Understanding the D'' zone: novel fluoride analogues to MgSiO3 post perovskite

对流系统的热边界层控制着对流和热化学历史的许多方面。对于硅酸盐地球，这些边界层是岩石圈和核幔边界（CMB）地幔侧的D“区，岩石圈的低温和高刚度导致板状下降。D“区域是羽状对流的来源，并调节从地核到硅酸盐地球的热交换。由于下地幔中最常见的矿物（镁硅酸盐钙钛矿）存在相变，下热边界变得更加复杂，这改变了CMB D“区域的性质。不幸的是，这些性质中的大多数不能在后钙钛矿相的稳定化的极端压力（120 GPa）下测量。限制它们的最佳机会是通过对低压模拟材料（具有相同的晶体结构但不同的化学成分）的测量和模拟和自然系统的从头计算模拟的组合。我们最近开发了一组ABF 3类似物，其性质比目前使用的CaBO 3类似物更类似于MgSiO 3。因此，我们建议使用这些改进的氟化物类似物来确定后钙钛矿的性质，其控制D“的动力学（相图、压力-温度-体积关系、粘度、滑移系统和热扩散率）。这些测量结果将有助于建立模型，准确预测地幔下部热边界层的行为。这将限制（1）内核的热收支、发电机功率和结晶的开始，（2）地幔柱的活力，（3）地幔底部加热与内部加热的比率，（4）硅酸盐地球的放射性元素收支。

The thermal boundary layers of a convecting system control many aspects of its style of convection and thermo-chemical history. For the silicate Earth these boundary layers are the lithosphere, whose low temperature and high rigidity induces slab-style downwellings, and the D'' region on the mantle side of the core-mantle-boundary (CMB). The D'' region is the source of plume-style convection and regulates heat exchange from the core to the silicate Earth. The lower thermal boundary is made more complex by the existance of a phase transition in the most common mineral in the lower mantle (magnesium-silicate perovskite) which changes the properties of the D'' region at the CMB. Unfortunately, most of these properties cannot be measured at the extreme pressures (120 GPa) of stabilisation of the post-perovskite phase. The best chance of constraining them is through a combination of measurements on low-pressure analogue materials (which have the same crystal structure but a different chemical composition) and ab initio simulations of both the analogue and natural systems. We have recently developed a set of ABF3 analogues whose properties are much more similar to MgSiO3 than are those of the CaBO3 analogues currently in use. We propose, therefore, to use these improved fluoride analogues to determine the properties of post-perovskite which control the dynamics of D'' (phase diagram, pressure-temperature-volume relations, viscosity, slip systems and thermal diffusivity). These measurements will allow models to be developed which accurately predict the behaviour of the lower thermal boundary layer of the mantle. This will place coinstraints on (1) the heat budget, dynamo power and start of crystallisation of the inner core, (2)the vigour of plumes, (3) the ratio of underside heating to internal heating in the mantle and, (4) the radioactive element budget of the silicate Earth.