HR: 1340h
AN: MR43A-0869    [Abstracts]
TI: The Fe-Ni-(S) System at 23 GPa: The Possibility of Strong Chemical Fractionation Between Phases in the Cores of the Earth, Mars and Mercury
AU: * Stewart, A J
EM: andrew.stewart@erdw.ethz.ch
AF: Institute for Mineralogy and Petrograph, ETH Zurich, Sonneggstrasse 5, Zurich, ZH 8092 Switzerland
AU: Schmidt, M W
EM: max.schmidt@erdw.ethz.ch
AF: Institute for Mineralogy and Petrograph, ETH Zurich, Sonneggstrasse 5, Zurich, ZH 8092 Switzerland
AB: The presence of nickel in the Earths core is widely accepted based on cosmochemical and seismological arguments. However, experimental studies into core compositions rarely include nickel, thus adding a degree of simplicity to otherwise complex experiments. Diamond-anvil cell studies have discovered that Fe-Ni alloys appear to separate into two phases upon heating above 10 GPa: from a single hexagonally close-packed (hcp) phase to the presence of both hcp and face centered cubic (fcc) phases (Lin {\it et al.}, 2002). Unfortunately, due to the small size of diamond-anvil cell samples, meaningful quantitative analysis is commonly impossible. We have conducted multi-anvil experiments at 23 GPa into the Fe-Ni system and have confirmed the presence of two phases in the sub-solidus system. The starting material for these experiments contains 6 wt% nickel, approximating the amount expected to be found in the Earths core (McDonough, 2003). In experiments to $1500\deg$C (the highest temperature thus far examined), electron microprobe analyses show dramatic phase fractionation with charges separating into an iron-rich phase containing less than 1 wt% Ni and a nickel-rich phase containing as much as 98 wt% Ni. We have observed the effect over a range of more than $500\deg$§C; further experiments are underway to determine whether these phases both persist toward the melting point of the alloy. Multi-anvil experiments at 23 GPa have also been conducted to examine the effect of nickel on the Fe-S system. Sulphur is an element favoured by many researchers as the light element component in the core of the Earth as well as that of Mars. Previous research has suggested that the addition of nickel to the Fe-S system results in the lowering of eutectic temperatures by about $75\deg$C (Pike {\it et al.}, 1999). The starting material for these experiments is the same as that used for the pure Fe-Ni experiments discussed above, with the addition of sulphur. Our results indicate a pseudo-binary, (Fe, Ni)-S, eutectic point lying slightly below $1200\deg$C, roughly consistent with the results of Pike {\it et al.} (1999). The measured eutectic liquid composition contains 4.4 wt% Ni and 15.8 wt% S. This liquid composition fits closely to the ideal composition of a (Fe, Ni)$_{3}$S compound (16.0 wt% S with 4.4 wt% Ni in the alloy), suggesting the possible importance of this structure in Fe-Ni-S melts. At subsolidus temperatures in the Fe-Ni-S system, our results become very interesting with each charge showing at least 3 coexisting phases. Based on these results, solid cores of Mercury and Mars containing iron, nickel and sulphur will hold at least 3 phases. Extrapolating our results to the inner core of the Earth would suggest that multiple phases occur in our planet as well.
DE: 3630 Experimental mineralogy and petrology
DE: 3672 Planetary mineralogy and petrology (5410)
DE: 1060 Planetary geochemistry (5405, 5410, 5704, 5709, 6005, 6008)
DE: 1507 Core processes (8115)
DE: 1015 Composition of the core
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting