Iron alloys of volatile elements in the deep Earth’s interior

Published in Earth & Environment and Physics
Iron alloys of volatile elements in the deep Earth’s interior

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The distribution of the trace elements in the Earth’s interior provides a key record for Earth evolution, core formation as well as the sustainment of Earth’s magnetic field. Certain elements in the silicate Earth exhibit varying degrees of depletion compared to their abundance in undifferentiated protoplanetary materials. The depletion in these elements could plausibly be ascribed either to volatilization into space or dissolution in the Earth’s core. In general, these two behaviors of the element are considered to be related to their 50% condensation temperature and siderophicity (iron-loving or rock-loving) of the elements. Therefore, the understanding of which types and what concentrations of elements are sequestered in the deep Earth through sedimentary processes is highly required.

It is well known that the predominant mineral in the Earth's core is iron (Fe). It is generally supposed that the existence of ‘light’ elements as the major alloying candidates within it. A recent work by two of our co-authors simulated possible chemical reactions between Fe and a series depleting elements using first principles crystal structure search method under high pressure1. That was a large, a comprehensively structure-searching study, in which many interesting Fe compounds with p-block elements were predicted at high pressures. As quantified by the formation enthalpies of the most stable compounds in each system, binding strength between the depleting elements and Fe are found to be inversely correlated with their abundances, showing that these reactions are unlikely to be the cause of the elemental depletion in the silicate Earth. Considering the abundance of metallic iron during core formation, it's expected that the trace elements would participate in reactions at low concentrations. The Fe compounds obtained by crystal structure search method may not be suitable for this scenario. This is reminiscent to another scenario that depleting elements could be incorporated into hcp-iron by forming Fe alloys.

In this study, we explored the reactions between Fe and depleting elements by forming alloys, where the latter elements were treated as impurities of the alloy. Here, we chose hcp-Fe alloys as the simulation models because more studies have suggested that hcp-Fe show seismic velocities close to the Preliminary Reference Earth Model (PREM) under the inner core conditions. At the beginning, two kinds of defect, interstitial and substitutional, are considered as candidates for our simulation. We calculated the thermodynamic stabilities of two configurations, in which impurity atoms occupy either interstitial or substitutional sites in hcp-Fe supercells under high pressures. The results show that the interstitial defect is energetically more favorable for smaller impurity atoms like hydrogen (H) and carbon (C), whereas the substitutional defect is preferable for larger atoms, which aligns with the physical intuition that the incorporation of extra atoms into the close-packed lattice interstices of Fe at high pressures is challenging. We thus chose the substitutional alloy as our final simulation configuration. Then, it is essential to ascertain the doping concentration of impurity elements within the alloy. To account for the abundance of impurity elements in the silicate Earth and our computational power, we made a supercell containing 128 Fe atoms from the primitive cell of hcp-Fe and the doping concentration was set to be 1/128 as shown in Figure 1.

Figure 1. The scenario on the depletion of volatile elements in the silicate Earth. 

By employing the constructed model, we conducted geometry optimizations on the Fe127X supercells (where X represents a trace element acting as an impurity in the alloy) at pressures of 20, 150, and 300 GPa. The formation energies for the substitutional reactions were collected for comparison with the elemental abundance. Our results indicate that numerous elements tend to spontaneously alloy with Fe as pressure increases, even if they are lithophile under ambient pressure conditions.

We computationally observed that some of these non-siderophile elements possess low condensation temperatures, indicating that their depletion likely results from volatilization into space according to conventional view. Upon examining the 'volatility trend' line commonly employed to explain the observed depletion of volatile elements, we found that certain of these volatile elements exhibit significant over-depletion within the silicate Earth. We wish that the present results could provide insights to account for such notable depletion in elemental abundance.


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1     Wang, X. et al. Reverse chemistry of iron in the deep Earth. Preprint at (2019).

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Physical Sciences > Earth and Environmental Sciences > Earth Sciences > Geophysics
Condensed Matter Physics
Physical Sciences > Physics and Astronomy > Condensed Matter Physics

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