A subsidiary fast-diffusing substitution mechanism of Al in forsterite investigated using diffusion experiments under controlled thermodynamic conditions
A subsidiary fast-diffusing substitution mechanism of Al in forsterite investigated using diffusion experiments under controlled thermodynamic conditions
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DOI：
10.1007/s00410-017-1365-x
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发表时间：
2017-06
影响因子：
3.5
通讯作者：
I. Zhukova;H. O’Neill;I. Campbell
中科院分区：
文献类型：
--
作者：
I. Zhukova;H. O’Neill;I. Campbell
研究了Al在常压(1100~1500℃)空气中沿[100]方向的扩散，缓冲了SiO_2、MgO和Al_2O_3(a_2O_2、a_2O_3和a_2O_3)的活性。在低aSiO_2条件下，缓冲液在所有温度下都是镁橄榄石+尖晶石+方镁石(fo_2+f_2O_3+P_2O_3)，而在高a_SiO_2和亚固相线条件下，在1100-1350℃使用各种含有镁橄榄石和来自尖晶石、堇青石、原顽辉石或蓝宝石的其他相的三相组合，在高a_SiO_2和1400℃下使用镁橄榄石+原辉绿岩+镁橄榄石熔体(fo_O_2+t_n+熔体)，以及在1500℃时，f_o_0+熔体。在扫描模式下，用LA-ICPMS分析所得到的扩散曲线。在高aSiO_2实验中，扩散剖面的长度一般为几百微米，但在低aSiO_2下的扩散比在相同温度下进行的高aSiO_2实验中的扩散慢三个数量级，所产生的短剖面只有几个微米长，接近分析方法的空间分辨率。通过外推扩散轮廓到晶体/缓冲界面获得的镁橄榄岩中Al的界面浓度仅为平衡时预期的一小部分，并根据(AAl_2O_3)1/2和(ASiO_2)3/4在不同的缓冲组合中有所不同，表明镁橄榄石中的Al被化学计量比为Al4/33+Va_2/3SiO_4的八面体空位偶联(OSVC)组分取代，而以前的平衡研究预计的主要替代将是2Al对Mg_2O+Al_2O_4的偶合取代，从而得到化学计量比为MgAl_2O_4。认为后一种取代在本实验的长度尺度上是看不到的，因为它需要在四面体位置上用Al取代Si，并因此受到Si的缓慢扩散率的限制。相反，通过OSVC机制的Al扩散相对较快，在高aSiO_2时，甚至比Fe-Mg互扩散更快。
Diffusion of Al in synthetic forsterite was studied at atmospheric pressure from 1100 to 1500 °C in air along [100] with activities of SiO2, MgO and Al2O3(aSiO2, aMgOand aAl2O3) buffered. At low aSiO2, the buffer was forsterite + spinel + periclase (fo + sp + per) at all temperatures, while at high aSiO2and 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 °C. Experiments at high aSiO2and 1400 °C used forsterite + protoenstatite + melt (fo + en + melt), and at 1500 °C, fo + melt. The resulting diffusion profiles were analysed by LA–ICP–MS in scanning mode. Diffusion profiles in the high aSiO2experiments were generally several hundred microns in length, but diffusion at low aSiO2was three orders of magnitude slower than in high aSiO2experiments 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 (aAl2O3)1/2and (aSiO2)3/4, pointing to the substitution of Al in forsterite by an octahedral-site, vacancy-coupled (OSVC) component with the stoichiometry Al4/33+vac2/3SiO4, 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 aSiO2, even faster than Fe–Mg interdiffusion.