Ti site occupancy in zircon
Ti site occupancy in zircon
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DOI：
10.1016/j.gca.2010.11.004
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发表时间：
2011-02
影响因子：
5
通讯作者：
N. Tailby;A. Walker;A. Berry;J. Hermann;K. Evans;J. Mavrogenes;H. O’Neill;I. S. Rodina;A. Soldatov;D. Rubatto;S. Sutton
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文献类型：
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作者：
N. Tailby;A. Walker;A. Berry;J. Hermann;K. Evans;J. Mavrogenes;H. O’Neill;I. S. Rodina;A. Soldatov;D. Rubatto;S. Sutton
钛占位锆石（ZrSiO 4）是基本的热压法，因为取代机制控制Ti含量-温度关系。在这里，我们描述了三个独立的方法来证明，钛替代硅，而不是锆在锆石的结果。在1bar和1300°C下，将氧化物粉末保持在Na 2 WO 4助熔剂中，合成了锆英石颗粒。金红石+方石英平衡的锆石晶粒的Ti含量（1201 ppm）几乎是四斜锆石（（Ti，Zr）O2）+四方氧化锆平衡的锆石晶粒的Ti含量（2640 ppm）的一半。在二氧化硅饱和条件下产生的锆石晶粒的较低的Ti含量表明Ti取代主要发生在Si位点上。此外，相对于在1GPa下的那些（457 ppm，Ferry和沃森，2007），在1bar（1201 ppm）下的二氧化硅饱和实验的较高的Ti含量指示对Ti在锆石中的溶解度的实质性压力效应。合成锆石颗粒的测量Ti K-α边X射线吸收近边结构（XANES）光谱显示出与四面体配位的含Ti标准矿物类似的能量和归一化强度，强烈表明Ti占据了Si位。密度泛函理论（DFT）计算证实，Ti取代最有可能发生在Si位上，并预测Ti-O键长为1.797（相比之下，Zr位上取代的平均值为2.160），与实验生长的锆石晶粒的X射线吸收精细结构（EXAFS）光谱非常一致，该光谱表明值为1.76（1）。用FEFF 8.4软件模拟了Si和Zr两个位置上Ti取代的DFT确定的缺陷结构的XANES谱。预测的光谱Ti的Si网站再现的实验锆石光谱的所有关键功能，而Ti的Zr网站是显着不同的。所有应用的方法都证实了钛在锆英石中取代了硅。因此，在给定的压力下，锆石的Ti含量不仅是温度的函数，而且会随着二氧化硅活性的降低而增加。由于激活或淬灭锆石中的阴极射线发光（CL）的元素被掺入Zr位点，因此预期CL与锆石中Si位点上掺入的Ti含量的解耦。这一假说已被系统的CL-微量元素研究的天然和实验锆石。
Ti site occupancy in zircon (ZrSiO4) is fundamental to thermobarometry because substitution mechanisms control Ti content–temperature relations. Here we describe the results of three independent methods used to demonstrate that Ti substitutes for Si and not Zr in zircon. Zircon grains were synthesized from oxide powders held in a Na2WO4flux at 1bar and 1300°C. Zircon grains equilibrated with rutile+cristobalite show Ti contents (1201ppm) nearly half that of zircon grains equilibrated with srilankite ((Ti,Zr)O2)+tetragonal zirconia (2640ppm). The lower Ti content of zircon grains produced at silica-saturated conditions indicates that Ti substitution predominately occurs on the Si site. Moreover, the higher Ti contents of silica-saturated experiments at 1bar (1201ppm), relative to those at 1GPa (457ppm, Ferry and Watson, 2007), indicates a substantial pressure effect on Ti solubility in zircon. Measured Ti K-α edge X-ray Absorption Near Edge Structure (XANES) spectra of synthetic zircon grains show energies and normalized intensities akin to those seen among tetrahedrally coordinated Ti-bearing standard minerals, strongly suggesting that Ti occupies the Si site. Density functional theory (DFT) calculations confirm that Ti substitution is most likely to occur on the Si site and predict a Ti–O bond length of 1.797Å (compared to an average of 2.160Å for substitution on the Zr site), in excellent agreement with X-ray Absorption Fine Structure (EXAFS) spectra of experimentally grown zircon grains which indicate a value of 1.76(1)Å. The software FEFF 8.4 was used to simulate XANES spectra from the defect structures determined by DFT for Ti substituting on both the Si and Zr sites. The predicted spectrum for Ti on the Si site reproduces all the key features of the experimental zircon spectra, whereas Ti on the Zr site is markedly different. All applied methods confirm that Ti substitutes for Si in zircon. Consequently, the Ti content of zircon at a given pressure is not only a function of temperature, but will increase with decreasing silica activity. Because elements that activate or quench cathodoluminescence (CL) in zircon are incorporated into the Zr site, a decoupling of CL from Ti contents – incorporated on the Si site in zircon is expected. This hypothesis has been verified by a systematic CL-trace element study of natural and experimental zircon.