14:15 〜 14:30
[SIT18-02] Density of iron-rich peridotitic melt: Implication for dense silicate melt layer above Mars' core
キーワード：Mars、Melt、Density、Iron
Seismological observations from the InSight mission proposed possible presence of basal silicate melt layer above Mars' core. The basal silicate melt layer may play important role in the geochemical and geophysical evolutions of Mars’ interior, such as acting as a thermal boundary layer that prevents cooling and solidification of molten core. Therefore, understanding the gravitational stability of the basal silicate melt layer at the core-mantle boundary is important in discussing the nature and evolution. The basal silicate melt layer is considered to be enriched in iron and heat-producing elements. The iron enrichment in SiO2-poor peridotitic silicate melt is considered as the key to form dense silicate melt layer gravitationally stable at the base of the Mars’ mantle. However, influence of iron content on the density of peridotitic melt has not been well understood, due to experimental difficulties.
Effect of compositions on the density of silicate melts has been investigated at ambient pressure by using the double-bob Archimedean method. However, previous studies have been carried our mostly for SiO2-rich silicate melts such as andesitic and basaltic silicate compositions. Density of iron-rich peridotitic melt has not been well investigated due to two experimental difficulties of high liquidus temperature of SiO2-poor peridotitic compositions and high reactivity of iron in silicate melt with container material such as platinum crucible.
In order to overcome these experimental difficulties, we utilize electrostatic levitation furnace (ELF) at the International Space Station (ISS). The ELF at the ISS is capable of high temperature experiment at more than 2000 K by laser heating, and containerless experiment by electrostatic levitation under the microgravity environment in the ISS overcomes the problem of chemical reaction between melt sample and container material. In this study, we conducted the density measurements of four iron-rich peridotitic melts (Mg0.8Fe0.2SiO3, Mg1.8Fe0.2SiO4, Mg0.7Fe1.2SiO4, and Mg0.9Fe1.6SiO4.5) at wide range of temperature conditions between 1235 and 2465 K. The results show that densities of iron-rich peridotitic melts are markedly different from those calculated by first principles simulation and those estimated by extrapolating a density model for SiO2-rich basaltic melts. The density differences increase with decreasing SiO2 content, probably due to uncertainty caused by extrapolation of the previous density model for SiO2-rich basaltic melt to SiO2-poor peridotitic compositions. In addition, we found marked differences in the temperature dependences of the densities of peridotitic melts between our experimental results and those estimated by the previous density model for basaltic melt.
Our new experimental density data and the determined density model for peridotitic melt provide important constraint on the gravitational stability of iron-rich peridotitic melt at the core-mantle boundary of Mars. We modelled densities of iron-rich peridotitic melts in the Mars’ mantle by using the effect of compositions and temperature on the density of peridotitic melt determined from our experimental results at 0 GPa and high temperatures, combined with a compression curve of a peridotite melt reported in a sink-float density measurement at high pressure by Suzuki et al. (1998). Our results show that the peridotitic melts with the Fe/(Mg+Fe) ratio more than 0.4-0.5 have higher densities than those of the solid mantle at ~1,500 km depth in Mars. The data indicate that iron-rich peridotitic melt with the Fe/(Mg+Fe) ratio more than 0.4-0.5 is gravitationally stable at the core-mantle boundary in Mars.
Effect of compositions on the density of silicate melts has been investigated at ambient pressure by using the double-bob Archimedean method. However, previous studies have been carried our mostly for SiO2-rich silicate melts such as andesitic and basaltic silicate compositions. Density of iron-rich peridotitic melt has not been well investigated due to two experimental difficulties of high liquidus temperature of SiO2-poor peridotitic compositions and high reactivity of iron in silicate melt with container material such as platinum crucible.
In order to overcome these experimental difficulties, we utilize electrostatic levitation furnace (ELF) at the International Space Station (ISS). The ELF at the ISS is capable of high temperature experiment at more than 2000 K by laser heating, and containerless experiment by electrostatic levitation under the microgravity environment in the ISS overcomes the problem of chemical reaction between melt sample and container material. In this study, we conducted the density measurements of four iron-rich peridotitic melts (Mg0.8Fe0.2SiO3, Mg1.8Fe0.2SiO4, Mg0.7Fe1.2SiO4, and Mg0.9Fe1.6SiO4.5) at wide range of temperature conditions between 1235 and 2465 K. The results show that densities of iron-rich peridotitic melts are markedly different from those calculated by first principles simulation and those estimated by extrapolating a density model for SiO2-rich basaltic melts. The density differences increase with decreasing SiO2 content, probably due to uncertainty caused by extrapolation of the previous density model for SiO2-rich basaltic melt to SiO2-poor peridotitic compositions. In addition, we found marked differences in the temperature dependences of the densities of peridotitic melts between our experimental results and those estimated by the previous density model for basaltic melt.
Our new experimental density data and the determined density model for peridotitic melt provide important constraint on the gravitational stability of iron-rich peridotitic melt at the core-mantle boundary of Mars. We modelled densities of iron-rich peridotitic melts in the Mars’ mantle by using the effect of compositions and temperature on the density of peridotitic melt determined from our experimental results at 0 GPa and high temperatures, combined with a compression curve of a peridotite melt reported in a sink-float density measurement at high pressure by Suzuki et al. (1998). Our results show that the peridotitic melts with the Fe/(Mg+Fe) ratio more than 0.4-0.5 have higher densities than those of the solid mantle at ~1,500 km depth in Mars. The data indicate that iron-rich peridotitic melt with the Fe/(Mg+Fe) ratio more than 0.4-0.5 is gravitationally stable at the core-mantle boundary in Mars.