melt inclusion research has increased the potential compositional factors to be considered to include H2O, OH-, CO2, HCO3-, CO32-, SO42-, PO43-, H3BO3, F, and Cl, as well as the elements Li, Na, K, Rb, Cs, and Be, herein described as melt structure modifiers. In this paper we emphasize that these elements act together in various combinations, and do not necessarily act in strictly linear, additive ways, making interpolation of experimental work problematic at best.
The combination and the amount of the different magmatic species, together with differences in the source region, and variations in pressure and temperature cause the great diversity of the pegmatites observed. Some volatile species, such as CO32- together with alkalies, often ignored in the past, have the capacity to increase the solubility of H2O in silicate melt to an extraordinary degree, to the extent that melt-melt-fluid immiscibility becomes inevitable. It is our view that the formation of pegmatites is connected with the complex interplay of many factors, although models of such processes have often been used in an all or nothing way. The vehement defense of any one model hinders the progress in the developing of proper understanding of pegmatite-formation.