SI IN THE CORE? NEW HIGH-PRESSURE AND HIGH-TEMPERATURE EXPERIMENTAL DATA

Abstract

High-pressure high-temperature experiments have been carried out up to 25 GPa and 2200°C in a multianvil press on assemblages made of silicates and iron-silicon alloys. At 20 GPa, silicon is extracted from the metal phase, forming stishovite reaction rims around metal grains. The silicon content in metal has been measured by analytical electron microscopy and electron microprobe. In contrast with earlier experiments, the present data were obtained by using silicon-rich metal alloys as starting materials instead of studying incorporation of silicon in initially silicon-free metal. As in most of previous studies carried out below 25 GPa, the silicon content in liquid metal increases with increasing pressure and with decreasing oxygen fugacity. The oxygen fugacity in most experiments was calculated by using two independent buffers: iron/ẅstite (IW) and SiO2/Si, allowing to link consistently the Fe contents in silicates, the Si contents in metal and the temperatures of the experiments. At oxygen fugacities 4 log units below IW, silicates are in equilibrium with Si-rich metallic alloys (up to 17 wt% of Si in metal at 20 GPa and 2200°C). Extrapolation to 2 log units below IW leads to less than 0.1 wt% Si in the metal phase. Presence of several wt% of silicon in the Earth’s core thus requires highly reduced initial materials that, if equilibrated at conditions relevant to small planets, should already contain significant amount of silicon dissolved in metal.