Transport properties of Fe-Ni-Si alloys at Earth's core conditions: Insight into the viability of thermal and compositional convection
Transport properties of Fe-Ni-Si alloys at Earth's core conditions: Insight into the viability of thermal and compositional convection
复制标题
DOI：
10.1016/j.epsl.2020.116614
复制
发表时间：
2021
影响因子：
5.3
通讯作者：
Youjun Zhang;M. Hou;Peter Edward Driscoll;N. Salke;Jin Liu;E. Greenberg;V. Prakapenka;Jung‐Fu Lin
中科院分区：
文献类型：
--
作者：
Youjun Zhang;M. Hou;Peter Edward Driscoll;N. Salke;Jin Liu;E. Greenberg;V. Prakapenka;Jung‐Fu Lin
地核的热对流和成分对流被认为是驱动地球发电机的主要动力源。地球历史上由热和成分驱动的对流的生存能力和强度取决于穿过核心-地幔边界（CMB）的绝热热流，这是由组成铁-镍轻元素合金在与核心相关的压力-温度（P-T）条件下的热导率决定的。硅通常被认为是一种丰富的轻元素，与铁和~ 5 wt%的Ni合金，但Fe-Ni- si合金的热输运性能在高- 3仍然很大程度上不确定。在激光加热的金刚石砧细胞实验中，我们使用四探针范德堡法测量了Fe-10wt%Ni和Fe-1.8wt%Si合金高达~ 142 GPa和~ 3400 K的电阻率。我们的研究结果表明，在给定的高压下，hcp- fe -1.8 si和Fe-10Ni的电阻率在~ 1500到3400 K之间呈准线性温度依赖性。在~ 138 GPa和4000 K条件下，在hcp- fe中加入~ 2 wt%的Si显著提高了~ 25%的电阻率，但在接近cmbp - t条件下，Fe-10wt%的Ni具有与纯cp- fe相似的电阻率。利用电阻率的测量值，我们通过Wiedemann-Franz定律模拟了导热系数，给出了最外层核的液态Fe-5Ni-8Si合金的标称导热系数为~ 50 W m−1K−1，这意味着绝热（导电）核心热流为~ 8.0 TW。由于在凝固的内核边界上的轻元素分化，外核的热导率比内核低得多。我们的研究表明，绝热地核热流低到足以使热对流在地球大部分甚至可能全部历史上驱动地球发电机，而成分对流的强度随着内核的增长而增加，占到当今地球发电机浮力通量的83%。
Thermal and compositional convection in Earth's core are thought to be the main power sources driving geodynamo. The viability and strength of thermally and compositionally-driven convection over Earth's history depend on the adiabatic heat flow across the core-mantle boundary (CMB) which is governed by the thermal conductivity of a constituent Fe-Ni-light element alloy at the pressure-temperature (P-T) conditions relevant to the core. Silicon is often proposed to be an abundant light element alloyed with Fe along with ∼5 wt% Ni, but the thermal transport properties of Fe-Ni-Si alloys at highP-Tremain largely uncertain. Here we measured the electrical resistivities of Fe-10wt%Ni and Fe-1.8wt%Si alloys up to ∼142 GPa and ∼3400 K using four-probe van der Pauw method in laser-heated diamond anvil cell experiments. Our results show that the resistivities ofhcp-Fe-1.8Si and Fe-10Ni display quasi-linear temperature dependence from ∼1500 to 3400 K at each given high pressure. Addition of ∼2 wt% Si inhcp-Fe significantly increases its resistivity by ∼25% at ∼138 GPa and 4000 K, but Fe-10wt%Ni has similar resistivity to purehcp-Fe at near CMBP-Tconditions. Using our measured values of electrical resistivities, we model thermal conductivities via the Wiedemann-Franz law, giving a nominal thermal conductivity of ∼50 W m−1K−1for liquid Fe-5Ni-8Si alloy at the topmost outer core, implying an adiabatic (conductive) core heat flow of ∼8.0 TW. The outer core has a much lower thermal conductivity than the inner core due to light-element differentiation across the solidifying inner-core boundary. Our studies imply that the adiabatic core heat flow is low enough to enable thermal convection to drive the geodynamo over most and possibly all of Earth's history, while the strength of compositional convection increases with the inner-core growth and accounts for ∼83% of the buoyancy flux to the present-day geodynamo.