Melting temperature of iron at megabar pressures: a new spectroscopic approach to estimate the Earth inner core boundary temperature

14-09-2015

Temperature, thermal history and dynamics of the Earth rely critically on the knowledge of the melting temperature of iron at the pressure conditions of the inner core boundary (ICB). The literature on this subject is overwhelming, and no consensus has been reached with a disagreement of the order of 2000 K at the ICB. An international team of scientists working at the ESRF now report new data on the melting temperature of iron in a laser heated diamond anvil cell (LH-DAC) with pressures up to 103 GPa obtained by X-ray absorption spectroscopy (XAS), a technique rarely employed at such conditions. The modifications of the onset of the absorption spectra are used as a reliable melting criterion regardless of the solid phase from which the solid to liquid transition takes place. These results show a melting temperature of iron of 3090 K at 103 GPa, in agreement with most previous studies up to 100 GPa.

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The interiors of Earth and other planets cannot be observed directly. No borehole has ever pierced Earth’s thin crust due to the high temperatures and pressures existing at depth. Yet we still know a great deal about Earth’s interior. Volcanoes bring material to the surface from depths of about 100 km. The rest of our information about the Earth's interior came from the study of earthquake shock waves, complemented by cosmochemistry and mineral physics. By studying how acoustic waves pass through Earth (travel times) a model for the internal structure of the planet has been worked out. Today, we know that the core is essentially made of Fe and that the inner core is solid while the outer core is liquid. Density profiles are known very precisely, but not the temperature profile (the geotherm). There is an uncertainty of about 2000 K for the value of the temperature in the centre of the Earth. The inner core boundary, ICB, is where solid and liquid Fe coexist (in a phase diagram, this is where the geotherm crosses the melting curve). Knowledge of the melting temperature of Fe at the border between the inner and outer core would give an important constraint to the geotherm, and is the Holy Grail of geophysicists.

Efforts to measure this temperature precisely date back to over 20 years, since Boehler gave a first estimate of the melting temperature of Fe at 200 GPa [1] suggesting a melting temperature of iron below 5000 K when extrapolated to ICB conditions.  Since then, there has been a long-standing controversy over the melting curve of Fe as determined from static laser heated diamond anvil cell (LH-DAC) and dynamic compression studies.  For example, recent XRD work [2] contradicts the first Boehler curve, pushing the ICB temperature up to above 6200 K.

In order to help understand the origin of this discrepancy, we proposed to use an alternative method, X-ray absorption spectroscopy (XAS), which had never been attempted before. XAS is sensitive to changes in the electronic structure through the features close to the absorption edge that probe the density of empty states close to the Fermi level. XAS also provides structural information within a few angstroms around the photoabsorbing atom, and therefore maintains the same sensitivity and accuracy regardless of the physical state of the investigated sample (crystalline, amorphous, or liquid). This is a considerable asset with respect to diffraction techniques in which the onset of melting appears as a weak diffuse halo superimposed on strong Bragg reflections from partially molten sample and sample environment.

Here we have used XAS as a criterion to detect melting under pressure.  Thanks to a major upgrade of beamline ID24, the first in situ laser heating facility for a diamond anvil cell (DAC) compatible with X-ray absorption spectroscopy has been developed. Figure 1 shows the changes that occur at the Fe K-edge as temperature is increased above the melting point at 68 GPa. An abrupt smoothing of the edge, at E ~ 7117 eV, is seen upon melting - evident in the derivative of the data shown in the inset.

XANES spectra of iron recorded at 68 GPa

Figure 1. XANES spectra of iron recorded at 68 GPa with increasing temperature through the γ-fcc to liquid phase transition. In the inset the derivatives of the XANES spectra are reported.

The spectral region near the K-edge of iron (XANES) involves, in the dipole approximation, the transition of an electron from the 1s to the empty 4p states. The observed smoother absorption at the onset of the liquid phase can therefore be attributed to a broader distribution of empty 4p states as a consequence of the disruption of crystalline order.

The present melting data are in agreement with most previous diamond anvil cells measurements, resulting in a flat melting curve around 100 GPa (Figure 2). However, it is in stark contrast with a recent X-ray diffraction study [2]. The latter reports a much steeper melting curve in that pressure range. Although an accurate extrapolation of the present data to Earth’s core conditions is difficult given the limited number of experimental data above the triple point, our results are in agreement with previous data to 200 GPa [1]. The latter suggests a melting temperature of iron below 5,000 K when extrapolated to ICB conditions.

This difference has significant implications for estimating the temperature in the Earth’s interior, and is key for calculating the core-mantle heat flow, inner core age, dynamo models, and cooling history of the Earth. For this reason, a collective effort is being made at the ESRF to understand the origin of the discrepancies from different experiments, this work and refs [2] and [3], as shown in Figure 2.

P-T conditions at which XANES spectra were collected.

Figure 2. P-T conditions at which XANES spectra were collected. Blue dots correspond to ε-hcp Fe. Green triangles correspond to γ-fcc Fe. Red squares correspond to liquid Fe. Phase boundaries for iron from other experimental studies are also shown (solid black line: Ref. [1]; dashed black line: Ref. [3]; dotted line: Ref. [2]; stars: Ref. [4]).

The combination of X-ray absorption spectroscopy and elevated pressure and temperature conditions  offers great potential to probe Earth's deep interior. The experiment reported here holds promise in particular for studies related to the structure and phase diagram of compressed melts, the investigation of structural complexity (polyamorphism) in the liquid phase or the extent of icosahedral ordering whose investigation has been limited until now to ambient conditions.

 

Principal publication and authors
Melting of iron determined by X-ray absorption spectroscopy to 100 GPa, G. Aquilanti (a), A. Trapananti (b), A. Karandikar (c,d), I. Kantor (e), C. Marini (e), O. Mathon (e), S. Pascarelli (e), R. Boehler (c), PNAS (2015); doi: 10.1073/pnas.1502363112.
(a) Elettra–Sincrotrone Trieste S.C.p.A., Trieste (Italy)
(b) CNR–Istituto Officina dei Materiali, OGG Grenoble c/o ESRF, Grenoble (France)
(c) Geophysical Laboratory, Carnegie Institution of Washington, Washington DC (USA)
(d) Geowissenschaften, Goethe-Universität, Frankfurt a.M. (Germany)
(e) ESRF

 

References
[1] R. Boehler, Nature 363, 534–536 (1993).
[2] S. Anzellini, A. Dewaele, M. Mezouar, P. Loubeyre, G. Morard, Science 340, 464–466 (2013).
[3] R. Boehler, D. Santamaría-Pérez, D. Errandonea, M. Mezouar, J Phys Conf Ser 121, 022018 (2008).
[4] J.M. Jackson, W. Sturhahn, M. Lerche, J. Zhao, Earth Planet Sci Lett 362,143–150 (2013).

 

Top image: I. Kantor setting up a laser-heating experiment at beamline ID24.