Large effect of water on Fe–Mg interdiffusion in garnet
Large effect of water on Fe–Mg interdiffusion in garnet
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DOI：
10.1016/j.epsl.2018.10.015
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发表时间：
2019-01
影响因子：
5.3
通讯作者：
Baohua Zhang;Bowen Li;Chengcheng Zhao;Xiaozhi Yang
中科院分区：
文献类型：
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作者：
Baohua Zhang;Bowen Li;Chengcheng Zhao;Xiaozhi Yang
在3gpa和1373-1673 K条件下，研究了水对Fe-Mg在石榴石中相互扩散动力学的影响，使用了pyrope和almandine团聚体的扩散偶，在< 7到~ 1260 wt. ppm的水含量范围内。用电子探针测量扩散曲线，用Boltzmann-Matano方程拟合所得数据。我们的研究结果表明，Fe - Mg互扩散系数（D Fe - Mg）几乎与石榴石的组成无关，在很宽的Fe/Mg比范围内（X Fe范围为0到1）。测定的dfe - mg可以用以下的Arrhenius关系式来描述：D Fe-Mg (m2 /s)= d0 C h2o r exp (- Δ H/ r T)，其中log (d0)= - 1.70±0.50,r= 1.38±0.06，H= 310±33 kJ/mol。指数r为1.38表明水的掺入机制是氢与金属和部分硅空位结合形成中性点缺陷配合物。同时也说明了水对铁-镁在石榴石中的相互扩散的促进作用。随着水含量从100 wt. ppm H 2 O增加到1260 wt. ppm H 2 O， Fe-Mg的互扩散率提高了约两个数量级，与干燥条件下温度升高约300℃的影响相当。如此大的水对石榴石动力学的影响，预计将对评估可用的地速和地热测量的闭合温度具有重要意义。
The effect of water on the kinetics of Fe–Mg interdiffusion in garnet was investigated at 3 GPa and 1373–1673 K using diffusion couples of pyrope and almandine aggregates, over a wide range of water content (C H 2 O) from< 7 up to∼ 1260 wt. ppm. Diffusion profiles were measured by electron microprobe, and the obtained data were fitted by the Boltzmann–Matano equation. Our results show that Fe–Mg interdiffusion coefficient (D Fe–Mg) is nearly independent of the composition of garnet over a wide range of Fe/Mg ratios (with X Fe ranging from 0 to 1). The determined D Fe–Mg can be described by the following Arrhenius relation: D Fe–Mg (m 2/s)= D 0 C H 2 O r exp(− Δ H/R T), where log(D 0)=− 1.70±0.50, r= 1.38±0.06, and H= 310±33 kJ/mol. The exponent r of 1.38 implies the incorporation mechanism of water is hydrogen associated with metal and partly silicon vacancies to form neutral point defect complexes. While at the same time, it also suggests a profound role of water in enhancing the Fe–Mg interdiffusion in garnet. With increasing water content from 100 to 1260 wt. ppm H 2 O, the Fe–Mg inter-diffusivity is enhanced by about two orders of magnitude, comparable to the effect of temperature increased by about 300° C under dry condition. Such large effect of water on the kinetics of garnet is expected to have important implications for evaluating the closure temperature of available geospeedometry and geothermometry.