New thermodynamic models and revised calibrations for the Ti-in-zircon and Zr-in-rutile thermometers期刊界 All Journals 搜尽天下杂志传播学术成果专业期刊搜索期刊信息化学术搜索

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New thermodynamic models and revised calibrations for the Ti-in-zircon and Zr-in-rutile thermometers

Authors:	J M Ferry E B Watson

Institution:	(1) Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD 21218, USA;(2) Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180, USA

Abstract:	The models recognize that ZrSiO₄, ZrTiO₄, and TiSiO₄, but not ZrO₂ or TiO₂, are independently variable phase components in zircon. Accordingly, the equilibrium controlling the Zr content of rutile coexisting with zircon is ZrSiO₄ = ZrO₂ (in rutile) + SiO₂. The equilibrium controlling the Ti content of zircon is either ZrSiO₄ + TiO₂ = ZrTiO₄ + SiO₂ or TiO₂ + SiO₂ = TiSiO₄, depending whether Ti substitutes for Si or Zr. The Zr content of rutile thus depends on the activity of SiO₂ $(a_{\text{SiO}_{2}})$ as well as T, and the Ti content of zircon depends on $a_{\text{SiO}_{2}}$ and $a_{\text{TiO}_{2}}$ as well as T. New and published experimental data confirm the predicted increase in the Zr content of rutile with decreasing $a_{\text{SiO}_{2}},$ and unequivocally demonstrate that the Ti content of zircon increases with decreasing $a_{\text{SiO}_{2}}$ . The substitution of Ti in zircon therefore is primarily for Si. Assuming a constant effect of P, unit $a_{\text{ZrSiO}_{4}},$ and that $a_{\text{ZrO}_{2}}$ and $a_{\text{ZrTiO}_{4}}$ are proportional to ppm Zr in rutile and ppm Ti in zircon, log(ppm Zr-in-rutile) + log $a_{\text{SiO}_{2}}$ ] = A₁ + B₁/T(K) and log(ppm Ti-in-zircon) + log $a_{\text{SiO}_{2}}$ − log $a_{\text{TiO}_{2}}$ ] = A₂ + B₂/T, where the A and B are constants. The constants were derived from published and new data from experiments with $a_{\text{SiO}_{2}}$ buffered by either quartz or zircon + zirconia, from experiments with $a_{\text{SiO}_{2}}$ defined by the Zr content of rutile, and from well-characterized natural samples. Results are A₁ = 7.420 ± 0.105; B₁ = −4,530 ± 111; A₂ = 5.711 ± 0.072; B₂ = −4,800 ± 86 with activity referenced to α-quartz and rutile at P and T of interest. The zircon thermometer may now be applied to rocks without quartz and/or rutile, and the rutile thermometer applied to rocks without quartz, provided that $a_{\text{SiO}_{2}}$ and $a_{\text{TiO}_{2}}$ are estimated. Maximum uncertainties introduced to zircon and rutile thermometry by unconstrained $a_{\text{SiO}_{2}}$ and $a_{\text{TiO}_{2}}$ can be quantitatively assessed and are ≈60 to 70°C at 750°C. A preliminary assessment of the dependence of the two thermometers on P predicts that an uncertainty of ±1 GPa introduces an additional uncertainty at 750°C of ≈50°C for the Ti-in-zircon thermometer and of ≈70 to 80°C for the Zr-in-rutile thermometer.

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