ZIRCON, XENOTIME AND MONAZITE STABILITY IN EVOLVING PEGMATITE FLUIDS: IMPLICATIONS FOR GEOCHRONOMETER STABILITY IN IGNEOUS AND METAMORPHIC SYSTEMS

The rare earth and high field-strength element bearing geochronometer minerals zircon, monazite and xenotime may be used to monitor multiple sequential petrogenetic processes because their mechanical, compositional and isotopic stability helps preserve grain domains from one geologic event to the next, and each domain may reflect the geologic conditions of its growth. Because diffusion in these minerals is very slow at all but the most extreme geologic conditions, fluids are perhaps the most important factor controlling mineral reactivity, for high fluid activity helps overcome kinetic barriers. Textural and compositional evidence for disequilibrium reactions, that occurred via dissolution-reprecipitation, from contrasting pegmatites and a carbonatite-alkali (nepheline) syenite system in Norway will be used to show that water and alkali-rich, halogen-CO₂-poor fluids drive net volume reactions in monazite and xenotime, with no reactivity observed in zircon. However, high alkalinity fluids in the presence of CO₂, promotes monazite growth and zircon dissolution-reprecipitation. A third example of a high-F-Ca pegmatite that has low-CO₂ and Cl activity provides an example of zircon and monazite instability and xenotime growth in response to disequilibrium reactions. The results demonstrate that the "strength", as well as the identity, composition and ratio of anionic ligands has a primary control over mineral reactivity and the development of diagnostic textures. The compositional and textural information and the comparative behavior of REE and HFSE's during the crystallization of high-volatile magmatic fluids may be combined with data from experimental work to show that, not only are the disequilibrium reactions geologically instantaneous, but the results are transferable to metamorphic systems. Furthermore, the results suggest that the importance of alkali fluids may be underestimated and they should be considered as important agents of metamorphic change during geologic processes.