HR: 1340h
AN: MR13A-0083    [Abstracts]
TI: The Effect of Ni on the Fe-S system to 36 GPa
AU: * Stewart, A J
EM: andrew.stewart@erdw.ethz.ch
AF: Institute for Mineralogy and Petrology, ETH Zurich, Sonneggstrasse 5, Zurich, ZH 8092 Switzerland
AU: Schmidt, M W
EM: max.schmidt@erdw.ethz.ch
AF: Institute for Mineralogy and Petrology, ETH Zurich, Sonneggstrasse 5, Zurich, ZH 8092 Switzerland
AU: van Westrenen, W
EM: wim.van.westrenen@falw.vu.nl
AF: Faculty of Earth and Life Sciences, Vrije Universiteit, De Boelelaan 1085, Amsterdam, 1081 Netherlands
AU: Liebske, C
EM: christian.liebske@erdw.ethz.ch
AF: Institute for Mineralogy and Petrology, ETH Zurich, Sonneggstrasse 5, Zurich, ZH 8092 Switzerland
AB: The metallic cores of the terrestrial planets are dominated by an iron-nickel-light element alloy. Sulfur is a major candidate for the dominant light element component, as such its phase relations with Fe-Ni metals are of paramount importance to understanding the nature of the cores of differentiated planets. Much previous work has focused on examining the phase diagram of the Fe-S binary, however only a few studies have yet included nickel as a major element. Here we present equilibrium multi-anvil research into metallic core phases to pressures equivalent to the center of Mars. In this work a series of experiments have been performed in both the Fe-S and Fe-Ni-S systems at 23 and 36 GPa and temperatures between 850 and 1600°C. The aim of these experiments was to determine the effect of nickel on the eutectic temperature and composition of the Fe-S system and to establish the Fe-S binary phase relations at much increased pressures. Four starting materials were prepared (in wt%): Fe₆₀Ni₃₄S₆; Fe₇₇Ni₁₇S₆; Fe₈₆Ni₈S₆ (roughly corresponding to the Earth's core Fe:Ni ratio) and Fe₉₄S₆ from pure iron, pure sulfur and a Fe₆₄Ni₃₆ pre-alloy. Multi-anvil experiments were run at ETH Zurich using a 10/3.5 assembly on WC cubes and a 7/2 assemblage on sintered diamond cubes in a true split sphere apparatus. Both assemblies are symmetrical with respect to the furnace' mid-plane and use a radial thermocouple, allowing for two experimental charges within one run. This setup greatly reduces the number of experiments to be performed. Results indicate that nickel has a strong effect on the eutectic temperature, but only a minor effect on the composition of eutectic liquids. At 23 GPa, increasing nickel quantities, to maximum 34 wt%, increase the amount of divergence of the eutectic point from the pure Fe-S system. We find the eutectic point for the Fe-S system at 23 GPa to occur at 14.5 wt% sulfur and a temperature of 1025°C while the addition of up to 34 wt% Ni progressively shifts the eutectic to 925°C. Eutectic liquid compositions show less consistent variability with increased Ni content and have 15-16 wt% S. Subsolidus phases at 23 GPa in both Ni-free and Ni-bearing charges are consistent with those described by Fei et al (2000, American Mineralogist), with (Fe,Ni) metal, (Fe,Ni)₃S and (Fe,Ni)₂S commonly present. However at 38 GPa, (Fe,Ni)₃S has not been observed, with pure (Fe,Ni) coexisting directly with (Fe,Ni)₂S, suggesting the limited stability and importance of the Fe₃S phase in planetary cores. Partitioning of nickel between the subsolidus phases shows a moderate preference for the sulfide phase over the iron-dominated metallic phase. The dP/dT slope of the eutectic in the Fe-S system is 18°C/GPa between 15 and 23 GPa, and seems to only slightly increase to 36 GPa, similar to the dP/dT slope of the Ni-bearing eutectic. Our results have important implications for planetary core chemistry, mineralogy and rheology with realistic nickel-bearing alloys showing a distinct decrease in the eutectic temperature of the system and with inner core phases having nickel partitioned into high pressure sulfides. Nickel cannot be ignored or assumed as an element equivalent to iron in this and other light alloy systems at high pressures.
DE: 1012 Reactions and phase equilibria (3612, 8412)
DE: 1015 Composition of the core
DE: 1027 Composition of the planets
DE: 1042 Mineral and crystal chemistry (3620)
DE: 3630 Experimental mineralogy and petrology
SC: Mineral and Rock Physics [MR]
MN: Fall Meeting 2005