A better understanding of the temperature profile of Earth's core gives insight into the major geophysical phenomena it influences. Many studies have suggested silicon as a candidate light element for the core of the Earth. However, the effect of silicon on the melting temperatures of core materials and the thermal profile of the core is poorly understood due to disagreements among melt detection techniques, uncertainties in sample pressure evolution during heating, and sparsity of studies investigating the combined effects of nickel and silicon on the phase diagram of iron. Researchers measured the high-pressure melting and solid phase relations of Fe0.8Ni0.1Si0.1, a composition compatible with recent hypotheses about the cores of the Earth and Mercury, at the U.S. Department of Energy’s Advanced Photon Source (APS). They found that adding 10 mol% Si to Fe0.9Ni0.1 to derive the alloy Fe0.8Ni0.1Si0.1 reduces melting temperatures by ∼500 K at pressures representative of the outermost layer of the Earth’s core. Overall, the results, published in the journal Earth and Planetary Science Letters, indicate strong reproducibility within synchrotron Mössbauer spectroscopy (SMS) and x-ray diffraction (XRD) measurements and close agreement between the two techniques, which can be used to explore other iron-bearing materials within planetary interiors.
The metallic cores of terrestrial planets in our solar system are thought to be composed of iron alloyed with nickel and a handful of light elements. A range of temperatures at the core has been imposed by high-pressure experimental studies on the melting curves of iron and iron alloys. Accurate estimates of this temperature range are essential for understanding major processes like inner core crystallization, magnetic field generation, and heat flow through the core-mantle boundary, as well as the compositions, phase relations, and dynamics of complex multiscale structures in Earth's lowermost mantle.
The presence of moderate amounts of light elements such as silicon (Si), oxygen (O), carbon (C), sulfur (S), and hydrogen (H) has consistently been shown to depress the melting temperatures of iron alloys, though researchers disagree on their precise effects on the temperatures. While evidence points to Si as a plausible light element for the cores of Earth and Mercury, few studies have investigated the combined effects of silicon and nickel on the high-pressure and temperature phase boundaries of iron.
Researchers in this team developed a multi-technique approach to measure the high-pressure melting and solid phase relations of the iron alloy Fe0.8Ni0.1Si0.1, a candidate composition for planetary cores that has been shown to satisfy seismic observational constraints of the density, bulk modulus, and bulk sound speed of Earth's inner core boundary.
First, they compressed Fe0.8Ni0.1Si0.1 samples in laser-heated diamond anvil cells. Next, at the X-ray Science Division Inelastic X-ray & Nuclear Scattering Group’s x-ray beamline 3-ID-B of the APS, an Office of Science user facility at Argonne National Laboratory, they used synchrotron Mössbauer spectroscopy to probe the atomic dynamics of the iron nuclei across the solid-liquid phase boundary. Then, at the GeoSoilEnviroCARS beamline 13-ID-D at the APS, they conducted x-ray diffraction experiments on the samples to detect the onset of the alloy’s liquid diffuse scattering, constrain the transition boundary between the face centered cubic (fcc) and hexagonal closed packed (hcp) phases, and measure in-situ thermal pressure evolution.
The researchers found that compared to their previous investigations of Fe0.9Ni0.1, adding 10 mol% Si to yield the alloy Fe0.8Ni0.1Si0.1 reduces melting temperatures by ∼250 K at low pressures (<60 GPa) and flattens the hcp-fcc phase boundary. Extrapolating their results constrains the location of the hcp-fcc-liquid quasi-triple point at 147±14 GPa and 3140±90 K, which implies a melting temperature reduction of 500 K at pressures expected at the outermost layer of the Earth’s core compared with Fe0.9Ni0.1.
In addition, the researchers observed that Fe0.8Ni0.1Si0.1 melting temperatures across a range of pressures (~25 GPa to ~80 GPa) demonstrated excellent agreement between the two independent techniques (Fig. 1).
The results indicate that differences in the way pressure was measured in previous studies cannot alone explain discrepancies in fcc melting temperatures of iron alloys among various techniques. Other explanations the researchers posit for these discrepancies include carbon contamination from the diamond anvils, variable sample thickness, differences in laser-heating methods, and differences in temperature measurements in the experimental set ups.
The approach deployed in this study demonstrates the advantages of combining complementary experimental techniques in measurements of melting at extreme conditions and can be applied to other iron-bearing materials relevant to planetary interiors.
With upcoming APS Upgrade, the focused x-ray beam size at APS beamlines gets smaller, from the current 10s of microns to less than 1 micron. It thus allows such a melting measurement of Fe-Ni alloys to even higher temperatures and pressures to attain conditions found in the Earth’s inner core. ― Chris Palmer
See: Vasilije V. Dobrosavljevic1*, Dongzhou Zhang2, Wolfgang Sturhahn1, Jiyong Zhao3, Thomas S. Toellner3, Stella Chariton4, Vitali B. Prakapenka4, Olivia S. Pardo1, and Jennifer M. Jackson1**, “Melting and phase relations of Fe-Ni-Si determined by a multi-technique approach,” Earth Planet. Sci. Lett. 584, 117358 (2022). DOI: 10.1016/j.epsl.2021.117358
Author affiliations: 1California Institute of Technology, 2University of Hawaii at Mãnoa, 3Argonne National Laboratory, 4The University of Chicago
The researchers are grateful to the National Science Foundation (NSF-EAR-1727020, NSF-EAR-CSEDI-2009935) for financial support of this research. GeoSoilEnviroCARS and Sector 3 operations are partially supported by COMPRES (NSF-EAR-1661511). GeoSoilEnviroCARS is supported by the National Science Foundation – Earth Sciences (NSF-EAR-1634415) and Department of Energy-GeoSciences (DE-FG02-94ER14466). This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science user facility, operated for the DOE Office of Science by Argonne National Laboratory under contract no. DE-AC02-06CH11357.
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