Melting relationships in the Fe–Fe3S system up to the outer core conditions
Highlights
► Melting relation in Fe–Fe3S system was investigated to 182 GPa based on in situ XRD. ► TICB was estimated based on the melting curve. ► TCMB was estimated by assuming the adiabatic temperature gradient in the outer core. ► A recovered sample from 123 GPa was observed by FE-SEM. ► A dendritic texure was observed.
Introduction
The Earth's core is considered to be composed of an iron alloy with light elements because of the density deficits of the Earth's liquid outer and solid inner cores (e.g., Birch, 1964, Dubrovinsky et al., 2000). Although there are many candidates for these light elements (e.g., Poirier, 1994), sulfur in particular has been considered as one of the most plausible candidates. Therefore, the phase relationships in the Fe–FeS system have been intensively studied (e.g., Campbell et al., 2007, Chudinovskikh and Boehler, 2007, Fei et al., 1997, Fei et al., 2000, Kamada et al., 2010, Li et al., 2001, Morard et al., 2008, Morard et al., 2011, Seagle et al., 2006, Walker et al., 2009).
The liquid outer core is considered to have a convective flow that drives the geodynamo to generate the Earth's magnetic field (e.g., Buffett, 2007). Since the outer core is liquid, the temperature at the core–mantle boundary (CMB) or the inner core boundary (ICB) should be constrained by the melting temperatures of the core forming materials, i.e. iron alloys with light elements. Therefore, it is necessary to investigate the phase and melting relationships in the Fe–FeS system in order to understand the physical and chemical properties of the core.
In previous studies, intermediate high pressure phases in the Fe–FeS system such as Fe3S2, Fe2S, and Fe3S have been reported (e.g., Fei et al., 1997, Fei et al., 2000, Li et al., 2001). The melting relationship in the Fe–FeS system does not show a simple monotonous increase due to the change of subsolidus phases. FeS coexists with Fe up to 14 GPa and then Fe coexists with Fe3S2 above 14 GPa. The eutectic temperature of the Fe–FeS system decreases up to 14 GPa (Fei et al., 1997). On the other hand, it increases after the appearance of Fe3S above 18 GPa (Fei et al., 2000), and Fe coexists with Fe3S above 18 GPa. Seagle et al. (2006) studied the stability of Fe3S up to 80 GPa and reported that there were no phase transitions in Fe3S. Kamada et al. (2010) confirmed that Fe3S was stable up to 220 GPa and up to 3300 K, and they also reported the absence of new transformations in the Fe–Fe3S system in the conditions of the outer core. The melting relationships of the Fe–FeS system were investigated above 21 GPa (Campbell et al., 2007, Chudinovskikh and Boehler, 2007, Kamada et al., 2010, Morard et al., 2008). The previous works confirmed the melting relationships of the Fe–FeS system based on X-ray diffraction up to 113 GPa (e.g., Campbell et al., 2007, Kamada et al., 2010, Morard et al., 2008, Morard et al., 2011). Chudinovskikh and Boehler (2007) and Kamada et al. (2010) reported the melting relationships in the Fe–FeS system up to 44 GPa and 86 GPa, respectively, based on textural observations. Campbell et al. (2007) reported a melting temperature depression of 700–900 K compared to the melting temperature of pure iron. They estimated that the temperature at the ICB was in the range of 4500–5900 K due to depression from the melting temperature of pure iron at the pressure of the ICB. On the other hand, Chudinovskikh and Boehler (2007) reported that the eutectic temperature in the system was 1000 K below the melting temperature of pure iron. They suggested that melting temperatures of pure iron would be depressed by several hundred degrees and the temperature at the CMB was substantially lower than 4000 K.
These previous studies of melting relationships in the Fe–FeS system are limited to 113 GPa. These pressure conditions are lower than the pressure of the CMB. It is, therefore, necessary to investigate the melting relationships of the Fe–FeS system to pressures corresponding to the outer core in order to discuss the temperature of the core. In this study, we investigated melting relationships in the Fe–FeS system up to the pressure relevant to the outer core conditions using a double-sided laser-heated diamond anvil cell (LHDAC) combined with synchrotron X-ray diffraction.
Section snippets
Experimental procedure
We used a symmetric DAC with beveled diamond anvils. The culet sizes of the diamond anvils were 75 and 150 μm. A mixture of powdered Fe (99.5% purity) and FeS (99.9% purity) containing 8.0 wt.% (13.2 at.%) sulfur was used as a starting material. The powder mixture was ground for 30 min using an agate mortar and a pestle. The typical grain size of the powder was 1–5 μm. We made a thin foil from the starting material with a thickness around 15 μm using a cold compression technique. The sample foil was
Results
In situ X-ray diffraction experiments were conducted in the pressure range of 56–175 GPa and the temperature range of 1400–3500 K. The experimental conditions are summarized in Table 1. Fig. 1 shows a typical example of two dimensional X-ray diffraction patterns of the sample at the highest pressure (around 170 GPa) and temperature (up to 3500 K) in this study. The X-ray diffraction peaks derived from ε-Fe and Fe3S can be observed at 174.7 GPa and 3190 K as shown in Fig. 1-A. This suggests that Fe3S
Estimation of the ICB temperature
The present results combined with the data from Kamada et al. (2010) were fitted by using the Simon equation (Simon and Glatzel, 1929) and Kraut–Kennedy law (Kraut and Kennedy, 1966) in order to extrapolate the melting temperature of the Fe–Fe3S system as shown in Fig. 2. The Simon equation is an empirically derived formulation and is expressed as follows:where Tm is a melting temperature at the experimental pressure, Tm,r is the melting temperature at a reference pressure,
Acknowledgments
The authors thank Prof. Stephane Labrosse for fruitful discussion. S.K. gratefully acknowledges the Japan Society for the Promotion of Science (JSPS) for providing a research fellowship. This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Science, Sport and Technology of the Japanese Government (nos. 18104009 and 22000002) to E.O., and a Grant-in-Aid for Young Scientists (B) (no. 21740374) to T.S. This work was conducted a part of the
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