Zircon Survival, Rebirth and Recycling during Crustal Melting, Magma Crystallization, and Mixing Based on Numerical Modelling

Abstract

Improved geochronological methods and in situ isotopic (O, Hf) and trace element studies of zircon require a new physical model that explains its behaviour during crustal melting. We present results of numerical modeling of zircon dissolution in melts of variable composition, water content, temperature, and thermal history. The model is implemented in spherical coordinates with two moving boundaries (for the crystal and the surrounding melt cell outer edge) using simplified mineral phase relationships, and accounting for melt proportion histories as a function of melting and crystallization of major minerals. We explore in detail the dissolution of variably sized zircons and zircon growth inside rock cells of different size, held at different temperatures and undersaturations, and provide an equation for zircon survivability. Similar modeling is performed for other accessory minerals: apatite and monazite. We observe the critical role of rock cell size surrounding zircons in their survivability. Diffusive fill away from a dissolving 100 μm zircon into a large >3 mm cell takes 10 2 –10 4 years at 750–950°C, but zircon cores may survive infinitely in smaller than 1 mm cells. Heating followed by cooling for a similar amount of time leads to dissolution followed by nucleation and growth, but new zircon growth remains smaller than the original within the cell. The final zircon size is also investigated as a function of microzircons crystallizing on a front of major minerals, leading to shrinking cell sizes and bulldozing of Zr onto the growing zircon surface. We explore in detail the survivability and regrowth of zircon inside and outside dikes and sills of different sizes and temperatures, and in different rock compositions, on timescales of their conductive cooling and heating, respectively. For zircon-rich rocks, only the largest >200 m igneous bodies are capable of complete dissolution–reprecipitation of typically sized zircons at significant distances from the intrusion. Smaller intrusions result in partial dissolution and rim overgrowth. Zircons captured near the contact of conductively cooling sills undergo more overgrowth than dissolution. In contrast, heat wave propagation from the sill will completely dissolve and reprecipitate zircons in Zr-poorer rocks many diameters of the sill away and often 10 3 –10 4 years after the sill intrusion. A single thermal spike and melting episode is capable of generating the observed complexity of isotopically diverse and geochronologically zoned zircons. A MATLAB program is presented for users to apply in their specific situations.