New thermodynamic models and revised calibrations for the Ti-in-zircon and Zr-in-rutile thermometers
New thermodynamic models and revised calibrations for the Ti-in-zircon and Zr-in-rutile thermometers
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DOI：
10.1007/s00410-007-0201-0
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发表时间：
2007-10-01
影响因子：
3.5
通讯作者：
Watson, E. B.
中科院分区：
文献类型：
--
作者：
Ferry, J. M.;Watson, E. B.
这些模型认识到，ZrSiO₄、ZrTiO₄和TiSiO₄是锆石中独立可变的相组分，而ZrO₂或TiO₂不是。因此，控制与锆石共存的金红石中Zr含量的平衡是ZrSiO₄ = ZrO₂（在金红石中）+ SiO₂。控制锆石中Ti含量的平衡要么是ZrSiO₄ + TiO₂ = ZrTiO₄ + SiO₂，要么是TiO₂ + SiO₂ = TiSiO₄，这取决于Ti是替代Si还是Zr。因此，金红石中的Zr含量取决于SiO₂的活度（a(SiO₂)）以及温度T，而锆石中的Ti含量取决于a(SiO₂)、a(TiO₂)以及T。新的和已发表的实验数据证实了随着a(SiO₂)降低，金红石中Zr含量预测的增加；并且明确表明锆石中的Ti含量随着a(SiO₂)降低而增加。因此，锆石中Ti的替代主要是针对Si。假设P的影响恒定，a(ZrSiO₄)为单位值；并且a(ZrO₂)和a(ZrTiO₄)分别与金红石中的Zr ppm和锆石中的Ti ppm成正比，[log(金红石中的ppm Zr) + loga(SiO₂)] = A(1) + B - 1/T(K)，[log(锆石中的ppm Ti) + loga(SiO₂) - loga(TiO₂)] = A(2) + B - 2/T，其中A和B是常数。这些常数是从已发表的数据以及新的数据推导出来的，这些数据来自于由石英或锆石 + 氧化锆缓冲aSiO₂的实验，来自于由金红石中Zr含量确定aSiO₂的实验，以及来自特征明确的天然样品。结果是A₁ = 7.420 ± 0.105；B - 1 = - 4530 ± 111；A(2) = 5.711 ± 0.072；B - 2 = - 4800 ± 86，活度参考相关P和T下的α - 石英和金红石。现在，锆石温度计可应用于没有石英和/或金红石的岩石，并且在估计a(SiO₂)和a(TiO₂)的情况下，金红石温度计可应用于没有石英的岩石。由未受约束的aSiO₂和aTiO₂给锆石和金红石测温带来的最大不确定性可以定量评估，在750°C时约为60 - 70°C。对这两个温度计对P的依赖性的初步评估预测，±1 GPa的不确定性在750°C时给锆石中Ti温度计带来约50°C的额外不确定性，给金红石中Zr温度计带来约70 - 80°C的额外不确定性。
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(SiO2)) as well as T, and the Ti content of zircon depends on a(SiO2) and a(TiO2) as well as T. New and published experimental data confirm the predicted increase in the Zr content of rutile with decreasing a(SiO2); and unequivocally demonstrate that the Ti content of zircon increases with decreasing a(SiO2). The substitution of Ti in zircon therefore is primarily for Si. Assuming a constant effect of P, unit a(ZrSiO4); and that a(ZrO2) and a(ZrTiO4) are proportional to ppm Zr in rutile and ppm Ti in zircon, [ log( ppm Zr-in-rutile) + loga(SiO2)] = A(1) + B-1/T( K) and [ log( ppm Ti-in-zircon) + loga(SiO2)-loga(TiO2)] = A(2) + B-2/T, where the A and B are constants. The constants were derived from published and new data from experiments with aSiO2 buffered by either quartz or zircon + zirconia, from experiments with aSiO2 defined by the Zr content of rutile, and from well-characterized natural samples. Results are A1 = 7.420 +/- 0.105; B-1 = - 4,530 +/- 111; A(2) = 5.711 +/- 0.072; B-2 = - 4,800 +/- 86 with activity referenced to alpha-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(SiO2) and a(TiO2) are estimated. Maximum uncertainties introduced to zircon and rutile thermometry by unconstrained aSiO2 and aTiO2 can be quantitatively assessed and are approximate to 60 to 70 degrees C at 750 degrees 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 degrees C of approximate to 50 degrees C for the Ti-in-zircon thermometer and of approximate to 70 to 80 degrees C for the Zr-in-rutile thermometer.