Here we determined the P wave velocity of liquid Fe₈₄Si₁₆ up to 56 GPa based on inelastic X‐ray scattering measurements in a laser‐heated diamond‐anvil cell. We found that silicon significantly increases the P wave velocity of liquid iron under high pressure. The equation of state (EoS) of liquid Fe‐Si is obtained from the present sound velocity measurements. When extrapolating the present data to core pressures with the EoS, the silicon concentration must be limited to less than  wt.% in the outer core to explain its P wave velocity. In contrast, if silicon is the sole light element, to  wt.% is necessary to account for the outer core density deficit. Thus, a liquid Fe‐Si cannot explain both the density and sound velocity simultaneously, suggesting that silicon is not the predominant light element in the core. Recent core formation models have predicted that the initial core contained 2 to 9 wt.% Si, which makes the P wave velocity much faster than that observed in the present‐day outer core. The core may have changed its composition by crystallizing SiO₂ or (Mg,Fe)SiO₃. It is possible that the initial core with <4 wt.% Si and ~5 wt.% O segregated from silicate under moderately oxidizing conditions and its composition evolved into a silicon‐poor one (<1.9 wt.% Si), which is compatible with the seismological observations. Such moderate Si content in the original core could also account for the high MgO/SiO₂ and ³⁰Si/²⁸Si ratios in Earth's mantle relative to chondritic values.

中文翻译：

---

液态铁硅合金的声速测量揭示了贫硅的当今地球外核

在此，我们基于在激光加热的金刚石-砧座中进行的非弹性X射线散射测量，确定了高达56 GPa的液态Fe ₈₄ Si ₁₆的P波速度。我们发现硅在高压下显着提高了液态铁的P波速度。液态Fe-Si的状态方程（EoS）可从当前的声速测量获得。当使用EoS将当前数据外推到堆芯压力时，必须将 外堆芯中的硅浓度限制为小于wt。％，以解释其P波速。相反，如果硅是唯一的轻元素，则  需要wt。％来解决外芯密度不足。因此，液态的Fe-Si不能同时解释密度和声速，这表明硅不是芯中主要的轻元素。最近的岩心形成模型预测，初始岩心中含有2至9 wt。％的Si，这使得P波速度比当今外核中观察到的要快得多。核心可能通过结晶SiO ₂或（Mg，Fe）SiO ₃改变了其成分。在中等氧化条件下，初始硅纤芯含量小于4％（重量）和O含量约为5％（重量）的硅酸盐有可能从硅酸盐中分离出来，并且其组成演变为贫硅（硅含量小于1.9％）。与地震观测。原始岩心中如此适中的Si含量也可能解释了地幔相对于软骨形态值的高MgO / SiO ₂和³⁰ Si / ²⁸ Si比。