Three water sites in upper mantle olivine and the role of titanium in the water weakening mechanism
Three water sites in upper mantle olivine and the role of titanium in the water weakening mechanism
复制标题
DOI：
10.1029/2006jb004620
复制
发表时间：
2007-05
影响因子：
--
通讯作者：
A. Walker;A. Walker;J. Hermann;A. Berry;H. O’Neill
中科院分区：
文献类型：
--
作者：
A. Walker;A. Walker;J. Hermann;A. Berry;H. O’Neill
[1]合成橄榄石的红外光谱已经确定，至少有四种不同的机制，氢被纳入晶体结构。在MgO-SiO2-H2O体系中存在两种机制，分别与硅空位和镁空位有关。第三种机制与三价阳离子取代有关，通常是天然橄榄石中的Fe 3+，而第四种机制是来自地球上地幔尖晶石-橄榄岩相的天然橄榄石中最普遍的机制，与Ti 4+有关[ Berry et al.，2005年]。在这里，基于密度泛函理论的第一性原理计算被用来推导纯MgO-SiO2-H2O系统中的两个缺陷的结构和相对能量，以及可能的含氢和不含氢的点缺陷在Ti 4+掺杂的镁橄榄石。计算的结构被用来比较的O-H键的预测方向与实验确定的极化。能量用于讨论化学环境、温度（T）、压力（P）以及水含量和水逸度（f（H2O））的不同制度如何影响不同羟基取代机制中的哪一种是热稳定的。我们发现，由于Ti杂质的存在下，最稳定的机制涉及到硅空位的形成含有两个质子电荷平衡的Ti 4+阳离子占据相邻的八面体网站。这种机制导致水介导的硅空位的形成。由于已知硅是橄榄石中扩散最慢的物质，这为橄榄石中观察到的水弱化效应提供了可信的解释。
[1] Infrared spectroscopy on synthetic olivines has established that there are at least four different mechanisms by which hydrogen is incorporated into the crystal structure. Two mechanisms occur in the system MgO-SiO2-H2O associated with silicon and magnesium vacancies, respectively. A third mechanism is associated with trivalent cation substitution, commonly Fe3+ in natural olivine, while the fourth mechanism, which is the one most prevalent in natural olivines from the spinel-peridotite facies of the Earth's upper mantle, is associated with Ti4+ [ Berry et al., 2005]. Here first principles calculations based on density functional theory are used to derive the structure and relative energies of the two defects in the pure MgO-SiO2-H2O system, and possible hydrogen-bearing and hydrogen-free point defects in Ti4+-doped forsterite. Calculated structures are used to compare the predicted orientation of the O-H bonds with the experimentally determined polarization. The energies are used to discuss how different regimes of chemical environment, temperature ( T), pressure ( P), and both water content and water fugacity (f(H2O)), impact on which of the different hydroxyl substitution mechanisms are thermodynamically stable. We find that given the presence of Ti impurities, the most stable mechanism involves the formation of silicon vacancies containing two protons charge balanced by a Ti4+ cation occupying an adjacent octahedral site. This mechanism leads to the water-mediated formation of silicon vacancies. As silicon is known to be the most slowly diffusing species in olivine, this provides a credible explanation of the observed water weakening effect in olivine.