MR31B-01 INVITED
Estimates of the Light Element Content of the Earth's Core From Models of Core Formation
It has been known for many years that the Earth's metallic Fe-Ni core contains up to 10-12 wt% light elements, with possible candidates being S, O, Si, C, N, and H. Because chemical interaction with the mantle has been very limited during Earth's history, the light element content is expected to have been established during core formation. We estimate the concentrations of oxygen and silicon in the core through a new multi- stage model of core formation that is constrained by the concentrations of both siderophile and lithophile elements in the Earth's mantle. The Earth accretes through a series of collisions with smaller planetary bodies that had already differentiated at low pressure (e.g. <1 GPa). Each impact results in a magma ocean in which the core of the impactor reequilibrates with silicate liquid at pressures that increase during accretion, e.g. from 3 to 90 GPa, before merging with the Earth's proto-core. The bulk compositions of the proto-Earth and impactors are chondritic with oxidation states (i.e. bulk oxygen contents) that can be varied. The compositions of coexisting liquid metal and liquid silicate in the magma ocean are determined by a mass balance calculation that is based on experimental determinations of the metal-silicate partitioning of FeO and Si, together with a range of trace elements including Ni, Co, V, Cr, Ta and Nb. Models that reproduce mantle geochemistry best involve heterogeneous accretion in which the oxygen content of the accreting material increases - which causes oxygen fugacity to increase significantly as the Earth grows. In addition, the final giant impacts involve only partial reequilibration of metal and silicate at high pressure (e.g. due to incomplete emulsification of the impactor cores). Current results indicate that both O and Si are present in the core in sub-equal concentrations (e.g. 2-3 wt% O and ~5 wt% Si). Together with approximately 2 wt% S, these concentrations are sufficient to satisfy recent estimates of the density deficit.
MR31B-02 INVITED
Experimental study of reaction between perovskite and molten iron to 146 GPa and implications for chemically-distinct buoyant layer at the top of the core
Partitioning of oxygen and silicon between molten iron and (Mg,Fe)SiO3 perovskite was investigated to 146 GPa and 3500 K by a combination of laser-heated diamond-anvil cell and analytical transmission electron microscope (TEM). The chemical compositions of co-existing quenched molten iron and perovskite were determined quantitatively with energy-dispersive X-ray spectrometry (EDS) and electron energy loss spectroscopy (EELS) for oxygen content. The results demonstrate that the quenched liquid iron in contact with perovskite contained substantial amounts of oxygen and silicon at such high pressure and temperature. Owing to the pretty high solubilities of oxygen and silicon into the molten iron, the chemical reaction at the core-mantle boundary (CMB) possibly results in the formation of chemically distinct layers both at the mantle side and the core side. Here we calculated the chemical equilibrium between perovskite, ferropericlase, and molten iron at the P-T conditions of the CMB, based on these experimental results and previous data on partitioning of oxygen between molten iron and ferropericlase to 134 GPa [Ozawa et al., 2008 GRL]. Note that perovskite is a predominant phase instead of post-perovskite above 3500 K at the CMB pressure. We found that molten iron should include oxygen and silicon much more than required to account for the core density deficit (less than 10%) when co-existing with both perovskite and ferropericlase at the CMB. These results suggest that the very bottom of the mantle may consist of either one of iron-depleted perovskite or ferropericlase. Alternatively, it is also possible that the bulk outer core liquid is not in direct contact with the mantle. The extensive dissolutions of oxygen and silicon from the mantle may form chemically-distinct buoyant layer at the top of the core. Indeed, seismological observations of a small P-wave velocity reduction in the topmost core have been repeatedly reported, suggesting the presence of such buoyant liquid layer. Such layer physically separates the mantle from the bulk outer core liquid, hindering the chemical reaction between them.
MR31B-03
Iron and silicate reactions and light elements in the core
Chemical reactions between metallic iron and silicates are an important mechanism for providing the light elements into the core. We have studied the several reactions introducing light elements into the core. Silicon and oxygen are the most important candidates for the light elements in the core. We studied the reactions of molten metallic iron and perovskite and post-perovskite phases and showed that a significant amount of Si up to 4 wt percent and O up to 6.3 wt percent are dissolved at the CMB conditions of 139 GPa and 3000 K. Thus, O and Si can be the major light elements in the molten outer core. Existence of these elements in the outer core strongly suggests that these elements could exist as light elements of the inner core. Thus, we studied stability and phase relations of Fe-Si alloy under the core conditions. The density deficit of the inner core might be explained by existence of about 2-5 wt percent Si in the inner core. Thus, we studied the stability of Fe-3.4 wt percent Si alloy up to 242 GPa and 3600 K, and found that the hcp phase is stable in this alloy under the present experimental conditions. This suggests that the Earthfs inner core is likely to be composed of hcp Fe-Si alloy, and the seismic anisotropy of the inner core can be explained by this phase in the inner core. Hydrogen is also an important candidate for the light element in the core, and it is also important regarding the global circulation of volatiles in whole earth including the core. We have studied series of reactions involving iron and hydrogen at high pressure and temperature. We studied the reactions of iron (or iron nickel alloy), and H2O, and several hydrous phases such as hydrous ringwoodite, hydrous phase delta (AlOOH). We observed that formation of FeHx in all of these reactions, suggesting metallic iron can be an important hydrogen absorber in the deep mantle and at the core-mantle boundary. This result implies that hydrogen could have been transported into the earthfs core during core formation in the magma ocean stage and by convective circulation including slab subduction during geological time.
MR31B-04 INVITED
Core formation in silicate bodies
Differentiation of a body into a metallic core and silicate mantle occurs most efficiently if temperatures are high enough to allow at least the metal to melt [1], and is enhanced if matrix deformation occurs [2]. Elevated temperatures may occur due to either decay of short-lived radio-isotopes, or gravitational energy release during accretion [3]. For bodies smaller than the Moon, core formation happens primarily due to radioactive decay. The Hf-W isotopic system may be used to date core formation; cores in some iron meteorites and the eucrite parent body (probably Vesta) formed within 1 My and 1-4~My of solar system formation, respectively [4]. These formation times are early enough to ensure widespread melting and differentiation by 26Al decay. Incorporation of Fe60 into the core, together with rapid early mantle solidification and cooling, may have driven early dynamo activity on some bodies [5]. Iron meteorites are typically depleted in sulphur relative to chondrites, for unknown reasons [6]. This depletion contrasts with the apparently higher sulphur contents of cores in larger planetary bodies, such as Mars [7], and also has a significant effect on the timing of core solidification. For bodies of Moon-size and larger, gravitational energy released during accretion is probably the primary cause of core formation [3]. The final stages of accretion involve large, stochastic collisions [8] between objects which are already differentiated. During each collision, the metallic cores of the colliding objects merge on timescales of a few hours [9]. Each collision will reset the Hf-W isotopic signature of both mantle and core, depending on the degree to which the impactor core re-equilibrates with the mantle of the target [10]. The re-equilibration efficiency depends mainly on the degree to which the impactor emulsifies [11], which is very uncertain. Results from N-body simulations [8,12] suggest that significant degrees of re- equilibration are required [4,10]. Re-equilibration is also suggested by mantle siderophile abundances [13], though simple partitioning models do not capture the likely complex P,T evolution during successive giant impacts. The timescale of Martian core formation is currently uncertain (0-10 My) [14], though it is clear that Martian core formation ended before that of the Earth. [1] Stevenson, in Origin of the Earth, 1990. [2] Groebner and Kohlstedt, EPSL 2006. [3] Rubie et al., Treatise Geophys. 2007. [4] Kleine et al., GCA submitted. [5] Weiss et al., LPSC 39, 2008. [6] Keil and Wilson, EPSL 1993 [7] Wanke and Dreibus, PTRSL, 1984. [8] Agnor et al. Icarus 1999 [9] Canup and Asphaug, Nature 2001 [10] Nimmo and Agnor, EPSL 2006. [11] Rubie et al., EPSL 2003 [12] O'Brien et al, Icarus 2006 [13] Righter, AREPS 2003. [14] Nimmo and Kleine, Icarus 2007.
MR31B-05
Connectivity of core forming melts: New constraints from an integrated experimental approach
The formation of a metallic core is one of the most profound events in the early evolution of a planet. There are still on-going controversies on the nature of the physical mechanisms that led to core-formation on the Earth, as well as on the smaller, differentiated planetesimals considered to be building blocks of Earth. Although there is much evidence that favors a very hot and deep magma ocean for the major core formation event on Earth, smaller planetesimals likely never became hot enough to generate the wide-scale melting required for such a scenario. Furthermore, there is evidence that planetesimal cores formed rapidly (within 3My). An inefficient percolative flow mechanism has been suggested to be viable for systems that have a metallic melt fraction in excess of the percolation threshold (approximately 5 vol%), provided that the permeability of these connected melts is high enough to remove the majority of the core liquid from the silicate matrix in such a relatively short time span (eg. Yoshino et al. 2004). More accurate knowledge of the permeability of core forming melts requires a detailed understanding of how the melt is connected in three dimensions, and the complex relationships between melt volume, connectedness and permeability. In this study, we calculated the permeability of core forming metallic liquids (FeS and Fe67S33) within a silicate matrix by lattice-Boltzmann simulations of flow through digital volumes generated from 3- dimensional, synchrotron-based x-ray tomographic images of experimental run samples. Mixtures of San Carlos olivine and sulfide liquids were synthesized in a piston-cylinder apparatus at conditions relevant to core segregation in planetesimals (1400°C and 1GPa for 24 hours). These conditions were determined to be sufficient for the samples to reach micro-textural equilibrium. Upon quench, the recovered samples were imaged at micron scale resolution using the dedicated tomography beam-line(8.3.2)at the Advanced Light Source (Lawrence Berkeley National Laboratory). Electrical conductivity measurements at 1 GPa and temperatures up to 1000°C on the same pre-synthesized samples were also conducted to independently determine the percolation (connectivity) threshold and begin developing a relationship between electrical conductivity and permeability in partially molten samples. The percolation threshold of these samples was determined to be close to earlier measurements (3-6 vol%) through both experimental methods, but the calculated permeability is substantially lower than previously estimated. As a consequence, although percolation still appears viable for some planetesimal sized objects, it may be a secondary mechanism acting in conjunction with flow induced through other processes such as deformation. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
MR31B-06
Highly Reducing Conditions During Core Formation on Mercury: Implications for Internal Structure, the Distribution of Heat-Producing Elements and the Origin of a Magnetic Field
According to previous models for the formation of Mercury, the estimated oxygen fugacity under which Mercurian core segregation took place was in the range 3 to 6 log units below the iron-wüstite buffer. These oxygen fugacities are in the same range as those suggested for enstatite meteorites. Based upon comparison with high pressure and high temperature experiments relevant to formation of enstatite meteorites, the core of Mercury may contain several wt% of Si, in addition to S. The presence of both Si and S is of importance as the ternary system Fe-S-Si has a vast immiscibility gap at low pressure, which is closed at high pressure (15 GPa–2000°C). It is possible that the Mercurian core has a shell structure comprising of: (i) an outer layer of Fe-S-Si liquid, rich in S; (ii) a middle layer of Fe-Si-S liquid, rich in Si; and (iii) an inner core of solid Fe-Si metal. We have explored the structure of the Mercurian mantle, assuming an EH-bulk composition, using the thermodynamic calculator p-MELTS. Quartz and orthopyroxene (opx) will be present throughout the Mercurian mantle, while garnet (gt) and cpx are only stable above 6GPa. Gt is the predicted liquidus phase at pressures > 6GPa, while it is Opx at pressures <6GPa. Both recently published metal/silicate partitioning data, as well as observations of U distribution in EH, suggest that this element behaves as a chalcophile element at low oxygen fugacity. We predict that U will be concentrated in the outer layer of the Mercurian core, and could thus act to maintain this part of Mercury's core molten, potentially contributing to the origin of Mercury's magnetic field.
MR31B-07
Sulfur- and Oyxgen(?)-Rich Cores of Large Icy Satellites
The internal structures of Jupiter's large moons, Io, Europa, Ganymede, and Callisto, and Titan once Cassini data is sufficiently analyzed, can be usefully compared with those of the terrestrial planets. With sufficient heating we expect not only separation of rock from ice, but also metal from rock. The internally generated dipole magnetic field of Ganymede is perhaps the strongest evidence for this separation, but the gravity field of Io also implies a metallic core. Nevertheless, the evolutionary paths to differentiation taken (or avoided in the case of Callisto) by these worlds are quite different from those presumed to have the governed differentiation of the terrestrial planets, major asteroids, and iron meteorite parent bodies. Several aspects stand out. Slow accretion in gas-starved protosatellite nebulae implies that neither giant, magma-forming impacts were likely, nor were short-lived radiogenic nuclei in sufficient abundance to drive prompt differentiation. Rather, differentiation would have relied on quotidian long-lived radionuclide heating and/or in the cases of Io, Europa, and possibly Ganymede, tidal heating in mean-motion resonances. The best a priori estimate for the composition of the "rock" component near Jupiter and Saturn is solar, and it is this material that is fed into the accretion disks around Jupiter and Saturn, across the gaps the planets likely created in the solar nebula. Solar composition rock implies a sulfur abundance close to the Fe-FeS eutectic (at appropriate pressures). The rocky component of these worlds was likely highly oxidized as well, based on carbonaceous meteorite analogues, implying relatively low Mg#s (by terrestrial standards), lower amounts of Fe metal available for core formation, or even oxidized Fe3O4 as a potential core component. The latter may be important, as an Fe-S-O melt wets silicate grains readily, and thus can easily percolate downward, Elsasser style, to form a core. Nevertheless, the amount of FeS alone available to form a core may have been considerable, and a picture emerges of large, relatively low-density cores (a far greater proportion of "light alloying elements" than in the Earth's core), and relatively iron-rich rock mantles. Ganymede, and possibly Europa, may even retain residual solid FeS in their rock mantles, depending on the tidal heating history of each. Large, dominantly fluid cores imply enhanced mantle tidal deformation and heating. Published models have claimed that the Galilean satellites are depleted in Fe compared to rock, and in the case of Ganymede, that it is either depleted or enhanced in Fe. Obviously Ganymede cannot be both, and detailed structural models show that the Galilean satellites can be explained in terms of solar composition, once one allows for abundant sulfur and hot (liquid) cores.
MR31B-08
Phase Diagrams of Iron Rich Alloys and Their Influence on the Chemical Structure of Planetary Cores
Many planetary bodies are thought to have metallic, iron rich cores, with a significant component of some 'light' alloying element(s). The identity of this light alloying component has a profound effect on the chemical properties of the core, including its melting/crystallization behavior, partitioning of minor and trace elements during core/mantle segregation and core crystallization, and other phase relations. Despite this importance, the light element component(s) of planetary bodies generally remain unknown, apart from those of a few iron meteorite parent bodies. Experimentally determined physical and chemical properties of iron-rich systems can be compared to observations and models of planetary interiors to constrain compositions of planetary cores. Here we summarize our recent high pressure, high temperature experiments on the phase diagrams of iron+light element (Fe-X) binaries, specifically iron-sulfide, iron-silicide, and iron-oxide systems. Melting as well as subsolidus phase relations have been determined in the laser heated diamond anvil cell, using either synchrotron X-ray diffraction or optical methods to establish phase boundaries. X-ray diffraction while laser heating the sample reveals the nature of structural transitions (including partial melting), and optical methods (such as temperature vs. emissivity and related methods) establish the phase boundaries with finer precision. Drawing on these and other recent experimental results, we compare and contrast the binary Fe-X phase diagrams to address such questions as: Which candidate light elements (S, Si, O, C) cause the largest melting point depression, and how does this change with pressure? Which can produce large density constrasts against crystallizing iron metal? and others. These results are compared to thermal and chemical models of terrestrial planet interiors (including Earth's), and important gaps and discrepancies in the available experimental data are highlighted.