Solving Controversies on the Iron Phase Diagram Under High Pressure

Abstract

As the main constituent of planetary cores, pure iron phase diagram under high pressure and temperature is of fundamental importance in geophysics and planetary science. However, previously reported iron-melting curves show large discrepancies (up to 1000 K at the Earth's core-mantle boundary, 136 GPa), resulting in persisting high uncertainties on the solid-liquid phase boundary. Here we unambiguously show that the observed differences commonly attributed to the nature of the used melting diagnostic are due to a carbon contamination of the sample as well as pressure overestimation at high temperature. The high melting temperature of pure iron under core-mantle boundary (4250 ± 250 K), here determined by X-ray absorption experiments at the Fe K-edge, indicates that volatile light elements such as sulfur, carbon, or hydrogen are required to lower the crystallization temperature of the Earth's liquid outer core in order to prevent extended melting of the surrounding silicate mantle.Plain Language Summary Iron is the main constituent of planetary cores; however, there are still large controversies regarding its melting temperature and phase diagram under planetary interior conditions. The present study reconciles different experimental approaches using laser-heated diamond anvil cell with different in situ X-ray diagnostics (absorption, diffraction, and Mossbauer spectroscopy). The main reason of discrepancies (over 1000 K at core-mantle boundary conditions) is attributed to carbon contamination from the diamond anvils and metrology issues related to thermal pressure overestimation. A high-melting temperature for iron at core-mantle boundary pressure would imply the presence of volatile elements in the liquid outer core, such as sulfur, carbon, or hydrogen, in order to lower its crystallization temperature and avoid extended melting of the surrounding silicate mantle.

A New Reference for the Thermal Equation of State of Iron

“…The diffraction patterns recorded between 30 GPa and 140 GPa at temperatures varying from ambient temperature to 3400 K, display how the face centered cubic (fcc) structure is stable at low pressure and high temperature and further transforms into the hexagonal closed packed (hcp) structure above 60 GPa. The recorded transition is consistent with most recent experimental results [9]. The cell parameters for the hexagonal closed packed structure have a linear and monotonous evolution with temperature, with no kink or inversion in the axial compressibility of c and a axis as a function of pressure.…”

“…Since the first pioneering work [1], a wealth of data has been produced on iron structure at extreme pressure and temperature, elastic properties or transport properties [2][3][4][5][6], so as to yield a precise model of the structure, composition and dynamics of the Earth's core. A lot of studies focus on the determination of a phase diagram for iron at relevant pressure and temperature conditions [4,[6][7][8][9]. Light elements have also been shown to be incorporated in the core (see [10] for a review), in order to match density and sound waves velocities obtained by seismological modelling.…”

Comparative study of the influence of pulsed and continuous wave laser heating on the mobilization of carbon and its chemical reaction with iron in a diamond anvil cell

“…carbon, which arises from the diamond anvils, resulting in the synthesis of iron carbide Fe 3 C. 11 As seen in Fig. 2, vanishing of the shoulder directly correlates with the distance from the center of the heated spot.…”

“…To take into account the additional component, a paramagnetic doublet was added to achieve the best fit of the experimental data. The CS of the doublet ranges from −0.18 (11) mm/s to −0.12(44) mm/s and quadrupole splitting (QS) from 0.64(4) mm/s to 0.77(6) mm/s. The hyperfine parameters of the doublet are in close agreement with the hyperfine parameters of Fe 3 C at similar pressure.…”

“…This is highly unwanted in melting experiments, since the presence of carbides or Fe-C solid solutions can significantly affect the melting temperature. [9][10][11] The carbon contamination problem is not limited to iron 12 but due to the important geological implications of its melting curve, iron is a highly investigated material.…”