Effect of Silicon, Carbon, and Sulfur on Structure of Liquid Iron and Implications for Structure‐Property Relations in Liquid Iron‐Light Element Alloys

Abstract

Planetary cores are considered to be composed of iron (Fe) and light elements. Extensive studies on structure and properties of crystalline Fe‐light element alloys have been carried out to discuss composition and structure of the solid inner core. In contrast, structure and properties of liquid Fe‐light element alloys, which are important to discuss nature and dynamics of the liquid outer core, remain poorly understood. Here we conducted series of liquid structure measurements to systematically understand effect of silicon (Si), carbon (C), and sulfur (S) on structure of liquid Fe. We found that incorporation of Si in liquid Fe shortens the nearest (r1) and second (r2) neighbor distances, while incorporation of C and small amount of S expands these distances. The different structural behavior is interpreted in view of different light‐element incorporation mechanisms: substitutional incorporation of Si and interstitial incorporation of C and S. All light elements lower density of liquid Fe regardless of shortening or expansion of the r1 and r2 distances, while P‐wave velocities of liquid Fe‐light element alloys show linear increase with shortening of the r1 and r2. The different light‐element incorporation behavior in the structure of liquid Fe and the resultant variation in the properties of liquid Fe‐light element alloys should be important in understanding behavior of liquid Fe‐light element alloys in the planetary cores.

Heat Flow in Earth's Core From Invariant Electrical Resistivity of Fe‐Si on the Melting Boundary to 9 GPa: Do Light Elements Matter?

“…On the other hand, we note that the evidence for such a transition is not observed in ρ along the melting boundary, which remains constant. We noted earlier that addition of even a small amount of Si stabilizes the bcc structure and reduces the nearest‐neighbor and second neighbor distances in liquid (e.g., Shibazaki & Kono, ). However, this fact alone is insufficient to address the observed electrical resistivity behavior of the liquid alloy in this study.…”

“…Indeed, seismic data indicate that the core is well mixed (Davies et al, ; Labrosse, ; Nimmo, ). However, compositional convection is still not fully resolved as there is significant uncertainty in terms of light element composition in the core (Morard et al, ; Mori et al, ; Ozawa et al, ; Shibazaki & Kono, ; Suehiro et al, ; Suer et al, ; Tateno et al, ). For example, it was recently proposed that the exsolution of Mg at the CMB is sufficient to drive compositional convection (O'Rourke et al, ; O'Rourke & Stevenson, ) from the top down.…”

“…A variety of light elements, such as S, C, and O, is expected to be in the liquid OC in relatively small quantities of a few percent (e.g., McDonough, ; Morard et al, ; Tateno et al, ; Zhang, Sekine, et al, ). It was established recently (e.g., Zhang, Sekine, et al, ) that the OC is unlikely to contain just a single light element, since it has been demonstrated that no one light element can satisfy the combined compositional, cosmochemical, and geophysical constraints in the OC near ICB (Morard et al, ; Mori et al, ; Ozawa et al, ; Shibazaki & Kono, ; Suehiro et al, ; Suer et al, ; Tateno et al, ). Even with the unknown compositional makeup of the light elements in the core, our study leads to the conclusion that the electrical resistivity and thermal conductivity of liquid Fe alloy will remain the same at ICB and likely at CMB, regardless of the light element identity for small contents.…”

“…Even with the unknown compositional makeup of the light elements in the core, our study leads to the conclusion that the electrical resistivity and thermal conductivity of liquid Fe alloy will remain the same at ICB and likely at CMB, regardless of the light element identity for small contents. The Fe alloys with candidate light element Si, S, C, and O have a stabilizing effect on the solid and form closely packed parent structures before melting at high pressures (e.g., Shibazaki & Kono, ). Thus, based on our results and the behavior of ISRO structures in Fe alloys, we postulate that other combinations of light elements alloyed with liquid Fe exhibit the same resistivity behavior in their liquid states as Fe‐4.5Si investigated here.…”

“…However, the challenge of better constraining the transport properties of the liquid Fe alloy in the core is multifaceted. The uncertainty in evaluation of both thermal conductivity and electrical resistivity of the Fe alloy in the OC is caused by (i) the unresolved compositional makeup and proportions of light elements in the liquid OC (e.g., Litasov & Shatskiy, ) although the consensus tilts toward Si as the major light element in the OC (e.g., Suer et al, ; Zhang, Sekine, et al, ), (ii) the exact effects of light element alloying on the transport properties of the liquid Fe (e.g., Shibazaki & Kono, ; Zhang, Sekine, et al, ), (iii) the amount of heat flow across the CMB (e.g., Ammann et al, ; Nimmo, ), and (iv) a large spread of estimated temperatures at the CMB and ICB (e.g., Andrault et al, ; Anzellini et al, ; Sakairi et al, ; Zhang, Sekine, et al, ). Lastly, from the experimental perspective, reaching and maintaining the core temperatures ( T ) and pressures ( P ) for sufficiently long time, while maintaining liquid sample geometry and purity to collect resistivity data, are still not experimentally achievable (e.g., Williams, ).…”