Melting of Fe and Fe0.9Ni0.1 alloy at high pressures

Abstract:

Cosmochemical studies suggest that the cores of terrestrial planets are primarily composed of Fe alloyed with about 5 to 10 wt% Ni, plus some light elements (e.g., McDonough and Sun 1995). Thus, the high pressure melting curve of Fe_0.9Ni_0.1 is considered to be an important reference for characterizing the cores of terrestrial planets. We have determined the melting points of fcc-structured Fe and Fe_0.9Ni_0.1 up to 86 GPa using an in-situ method that monitors the atomic dynamics of the Fe atoms in the sample, synchrotron Mössbauer spectroscopy (Jackson et al. 2013). A laser heated diamond anvil cell is used to provide the high pressure-high temperature environmental conditions, and in-situ X-ray diffraction is used to constrain the pressure of the sample. To eliminate the influence of temperature fluctuations experienced by the sample on the determination of melting, we develop a Fast Temperature Readout (FasTeR) spectrometer. The FasTeR spectrometer features a fast reading rate (>100 Hz), a high sensitivity, a large dynamic range and a well-constrained focus. By combining the melting curve of fcc-structured Fe_0.9Ni_0.1 alloy determined in our study and the fcc-hcp phase boundary from Komabayashi et al. (2012), we calculate the fcc-hcp-liquid triple point of Fe_0.9Ni_0.1. Using this triple point and the thermophysical parameters from a nuclear resonant inelastic X-ray scattering study on hcp-Fe (Murphy et al. 2011), we compute the melting curve of hcp-structured Fe_0.9Ni_0.1. We will discuss our new experimental results with implications for the cores of Venus, Earth and Mars.

Select references:

McDonough & Sun (1995): The composition of the Earth. Chem. Geol. 120, 223–253.

Jackson et al. (2013): Melting of compressed iron by monitoring atomic dynamics, EPSL, 362, 143-150.

Komabayashi et al. (2012): In situ X-ray diffraction measurements of the fcc-hcp phase transition boundary of an Fe-Ni alloy in an internally heated diamond anvil cell, PCM, 39, 329-338.

Murphy et al. (2011): Melting and thermal pressure of hcp-Fe from the phonon density of states, PEPI, 188, 114-120.