Application of Proxy Minerals for Evaluation of the Oxygen Isotope Composition of Felsic Melts

The problems associated with the use of quartz and zircon as proxy minerals for the reconstruction of δ¹⁸O values in acidic melts are considered. It is shown that the correction values Δ(Qz–R) and Δ(R–Zrn) used for the reconstructions are not strictly constant and depend on the mineral composition of the rock and the closure temperature of the oxygen isotopic system of the proxy mineral (T_q, T_z–closure temperature of quartz and zircon, respectively). The applicability of quartz was estimated using an equation for Δ(Qz–R) calculation, which takes into account T_q and the mineral composition of rocks. Using the leucogranites of the Raumid massif (South Pamirs) as an example, it was shown that this correction can be approximated by constant value only under definite conditions. The value of Δ(R–Zrn) was estimated using approach based on calculating the weighted average fractionation coefficient and independent estimates Tz using a zirconium thermometer. It was shown for leucogranite porphyries of the Omsukchan trough that this correction for identical rocks varies from 1.3 to 1.9 ‰, unlike Δ(R–Zrn) = 2.1 determined by the dependence on the SiO₂ content (Lackey et al., 2008). The advantages and limitations on the application of quartz and zircon as proxy minerals have been analyzed.