Determining the melting curves of NiSi using the laser-heated diamond anvil cell and the multi-anvil press
Abstract
It is believed that the cores of terrestrial planets consist primarily of an iron-nickel alloy with a small fraction of light elements1. In the case of the Earth, the possible candidates for the light elements are constrained by cosmochemical arguments2. However, although the exact nature of the light element is in dispute, it is widely believed that Si is a significant light element in the core3. Research into the iron-nickel-silicon ternary system is therefore invaluable for our understanding of the composition of the Earth's core and of planetary cores in general. We have initially focused on the FeSi and NiSi end-members as a first step in understanding the ternary system. Recent work on NiSi4,5 has revealed a more complicated phase diagram than that of FeSi, with a range of stable phases found at high pressure and temperature. In order to constrain the liquidus of NiSi, we have carried out experiments in the laser-heated diamond anvil cell (LHDAC) using perturbations in the power versus temperature function as the melting criterion6. Thus far we have determined the melting curve of the room-pressure MnP structured phase to ~20 GPa, which agrees closely with an in situ multi-anvil press experiment in which the melting criteria were 1) the appearance of diffuse scattering during X-ray diffraction and 2) the appearance of convection during X-ray videography. We have also detected the break-in-slope of the melting curve associated with the MnP+ɛ-FeSi+liquid triple point, and extended the melting curve of the ɛ-FeSi structure of NiSi to 50 GPa. We are currently undertaking further experimental work to extend the melting curve above 100 GPa, beyond the pressure at which the CsCl structure becomes the liquid phase. Previous studies indicate that the CsCl structure is likely stable to inner core conditions4,5 making the results of relevance to planetary cores including that of the Earth. (1) Birch, F. Journal of Geophysical Research 1952, 57, 227. (2) Poirier, J. P. Physics of the Earth and Planetary Interiors 1994, 85, 319. (3) Gessmann, C. K.; Wood, B. J.; Rubie, D. C.; Kilburn, M. R. Earth and Planetary Science Letters 2001, 184, 367. (4) Lord, O. T.; Voccadlo, L.; Wood, I. G.; Dobson, D. P.; Clark, S. M.; Walter, M. J. J. Appl. Cryst. 2012, 45, 726 (5) Vočadlo, L.; Wood, I. G.; Dobson, D. J Appl Cryst. 2012, 45, 186. (6) Lord, O. T., Walter, M. J., Dobson, D. P., Armstrong, L., Clark, S. M., Kleppe, A. J. Geophys. Res., 2010, 115, B06208.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFMMR11B2487W
- Keywords:
-
- 3924 MINERAL PHYSICS / High-pressure behavior;
- 3954 MINERAL PHYSICS / X-ray;
- neutron;
- and electron spectroscopy and diffraction