Melting properties and equations of state of Fe-Ni-S and Fe-Ni-Si liquid alloys up to megabar pressure
Abstract
It is well established that the Earth's liquid outer core is less dense than a pure Fe-Ni liquid alloy. The so-called "core density deficit" is currently estimated around 5-10 wt %[1] and is attributed to the presence of light elements dissolved in an iron-rich liquid alloy. Hence, it is important to investigate the evolution of the Fe-FeX phase diagrams, with X an element of lower molar mass than iron, and the physical properties of the liquid iron alloys with respect to pressure, temperature and light element content. We studied the melting properties of several alloys, Fe-5%wtNi-15%wtSi; Fe-5%wtNi-10%wtSi; Fe-5%wtNi-12%wtS up to megabar pressures in-situ in a laser-heated diamond-anvil cell at a synchrotron x-ray diffraction beamline [2]. Scrupulous attention to the synthesis and characterization of the starting material was fundamental to accurately control the chemical compositions in the laser-heated spot. The appearance of a diffuse X-ray scattering signal at wavevectors of about 30 nm-1 was used to determine the onset of melting. Extrapolations of the such measured melting curve up to the core-mantle boundary pressure yielded values of 3,600-3,750 K for the freezing temperature of plausible outer core compositions. This implies that partial melting of the silicate mantle could have extended in the form of a basal magma ocean [3] and could reasonably still be present in some mantle regions nowadays [4]. We extracted densities and compressibility from the diffuse X-ray signal scattered by the liquid up to megabar conditions[5], using a method developed for diamond anvil cells by Eggert and collaborators[6]. These equation of state results indicate that sulfur, and not silicon, can more easily account for the differences in density and bulk modulus between pure iron and a reference Earth seismic model. This challenges traditional Earth's accretion and differentiation models, that do not foresee S as major light element in the core. These results thus rather argue for strong disequilibrium Earth formation mechanisms. REFERENCES [1] Anderson, O. L.; Isaak, D. G., Phys. Earth Plan. Int., (2002) 131, 19. [2] Morard, G.; Andrault, D.; Guignot, N.; Siebert, J.; Garbarino, G.; Antonangeli, D., Phys. Chem. Minerals, (2011) 38, 767. [3] Labrosse, S.; Hernlund, J. W.; Coltice, N., Nature, (2007) 450, 866. [4] Lay, T.; Garnero, E. J.; Williams, Q., Phys. Earth Plan. Int., (2004) 146, 441. [5] Morard, G.; Antonangeli, D.; Andrault, D.; Guignot, N.; Siebert, J.; Guyot, F.; Garbarino, G., (2012), Submitted. [6] Eggert, J. H.; Weck, G.; Loubeyre, P.; Mezouar, M., Phys. Rev. B, (2002) 65, 174105.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMMR11B2477M
- Keywords:
-
- 1027 GEOCHEMISTRY / Composition of the planets;
- 3919 MINERAL PHYSICS / Equations of state;
- 3954 MINERAL PHYSICS / X-ray;
- neutron;
- and electron spectroscopy and diffraction