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Mineralogy and dynamics of a pyrolite lower mantle

机译：黄铁矿下地幔的矿物学和动力学

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There is a growing consensus that the Earth's lower mantle possesses a bulk composition broadly similar to that of the upper mantle (known as pyrolite). But little is known about lower-mantle mineralogy and phase chemistry, especially at depth. Here we report diamond-anvil cell experiments at pressures of 70 and 135 GPa (equivalent to depths within the Earth of about 1,500 and 2,900 km, respectively) which show that pyrolite would consist solely of magnesian-silkate perovskite (MgPv), calcium-silicate perovskite (CaPv) and magnesiowuestite (Mw). Contrary to recent speculation, no additional phases or dis-proportionations were encountered and MgPv was found to be present at both pressures. Moreover, we estimate that, at ultra-high pressures where thermal expansivities are low, buoyancy forces inherent in subducted slabs because of their lithology will be of similar magnitude to those required for thermally driven upwelling. So slabs would need to be about 850℃ cooler than their surroundings if they are to sink to the base of the mantle. Furthermore, initiation of plume-like upwellings from the core-mantle boundary, long attributed to superheating, may be triggered by lithologically induced buoyancy well before thermal equilibration is attained. We estimate that ascent would commence within ～0.5 Gyr of the slab reaching the core-mantle boundary, in which case the lowermost mantle should not be interpreted as a long-term repository for ancient slabs.
机译：人们越来越一致地认为，地球的下地幔具有与上地幔大致相同的体积组成（称为黄铁矿）。但是对下地幔的矿物学和相化学，尤其是深度化学知之甚少。在这里，我们报告了在70和135 GPa（分别相当于地球内部大约1,500和2,900 km的深度）的压力下进行的金刚石-砧细胞实验，该实验表明，黄铁矿将仅由镁硅酸盐钙钛矿（MgPv），硅酸钙组成钙钛矿（CaPv）和镁铝辉石（Mw）。与最近的推测相反，没有遇到其他相或歧化，并且发现在两种压力下都存在MgPv。此外，我们估计，在热膨胀系数较低的超高压下，俯冲板固有的浮力将因其岩性而与热驱动上升流所需的浮力相似。因此，要下沉到地幔底部，它们的温度必须比周围环境低850℃。此外，长期以来归因于过热，从岩心-地幔边界开始出现羽状上升流，这可能是由岩性诱发的浮力在达到热平衡之前触发的。我们估计，上升将在平板到达岩心幔边界的约0.5 Gyr内开始，在这种情况下，最下层的幔不应被解释为古代平板的长期储藏库。

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