CONTROLS ON ZIRCON GROWTH IN MIGMATITES: A PHASE DIAGRAM MODELING APPROACH
New phase equilibria modeling work using THERMOCALC (Powell & Holland, 1998) is aiming to understand better the controls on zircon growth from anatectic melts. With the recent addition of zirconium to thermodynamic datasets (Kelsey et al., 2011), it is now possible to integrate zircon saturation models within the framework of an internally consistent dataset to fully evaluate the roles of melt and mineral composition on zircon growth in a more holistic way. Phase diagrams have been calculated to directly assess the role of water on zircon growth under conditions equivalent to those experienced in metasedimentary migmatites. Using the composition of melt generated at granulite facies conditions (P~7kb, T~860°C), T-XH2Opseudosections were constructed to simulate the effects of reactions between hydrous melt and comparatively anhydrous wall rocks during melt ponding in leucosomes. Results show that loss of water from the melt leads to immediate saturation of zircon, and continued zircon growth is possible without cooling. Multiple “ponding” events lead to additional zircon growth from melt significantly reducing melt-zirconium content. Results are consistent with observations on deep crustal migmatites, and provide a mechanism for trace element depletion in S-type granites, and relative enrichment of the granulite residue.
Kelsey et al. (2011), J. Met. Geol., 29, 151-166. Powell et al. (1998), J. Met. Geol., 16, 577–588.