Jacobsen Steven D, Spetzler Hartmut, Reichmann Hans J, Smyth Joseph R
Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany.
Proc Natl Acad Sci U S A. 2004 Apr 20;101(16):5867-71. doi: 10.1073/pnas.0401564101. Epub 2004 Apr 12.
The emerging picture of Earth's deep interior from seismic tomography indicates more complexity than previously thought. The presence of lateral anisotropy and heterogeneity in Earth's mantle highlights the need for fully anisotropic elasticity data from mineral physics. A breakthrough in high-frequency (gigahertz) ultrasound has resulted in transmission of pure-mode elastic shear waves into a high-pressure diamond-anvil cell using a P-to-S elastic-wave conversion. The full elastic tensor (c(ij)) of high-pressure minerals or metals can be measured at extreme conditions without optical constraints. Here we report the effects of pressure and composition on shear-wave velocities in the major lower-mantle oxide, magnesiowüstite-(Mg,Fe)O. Magnesiowüstite containing more than approximately 50% iron exhibits pressure-induced c(44) shear-mode softening, indicating an instability in the rocksalt structure. The oxide closer to expected lower-mantle compositions ( approximately 20% iron) shows increasing shear velocities more similar to MgO, indicating that it also should have a wide pressure-stability field. A complete sign reversal in the c(44) pressure derivative points to a change in the topology of the (Mg,Fe)O phase diagram at approximately 50-60% iron. The relative stability of Mg-rich (Mg,Fe)O and the strong compositional dependence of shear-wave velocities (and partial differential c(44)/ partial differential P) in (Mg,Fe)O implies that seismic heterogeneity in Earth's lower mantle may result from compositional variations rather than phase changes in (Mg,Fe)O.
地震层析成像技术所呈现的地球深部内部新图景显示，其复杂性超出此前预期。地球地幔中横向各向异性和非均质性的存在，凸显了获取矿物物理学中完全各向异性弹性数据的必要性。高频（吉赫兹）超声技术的一项突破，实现了利用纵波到横波的弹性波转换，将纯模式弹性剪切波传输到高压金刚石对顶砧室中。在极端条件下，无需光学限制即可测量高压矿物或金属的完整弹性张量（c(ij)）。在此，我们报告了压力和成分对下地幔主要氧化物——方镁铁矿（Mg,Fe）O中剪切波速度的影响。含铁量超过约50%的方镁铁矿呈现出压力诱导的c(44)剪切模式软化，这表明岩盐结构存在不稳定性。更接近预期下地幔成分（约20%铁）的氧化物，其剪切波速度增加，更类似于MgO，这表明它也应具有较宽的压力稳定场。c(44)压力导数的完全符号反转表明，在含铁量约为50 - 60%时，（Mg,Fe）O相图的拓扑结构发生了变化。富镁（Mg,Fe）O的相对稳定性以及（Mg,Fe）O中剪切波速度（以及∂c(44)/∂P）对成分的强烈依赖性意味着，地球下地幔中的地震非均质性可能源于成分变化，而非（Mg,Fe）O中的相变。
