MR32A-01
An Experiment-Based Model Describing the Partitioning of Oxygen Between Earths Mantle and Core
Under the high temperature conditions favoured by many core formation scenarios a significant portion of oxygen is likely to have entered the Earth's core. Modelling core-mantle reactions during core formation and at the present day core mantle boundary requires information on the partitioning of O between liquid Fe metal and silicate minerals and melts. Experimental problems with encapsulating silicate liquids at the very high temperatures of interest can be overcome by using magnesiowustite as a proxy for silicate materials. Multianvil and diamond anvil cell experiments on the partitioning of O between liquid Fe metal and magnesiowustite show that the nominally fo2 independent O distribution coefficient KD (defined as KD=(XFe.XO)/XFeO where XFe and XO are the mole fractions of Fe and O in liquid Fe and XFeO is the mole fraction of FeO in magnesiowustite) is a strong function of temperature but a relatively weak function of pressure. However, diamond anvil cell experiments also indicate that KD is no longer independent of fo2 at pressures of 70 GPa. The fo2 dependence to KD has been modelled by fitting experimental data on the Fe-FeO liquid immiscibility gap at high pressure to obtain activity compiosition relations for O in Fe metal. Using this model the partitioning of oxygen between magnesiowustite and Fe-rich metal can be calculated as a function of pressure, temperature and fo2. The calculation is in excellent agreement with multianvil and diamond anvil cell partitioning results. Calculations performed at the present day core mantle boundary using this model indicate that the mantle adjacent to the core will be strongly depleted in FeO.
MR32A-02
How Deep and Hot was Earth's Magma Ocean? Combined Experimental Datasets for the Metal-Silicate Partitioning of 11 Siderophile Elements – Ni, Co, Mo, W, P, Mn, V, Cr, Ga, Cu and Pd.
Since ~1990 high pressure and temperature (PT) experiments on metal-silicate systems have showed that partition coefficients (D) for siderophile (iron-loving) elements are much different than those measured at low PT conditions. The high PT data have been used to argue for a magma ocean during growth of the early Earth. Initial conclusions were based on experiments and calculations for a small number of elements such as Ni and Co. However, for many elements only a limited number of experimental data were available then, and they only hinted at values of metal-silicate D's at high PT conditions. In the ensuing decades there have been hundreds of new experiments carried out and published on a wide range of siderophile elements. At the same time several different models have been advanced to explain the siderophile elements in the earth's mantle: a) intermediate depth magma ocean; 25-30 GPa, b) deep magma ocean; up to 50 GPa, and c) early reduced and later oxidized magma ocean. Some studies have drawn conclusions based on a small subset of siderophile elements, or a set of elements that provides little leverage on the big picture (like slightly siderophile elements), and no single study has attempted to quantitatively explain more than 5 elements at a time. The purpose of this abstract is to update the predictive expressions outlined by Righter et al. (1997) with new experimental data from the last decade, test the predictive ability of these expressions against independent datasets (there are more data now to do this properly), and to apply the resulting expressions to the siderophile element patterns in Earth's upper mantle. The predictive expressions have the form: ln D = alnfO2 + b/T + cP/T + d(1-Xs) + e(1-Xc) + Σ fiXi + g. These expressions are guided by the thermodynamics of simple metal-oxide equilibria that control each element, include terms that mimic the activity coefficients of each element in the metal and silicate, and quantify the effect of variable oxygen fugacity. Preliminary results confirm that D(Ni) and D(Co) converge at pressures near 25-30 GPa and ~2200 K, and show that D(Pd) and D(Cu) become too low at the PT conditions of the deepest models. Furthermore, models which force fit V and Cr mantle concentrations by metal-silicate equilibrium overlook the fact that at early Earth mantle fO2, these elements will be more compatible in Mg-perovskite and (Fe,Mg)O than in metal. Thus an intermediate depth magma ocean, at 25-30 GPa, 2200 K, and at IW-2, can explain more mantle siderophile element concentrations than other models.
MR32A-03
Experimental Evidence for Iron and Silicon Isotope Fractionation during Earth's Core Formation
Several recent studies have shown that comparisons between meteorites and terrestrial stable isotope ratios can be used to elucidate the composition of Earth's core if fractionation factors between silicate and metal are known. In order to understand the fractionation that might occur at core conditions, isotopic experiments need to be conducted at high pressure and temperature. We have developed a method for tracing the evolution towards isotopic equilibrium in piston cylinder experiments based on the so-called three- isotope method. Our implementation of the three-isotope method utilizes mono-isotopic spikes for the elements of interest (e.g., 54Fe, 28Si) in one of the participating phases. We present Fe and Si isotope ratio data from one such set of three-isotope experiments conducted in a piston cylinder apparatus at 2100 K and 1 GPa. In these experiments silicates were spiked with both 54Fe and 28Si. The goal was to determine if core formation could have resulted in isotopic fractionation in both the Fe and Si systems at high pressures and temperatures. Preliminary results suggest that there is a small 57/54Fe fractionation at these conditions between silicate and iron metal of ~ +0.15 ‰ (silicate > metal). These results are inconsistent with some earlier findings (Poitrasson and Roskosz, 2007 and Williams et al. 2008) but are in agreement with estimates based on differences between meteoritical and terrestrial Fe isotope ratios (Georg et al. 2007). We also find a larger silicon isotope fractionation between Si in metal and Si in silicate of ~ 2 ‰ in 30Si/28Si, suggesting that there is approximately 4 wt % Si in the core. These results indicate that isotope fractionation of major elements at high pressure and temperature can be significant, and suggest that experimental calibrations of fractionations between metal and silicate/oxide are essential for interpreting stable isotope data on the planetary scale.
MR32A-04
No iron isotope fractionation between molten alloys and silicate melt to 2000 °C and 7.7 GPa: Experimental evidence and implications for planetary differentiation
Whether core-mantle differentiation of terrestrial planets fractionates iron isotope is currently a debated issue. Melting experiments corresponding to the conditions inferred for core differentiation in an early silicate magma ocean were performed at 1750 and 2000 °C, and from 1.0 to 7.7 GPa to address this question. The starting mixtures correspond to a devolatilized CI chondrite composition and oxygen fugacity conditions were about 2 log units below the iron wüstite buffer. Scanning electron microscopy observations, electron microprobe chemical analyses and plasma source mass spectrometric isotope analyses of the experimental charges show that chemical and iron isotope equilibrium was reached at 2000 °C within 100 seconds. No Fe isotope fractionation was found between the Fe-Ni alloy and the ultramafic silicate melt at this temperature. This result holds within the 1.0-7.7 GPa pressure range and is likely to remain valid at higher pressures and temperatures. The addition of sulfur to the system, hence to the molten alloy, does not alter this conclusion. Our results suggest that significant iron isotope fractionation is unlikely during equilibration of molten core-forming materials in a deep magma ocean. This process therefore cannot explain the heavier Fe isotope composition of the Moon relative to the Earth, itself heavier than Mars, Vesta and the chondrite parent bodies.