Decomposition of Fe₃S above 250 GPa

Abstract

We have determined subsolidus phase relations in the Fe–FeS system up to 271 GPa using laser‐heated diamond‐anvil cell techniques. In situ synchrotron X‐ray diffraction (XRD) measurements performed at high pressure and high temperature demonstrate the coexistence of hexagonal close‐packed (hcp) Fe and tetragonal Fe3S up to 241 GPa and 2510 K. In contrast, the XRD data obtained above 250 GPa show that the hcp phase coexists with the CsCl (B2)‐type phase for three different Fe–S bulk compositions (10, 16, and 20 atm% S). Furthermore, chemical analyses using a scanning transmission electron microscope on a retrieved sample indicate that Fe3S sample decomposes into two phases at 271 GPa and 2530 K, consistent with the XRD data. Theory predicts the presence of extensive solid solution between Fe and FeS at inner core conditions, whereas our results suggest that the Fe–FeS system remains eutectic at least to 271 GPa.

The structure of Fe–Si alloy in Earth's inner core

“…On the other hand, the previous DAC study by Kamada et al (2012) showed that maximum ∼4 wt.% sulfur dissolves into hcp iron at 123 GPa. More recent experiments by Ozawa et al (2013) demonstrated that the hcp phase formed in the Fe-FeS system at 270 GPa, although they reported only the upper bound of sulfur content (7.5 wt.% sulfur) in hcp iron. The effect of sulfur on stable crystal structure remains to be examined experimentally at inner core P-T conditions.…”

Melting Experiments on Liquidus Phase Relations in the Fe‐S‐O Ternary System Under Core Pressures

“…Based on this observation and the binary eutectic liquid compositions inferred from previous studies (Komabayashi, 2014;Morard et al, 2017;Mori et al, 2017), we estimate the liquidus field of Fe in the Fe-S-O ternary at 330 GPa, which extends to relatively S-poor and O-rich compositions (Figure 3d). Although our experiments were performed below the decomposition pressure of Fe 3 S (Ozawa et al, 2013), it does not cause a discontinuous jump in the eutectic liquid composition and should not change our estimate of the eutectic composition at 330 GPa remarkably (Mori et al, 2017). Badro et al (2014) argued that the outer core is sulfur-poor (0-2 wt% S) and oxygen-rich (4-5.5 wt% O) in order to account for seismological observations of its density and bulk sound velocity when S and O are the core light elements.…”

Stability of B2‐type FeS at Earth's inner core pressures

“…The presence of these light elements explains the density deficit of the core. Although the concentration and the chemical form of the light elements remain open questions, S, Si, and O are the primary candidates for light elements contained in the core [ Chen et al, ; Kamada et al, , ; Ozawa et al, ; Seagle et al, ].…”

“…However, high‐temperature experiments [ Sata et al, ] indicate that above 180 GPa, FeS adopts B2 structure under heating. High‐pressure experiments on Fe 3 S [ Ozawa et al, ] indicate that this compound decomposes into FeS‐B2 and Fe‐hcp when compressed to 264 GPa and heated to 2540 K.…”

“…If such phases have a Gibbs energy low enough, they will crystallize from the (Fe,S) solid solution even at a low sulfur concentration. [Ozawa et al, 2013] indicate that this compound decomposes into FeS-B2 and Fe-hcp when compressed to 264 GPa and heated to 2540 K.…”