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

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Highlights

  • Phase relations of Fe–Si alloys were studied by laser-heated DAC to 407 GPa and 5960 K.

  • The EOS of hcp Fe–9 wt.% Si was obtained on the basis of compression data to 305 GPa.

  • Fe–7 wt.% Si crystallizes hcp as a single phase at inner core conditions.

  • Incorporation of silicon in iron stabilizes hcp relative to fcc.

Abstract

Phase relations of iron–silicon alloy (Fe–6.5 wt.% Si and Fe–9 wt.% Si) were investigated up to 407 GPa and 5960 K in a laser-heated diamond–anvil cell, which likely covers the entire pressure and temperature conditions of the Earth's inner core. Synchrotron X-ray diffraction measurements show that Fe–9 wt.% Si with a hexagonal close-packed (hcp) structure is stable to 4800 K at 330 GPa, corresponding to the pressure at the inner/outer core boundary, and decomposes into a mixture of Si-poor hcp and Si-rich CsCl-type (B2) phases at higher temperatures. We also found that the solubility of silicon in solid iron is relatively insensitive to temperature, decreasing from 9 to >6.5 wt.% over a range of 1500 K at 70 GPa. These suggest that the inner core is composed solely of the hcp phase, when the silicon content is up to 7 wt.% that likely accounts for the inner core density deficit as well as for the Mg/Si ratio and the Si isotopic composition of the mantle. Additionally, the present experiments demonstrate that the incorporation of silicon in iron expands the stability of hcp with respect to that of fcc.

Introduction

Crystal structure in the Earth's inner core is of great importance to understand its elastic and transport properties, and dynamics (de Koker et al., 2012, Pozzo et al., 2012, Gomi et al., 2013). While a variety of different crystal structures have been proposed for iron at high pressure and temperature (PT) (Saxena et al., 1995, Belonoshko et al., 2003, Mikhaylushkin et al., 2007), recent static experiments (Tateno et al., 2010) revealed that the hcp structure is a stable form of iron up to 377 GPa and 5700 K. Such experimental result is also supported by the most recent ab initio calculations (Stixrude, 2012). Nevertheless, the terrestrial core is not pure iron. Silicon is likely to be a major light component in the core (∼6 wt.%, Hirose et al., 2013), which is strongly favored by cosmochemical and geochemical arguments based on 1) the depletion in silicon in the Earth's mantle relative to solar abundance and primitive meteorites (e.g., Allègre et al., 1995) and 2) the difference in 30Si/28Si ratio between meteorites and terrestrial rocks (Georg et al., 2007, Fitoussi et al., 2009, Shahar et al., 2009, Ziegler et al., 2010). Such high concentration of silicon in the core has been supported by metal–silicate partitioning experiments (Takafuji et al., 2005, Sakai et al., 2006, Ozawa et al., 2009 Rubie et al., 2011), although some experimental and theoretical studies argued for a subordinate role of silicon (<4.5 wt.%) (Antonangeli et al., 2010, Siebert et al., 2013, Badro et al., 2014). Since only small fractionation of silicon is known to occur between liquid and solid iron, the inner core may possess similar amount of silicon to that in the bulk core (Kuwayama and Hirose, 2004; Alfè et al., 2002; Fischer et al., 2013). Indeed, previous density and sound velocity measurements of Fe and Fe–Si alloy at high pressure suggested that iron alloyed with silicon can explain seismic observations in the inner core (Badro et al., 2007, Mao et al., 2012).

Phase relations in Fe–Si alloys, however, have been examined by earlier experiments only to 257 GPa and 2400 K (Asanuma et al., 2008), while the inner core is under >330 GPa and ∼5000 K. The phase diagram of Fe–9 wt.% Si was extensively studied under pressure in a DAC (Kuwayama et al., 2009, Lin et al., 2009, Fischer et al., 2013). These previous experiments consistently demonstrated that hcp-Fe containing 9 wt.% silicon decomposes into a mixture of hcp and B2 phases with increasing temperature. It suggests a possibility of two-phase mixture in the inner core (Brosh et al., 2009, Fischer et al., 2013), although the boundary between hcp and hcp + B2 has never been constrained above 240 GPa. On the other hand, ab initio calculations argued that the incorporation of small amount of silicon in iron stabilizes body-centered cubic (bcc) or fcc phase with respect to the hcp phase (Vočadlo et al., 2003; Côté et al., 2008a, Côté et al., 2010), but it has never been verified by experiments.

In this study, we examine the phase diagram and the equation of state (EoS) of Fe–Si alloys at multimegabar pressures by a combination of laser-heated diamond–anvil cell (DAC) techniques and synchrotron X-ray diffraction (XRD) measurements. The phase relation in Fe–9 wt.% Si (Fe0.84Si0.16 in atomic ratio) was determined up to 407 GPa and 5960 K, the highest static PT ever reported. The pressure–volume data for hcp Fe–9 wt.% Si was also obtained to 305 GPa at 300 K. In addition, the stabilities of hcp and fcc Fe–6.5 wt.% Si (Fe0.88Si0.12 in atomic ratio) were examined up to melting temperature at relatively low pressures (<70 GPa). Based on these new data, we discuss the stable crystal structure of Fe–Si alloy in the Earth's inner core.

Section snippets

High-P/room-T experiments for Fe–9 wt.% Si

High-pressure/room-temperature experiments were performed in a DAC to obtain the EoS of hcp Fe–9 wt.% Si (runs #1–4) (Table S1). We used a powder sample (Goodfellow) as starting material. We examined unheated portion of a recovered sample under scanning transmission electron microscope and confirmed the starting composition and the chemical homogeneity at micrometric scale. It was loaded into a hole in a pre-indented rhenium gasket together with MgO (pressure standard) and pressure medium (Ar

Equation of state of hcp Fe–9 wt.% Si

Four separate sets of experiments were performed to determine the EoS of hcp-structured Fe–9 wt.% Si at 300 K between 13 and 305 GPa (runs #1–4). The observed unit-cell parameters and volumes of hcp Fe–9 wt.% Si and MgO are summarized in Table S1. A representative XRD pattern collected at 305 GPa at room temperature is shown in Fig. 1a, in which the diffraction peaks from hcp Fe–9 wt.% Si and NaCl-type (B1)-MgO were found. The diffraction peaks of SiO2 were not observed through the present

Crystal structure of Fe–Si alloy in the inner core

The density of the inner core deduced from seismological observations is smaller by 5 to 6% than that of pure iron (Dziewonski and Anderson, 1981, Dewaele et al., 2006) when ICB temperature is 4900–5700 K (Alfè et al., 2007; Anzellini et al., 2013, Nomura et al., 2014) (Fig. 10). The EoS of Fe–9 wt.% Si obtained in this study indicates that such inner core density deficit is reconciled with an hcp iron-alloy containing 6 to 8 wt.% Si at the ICB. Additionally, the high Mg/Si ratio (∼1.3) in the

Conclusions

The phase relations and the EoS of Fe–9 wt.% Si were examined to a multimegabar pressure range. Experiments were performed up to 407 GPa and 5960 K in a laser-heated DAC, the highest static PT condition ever reported. The results demonstrate that iron with an hcp structure contains up to 9 wt.% Si at 4800 K and 330 GPa of ICB pressure. The Earth's inner core may include about 7 wt.% Si to account for its density deficit from pure iron according to the present density measurements, which is

Acknowledgments

We thank N. Hirao, H. Ozawa, and Y. Kudo for their technical assistance. T. Komabayashi is thanked for fruitful discussions. The comments from two anonymous reviewers were helpful to improve the manuscript. The synchrotron X-ray diffraction measurements were conducted at BL10XU of SPring-8 (proposal Nos. 2011B0087, 2011B1405, 2012A0087, 2012B1212, and 2013B0087).

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