Melting diagrams of Fe-rich alloys determined from synchrotron in situ measurements in the 15–23GPa pressure range

Abstract

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.Page 1 of 34A c c e p t e d M a n u s c r i p t We report in situ observations of the melting behaviour of iron alloyed with 10 to 20 atom% 14 C, O, S, or Si at pressures between 15 and 24 GPa, using X-ray diffraction in a multi-anvil 15 press (SPring8). The degree of partial melting of the iron-alloys has been quantified from 16 analysis of the intensity of diffuse x-ray scattering of molten iron as a function for decreasing 17 temperature with a 50 degrees step. Coupled with microanalysis of recovered samples, the in-18 situ observations bring direct constraints on shape and positions of liquidus and solidus in the 19 melting diagrams. 20For the Fe-S system, our results are in good agreement with previous works. We observe 21 that the eutectic temperature increases from 1023 K at 15 GPa to 1123 K at 20.6 GPa and that 22 the eutectic composition decreases with increases pressure. Concerning the Fe-C system the 23 eutectic temperature of 1460 K at 20.7 GPa falls slightly below a linear extrapolation of the 24 previous work. In the case of the Fe-Ni-Si system and the Fe-O system, we find eutectic 25 temperatures significantly lower than previously reported. For the two systems, both eutectic 26 temperature and composition increase with increasing pressure in the 15-20 GPa range. 27Compare to previous work, we observe eutectic compositions (a) richer in light elements in 28 the Fe-O system, with 9.0 and 10.5 wt% O at 16.5 and 20.5 GPa, respectively, and (b) poorer 29 in the Fe-Ni-Si system with 11.5 wt% Si at 16.9 GPa. 30We confirm very high solubility of Si and C with solid iron, and report a Si partitioning 31 coefficient of 1.3(2) at 16.9 GPa. The S and O solubility in solid iron appears very small. 32 Therefore, both S and/or O could explain density jumps between liquid outer and solid inner 33 parts of planetary cores, at least up to ~25 GPa. 34

Melting phase relations in the Fe-S and Fe-S-O systems at core conditions in small terrestrial bodies

“…We observe a steep slope of the liquidus curve between 2000 and about 1600°C and a relatively flat slope near 1500°C. Our data would be consistent with an inflection point at about 7wt.% S. This has previously been observed in the Fe-S system over a wide pressure range for a similar sulfur content (at 14 GPa, Chen et al, 2008a; from 15 to 20.6 GPa, Andrault et al, 2009; from 1 bar to 10 GPa, Buono and Walker, 2011 and references therein) and…”

Phase Relations of Earth’s Core-Forming Materials

“…The above discussions are based on the stability of crystal structures under subsolidus conditions (see Figure 11b-d), but the liquidus phase of the outer core that crystallises to form the inner core depends on the eutectic composition. Ozawa et al [98] placed constraints on the eutectic composition in the Fe-Si system from textural observations made on samples recovered from DAC experiments, which is compared with existing data in Figure 15 [95,97,98,100,105]. Their results show that the Si content in eutectic melt decreases with increasing pressure and is less than 1.5 ± 0.1 wt% Si at 127 GPa (Figure 15).…”

Geodesy constraints on the interior structure and composition of Mars

“…Boehler, 1993;Shen et al, 1998) and to a lesser extent the melting temperature at the eutectic concentration (e.g. Fei et al, 2000;Li et al, 2001;Stewart et al, 2007;Chudinovskikh and Boehler, 2007;Chen et al, 2008;Morard et al, 2008;Andrault et al, 2009), melting temperatures of the Fe − S systems for intermediary sulfur concentrations have only been measured at a few discrete pressures and for a small set of sulfur concentrations (e.g. Fei et al, 2000;Li et al, 2001;Stewart et al, 2007;Andrault et al, 2009).…”