Title: Physical and chemical constraints on core - mantle differentiation in terrestrial planets.
Abstract: In this study a physical mechanism and geochemical parameters have been examined in high pressure (P) and high temperature (T) experiments in order to place constraints on the conditions and the manner by which core-mantle differentiation occurred on Earth and terrestrial planets. The wetting characteristics of liquid Fe-Si alloys in a matrix of the respective predominating stable silicate mantle mineral (forsterite, silicate perovskite) at pressures of 2-5 and 25 GPa and temperatures of 1600-2000°C have been studied by determining the liquid metal - solid silicate contact angles. The median angle values from texturally-equilibrated samples were found to be independent of P, T, silicate mineralogy and the Si content in the metal fraction and range between 130° and 140° which is far above the critical wetting boundary of 60°. Therefore, within the studied range of conditions dissolved Si does not lower the surface energies between Fe-rich liquids and silicate mantle grains. As a consequence, under reducing conditions the presence of Si in the metal phase of planetary bodies would not have enhanced percolative flow as an effective metal-silicate separation process. The effects of P, T and oxygen fugacity on the liquid metal - liquid silicate partitioning behaviour of the elements Ta, Nb, V, Cr, Si, Mn, Ga, In and Zn have been studied experimentally over a wide range of high-P and high-T conditions of 2-24 GPa, 1750-2600°C and at low oxygen fugacities of -1.3 to -4.2 log units below the iron wustite buffer. With the derived parameters the respective element depletions in the mantle can be tested under various conditions suggested in core formation models. These data indicate that Nb can serve as an important constraint on oxygen fugacity and P for metal-silicate equilibration. Core formation must have occurred at conditions significantly greater than 20 GPa in order for Nb not to have been massively depleted under conditions necessary to deplete the weakly siderophile element V. Moreover, our study shows that the volatile elements Mn and Ga, would experience strong fractionations in any core-mantle equilibration scenario at pressures below 60 GPa and temperatures at least as high as the peridotite liquidus, while their observed abundances in the mantle is near-chondritic. To a more extreme extent such an observation has been made for the elements Zn and In for which pressures over 80 GPa may be required to explain their near-chondritic ratio in the mantle. Based on these observations we find strong support for the existence of a deep magma ocean during metal-silicate separation, which is an essential component in current polybaric multi-stage core formation models. Although these models succeed in reproducing the observed mantle abundances of many siderophile elements, and can be constrained based on the partitioning behaviour of elements such as Nb, the observed behaviour of the volatile elements Mn, Ga, Zn and In may call for an additional process. Such a process may be the late accretion of volatiles in material that…
Publication Year: 2007
Publication Date: 2007-01-01
Language: en
Type: dissertation
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Cited By Count: 1
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