Feedbacks between mineral reactions and mantle convection

The evolution of the solid Earth and many surface features are controlled by movements deep within. We aim to transform our understanding of those movements through a new understanding of mineral behaviour. Rocks in the mantle, the outer half of the Earth, can flow despite being solid, in the same way that a glacier flows. This flow is driven by contrasts in density, for example dense material sinks. One control on density is mineralogy, so we need to understand the controls on mineral changes. Pressure is key, for example, graphite (a form of carbon) transforms into diamond (a denser form of carbon) with increasing pressure. Pressure increases with depth in the Earth, in the same way as it does in the deep oceans. However, in a flowing system, pressure may not relate simply to depth. Another control on mineralogy is stress (different force per unit area in different directions), which prevails in the mantle as it deforms.

These ideas are illustrated by a simple analogy with clouds. On a calm day, the bases of clouds often appear undisturbed at a particular level, above which water is condensing. On a day of livelier weather, the cloud bases can be disturbed, as the wind wafts them up and down, and it takes time for water to evaporate or condense in response. Thus, looking at the bases of the clouds from a distance tells us something about the on-going dynamics in the atmosphere. Similarly, in the mantle, we have mineral changes at specific levels which we can "see" using seismic waves. In places the levels vary sideways, sometimes explained in terms of varying chemistry. We propose that this may in some places be like the effects on the cloud bases, in which case the observed levels are an imprint of the on-going dynamics.

We aim to understand the pressures and stresses in a flowing mantle and predict their effects on mineralogy. The changing mineralogy will affect density, which in turn affects the flow patterns. Changing mineralogy affects flow, and flow...