Phase relations and equation-of-state of aluminous Mg-silicate perovskite and implications for Earth's lower mantle
Phase relations and equation-of-state of aluminous Mg-silicate perovskite and implications for Earth's lower mantle
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DOI：
10.1016/j.epsl.2004.03.014
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发表时间：
2004-05
影响因子：
5.3
通讯作者：
M. Walter;A. Kubo;T. Yoshino;J. Brodholt;K. Koga;Y. Ohishi
中科院分区：
文献类型：
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作者：
M. Walter;A. Kubo;T. Yoshino;J. Brodholt;K. Koga;Y. Ohishi
我们研究了Al~(3+)对钙钛矿室温压缩性的影响，得到了化学计量比为25%的Al_2O_3-Al_2O_5-Al_2O_3-Al_2O_3-Al_2O_3体系。以玻璃为原料合成了铝镁钙钛矿，在∼2 0 0 0~2 6 0 0K温度范围内，在∼30~10 0 Gpa范围内保持稳定相，用原位X射线衍射法测定了其晶格参数。在钙钛矿结构中加入Al3+，在环境条件下(V0)，增加了正交形变和晶胞体积。压缩导致轴长各向异性减小，a轴比b轴和c轴分别压缩约25%和3%。正交形变的幅度随着压力的增加而增加，但铝钙钛矿在至少100 Gpa的压力下保持稳定。结果表明，Al~(3+)的取代使材料的压缩性能略有提高，体积弹性模量(K_0)以−67±35 Gpa/Xal的速度下降。K0的这种降低与最近的理论计算是一致的，如果基本上所有的Al3+都平等地通过与Mg2+和Si4+的电荷耦合取代进入六重和八重位置。相反，在一些研究中报道的添加微量Al的压缩系数的大幅增加与Al3+通过形成氧空位的取代反应进入六重位是一致的。MgSiO_3-AlO_(1.5)-SiO_2三元系中的相关系表明，三元缺陷镁钙钛矿的稳定场在上下地幔条件下应该是稳定的。根据最近的实验结果，将相关系扩展到四元系的MgSiO_3-AlO_(1.5)-FeO_(1.5)-SiO_2，表明存在由电荷耦合和氧空位的Al~(3+)和Fe~(3+)取代组成的镁钙钛矿固溶体的复杂多面体体积。原始地幔的AlO1.5摩尔分数约为5%，Fe~(3+)/(Fe~(3++)Fe~(2+))比值约为∼~(2+)=0.5，在下地幔最上部的条件下，可能由铁方镁矿与镁钙钛矿共生，镁钙钛矿具有相当大的Al~(3+)和Fe~(3+)缺位。压力的增加可能比空位形成反应更有利于电荷耦合的取代反应，这样在下地幔中可能存在一个替代机制具有梯度的区域。在这种情况下，我们预计镁钙钛矿的物理和输运性质将随着深度的变化而变化，在下地幔顶部，可能是更软的，可能是更多的水合，缺陷主导的镁钙钛矿，在更深的地方逐渐变成更硬的，脱水的，电荷耦合的钙钛矿主导的镁钙钛矿。
We have investigated the effect of Al3+on the room-temperature compressibility of perovskite for stoichiometric compositions along the MgSiO3–AlO1.5join with up to 25 mol% AlO1.5. Aluminous Mg-perovskite was synthesized from glass starting materials, and was observed to remain a stable phase in the range of ∼30–100 GPa at temperatures of ∼2000 to 2600 K. Lattice parameters for orthorhombic (Pbnm) perovskite were determined using in situ X-ray diffraction at SPring8, Japan. Addition of Al3+into the perovskite structure increases orthorhombic distortion and unit cell volume at ambient conditions (V0). Compression causes anisotropic decreases in axial length, with the a axis more compressive than the b and c axes by about 25% and 3%, respectively. The magnitude of orthorhombic distortion increases with pressure, but aluminous perovskite remains stable to pressures of at least 100 GPa. Our results show that substitution of Al3+causes a mild increase in compressibility, with the bulk modulus (K0) decreasing at a rate of −67±35 GPa/XAl. This decrease in K0is consistent with recent theoretical calculations if essentially all Al3+substitutes equally into the six- and eight-fold sites by charge-coupled substitution with Mg2+and Si4+. In contrast, the large increase in compressibility reported in some studies with addition of even minor amounts of Al is consistent with substitution of Al3+into six-fold sites via an oxygen-vacancy forming substitution reaction. Schematic phase relations within the ternary MgSiO3–AlO1.5–SiO2indicate that a stability field of ternary defect Mg-perovskite should be stable at uppermost lower mantle conditions. Extension of phase relations into the quaternary MgSiO3–AlO1.5–FeO1.5–SiO2based on recent experimental results indicates the existence of a complex polyhedral volume of Mg-perovskite solid solutions comprised of a mixture of charge-coupled and oxygen-vacancy Al3+and Fe3+substitutions. Primitive mantle with about 5 mol% AlO1.5and an Fe3+/(Fe3++Fe2+) ratio of ∼0.5 is expected to be comprised of ferropericlase coexisiting with Mg-perovskite that has a considerable component of Al3+and Fe3+defect substitutions at conditions of the uppermost lower mantle. Increased pressure may favor charge-coupled substitution reactions over vacancy forming reactions, such that a region could exist in the lower mantle with a gradient in substitution mechanisms. In this case, we expect the physical and transport properties of Mg-perovskite to change with depth, with a softer, probably more hydrated, defect dominated Mg-perovskite at the top of the lower mantle, grading into a stiffer, dehydrated, charge-coupled substitution dominated Mg-perovskite at greater depth.