AbstractsBiology & Animal Science

Siderophile Element Partitioning at high Pressures and Temperatures : Implications for Core Formation Processes

by Antje Kathrin Vogel




Institution: Universität Bayreuth
Department: Biologie, Chemie und Geowissenschaften
Degree: PhD
Year: 2015
Record ID: 1105565
Full text PDF: https://epub.uni-bayreuth.de/2039/


Abstract

Abstract Siderophile (Fe-loving) elements are depleted in the Earth´s mantle because they have been extracted into the Earth´s core by core formation processes. Metal – silicate partitioning of siderophile elements can provide major constraints on the conditions that prevailed during core formation of the Earth because partitioning is dependent on pressure, temperature, oxygen fugacity and the silicate and metal compositions. The liquid metal – liquid silicate partitioning behaviour of the non-volatile elements Ni, Co, Mo and W and the volatile elements Cu, Sn, Sb, Ge, Pb, Ag, Au, P and As has been studied at pressures between 11 and 23 GPa and temperatures of 2342 K to 2911 K by performing partitioning experiments in a multi-anvil apparatus. This work has been primarily focused on volatile elements, because previous studies and therefore core formation models lack data for this group of elements. The silicate starting material of the experiments had a peridotitic composition and the starting Fe-rich metal has been varied by adding S in the case of non-volatile elements and Si or S when investigating volatile element partitioning. This enables the influence of these light elements, that potentially contribute to the Earth´s core density deficit, on the partitioning of siderophile elements to be quantified. The metal, ferropericlase and silicate phases of 104 samples have subsequently been analysed with the electron probe micro-analyser for the first two phases and the laser ablation inductively coupled plasma mass spectrometer in the case of silicate. Subsequently partition and exchange coefficients have been calculated on a molar basis. The partitioning was analysed by employing the ε-approach as formulated by Ma et al. (2001). It was found that over the experimentally investigated pressure and temperature range the partitioning of all elements studied does not change significantly. The addition of Si to the starting metal powder resulted in decreased siderophility for all volatile elements. Since the addition of Si is accompanied by a decrease in oxygen fugacity this implies that for each volatile element studied the interaction of Si with these elements in the metal counteracts the effect of low fO2 which normally results in increased siderophile behaviour. This shows that reducing conditions in the early stages of core formation do not necessarily result in complete or even strong depletion of siderophile elements when Si is present as a light element in the metal phase. The quantification of the effect of Si on the metal – silicate partitioning of volatile elements furthermore facilitates the identification of the valence states of these elements in the silicate phase. It was found that Cu and Ag are present as 1+ cations, Au and Pb are divalent, Sn and Ge have a valence of 3+ and Sb and As are tetravalent and pentavalent respectively. The addition of S to the metal phase of the experiments resulted in increased siderophility for the elements Ag, Cu, Pb and Ni and vise versa for all other elements studied.…