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类型和热化学史的许多方面。对于硅酸盐地球来说，这些边界层是岩石圈和核幔边界(CMB)地幔一侧的D‘’区，岩石圈的低温和高刚性导致板条式下沉。D‘’区是羽状对流的源头，调节着从地核到硅酸盐地球的热交换。下地幔中最常见的矿物(镁硅酸盐钙钛矿)的相变改变了CMB中D‘’区的性质，使下热界变得更加复杂。不幸的是，这些性质中的大多数不能在钙钛矿后相稳定的极端压力(120 Gpa)下测量。限制它们的最好机会是结合对低压模拟材料(它们具有相同的晶体结构，但化学成分不同)的测量，以及对模拟和自然系统的从头计算模拟。我们最近开发了一组ABF3类似物，它们的性质与目前使用的CaBO3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控制D‘’动力学的后钙钛矿性质(相图、压力-温度-体积关系、粘度、滑移系和热扩散率)。这些测量将使开发出准确预测地幔下层热边界层行为的模型成为可能。这将对(1)热收支、发电机功率和内核结晶的开始，(2)羽流的活力，(3)地幔底部加热与内部加热的比率，以及(4)硅酸盐地球的放射性元素收支进行关联。

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.