Aluminum partitioning between zircon and haplogranitic melts: The influence of temperature and melt composition

Abstract

We present measurements of Al concentrations in zircon (XAlZrc, in ppm) grown from one peralkaline, one metaluminous, and two peraluminous melts in the Na2O-K2O-CaO-Al2O3-SiO2 system (with ~10 wt% H2O). Twenty experiments were conducted at 1 GPa from 1100 to 1400 °C and we infer two main conclusions. First, XAlZrc is strongly dependent on melt aluminum saturation index (ASI = molar ratio of Al2O3 / [CaO + Na2O + K2O]) and M (molar ratio of [K + Na + 2Ca]/[Si × Al]). That is, as melt ASI values increase from 0.50 to 1.38 – or M values decrease from 3.25 to 1.17 − XAlZrc increase correspondingly. Second, for similar melt compositions, XAlZrc is positively related to temperature. Our data confirm that the melt parameter ASI or M are appropriate compositional proxies that describe Al concentration in zircon. Our preferred model which relates melt ASI composition (ASImelt) and temperature (T, in K) to Al-in-zircon (XAlZrc,in ppm) is given by:logXAlZrcinppm=0.98±0.07×ASImelt−4334±459/TK+4.09±0.31This model lays the groundwork to define natural zircon parent melt composition(s), including changes in melt composition, during zircon crystallization. We extrapolate our experimental results to previously measured XAlZrc data from rocks of known parentage. When using whole rock as a proxy for the melt composition, Al concentrations in peraluminous zircon are in broad agreement with the empirical model. However, the model predicts lower ASI values when applied to metaluminous natural zircons. The model presented here is derived from high temperature and high H2O content experiments (1100–1400 °C; H2O = ~10 wt%) and a compositionally limited suite of melt compositions and will therefore require further development for more accurate extrapolation to lower temperatures. These results demonstrate feasibility of this approach to reveal zircon parent melt composition.