Sound velocity and density of liquid Fe-Ni-Si under pressure: Application to the composition of planetary molten core
Abstract:The cores of Mercury, Mars and Moon are reported to be partially/totally molten (e.g., Margot et al. 2007, Yoder et al. 2003, Williams et al. 2001). In order to constrain the core compositions of those bodies from observed and future-planned seismic data, sound velocity and density of the core material, i.e., liquid Fe-alloy, are necessary. In this study, we have performed simultaneous measurements on these physical properties of liquid Fe-Ni–Si alloys, which is one of the major candidates for the core constituent. The effects of pressure and Si content on these properties were studied.
High pressure experiments were performed using 80-ton uniaxial press designed for CT measurement or 180-ton cubic type multi-anvil press installed at BL20XU and BL22XU beamlines of SPring-8 synchrotron facility, respectively. Used samples were Fe-Ni-Si with Si content of 10-30 at%. The sample pellet was sandwiched by the single crystal sapphire buffer rod for sound velocity measurement. P-wave sound velocity was measured using pulse-echo overlapping ultrasonic method. LiNbO3 transducer was attached to the backside of the anvil to generate and receive elastic wave signals. Density was determined based on 3D volume data obtained from CT measurement or X-ray absorption profile.
The P-wave velocity (VP) and density of liquid Fe-Ni-Si were successfully measured up to 2.5 GPa and 1773 K. Obtained VP of the Fe-Ni-Si is found to increase rapidly with pressure below 1 GPa and increase gradually above 1 GPa. It is also found that VP increases slightly with Si content on the density-VP plot. These trends provide a constraint on the core composition of the planets and moon by comparing with observed data.