Photon Factory Activity Report 2014 #32 (2015) BPF-AR-NE7A/2013G609

High Pressure and High Temperature In-situ X-ray Radiography of the Segregation ofMolten Iron from Partially Molten Silicate
Hirotada Gotou1,*, Takehiko Yagi2, Riko Iizuka3, Akio Suzuki4
1Institute for Solid State Physics, The University of Tokyo, Kashiwanoha, Kashiwa, 277-8581, Japan
2
Geochemical Research Center, Graduate School of Science, The University of Tokyo, Hongo,Bunkyo-ku, 113-0033, Japan
3
Geodynamics Research Center, Ehime University, Bunkyo-cho, Matsuyama, 790-8577, Japan
4
Department of Earth and Planetary Materials Science, Tohoku University, Aoba-ku, Sendai 9808578, Japan
1 Introduction
X-ray radiography combined with synchrotron
radiation provides unique information for studying thebehavior of samples at high pressure and hightemperature. In geophysical sciences, this technique has
been applied, for example, to determine the viscosity ofmolten silicate [1-3], the interfacial tension of molten ironalloy using the sessile drop method [4, 5], and the totalstrain of specimens under deformation experiments [6-8].
The Earth’s core is believed to have been formed by thesegregation of molten iron from the silicate mantle of theundifferentiated proto-Earth. Although much work has
been conducted regarding the segregation process, theresults were based on indirect evidence, such as texturalobservations of recovered samples [9,10] by X-ray CT
observations [11, 12], or by in-situ electrical resistancemeasurements [13]. Accordingly, there is a lack of studies
based on in situ dynamic observations of the separation
process at high pressure and high temperature. Therefore,
we have attempted to conduct such observations for theseparation of molten iron from silicate using X-ray
radiography. Based on these observations, wedemonstrate that X-ray radiography is an extremely
useful tool for studying such phenomena.
2 Experiment
Two types of starting mixtures were prepared to obtainhydrogen-containing and sulfur-containing samples. The
hydrogen-containing sample was a mixture of Fe, SiO2,and Mg(OD)2 powder at a molar ratio of 2:1:1. The
sulfur-containing sample was a mixture of Fe, FeS, SiO2,and MgO powder at a molar ratio of 0.66:1.17:1:1.
High-pressure and high-temperature in situ X-ray
radiography experiments were performed usingsynchrotron radiation at the AR-NE7 beamline of the PFin KEK. The setup of the X-ray radiography system issummarized in Ref. 14. The radiography images wereobtained using an exposure time of 0.5 s/frame, and wererecorded to hard disk every 1.5 s. Monochromatic X-raysof 35 keV produced with a double crystal spectrometer(Si(111)) were used to obtain the best absorption image
contrast from the sample.
High pressure was generated with a double-stage multianvil system using the MAX-III 700 ton press of AR-NE7.

Fig. 1 X-ray radiography images of hydrogencontaining sample observed at 5 GPa with increasing
temperature.

Fig. 2 X-ray radiography images of sulfur-containing
sample observed at 5 GPa with increasing
temperature.
Tungsten carbide cubes with a truncated edge length of 12mm were used as second-stage anvils. The sample was
first pressurized to 5 GPa at room temperature and thenheated to 1827 K (for the hydrogen-containing samples)or 1973 K (for the sulfur-containing samples) at a rate of
approximately 100 K/min while maintaining the load.
3 Results and Discussion
A series of screenshots of the X-ray radiography
images, obtained during heating at 5 GPa from a video ofthe system, are shown in Figs. 1 and 2. As thetemperature of the hydrogen-containing sample increased,cohesion of the molten iron occurred at first and small
iron droplets were formed. These iron droplets then sankto the bottom of the sample chamber, where they
eventually merged into a single, large molten iron droplet.
However, the small iron droplets did not move only
vertically down during heating; movement also occurredin random directions. This behavior was markedly
different from that observed in the sulfur-containing
sample, where the molten iron flowed slowly to the wallof the sample chamber as the temperature of the sample
gradually increased. It is thought that such motion mayPhoton Factory Activity Report 2014 #32 (2015) B

have been driven by a temperature or pressure gradient.
These observations demonstrate that the behavior ofmolten iron changes according to the dissolved elements
present.
As demonstrated in the present study, a great deal ofinformation regarding the process of molten iron
segregation from partially molten silicate during heating
can be obtained using in situ X-ray radiography. Thislevel of information cannot be obtained using quench
experiments alone. The combination of such in situ X-ray
radiography with tomography observations of thequenched sample will provide rich information about theentire segregation process. This study has also
demonstrated that the behavior of molten iron changes
dramatically depending on which light element isincorporated into the system. The in situ observation
method described here, particularly when combined withrecovered experiments or tomography techniques using
quench samples, will act as a very useful and powerfultool that will aid future studies of the core formation
mechanism of the Earth and other planets.
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* hgotou@issp.u-tokyo.ac.jp