热覆盖对流及其对大陆岩石圈的意义
Heat-blanketed convection and its implications for the continental lithosphere
摘要：
地球各大洲的特点是长寿命放射性同位素高度富集。最近的估计表明，它们贡献了地球表面释放的33%的热量，而只占地幔的不到1%。这种独特的特征通过影响下伏地幔的热结构和热量传送而对下伏地幔产生深刻的影响。然而，富含生热元素的陆壳对下伏地幔的影响尚未得到系统的研究。在这里，我们进行了初步的调查，通过考虑一个简化的对流系统组成的混合加热的流体中，所有的内部加热集中在顶层的厚度$d_{HL}$（称为“热覆盖对流”）。我们进行24个数值模拟在三维笛卡尔几何四个特定的设置和各种值的$d_{HL}$。我们的研究结果表明，加热层的影响强烈地依赖于它的厚度相对于均匀加热情况下（$d_{HL} = 1.0$）的热边界层（$δ_{TBL}$）的厚度。更具体地说，对于$d_{HL} > δ_{TBL}$，由加热层引起的影响是相当适度的，而对于$d_{HL} < δ_{TBL}$，对流系统的性质随着$d_{HL}$的减小而强烈改变。特别是，表面热通量和对流活力显着增强非常薄的加热层相比，情况$d_{HL} = 1.0$。生热元素的垂直分布可能对地幔动力学起关键作用。然而，对于地球来说，大陆的存在应该不会显著影响表面热通量，从而影响地球的冷却速率。
英文摘要：
Earth's continents are characterized by a strong enrichment in long-lived radioactive isotopes. Recent estimates suggest that they contribute to 33\% of the heat released at the surface of the Earth, while occupying less than 1\% of the mantle. This distinctive feature has profound implications for the underlying mantle by impacting its thermal structure and heat transfer. However, the effects of a continental crust enriched in heat-producing elements on the underlying mantle have not yet been systematically investigated. Here, we conduct a preliminary investigation by considering a simplified convective system consisting in a mixed heated fluid where all the internal heating is concentrated in a top layer of thickness $d_{HL}$ (referred to as "heat-blanketed convection"). We perform 24 numerical simulations in 3D Cartesian geometry for four specific set-ups and various values of $d_{HL}$. Our results suggest that the effects of the heated layer strongly depend on its thickness relative to the thickness of the thermal boundary layer ($δ_{TBL}$) in the homogeneous heating case ($d_{HL} = 1.0$). More specifically, for $d_{HL} > δ_{TBL}$, the effects induced by the heated layer are quite modest, while, for $d_{HL} < δ_{TBL}$, the properties of the convective system are strongly altered as $d_{HL}$ decreases. In particular, the surface heat flux and convective vigour are significantly enhanced for very thin heated layers compared to the case $d_{HL} = 1.0$. The vertical distribution of heat producing elements may therefore play a key role on mantle dynamics. For Earth, the presence of continents should however not affect significantly the surface heat flux, and thus the Earth's cooling rate.
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