The effect of light elements on metal/silicate partitioning

Thesis

The accretion of the Earth was marked by the high-pressure segregation of most of its core, accompanied by dissolution of about 10% of one or more “light” elements into the metallic phase. Various light elements have been proposed including S, Si, C and O, with each having an effect on the partitioning behaviour of the trace elements. Metallurgical data indicate that dissolution of even small amounts of light elements in liquid Fe can have profound effects on the activities of some trace components. For instance, significant partitioning of Si into the core of the growing Earth should have affected the observed Mo content of the mantle (Ono-Nakazato et al., 2007).

Here, I use the epsilon model of non-ideal interactions in Fe liquids (ε. I present interaction parameters (ε, derived from 1.5GPa, 1550-1650oC metal-silicate equilibration experiments, for W, Ni, Co and Mo in liquid Fe alloyed with C, and W, Ni, Co, Mo, Cu, Mn, Ag, Sb, Cd, In, Tl, Pb, Ga, Ge, Cr, V and Zn in liquid Fe alloyed with S. Additional epsilon values were taken from the steelmaking sourcebook when necessary.

I used this new data in conjunction with published metal–silicate partitioning results to develop a model of continuous accretion and core segregation taking explicit account of the partitioning of Si (this study) and O (from Ozawa et al., 2008) between metal and silicate and their effects on metal–silicate partitioning of siderophile elements. The best model for explaining the Earth’s formation including the Mo:W ratio of the silicate Earth posits that the Earth’s oxidation state increased in steps from 1 to 6.26% FeO, increasing Si in the Earth’s core and the light elements C and S being added to the planet in the last ~20% of accretion material.

Authors: Mirolo, F

• Publication date: 2013
• Type of award: MSc by Research
• Level of award: Masters
• Awarding institution: University of Oxford
• Language: English
• Keywords: metal/silicate partitioning
• Subjects: Petrology; Geochemistry; Earth