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

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. Finally, the parameterization of the metal – silicate partitioning behaviour of all elements – in the case of non-volatile elements including literature - was incorporated into a single stage core formation model, in which oxygen fugacity was set by the present day FeO content of the Earth´s mantle and the temperature was fixed by the peridotitic liquidus at a given pressure after Liebske et al. (2012). For the non-volatile elements it was attempted to match their absolute abundances in the Earth´s mantle or if not possible the ratio between the partition coefficients of two elements. In the case of volatile elements the ratio of the respective mantle abundances of two elements with similar 50% condensation temperature was tried to explain in order to be able to neglect any assumptions about the degree of volatility these elements might have experienced. It was found that increasing pressure facilitates the achievement of the correct relative abundances of Mo/W (29 GPa), Cu/Au (12 GPa), Cu/As (5 GPa) and Sn/Pb (21 GPa). It was furthermore possible to explain the absolute abundances of Ni and Co in the Earth by enormeously high pressures of 126 and 116 GPa respectively. Incorporating the influence of S on the partitioning behaviour of the elements of interest extended the pressure range that can explain the relative abundances of Ni/Co, Cu/Au, Cu/As, and Ag/Sb. In particular it has been found that the solutions for the element pairs Cu/Au and Cu/As intersect at 7 GPa and 4.5 wt% S, Cu/Au and Ag/Sb have the same solution at 2 GPa and 9 wt% S and the trends for Cu/As and Ag/Sb intersect at 9 GPa and 12 wt% S. These results clearly exceed the assumed abundance of around 2 wt % S in the Earth´s core (Dreibus and Palme, 1996), but are not in conflict with the theory of a late S-rich accretionary component as proposed by O´Neill (1991). For each pair of volatile elements (Cu/Au, Cu/As, Ag/Sb and Sn/Pb) a combination of S and Si in the metal prevents their fractionation from one another at various pressures. In particular it was found that S and Si contents of each less than 1 wt % at pressures between 6 and 9 GPa can explain the relative abundances of Cu, Au and As simultaneously. However a unique solution that is capable of explaining even the relative abundances of all elements studied has not been found. In summary a single stage core formation event can be excluded by the results of this study. It seems that increasing pressure and with it increasing temperature, as well as increasing S-contents of the metal facilitate the achievement of the relative mantle abundances of various elements. Best results have been obtained when the effects of Si- and S- metal contents on siderophile element partitioning have been taken into account simultaneously.

Level thesis:degree_level

thesis.doctoral

Grantor dc:publisher

Universität Bayreuth

Year

2015

Author dc:creator

• Vogel, Antje Kathrin

Contributors dc:contributor

• Rubie, David C.

Repository record source_url

https://epub.uni-bayreuth.de/id/eprint/2039/

OAI identifier oai:identifier

oai:epub.uni-bayreuth.de:2039