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首页> 外文期刊>Contributions to Mineralogy and Petrology >A subsidiary fast-diffusing substitution mechanism of Al in forsterite investigated using diffusion experiments under controlled thermodynamic conditions
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A subsidiary fast-diffusing substitution mechanism of Al in forsterite investigated using diffusion experiments under controlled thermodynamic conditions

机译：使用控制热力学条件下的扩散实验研究了Al中Al中Al的辅助快速扩散替代机制

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Diffusion of Al in synthetic forsterite was studied at atmospheric pressure from 1100 to 1500 degrees C in air along [ 100] with activities of SiO2, MgO and Al2O3 (a(SiO2), a(MgO) and a(Al2O3)) buffered. At low a(SiO2), the buffer was forsterite + spinel + periclase (fo + sp + per) at all temperatures, while at high a(SiO2) and subsolidus conditions a variety of three-phase assemblages containing forsterite and two other phases from spinel, cordierite, protoenstatite or sapphirine were used at 1100-1350 degrees C. Experiments at high a(SiO2) and 1400 degrees C used forsterite + protoenstatite + melt (fo + en + melt), and at 1500 degrees C, fo + melt. The resulting diffusion profiles were analysed by LA-ICPMS in scanning mode. Diffusion profiles in the high a(SiO2) experiments were generally several hundred microns in length, but diffusion at low a(SiO2) was three orders of magnitude slower than in high a(SiO2) experiments carried out at the same temperature, producing short profiles only a few microns in length and close to the spatial resolution of the analytical method. Interface concentrations of Al in the forsterite, obtained by extrapolating the diffusion profiles to the crystal/buffer interface, were only a fraction of those expected at equilibrium, and varied among the differing buffer assemblages according to (a(Al2O3))(1/2) and (a(siO2))(3/4), pointing to the substitution of Al in forsterite by an octahedral-site, vacancy-coupled (OSVC) component with the stoichiometry Al(4/3)(3+)vac(2/3)SiO(4), whereas the main substitution expected from previous equilibrium studies would be the coupled substitution of 2 Al for Mg + Si, giving the stoichiometry MgAl2O4. It is proposed that this latter substitution is not seen on the length scales of the present experiments because it requires replacement of Si by Al on tetrahedral sites, and is accordingly rate-limited by the slow diffusivity of Si. Instead, diffusion of Al by the OSVC mechanism is relatively fast, and at high aSiO(2), even faster than Fe-Mg interdiffusion.
机译：在沿[100]的空气中，在大气压下在大气压中研究了Al在合成叉段中的扩散，并在[100]中，用SiO 2，MgO和Al 2 O 3（A（SiO 2），A（MgO）和A（Al 2 O 3））的活性。在低A（SiO 2）中，缓冲液在所有温度下啮齿机+尖晶石+蠕变（FO + SP + PER），而在高A（SiO 2）和子杂化条件下，各种三相组件含有叉炉和另外两阶段在1100-1350℃下使用尖晶石，堇青石，原生毛酸盐或蓝宝石。高A（SiO2）和1400℃的矿石+原型+熔体（FO + EN +熔体），以及1500℃，FO +熔体的实验。通过La-ICPMS在扫描模式下分析所得到的扩散轮廓。高A（SiO 2）实验中的扩散曲线通常为几百微米，但低A（SiO 2）的扩散比在相同温度下进行的高A（SiO 2）实验中的三个数量级较慢只有几微米的长度，接近分析方法的空间分辨率。通过将扩散曲线推断到晶体/缓冲界面来获得的插入族浓度仅是预期在平衡时的一部分，并且根据（A（AL2O3））（1/2）的不同缓冲组合）（A（SiO 2））（3/4），用八面体 - 位点（4/3）（3 +）VAC（3/3）（ 2/3）SiO（4），而预期的先前平衡研究的主要取代是2 Al的偶联替代，给予化学计量mgAl2O4。提出，在本实验的长度尺度上没有看到后一种取代，因为它需要在四面体位点上用Al替换Si，因此通过Si的缓慢扩散性速率限制。相反，通过OSVC机制的扩散是相对较快的，并且在高ASIO（2）中，甚至比FE-MG间隔快。

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