Crystallization of silicon dioxide and compositional evolution of the Earth's core

The Earth's core is about ten per cent less dense than pure iron (Fe), suggesting that it contains light elements as well as iron. Modelling of core formation at high pressure (around 40-60 gigapascals) and high temperature (about 3,500 kelvin) in a deep magma ocean(1-5) predicts that both silicon (Si) and oxygen (O) are among the impurities in the liquid outer core(6-9). However, only the binary systems Fe-Si and Fe-O have been studied in detail at high pressures, and little is known about the compositional evolution of the Fe-Si-O ternary alloy under core conditions. Here we performed melting experiments on liquid Fe-Si-O alloy at core pressures in a laser-heated diamond-anvil cell. Our results demonstrate that the liquidus field of silicon dioxide (SiO2) is unexpectedly wide at the iron-rich portion of the Fe-Si-O ternary, such that an initial Fe-Si-O core crystallizes SiO2 as it cools. If crystallization proceeds on top of the core, the buoyancy released should have been more than sufficient to power core convection and a dynamo, in spite of high thermal conductivity(10,11), from as early on as the Hadean eon(12). SiO2 saturation also sets limits on silicon and oxygen concentrations in the present-day outer core

Strong inheritance of texture between perovskite and post-perovskite in the D′′ layer

The main mineral in the lower mantle, magnesium-silicate perovskite, transforms into a high-pressure, post-perovskite, phase at pressures and temperatures corresponding to the D'' seismic discontinuity approximately 200 km above the core-mantle boundary. The strong elastic anisotropy of post-perovskite has been invoked to explain the observed seismic anisotropy and to infer flow in the D'' region, based on models of textured post-perovskite. Such inferences rely on a knowledge of the mechanisms by which the post-perovskite can obtain texture. It is generally thought that seismic anisotropy in D'' is produced from lattice-preferred orientation generated during plastic deformation; however, it is difficult to explain all of the observed seismic anisotropy in D'' using a single deformation mechanism in post-perovskite. Here we show that strong texture inheritance is possible during transformation from perovskite to post-perovskite using a recently developed fluoride analogue system. If a similar transformation mechanism operates in the Earth, post-perovskite will inherit textures from deformed perovskite and vice versa as lower-mantle material passes into and out of regions of post-perovskite stability. This texture inheritance during the transition from post-perovskite to perovskite, combined with a single slip system in post-perovskite, can explain the seismic anisotropy of the lowermost mantle

Anisotropy of Earth's D '' layer and stacking faults in the MgSiO3 post-perovskite phase

The post-perovskite phase of (Mg,Fe)SiO3 is believed to be the main mineral phase of the Earth\u2019s lowermost mantle (the D 00 layer). Its properties explain1\u20136 numerous geophysical obser- vations associated with this layer \u2014 for example, the D 00 discon- tinuity7, its topography8 and seismic anisotropy within the layer9. Here we use a novel simulation technique, \ufb01rst-principles metadynamics, to identify a family of low-energy polytypic stacking-fault structures intermediate between the perovskite and post-perovskite phases. Metadynamics trajectories identify plane sliding involving the formation of stacking faults as the most favourable pathway for the phase transition, and as a likely mechanism for plastic deformation of perovskite and post- perovskite. In particular, the predicted slip planes are {010} for perovskite (consistent with experiment10,11) and {110} for post- perovskite (in contrast to the previously expected {010} slip planes1\u20134). Dominant slip planes de\ufb01ne the lattice preferred orientation and elastic anisotropy of the texture. The {110} slip planes in post-perovskite require a much smaller degree of lattice preferred orientation to explain geophysical observations of shear-wave anisotropy in the D 00 layer

First-principles constraints on diffusion in lower-mantle minerals and a weak D'' layer

Post-perovskite MgSiO3 is believed to be present in the D '' region of the Earth's lower most mantle(1-4). Its existence has been used to explain a number of seismic observations, such as the D '' reflector and the high degree of seismic anisotropy within the D '' layer(5-8). Ionic diffusion in post-perovskite controls its viscosity, which in turn controls the thermal and chemical coupling between the core and the mantle, the development of plumes and the stability of deep chemical reservoirs(9). Here we report the use of first-principles methods to calculate absolute diffusion rates in post-perovskite under the conditions found in the Earth's lower mantle. We find that the diffusion of Mg2+ and Si4+ in post-perovskite is extremely anisotropic, with almost eight orders of magnitude difference between the fast and slow directions. If post-perovskite in the D '' layer shows significant lattice-preferred orientation, the fast diffusion direction will render post-perovskite up to four orders of magnitude weaker than perovskite. The presence of weak postperovskite strongly increases the heat flux across the core-mantle boundary and alters the geotherm(9). It also provides an explanation for laterally varying viscosity in the lowermost mantle, as required by long-period geoid models(10). Moreover, the behaviour of very weak post-perovskite can reconcile seismic observation of a D '' reflector with recent experiments showing that the width of the perovskite-to-post-perovskite transition is too wide to cause sharp reflectors(11). We suggest that the observed sharp D '' reflector is caused by a rapid change in seismic anisotropy. Once sufficient perovskite has transformed into post-perovskite, post-perovskite becomes interconnected and strain is partitioned into this weaker phase. At this point, the weaker post-perovskite will start to deform rapidly, thereby developing a strong crystallographic texture. We show that the expected seismic contrast between the deformed perovskite-plus-post-perovskite assemblage and the overlying isotropic perovskite-plus-post-perovskite assemblage is consistent with seismic observations

Structure and density of basaltic melts at mantle conditions from first-principles simulations

The origin and stability of deep-mantle melts, and the magmatic processes at different times of Earth's history are controlled by the physical properties of constituent silicate liquids. Here we report density functional theory-based simulations of model basalt, hydrous model basalt and near-MORB to assess the effects of iron and water on the melt structure and density, respectively. Our results suggest that as pressure increases, all types of coordination between major cations and anions strongly increase, and the water speciation changes from isolated species to extended forms. These structural changes are responsible for rapid initial melt densification on compression thereby making these basaltic melts possibly buoyantly stable at one or more depths. Our finding that the melt-water system is ideal (nearly zero volume of mixing) and miscible (negative enthalpy of mixing) over most of the mantle conditions strengthens the idea of potential water enrichment of deep-mantle melts and early magma ocean