Element Partitioning

Element Partitioning at Earth’s Deep Chemical Boundaries

The Earth’s core-mantle boundary (CMB) and inner core boundary (ICB) are the most important deep chemical boundaries that define the dynamics of the interior and control the generation of Earth magnetic field. The boundaries reset the element redistribution through metal-silicate differentiation and inner crystallization. The goal of this study is to understand the behavior of element partitioning at these boundaries using newly developed high-pressure and temperature techniques and state-of-the-art analytical tools that allow simulating the conditions of the boundaries and analyzing the chemical compositions of the coexisting phases in the recovered samples. The research will significantly advance our knowledge of Earth’s deep processes and experimental techniques to investigate element redistribution at deep chemical boundaries.

Element partitioning between molten silicate and iron up to CMB pressure

It is important to understand the behaviour of the proposed light elements during core formation. Silicon and oxygen are major elements in the bulk Earth, and were readily available in large quantities during all stages of planet accretion. It is therefore fundamental to study their partitioning between molten metal and silicate melts at pressure, temperature and redox conditions relevant to core formation.

Fig. 1. Image of a recovered sample after laser-heating in DAC at 42 GPa. Five heating spots were produced at different super-liquid temperatures up to 4000 K to examine metal-silicate partitioning. (Cottrell and Fei, unpublished data). The polished heating spots can clearly see coexisting liquid metal and molten silicate. Representative X-ray diffraction patterns during a heating cycle are also shown, indicating clear melting at super-liquidus temperatures.

Light element partitioning between liquid and solid iron up to ICB pressure

We have been developing techniques and determine sulfur partitioning between solid and liquid in the Fe-FeS system at high pressure and temperature, using both multi-anvil apparatus and laser-heating diamond-anvil cell. We have made significant progress towards obtaining high-quality partitioning data as highlighted in the previous section. We have demonstrated that experimental techniques and procedure from sample recovering to quantitative analysis are well established. It is the time to extend the experiments to the Fe-S-Si-O system. We also set a new challenge to recover sample at the ICB conditions and examine Si, O, and S partitioning between the coexisting phases.

Fig. 2. Element mapping of the recovered Fe-melting sample at 45 GPa and 2200 K with field-emission SEM EDS, showing existence of Fe with Fe-O melt (Fei et al., unpublished data).