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Powering Earth's dynamo with magnesium precipitation from the core

机译：通过核心中的镁沉淀为地球发电机供电

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Earth's global magnetic field arises from vigorous convection within the liquid outer core. Palaeomagnetic evidence reveals that the geodynamo has operated for at least 3.4 billion years(1), which places constraints on Earth's formation and evolution. Available power sources in standard models include compositional convection (driven by the solidifying inner core's expulsion of light elements), thermal convection (from slow cooling), and perhaps heat from the decay of radioactive isotopes. However, recent first-principles calculations(2,3) and diamond-anvil cell experiments(4,5) indicate that the thermal conductivity of iron is two or three times larger than typically assumed in these models. This presents a problem: a large increase in the conductive heat flux along the adiabat (due to the higher conductivity of iron) implies that the inner core is young (less than one billion years old(4)), but thermal convection and radiogenic heating alone may not have been able to sustain the geodynamo during earlier epochs. Here we show that the precipitation of magnesium-bearing minerals from the core could have served as an alternative power source. Equilibration at high temperatures in the aftermath of giant impacts allows a small amount of magnesium (one or two weight per cent) to partition into the core while still producing the observed abundances of siderophile elements in the mantle and avoiding an excess of silicon and oxygen in the core. The transport of magnesium as oxide or silicate from the cooling core to underneath the mantle is an order of magnitude more efficient per unit mass as a source of buoyancy than inner-core growth. We therefore conclude that Earth's dynamo would survive throughout geologic time (from at least 3.4 billion years ago to the present) even if core radiogenic heating were minimal and core cooling were slow.
机译：地球的整体磁场是由液态外核内的强对流引起的。古地磁学证据表明，地磁发电机已经运行了至少34亿年（1），这对地球的形成和演化施加了限制。标准模型中可用的电源包括成分对流（由固化的内芯排出的轻元素驱动），热对流（缓慢冷却），以及放射性同位素衰变产生的热量。但是，最近的第一性原理计算（2,3）和金刚石-砧室实验（4,5）表明，铁的导热率是这些模型中通常假定的两倍或三倍。这就提出了一个问题：沿着绝热体的传导热通量大幅度增加（由于铁的电导率更高）意味着内芯很年轻（小于十亿岁（4）），但是热对流和放射致热在较早的时期，仅靠地球可能无法维持土力发电。在这里，我们表明，岩心中含镁矿物的沉淀可以作为替代能源。巨大撞击后在高温下达到平衡，使少量的镁（1％或2％重量）分配到岩心中，同时仍在地幔中产生大量的嗜铁亲铁元素，并避免了硅中过量的硅和氧核心。作为冷却源的镁作为氧化物或硅酸盐从冷却核到地幔下的输送，比内核的生长要高一个数量级。因此，我们得出结论，即使核心放射源加热极少且核心冷却速度较慢，地球的发电机仍将在整个地质时间内（至少从34亿年前到现在）生存。

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  • 来源
    《Nature》 |2016年第7586期|387-389|共3页
  • 作者

    ORourke Joseph G.; Stevenson David J.;

  • 作者单位

    CALTECH, Div Geol & Planetary Sci, Pasadena, CA 91125 USA;

    CALTECH, Div Geol & Planetary Sci, Pasadena, CA 91125 USA;

  • 收录信息 美国《科学引文索引》(SCI);美国《工程索引》(EI);美国《生物学医学文摘》(MEDLINE);美国《化学文摘》(CA);
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