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[SIT17-P03] Sound velocity of liquid Fe-S at high-pressure
Keywords:Core light-element, Liquid Fe alloys, P-wave velocity, High-pressure
The Earth’s liquid outer core consists of iron alloy with ~5 wt.% nickel and ~10 wt.% lighter elements [1]. Sulfur (S) is one of the candidates for the core light-elements. Iron-sulfides are commonly found in meteorites, and the concentrations of S are relatively high as 2-6 wt% S in a variety of chondrites [2]. On the other hand, S is depleted in Earth’s mantle [3]. A recent study on the element partitioning between magma ocean and core-forming metal proposed an upper limit of S in the core to be 6.3 wt% [4]. To examine the possibility of sulfur in the core, we have measured the P-wave velocity of liquid Fe-S alloy up to 100 GPa and 3000 K using inelastic X-ray scattering (IXS) from a sample in a laser-heated diamond-anvil cell (LH-DAC).
We carried out IXS measurements at the RIKEN Quantum NanoDynamics beamline BL43LXU of SPring-8 [5]. The starting specimen was a foil of Fe83S17, which was loaded in LH-DAC together with single-crystal Al2O3 discs acting as the thermal- and chemical-insulator. The sample melting was confirmed upon laser-heating before and after each IXS measurement based on X-ray diffraction patterns. The IXS spectra were collected in a momentum transfer range of 3-5.7 nm-1 with an energy resolution of ~2.8 meV at 17.79 keV. The P-wave velocity of liquid Fe83S17 was determined from the dispersion relation of the longitudinal acoustic phonon mode of the liquid. Sulfur was found to increase the P-wave velocity of liquid Fe [6], while the effect is gentle. Using the present P-wave data-set, we constructed an equation of state (EoS) for liquid Fe83S17 and estimated the P-wave velocity and density under the core P-T conditions. Comparing with seismological observations of the outer core, we found liquid Fe alloying with 6-7 wt% S to satisfy both P-wave velocity and density of the outer core simultaneously, which is similar to the S content based on previous element-partitioning experiments [4]. The present results imply that S can be the dominant light-element in the Earth’s outer core.
[1] Stevenson, Science 214, 611-619 (1981).
[2] Wasson and Kallemeyn, Phil. Trans. R. Soc. Lond. A 325, 535-544 (1988).
[3] McDonough. Treatise on Geochemistry (Second Edition), 3, 559–577 (2014).
[4] Mahan et al. Geohim. Cosmochim. Ata 196, 252-270 (2017).
[5] Baron, SPring-8 Inf. Newsl. 15, 14-19 (2010).
[6] Kuwayama et al. Phys. Rev. Lett. 124(16), 165701 (2020).
We carried out IXS measurements at the RIKEN Quantum NanoDynamics beamline BL43LXU of SPring-8 [5]. The starting specimen was a foil of Fe83S17, which was loaded in LH-DAC together with single-crystal Al2O3 discs acting as the thermal- and chemical-insulator. The sample melting was confirmed upon laser-heating before and after each IXS measurement based on X-ray diffraction patterns. The IXS spectra were collected in a momentum transfer range of 3-5.7 nm-1 with an energy resolution of ~2.8 meV at 17.79 keV. The P-wave velocity of liquid Fe83S17 was determined from the dispersion relation of the longitudinal acoustic phonon mode of the liquid. Sulfur was found to increase the P-wave velocity of liquid Fe [6], while the effect is gentle. Using the present P-wave data-set, we constructed an equation of state (EoS) for liquid Fe83S17 and estimated the P-wave velocity and density under the core P-T conditions. Comparing with seismological observations of the outer core, we found liquid Fe alloying with 6-7 wt% S to satisfy both P-wave velocity and density of the outer core simultaneously, which is similar to the S content based on previous element-partitioning experiments [4]. The present results imply that S can be the dominant light-element in the Earth’s outer core.
[1] Stevenson, Science 214, 611-619 (1981).
[2] Wasson and Kallemeyn, Phil. Trans. R. Soc. Lond. A 325, 535-544 (1988).
[3] McDonough. Treatise on Geochemistry (Second Edition), 3, 559–577 (2014).
[4] Mahan et al. Geohim. Cosmochim. Ata 196, 252-270 (2017).
[5] Baron, SPring-8 Inf. Newsl. 15, 14-19 (2010).
[6] Kuwayama et al. Phys. Rev. Lett. 124(16), 165701 (2020).