The Fe-Si and Fe-S Binary Systems at 15 Gpa: Important Relations for the Light Element Constituent of the Earth's Core.

The identity of the light element, or combination of light elements, within the EarthAŸA›A›ƒ_sAªA›ƒ_zA›s core remains elusive. Two commonly proposed alloying elements which may explain the density of the core are silicon and sulphur. Multi-anvil experiments at 15 GPa have been conducted to examine the iron-rich portions of the Fe-Si and Fe-S phase diagrams. The Fe-Si system has proven difficult to study experimentally due to silicon reactivity with oxygen as well as commonly used sample capsule materials. We have tested different capsule materials and have found the use of fused silica capsules, transforming to stishovite at experimental conditions, to be the most effective of those tried. The oxygen within these charges will be buffered against the highly reducing redox reaction Si + O₂ = SiO₂. Initial results have verified the presence of two coexisting subsolidus iron-silicon phases between 6 and 8 wt% silicon to 1300° C as discussed by Lin et al. (Science, 2002). Experiments above the solidus have also been conducted and are continuing, however, they have yet to yield reliable data on the liquidus loop in the iron-rich portion of the Fe-Si system. The Fe-S system has been more readily studied using MgO capsules. Measurements of iron wAŸ’'A,A¬stite content in the MgO capsule contacting the experimental charge specify oxygen fugacities below IW-2. Results indicate the presence of a eutectic at about 900° C and 21.5wt% sulphur at 15 GPa. At eutectic temperatures, over 1wt% sulphur is found dissolved in the solid iron-rich phase. Experiments below the eutectic temperature confirm the existence of the high-pressure Fe₃S₂ phase described by Fei et al. (Science, 1997). Experiments at higher pressures and into the Fe-S-Si ternary system are underway.