Abstract

Oxygen isotope fractionation between synthesised zircon and water has been experimentally quantified at 700, 800, 900, and 1000 °C. The results are interpolated by: Δzm-H20 = −3.70 + 2.74 ± 0.19x2, where x = 103/T (K). Combined with the fractionation between quartz and water (Bottinga & Javoy, 1973) this yields: Δqtz-zm= 1.36x2.

Theoretical evaluations of the reduced partition function ratios for zircon and two (a- and β-) modifications of quartz are expressed in terms of the following polynomials: 1000 lnfzm= 8.3306x2+ 1.9402 x − 0.6896 (400 < T < 1100°C) 1000 lnfa-qtz= 7.8963 x2+7.4091 x-3.6015 (200°C < T < a-quartz stability field) 1000 lnfβ-qtz= 9.3362 x2+ 2.4514 x − 0.7844 (β-quartz stability field up to 1100 °C).

These expressions are in excellent agreement both with the experimentally derived factors of oxygen isotope fractionation for β-quartz and zircon, and the incremental calibrations for a-quartz and zircon (Hoffbauer et al., 1994). The effect of a-β-quartz transition on oxygen isotope fractionation implies, that those calculations, anchored to the theoretically evaluated reduced partition function ratios of quartz (e.g.,Zheng, 1993), can predict fractionations only within the P-T stability field of the respective modification of quartz (i.e. a-quartz).

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