Zircon saturation in silicate melts: a new and improved model for aluminous and alkaline melts

Abstract

The importance of zircon in geochemical and geochronological studies, and its presence not only in aluminous but also in alkaline rocks, prompted us to think about a new zircon saturation model that can be applied in a wide range of compositions. Therefore, we performed zircon crystallization experiments in a range of compositions and at high temperatures, extending the original zircon saturation model proposed by Watson and Harrison (Earth Planet Sci Lett 64:295–304, 1983) and Boehnke et al. (Chem Geol 351:324–334, 2013). We used our new data and the data from previous studies in peraluminous melts, to describe the solubility of zircon in alkaline and aluminous melts. To this effect, we devised a new compositional parameter called G $$ \left[ {\left( {3 \cdot {\text{Al}}_{2} {\text{O}}_{3} + {\text{SiO}}_{2} )/({\text{Na}}_{2} {\text{O}} + {\text{K}}_{2} {\text{O}} + {\text{CaO}} + {\text{MgO}} + {\text{FeO}}} \right)} \right] $$ (molar proportions), which enables to describe the zircon saturation behaviour in a wide range of rock compositions. Furthermore, we propose a new zircon saturation model, which depends basically on temperature and melt composition, given by (with 1σ errors): $$ \ln \left[ {\text{Zr}} \right] = \left( {4.29 \pm 0.34} \right) - \left( {1.35 \pm 0.10} \right) \cdot \ln G + \left( {0.0056 \pm 0.0002} \right) \cdot T\left( {^\circ C} \right) $$ where [Zr] is the Zr concentration of the melt in µg/g, G is the new parameter representing melt composition and T is the temperature in degrees Celsius. The advantages of the new model are its straightforward use, with the G parameter being calculated directly from the molar proportions converted from electron microprobe measurements, the temperature calculated given in degrees Celsius and its applicability in a wider range of rocks compositions. Our results confirm the high zircon solubility in peralkaline rocks and its dependence on composition and temperature. Our new model may be applied in all intermediate to felsic melts from peraluminous to peralkaline compositions.