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Zr and Hf have the same charge and similar ionic radii; consequently the geochemical behaviour of these elements is similar for a variety of different geological processes i.e, for many suites of igneous rocks the Zr/Hf ratio remains nearly constant with magmatic evolution. By contrast in other suites, peraluminous granites in particular, the Zr/Hf ratio decreases with fractionation, as does the Zr/Hf ratio of zircon-hafnon. The solubilities of zircon (ZrSiO₄) and hafnon (HfSiO₄) were therefore determined for water-saturated haplogranitic melts with peraluminous to peralkaline compositions in order to constrain the interpretation of the distribution of Zr-Hf in igneous environments.

Zircon and hafnon solubilities at 1035°C, 2 kbar and H₂O-saturation conditions increase dramatically with the alkali content of the melt: from 0.25 wt% to 6.9 wt% HfO₂ and from 0.09 wt% to 4.5 wt% ZrO₂ for Al/(Na+K) compositions of 1.2 to 0.6, respectively. The mole percent of SiO₂ of the melts is constant, thus the activity of SiO₂ in peralkaline to peraluminous compositions is nearly constant, and the increases in zircon and hafnon solubilities with alkali content is largely a consequence of decreases of Zr and Hf activity coefficients. Futhermore, for a given Al/(Na+K) ratio, the division of the Zr and Hf molar abundances gives a value that is inversely proportional to the Zr/Hf activity coefficient ratio. The molar Zr/Hf solubility ratio is shown on Fig. 3.6-9. For strongly peralkaline
 

Fig. 3.6-9: Molar ratio of the solubility of zircon/hafnon in water-saturated peralkaline to peraluminous granitic melts at 1035°C and 2 kbar.

[Al/(Na+K)=0.6 to 0.8] compositions this ratio is close to one. It follows that the Zr and Hf activity coefficients are similar and that the fractionation of alkaline granitic liquids should not result in a significant change of the Zr/Hf ratio. This agrees well with the natural data. In general the Zr/Hf ratio is also nearly constant for suites of and basaltic rocks, and thus it can be inferred that the Zr/Hf activity coefficient ratio of mafic melts is also near unity. By contrast the Zr/Hf solubility ratio is less than one for highly polymerised metaluminous [Al/(Na+K)=1.0] and peraluminous [Al/(Na+K)=1.2] melt composition, indicating that the Zr/Hf activity coefficient ratio is greater than one. This explains why for such melt compositions there are systematic decreases in the Zr/Hf ratio of zircon and of whole-rock values with increasing fractionation.