OS3-8
かんらん岩組成融体の密度に鉄が及ぼす影響
Effect of iron on the density of peridotitic melt
河野義生 1, 小山千尋 2, 近藤望 1,3, 尾原幸治 4, 桑原秀治 1, 中田亮一 5, 渡邉勇基 6, 織田裕久 2,石川毅彦 2,Yoshio KONO1, Chihiro KOYAMA2, Nozomi KONDO1,3, Koji Ohara4, Hideharu KUWAHARA1, Roichi
NAKADA5, Yuki WATANABE6, Hirohisa ODA2, Takehiko ISHIKAWA2
愛媛大学，Ehime University.
宇宙航空研究開発機構，JAXA.
3 岡山大学，Okayama University.
4 島根大学，Shimane University.
5 高知大学，Kochi University.
6 株式会社エイ・イー・エス，Advanced Engineering Services Co., Ltd.
121. Introduction
Knowledge of the density of peridotitic melt is fundamental to discuss nature and dynamics of the magma
ocean in the early Earth and its evolution. In particular, possible density crossover between magma andcrystallized mantle minerals in the magma ocean is one of the most important issues to understand formationof basal magma ocean at the base of the Earth’s mantle1). Preferential partitioning of iron in silicate melt at thedeep mantle condition is considered to be the key to cause density crossover between silicate melt and mantle
minerals2). Petitgirard et al. (2015)3) discussed density difference between silicate melt and bridgmanite, whichis the major mineral constituent of the Earth’s lower mantle, in the experimentally observed range of thepartitioning coefficient of iron of ~0.1-0.5 between silicate melt and bridgmanite2,4), and suggested that silicate
melt is denser than the surrounding mantle mineral in the lowermost mantle condition. However, it isimportant to note that effect of iron on the density of silicate melt with peridotitic composition has not been
experimentally investigated. Petitgirard et al. (2015)3) used density of Fe2SiO4 melt calculated by first principles
simulation3) to estimate effect of iron on the density of MgSiO3-FeSiO3 melts. Experimental determination of
the effect of iron on the density of peridotitic melt is essential to clarify the possible occurrence of the density
crossover between MgSiO3-FeSiO3 melt and bridgmanite in the deep mantle conditions.
Efforts have been made to understand the effect of major chemical compositions on the density of silicate
melts6-8). However, previous density measurements were conducted for silicate samples with relatively high
SiO2+Al2O3 compositions (more than ~50 wt.%)6-8) such as basaltic and andesitic compositions (Fig. 1). On theother hand, densities of SiO2-poor peridotitic melts have not been experimentally investigated, due toexperimental difficulties of the high melting temperature peridotitic compositions (for example, melting
temperature of Mg2SiO4 olivine composition at 2161 K9)). In addition, effect of iron on the density of silicate
melts has not been well studied, due to experimental difficulty by high reactivity of iron with typical crucible
material such as platinum. Only Guo et al. (2014)8) investigated density of iron-bearing silicate samples withbasaltic compositions in molybdenum capsule.
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Figure 1. Sample compositions and the temperature conditions of the density measurements in this and previous studies.

Here we show density measurements of four iron-rich peridotitic melts (Mg0.8Fe0.2SiO3, Mg1.8Fe0.2SiO4,Mg0.7Fe1.2SiO4, and Mg0.9Fe1.6SiO4.5) at the temperature conditions up to 2465 K (Fig. 1) by using electrostatic
levitation furnace (ELF) at the international space station (ISS). ELF at the ISS enables us to melt high melting
temperature peridotitic compositions by laser heating, and to carry out density measurements for levitatedmelt samples without container material, which avoids reaction of iron in melt with capsule material. This
technique opens new way to experimentally investigate density of iron-rich peridotitic melts at hightemperature conditions.
2. Results and discussion
We measured densities of Mg0.8Fe0.2SiO3 melt at 1235-2206 K, of Mg1.8Fe0.2SiO4 melt at 1718-2465 K, ofMg0.7Fe1.2SiO4 melt at 1348-2360 K, and Mg0.9Fe1.6SiO4.5 melt at 1586-2297 K under Ar gas environment in theELF at the ISS. Our obtained densities of Mg0.8Fe0.2SiO3 melt at 1500 and 2200 K is 6.1 % and 7.3 % higher thanthose calculated by first principles simulation10), respectively. On the other hand, our obtained densities ofMg0.8Fe0.2SiO3 melt at 1500 and 2200 K show only 0.5 % and 2.8 % differences, respectively, from thoseestimated by the previous density model, which is determined based on the experimental density results ofbasaltic compositions8). In contrast, temperature dependence of Mg0.8Fe0.2SiO3 melt obtained in this study (1.4×10-4 g/cm3 K) is similar to that of the first principles simulation study (-1.8×10-4 g/cm3 K)10), while ismarkedly lower than that of the previous density model (-2.3×10-4 g/cm3 K)8).
In order to construct density model of peridotitic melts, we fit the experimentally observed density (ρ) ofMg0.8Fe0.2SiO3, Mg1.8Fe0.2SiO4, Mg0.7Fe1.2SiO4, and Mg0.9Fe1.6SiO4.5 melts at the temperature conditions of 12352465 K into the following equation:𝜌𝜌 = � 𝑋𝑋𝑖𝑖 𝑀𝑀𝑖𝑖 �� 𝑋𝑋𝑖𝑖 �𝑉𝑉𝑖𝑖,𝑇𝑇𝑟𝑟𝑟𝑟𝑟𝑟 +𝛿𝛿𝑉𝑉𝑖𝑖
�𝑇𝑇 − 𝑇𝑇𝑟𝑟𝑟𝑟𝑟𝑟 ��,𝛿𝛿𝛿𝛿

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where Xi and Mi are the mole fraction and molecular weight of each oxide component, respectively. Vi,Tref isthe partial molar volume of each oxide component at a reference temperature condition (Tref=1723 K), andδVi/δT is partial thermal expansivity of each oxide component. The partial molar volumes of SiO2, MgO, andFeO determined from the our obtained density data of peridotitic melts are similar to those of the previousdensity model derived from basaltic melts data8), while the partial thermal expansivities obtained in this study
are markedly lower than those of the previous density model8). The result indicates that the effect of iron on
the density of peridotitic melt is similar to those of basaltic melts, while there is marked difference in thethermal expansivity between peridotitic and basaltic melts. In addition, effect of iron on the density of(Mg,Fe)SiO3 melt calculated by first principles simulation10) is similar to that obtained in this study.
In summary, we succeeded in determining densities of SiO2-poor and iron-rich peridotitic melts at thetemperature conditions up to 2465 K. Our obtained densities of peridotitic melts are higher than those ofprevious studies. The new density data of iron-rich peridotitic melts provide important constraints on thedensity crossover between silicate melts and mantle minerals in the Earth’s deep mantle.
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