The effect of silica activity on the diffusion of Ni and Co in olivine
The effect of silica activity on the diffusion of Ni and Co in olivine
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DOI：
10.1007/s00410-014-1029-z
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发表时间：
2014-08
影响因子：
3.5
通讯作者：
I. Zhukova;H. O’Neill;I. H. Cambell;M. Kilburn
中科院分区：
文献类型：
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作者：
I. Zhukova;H. O’Neill;I. H. Cambell;M. Kilburn
测量了常压下合成单晶镁橄榄石(Mg_2SiO_4)中Ni和Co的扩散，温度范围为1200~1500℃，氧逸度为[100]，SiO_2和MgO的活度由镁橄榄石+镁橄榄石(fO_2+Al_2O_3/缓冲液)或镁橄榄石+原辉石(fO_2+H_2O_2缓冲液)确定。用激光烧蚀电感耦合等离子体质谱、纳米二次离子质谱仪和电子探针三种方法测量了扩散谱，结果表明，三种方法之间具有较好的一致性。对于Ni和Co，在给定温度下，原顽火辉石缓冲实验的扩散速度比方镁石缓冲实验快一个数量级。组合数据集的扩散系数Dm(M=M i或Co)可拟合式为：$$\log\，D_{\Text{M}}\，\Left({{\Text{In}}\，{\Text{m}}^{2}\，{\TEXT{S}}^{-1}}\右)=-6.77(\PM0.33)+\Delta E_{\TEXT{a}}(M)/RT+2/3\loga_{SiO_2}}$，其中Ea(Ni)=−=284.3 kJ·moL−1，Ea(Co)=kJ·moL−1，不确定度为±10.2kJ·moL−1。扩散对扩散的依赖关系符合点缺陷模型，在点缺陷模型中，镁空位由硅间隙电荷平衡。在1300℃与成分为Mg1.8Fe0.2SiO4的圣卡洛斯橄榄石的对比实验表明，对D()的依赖程度较小，这可能是因为在含铁橄榄石中掺入Fe3+，镁位空位增加。但对fO2的依赖性较小，D(FO2)为0.12±0.12。这些结果表明，在设计扩散实验时，有必要限制一个相的所有化学计量组分的化学势。同样，在将实验数据应用于天然矿物以限制地质过程的速率时，必须考虑主要元素组分的化学势。例如，在相同的温度和fO2下，来自低二氧化硅岩浆(如金伯利岩或碳酸岩)的橄榄石中二价元素的扩散将比来自高二氧化硅岩浆(如拉斑玄武岩)的橄榄石慢一个数量级。
The diffusion of Ni and Co was measured at atmospheric pressure in synthetic monocrystalline forsterite (Mg2SiO4) from 1,200 to 1,500 °C at the oxygen fugacity of air, along [100], with the activities of SiO2and MgO defined by either forsterite + periclase (fo + per buffer) or forsterite + protoenstatite (fo + en buffer). Diffusion profiles were measured by three methods: laser-ablation inductively-coupled-plasma mass-spectrometry, nano-scale secondary ion mass spectrometry and electron microprobe, with good agreement between the methods. For both Ni and Co, the diffusion rates in protoenstatite-buffered experiments are an order of magnitude faster than in the periclase-buffered experiments at a given temperature. The diffusion coefficientsDM(M = Ni or Co) for the combined data set can be fitted to the equation:$$\log \,D_{\text{M}} \,\left( {{\text{in}}\,{\text{m}}^{2} \,{\text{s}}^{ - 1} } \right) = - 6.77( \pm 0.33) + \Delta E_{\text{a}} (M)/RT + 2/3\log a_{{SiO_{2} }}$$with Ea(Ni) = − 284.3 kJ mol−1and Ea(Co) = − 275.9 kJ mol−1, with an uncertainty of ±10.2 kJ mol−1. This equation fits the data (24 experiments) to ±0.1 in logDM. The dependence of diffusion onis in agreement with a point-defect model in which Mg-site vacancies are charge-balanced by Si interstitials. Comparative experiments with San Carlos olivine of composition Mg1.8Fe0.2SiO4at 1,300 °C give a slightly small dependence on, withD(), presumably because the Mg-site vacancies increase with incorporation of Fe3+in the Fe-bearing olivines. However, the dependence onfO2is small, withD(fO2)0.12±0.12. These results show the necessity of constraining the chemical potentials of all the stoichiometric components of a phase when designing diffusion experiments. Similarly, the chemical potentials of the major-element components must be taken into account when applying experimental data to natural minerals to constrain the rates of geological processes. For example, the diffusion of divalent elements in olivine from low SiO2magmas, such as kimberlites or carbonatites, will be an order of magnitude slower than in olivine from high SiO2magmas, such as tholeiitic basalts, at equal temperatures andfO2.