[Silicate Melt]
Measurement of temperature dependence of viscosity and density of depolymerized silicate melts

RESEARCH OVERVIEW

• Knowledge of viscosity and density of SiO₂-poor depolymerized silicate melts is important in the understanding the nature and dynamics of silicate magmas in the magma ocean of the early Earth, and deep magmas in the current Earth’s deep interiors. However, temperature dependence of the viscosity and the density of depolymerized silicate melts remain poorly understood due to experimental difficulties such as high melting temperature, high reactivity, and/or low viscosity.
• The electrostatic levitation furnace onboard the International Space Station overcomes the experimental difficulties to investigate viscosity and density of SiO₂-poor depolymerized silicate melts, and provides important information about temperature dependence of viscosity and density of the depolymerized silicate melts with varying SiO₂ content and Mg/Fe ratio.
• Results from the Measurement of Temperature Dependence of Viscosity and Density of Depolymerized Silicate Melts (ELF-Silicate Melt) investigation, in conjunction with the results from experiments on Earth, provide important information to understand the nature and dynamics of the magma ocean of the early Earth and the current deep magmas in the Earth’s interior.

DESCRIPTION

Knowledge of the viscosity and density of silicate melts is important in a wide range of geoscientific research, including understanding the nature and dynamics of the magmas inside the Earth and other planets. In particular, most of the Earth was molten in the early stage of the formation of the Earth (magma ocean), and therefore understanding the nature and dynamics of such a magma ocean is important to discuss the formation and evolution of the Earth, as well as other planets.

Silicate melts have a polymerized structure. The degree of polymerization varies mainly by changing the amount of silicon dioxide (SiO₂) content, and as a result, the physical properties such as viscosity and density significantly change with varying degree of polymerization. There are previous studies for the viscosity of SiO₂-rich polymerized silicate melts, such as those found in current surface volcanoes. On the other hand, silicate melts in the magma ocean of the early Earth and magmas in the current Earth’s deep interiors are considered to be SiO₂-poor depolymerized melts. However, viscosity and density of such SiO₂-poor depolymerized silicate melts remain poorly understood, due to experimental difficulties such as high melting temperature, high reactivity, and/or low viscosity. There are some high-pressure viscosity measurements for depolymerized silicate melts that use the falling sphere viscometry technique in synchrotron X-ray facilities. Yet, these experiments are limited only at just above the melting temperature, because the probing sphere immediately falls upon melting. Therefore, it is difficult to investigate the temperature dependence of viscosity and density of depolymerized silicate melts by such high-pressure experiments. In order to discuss the nature and behavior of magmas at very high temperature conditions (e.g. the magma ocean of the early Earth), it is critical to understand the temperature dependence of the viscosity and density of depolymerized silicate melts far above the melting temperature.

The purpose of the Measurement of Temperature Dependence of Viscosity and Density of Depolymerized Silicate Melts (ELF-Silicate Melt) investigation is to determine the temperature dependence of viscosity and density of depolymerized silicate melts with different degrees of SiO₂ content, and different Magnesium/Iron (Mg/Fe) ratios at high temperatures to supercooled temperature range. The electrostatic levitation furnace onboard the International Space Station can melt high-melting-point olivine (a major component of the Earth's and planetary mantle), which is difficult to melt with a general electric furnace, and it enables the ability to continuously observe temperature-dependent data. In addition, this containerless method solves the problem of high reactivity of iron-rich silicate melts. Furthermore, silicate melts are highly volatile, and evaporation of sample is a problem in high-vacuum electrostatic levitation furnaces on Earth. Some testing has been conducted using a pressurized electrostatic levitation furnace on the ground, but the complete levitation experiment in the molten state has not been successful. Therefore, the experiment in the microgravity environment of space is required.

The results of the temperature dependence of the viscosity and density of the silicate melts obtained in the space experiment, combined with the results under high pressure conditions by the experiment on the ground, provides important information to better understand the dynamics of the magmas inside the Earth. In addition, the temperature dependence of viscosity and density of silicate melts may also provide important information on discussing other geoscientific phenomena, such as the slip behavior of seismic faults with a silicate melt layer generated by frictional melting of rocks. Furthermore, understanding the changes in viscosity and density of silicate melts from high temperatures to the supercooled state, and their vitrification/crystallization processes are considered to be important not only in view of Earth science, but also in physics and materials science.

SPACE APPLICATIONS

The results of the temperature dependence of the viscosity and density of the depolymerized silicate melts contribute to better understanding the behavior of molten rock compositions under high temperature conditions in the processes of the solar system formation and planetary evolution.

EARTH APPLICATIONS

Information about the viscosity and density of silicate melts could improve understanding of a wide range of geoscientific phenomena, such as the nature and dynamics of the magma ocean of early Earth, current volcanic processes on Earth, and frictional melting in seismic faults. These processes also are important to physics and materials science.

OPERATIONAL REQUIREMENTS AND PROTOCOLS

The ELF instrument is assembled and installed in the Multi-Purpose Small Payload Rack (MSPR/MSPR2) in Kibo. After setup, the investigation is operated from ground control as required by investigators at Space Station Integration and Promotion Center (SSIPC), Tsukuba Space Center. The investigation procedure is as follows:

• A crew member prepares the investigation by inserting the Sample Holder into the Sample Cartridge and inserting the Sample Cartridge into the ELF chamber. The ELF is then activated and configured for operation.
• To begin investigation operations, the sample is released into Experiment Volume by the Sample Release Rod. The Sample is charged, position controlled, heated, and melted using electrodes and Power Lasers. During operations, the sample is measured through sensors and cameras.
• At the completion of investigation operations, recorded video, pictures, and data are downlinked to Earth.
• The investigation is closed out by the deactivation of ELF.

KONO Yoshio [Ehime University]