Olivine water contents in the continental lithosphere and the longevity of cratons
Olivine water contents in the continental lithosphere and the longevity of cratons
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DOI：
10.1038/nature09317
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发表时间：
2010-09
期刊：
影响因子：
64.8
通讯作者：
A. Peslier;A. Woodland;D. Bell;M. Lazarov
中科院分区：
文献类型：
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作者：
A. Peslier;A. Woodland;D. Bell;M. Lazarov
古陆是大陆的古老核心，包含地球上最古老的地壳和地幔（>2 Gyr）。它们横向延伸数百公里，并在180-250公里深处由地幔根覆盖，这些地幔根在化学和物理上与周围的地幔不同。它们形成了地球上最厚的岩石圈，就像与流动的软流圈隔离的刚性龙骨;然而，这仍然是一个悬而未决的问题，这些地幔的大部分如何能够保持与地幔对流隔离这么长时间。被认为有助于这种寿命的关键物理特性包括由于高度的熔体耗尽而产生的化学浮力和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，这些特征不足以阻止岩石圈地幔在数十亿年的地幔对流过程中被夹带。水含量的差异是一个潜在的来源，额外的粘度差异之间的岩石根和周围的地幔，由于公认的水解弱化作用的橄榄石，，上地幔最丰富的矿物。然而，含水量的dexanonic地幔根迄今为止受到很大的限制。在这里，我们表明，橄榄石橄榄岩包体的岩石圈软流圈边界区域的卡普瓦尔克拉通地幔根贫水，并提供足够的粘度与底层软流圈的对比，以满足地球动力学计算所需的稳定性标准。我们的研究结果为板块构造的一个令人困惑的谜团提供了解决方案，即为什么最古老的大陆，与短命的海洋板块相比，一直抵制再循环到我们的构造动态星球的内部。
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 asthenosphere; 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.