Compressional Wave Velocities in Fe-Ni-Si Melts to 1650° C: Contrasting Results Compared to Fe-Ni-S Melts

Using high-precision high temperature ultrasonic interferometric techniques we have measured V_P in three molten Fe-Ni-Si alloys containing varying amounts of Fe (75-89 wt % Fe), 5 wt % Ni, and varying amounts of Si (6-20 wt %) in the temperature range 1190-1650° C and in the frequency range 9-13 MHz. The temperature dependencies of V_P for all the compositions are linear, and no dispersion is observed in the temperature and frequency ranges of the experiment. However, increasing amounts of Si cause a decrease in V_P and an increase in the (dV_P/dT) value. The results for the Fe-Ni-Si melts are also compared with those previously obtained for Fe, Fe-Ni and Fe-Ni-S melts. Whereas in the Fe-Ni-S melts, addition of S was observed to cause anomalous elastic behavior (+dV_P/dT), the behavior in Fe-Ni-Si is normal. The results from these studies, although carried out at ambient pressure, may provide useful constraints on the composition of the Earth`s outer core.