Abstract
The saturation surfaces of rutile (TiO2), zircon (ZrSiO4), and hafnon (HfSiO4) were determined in anhydrous, peraluminous, high silica liquids of the system SiO2-Al2O3-Na2O-K2O as functions of silica concentration at 1,400° C in air. The saturation concentrations of TiO2, ZrO2, and HfO2 in rutile, zircon, and hafnon-saturated liquids, respectively, decrease smoothly and gradually as functions of increasing silica concentration. Thermodynamic analyses of the data demonstrate that the activity coefficients of TiO2, ZrO2, and HfO2 increase smoothly and gradually as silica concentration is increased from 67 wt-% to 80 wt-%, and that changes in SiO2 of 1 or 2 wt-% result in small changes in the saturation concentrations and activity coefficients of +4 cations. Because the solution behavior of +4 cations in highly siliceous liquids (>75 wt-% SiO2) is predictably different than in less siliceous liquids (70 to 75 wt-% SiO2), classification of highly-siliceous igneous rocks on the basis of silica concentration alone should not be interpreted to mean that their solution chemistry differs significantly from that of less siliceous rocks. The results of this study are compared with other studies of +4 cation solution behavior. From this it is concluded that variations in liquid compositions observed in cogenetic suites of high silica rhyolites cannot cause the observed changes in +4 cation concentrations. Thus, even if a large change in solution behavior of +4 cations is inferred from the large variations in their concentrations, it cannot be due to changes in bulk composition of the parental liquid. In addition, the similarity in the solution behavior of Zr and Hf seen in this study suggests that their solution mechanisms are similar. It is thus unlikely that liquid-state processes can fractionate one with respect to another, and variations in Zr/Hf ratios in suites of extrusive rocks are likely due to crystal-liquid equilibria, e.g., zircon fractionation.
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Ellison, A.J., Hess, P.C. Solution behavior of +4 cations in high silica melts: petrologic and geochemical implications. Contr. Mineral. and Petrol. 94, 343–351 (1986). https://doi.org/10.1007/BF00371443
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DOI: https://doi.org/10.1007/BF00371443