Rare-earth diffusion in zircon
Rare-earth diffusion in zircon
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DOI：
10.1016/s0009-2541(96)00098-8
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发表时间：
1997-01-14
期刊：
影响因子：
3.9
通讯作者：
Watson, EB
中科院分区：
文献类型：
--
作者：
Cherniak, DJ;Hanchar, JM;Watson, EB
测定了三种稀土元素（Sm、Dy、Yb）在合成锆石和天然锆石中的扩散速率。以稀土-磷酸盐粉末为扩散剂源，利用卢瑟福后向散射光谱（RES）测量稀土深度剖面。在1150 ~ 1400℃的温度范围内，得到了以下的Arrhenius关系(扩散系数以m（2) s（-1）为单位）：log D-Yb = (7.40 +/- 1.15) + (-769 +/- 34 kJ mol(-1)/2.303 RT)log D-Dy = (5.36 +/- 0.21) + (-734 +/- 35 kJ mol(-1)/2.303 RT)log D-Sm = (8.46 +/- 1.61) + (-841 +/- 57 kJ mol(-1)/2.303 RT)合成锆石和天然锆石的结果非常相似，在顺C轴和顺C轴的输移对比中没有发现明显的各向异性。数据表明，随着离子半径的减小，扩散率有系统地增加（即较重的ree的扩散速率更快）。考虑到这些趋势，Lu和La的扩散速率应该相差两个数量级以上。扩散分选在Sm-Nd体系中不太可能发生，因为扩散系数的差异相对较小，但在Lu-Hf体系中可能是一个因素，因为四价阳离子的扩散速率要慢得多。对稀土元素所测得的非常缓慢的扩散速率表明，它们在大多数地质条件下基本上是不动的，因此可以保留继承岩心的精细化学分带和同位素特征。
Diffusion rates for three rare-earth elements (REEs: Sm, Dy, Yb) have been measured in synthetic and natural zircon. REE-phosphate powders were used as the source of diffusant, with Rutherford backscattering spectrometry (RES) used to measure REE depth profiles.Over the temperature range 1150-1400 degrees C, the following Arrhenius relations were obtained (diffusion coefficients in m(2) s(-1)):log D-Yb = (7.40 +/- 1.15) + (-769 +/- 34 kJ mol(-1)/2.303 RT)log D-Dy = (5.36 +/- 0.21) + (-734 +/- 35 kJ mol(-1)/2.303 RT)log D-Sm = (8.46 +/- 1.61) + (-841 +/- 57 kJ mol(-1)/2.303 RT)Results for synthetic and natural zircons were quite similar, and no evidence of significant anisotropy was observed when comparing transport normal and parallel to the c-axis.The data show a systematic increase in diffusivity with decreasing ionic radius (i.e. faster diffusion rates for the heavier REEs). Given these trends, the diffusion rates of Lu and La should differ by over two orders of magnitude. Diffusive fractionation is unlikely in the Sm-Nd system because differences in diffusivities are relatively small, but may be a factor in the Lu-Hf system given the much slower diffusion rates of tetravalent cations.The very slow diffusion rates measured for the REEs suggest that they are essentially immobile under most geologic conditions, thus permitting the preservation of fine-scale chemical zoning and isotopic signatures of inherited cores.