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Olivine water contents in the continental lithosphere and the longevity of cratons

机译：大陆岩石圈的橄榄石含水量和克拉通的寿命

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摘要

Cratons, the ancient cores of continents, contain the oldest crust and mantle on the Earth (>2 Gyr old). They extend laterally for hundreds of kilometres, and are underlain to depths of 180-250 km by mantle roots that are chemically and physically distinct from the surrounding mantle. Forming the thickest lithosphere on our planet, they act as rigid keels isolated from the flowing astheno-sphere; however, it has remained an open question how these large portions of the mantle can stay isolated for so long from mantle convection. Key physical properties thought to contribute to this longevity include chemical buoyancy due to high degrees of melt-depletion and the stiffness imparted by the low temperatures of a conductive thermal gradient. Geodynamic calculations, however, suggest that these characteristics are not sufficient to prevent the lithospheric mantle from being entrained during mantle convection over billions of years. Differences in water content are a potential source of additional viscosity contrast between cratonic roots and ambient mantle owing to the well-established hydrolytic weakening effect in olivine, the most abundant mineral of the upper mantle. However, the water contents of cratonic mantle roots have to date been poorly constrained. Here we show that olivine in peridotite xenoliths from the lithosphere-asthenosphere boundary region of the Kaapvaal craton mantle root are water-poor and provide sufficient viscosity contrast with underlying asthenosphere to satisfy the stability criteria required by geodynamic calculations. Our results provide a solution to a puzzling mystery of plate tectonics, namely why the oldest continents, in contrast to short-lived oceanic plates, have resisted recycling into the interior of our tectonically dynamic planet.
机译：Cratons是大陆的古老核心地带，拥有地球上最古老的地壳和地幔（> 2 Gyr年纪）。它们横向延伸数百公里，并在地幔底下深度达180-250公里，这些地幔在化学和物理上与周围的地幔不同。它们形成了我们星球上最厚的岩石圈，起着刚性龙骨的作用，与流动的软流圈隔离开来。然而，仍然存在一个悬而未决的问题，即如何使地幔的这些大部分与地幔对流保持如此长的时间。被认为有助于延长使用寿命的关键物理性能包括由于高度的熔体损耗而产生的化学浮力，以及由传导热梯度的低温赋予的刚度。然而，地球动力学计算表明，这些特征不足以防止数十亿年的地幔对流过程中夹带岩石圈地幔。含水量的差异是克拉通根与周围地幔之间额外的粘度反差的潜在来源，这是由于在上地幔最丰富的矿物橄榄石中已建立的水解减弱作用。但是，迄今为止，克拉通地幔根的水分含量受到限制。在这里，我们表明，Kaapvaal克拉通地幔根的岩石圈-软流圈边界区域的橄榄岩异岩中的橄榄石是贫水的，并且与下面的软流圈提供足够的粘度对比，以满足地球动力学计算所需的稳定性标准。我们的结果提供了一个令人困惑的板块构造奥秘的解决方案，即为什么最古老的大陆与寿命短的海洋板块相反，却阻止了再循环进入构造动态行星的内部。

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  • 来源
    《Nature》 |2010年第7311期|P.78-81|共4页
  • 作者

    Anne H. Peslier; Alan B. Woodland; David R. Bell; Marina Lazarov;

  • 作者单位

    Jacobs Technology, ESCG, Mail Code JE23, 2224 Bay Area Boulevard, Houston, Texas 77058, USA Astromaterials Research and Exploration Science, NASA-Johnson Space Center, Houston, Texas 77058, USA;

    rnInstitute of Geosciences, University of Frankfurt, Altenhoeferallee 1, D-60438 Frankfurt am Main, Germany;

    rnSchool of Earth & Space Exploration, Arizona State University, 550 East Tyler Mall, Tempe, Arizona 85287, USA;

    rnInstitute of Geosciences, University of Frankfurt, Altenhoeferallee 1, D-60438 Frankfurt am Main, Germany;

  • 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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