Seismological and cosmochemical observations indicate that the main constituent in Earth's core is iron, which is thought to be alloyed with ~10 wt% nickel and some light elements (e.g. S, Si, O, C, H). Earth's core consists of a solid inner region surrounded by a liquid outer core. The melting temperature of iron at high-pressure provides an important reference point for the temperature distribution within Earth's core and affects a number of important geophysical quantities related to this region: heat flow across the core-mantle boundary (CMB), the temperature gradient within the thermal boundary layer above the CMB, the phase relations, and expected seismic wave speeds of candidate phases. More specifically, the melting point of iron at the boundary between the liquid outer core and solid inner core provides an upper bound of the temperature at that interface, because studies thus far indicate that all plausible outer core liquids coexist with corresponding inner core alloy solids at or below the melting point of pure iron. Previously, melting studies of iron at high-pressures were performed by shock-compression, resistive- and laser-heating in diamond anvil cells using visual observations or synchrotron x-ray diffraction, and theoretical methods. However, the melting curve of iron with respect to its alloys remains uncertain, especially at pressures above 70 GPa.
The PI has developed a novel metric for detecting the solid-liquid phase boundary of iron-bearing materials at high-pressures using 57Fe synchrotron Mössbauer spectroscopy (SMS), also known as nuclear forward scattering. Focused synchrotron radiation with 1 meV bandwidth passes through a laser-heated 57Fe-bearing sample inside a diamond anvil cell. The characteristic SMS time signature vanishes when melting occurs. This process is described by the Lamb-Mössbauer factor, a quantity that is directly related to the mean-square displacement of the iron atoms. Therefore, we measure the dynamics of the atoms in the material, in contrast to a static diffraction measurement. As this method monitors the dynamics of the atoms, the SMS technique provides a new and independent means of melting point determination for materials under high-pressure. We have successfully performed such melting investigations on pure iron up to 82 GPa. Our data define a different melting trend compared with previous studies, thus providing important rationale to proceed further with this method on iron and iron-alloys at higher pressures. The melting experiments will be conducted to pressures of at least 80 GPa for the alloys and higher than 80 GPa for pure iron, at sector 3-ID-B of the Advanced Photon Source at Argonne National Laboratory. By constraining the relative melting curves of these iron-alloys at high-PT conditions, we will provide a new constraint on the effect of select light-element alloying to iron?s melting behavior. These outcomes will represent a significant step towards understanding the melting of iron-rich alloys at conditions of terrestrial-type planetary cores and thus provide necessary temperature constraints of their respective interiors.
