03_Abstract (part 4: nature & science) - Yiwei666/08_computional-chemistry-learning-materials- GitHub Wiki
- 主要是2000年之前发表在Nature、Science上有关Element partitioning论文
1. Partitioning of Tungsten and Molybdenum Between Metallic Liquid and Silicate Melt (Science, 1995)
The "excess" of siderophile elements in Earth's mantle is a long-standing problem in understanding the evolution of Earth. Determination of the partitioning behavior of tungsten and molybdenum between liquid metal and silicate melt at high pressure and temperature shows that partition coefficients (Dmetal/silicate) vary by two orders of magnitude depending on whether metal segregated from a basaltic or peridotitic melt. This compositional dependence is likely a response to changes in the degree of polymerization of the silicate melt caused by compositional variations of the network-modifying cations Mg2+ and Fe2+. Silicate melt compositional effects on partition coefficients for siderophile elements are potentially more important than the effects of high pressure and temperature.
2. High-Pressure and High-Temperature Experiments on Core-Mantle Segregation in the Accreting Earth (Science, 1994)
The abundances of siderophile elements in the Earth's silicate mantle are too high for the mantle to have been in equilibrium with iron in the core if equilibrium occurred at low pressures and temperatures. It has been proposed that this problem may be solved if equilibrium occurred at high pressures and temperatures. Experimental determination of the distribution of siderophile elements between liquid metal and liquid silicate at 100 kilobar and 2000°C demonstrates that it is unlikely that siderophile element abundances were established by simple metal-silicate equilibrium, which indicates that the segregation of the core from the mantle was a complex process.
3. Noble Gas Partitioning Between Metal and Silicate Under High Pressures (Science, 1993)
Measurements of noble gas (helium, neon, argon, krypton, and xenon) partitioning between silicate melt and iron melt under pressures up to 100 kilobars indicate that the partition coefficients are much less than unity and that they decrease systematically with increasing pressure. The results suggest that the Earth's core contains only negligible amounts of noble gases if core separation took place under equilibrium conditions.
4. Hydrogen Partitioning into Molten Iron at High Pressure: Implications for Earth's Core (Science, 1997)
Because of dissolution of lighter elements such as sulfur, carbon, hydrogen, and oxygen, Earth's outer core is about 10 percent less dense than molten iron at the relevant pressure and temperature conditions. To determine whether hydrogen can account for a major part of the density deficit and is therefore an important constituent in the molten iron outer core, the hydrogen concentration in molten iron was measured at 7.5 gigapascals. From these measurements, the metal-silicate melt partitioning coefficient of hydrogen was determined as a function of temperature. If the magma ocean of primordial Earth was hydrous, more than 95 mole percent of H2O in this ocean should have reacted with iron to form FeHx, and about 60 percent of the density deficit is reconciled by adding hydrogen to the core.
5. Evidence for a late chondritic veneer in the Earth's mantle from high-pressure partitioning of palladium and platinum (Nature, 2000)
The high-pressure solubility in silicate liquids of moderately siderophile ‘iron-loving’ elements (such as nickel and cobalt) has been used to suggest that, in the early Earth, an equilibrium between core-forming metals and the silicate mantle was established at the bottom of a magma ocean1,2. But observed concentrations of the highly siderophile elements—such as the platinum-group elements platinum, palladium, rhenium, iridium, ruthenium and osmium—in the Earth's upper mantle can be explained by such a model only if their metal–silicate partition coefficients at high pressure are orders of magnitude lower than those determined experimentally at one atmosphere (refs 3,4,5,6,7,8). Here we present an experimental determination of the solubility of palladium and platinum in silicate melts as a function of pressure to 16 GPa (corresponding to about 500 km depth in the Earth). We find that both the palladium and platinum metal–silicate partition coefficients, derived from solubility, do not decrease with pressure—that is, palladium and platinum retain a strong preference for the metal phase even at high pressures. Consequently the observed abundances of palladium and platinum in the upper mantle seem to be best explained by a ‘late veneer’ addition of chondritic material to the upper mantle following the cessation of core formation.
6. Partitioning of nickel and cobalt between silicate perovskite and metal at pressures up to 80 GPa (Nature, 1999)
The high abundance of both nickel and cobalt and the chondritic Ni/Co ratio found in samples derived from the Earth's mantle are at odds with results from laboratory-based partitioning experiments conducted at pressures up to 27 GPa (refs 1,2). The laboratory results predict that the mantle should have a much lower abundance of both Ni and Co and a considerably lower Ni/Co ratio owing to the preferential partitioning of these elements into the iron core. Two models have been put forward to explain these discrepancies: homogeneous accretion3,6 (involving changes of the Ni and Co partition coefficients with oxygen and sulphur fugacities, pressure and temperature) and heterogeneous accretion7,9 (the addition of chondritic meteorites to the mantle after core formation was almost complete). Here we report diamond-cell experiments on the partitioning of Ni and Co between the main lower-mantle mineral ((Mg,Fe)SiO3-perovskite) and an iron-rich metal alloy at pressures up to 80 GPa (corresponding to a depth of ∼1,900 km). Our results show that both elements become much less siderophilic with increasing pressure, such that the abundance of both Ni and Co and the Ni/Co ratio observed in samples derived from the Earth's mantle appear to indeed be consistent with a homogeneous accretion model.
7. Geochemistry of mantle–core differentiation at high pressure (Nature, 1996)
THE apparent excess of siderophile (iron-Ioving) elements in the Earth's mantle has been a long-standing enigma in the geochemistry of mantle–core differentiation1,2. Although current models have proved successful in explaining some aspects of this problem3–7, important questions remain. In particular, the mantle's near-chondritic ratio of nickel to cobalt (close to that expected for the material from which the Earth formed) is hard to explain, given the markedly different ambient-pressure partitioning behaviour of these elements between iron-alloy and silicate melts3–8. Here we report experimental results which show that both elements become less siderophile with pressure, but the effect is much more pronounced for Ni, so that the partition coefficients of the two elements become essentially equivalent at an extrapolated pressure of ∼28 GPa. The absolute and relative abundances of Ni and Co in the mantle are therefore consistent with alloy–silicate chemical equilibrium at high pressure, indicating that core formation may have taken place in a magma ocean with a depth of 750–1,100 km. We also find that, unlike Ni and Co, sulphur becomes more siderophile with pressure. Sulphur's increased affinity for iron with depth could make it the dominant light element in the Earth's core.
8. The Earth's ‘missing’ niobium may be in the core (Nature, 2001)
As the Earth's metallic core segregated from the silicate mantle, some of the moderately siderophile (‘iron-loving’) elements such as vanadium and chromium1,2 are thought to have entered the metal phase, thus causing the observed depletions of these elements in the silicate part of the Earth. In contrast, refractory ‘lithophile’ elements such as calcium, scandium and the rare-earth elements are known to be present in the same proportions in the silicate portion of the Earth as in the chondritic meteorites—thought to represent primitive planetary material1,3. Hence these lithophile elements apparently did not enter the core. Niobium has always been considered to be lithophile and refractory yet it has been observed to be depleted relative to other elements of the same type in the crust and upper mantle4,5. This observation has been used to infer the existence of hidden niobium-rich reservoirs in the Earth's deep mantle5. Here we show, however, that niobium and vanadium partition in virtually identical fashion between liquid metal and liquid silicate at high pressure. Thus, if a significant fraction of the Earth's vanadium entered the core (as is thought), then so has a similar fraction of its niobium, and no hidden reservoir need be sought in the Earth's deep mantle.
9. High-pressure geochemistry of Cr, V and Mn and implications for the origin of the Moon (Nature, 1990)
CHROMIUM, vanadium and manganese are present in similar abundances in the Earth's mantle and the Moon, and are substantially depleted relative to their Mg-normalized primordial abundances1–6. Experimental studies7,8 of the partitioning of chromium, vanadium and manganese between molten iron and silicates show that these elements are lithophile at the pressures, temperatures and oxygen fugacities prevailing in the Earth's upper mantle and in the Moon. Here, we show that at much higher pressures, corresponding to those in the Earth's lower mantle, the partitioning behaviour of Cr, V and Mn changes owing to increasing solubility of oxygen in molten iron. Cr and V (and perhaps Mn) are preferentially partitioned into molten iron under these conditions. We therefore attribute the depletions of these elements in the Earth's mantle to their siderophile behaviour during formation of the Earth's core, at pressures that were sufficiently high to cause substantial amounts of oxygen to dissolve in molten metallic iron. Similar depletion patterns of Cr, V and Mn in the Earth's mantle and the Moon strongly suggest that a large proportion of the Moon was derived from the Earth's mantle after the Earth's core had segregated.
10. Superheating Effects on Metal-Silicate Partitioning of Siderophile Elements (Science, 1993)
Liquid metal—liquid silicate partition coefficients for several elements at 100 kilobars and temperatures up to about 3000 kelvin in carbon capsules experimentally converge on unity with increasing temperature. The sense of change of the partition coefficients with temperature resembles the extrapolation of Murthy and may partially contribute to, but by no means provide a complete resolution of, the "excess" siderophile problem in the Earth's mantle. Sulfur and perhaps carbon successfully compete with oxygen for sites in the metallic liquid at these temperatures and pressures. This observation casts doubt upon the hypothesis that oxygen is the light element in the Earth's core.
11. Effect of pressure on rare earth element partition coefficients in common magmas (Nature, 1983)
Geochemical modelling of magma generation and crystal fractionation processes at pressures corresponding to those of the lower crust1 and upper mantle2 is critically dependent on the trace element partition coefficients used in the calculations. The effects of temperature and melt composition on partition coefficients are reasonably well established3,4, but the possible effect of pressure has not been closely evaluated, apart from some work on nickel partitioning5. Rare earth elements (REE) are important in geochemical modelling, and the need for data on the pressure effect on REE partitioning has been pointed out3. We have done a series of experiments in which the role of pressure was isolated from that of temperature and melt composition (or melt structure), and we are able to demonstrate a systematic increase in La, Sm, Ho and Lu partition coefficients for sphene/silicate liquid and clinopyroxene/silicate liquid as pressure increases from 7.5 to 30 kbar.
12 . Constraints on the composition of the Earth's core from ab initio calculations (Nature, 2000)
Knowledge of the composition of the Earth's core1,2,3 is important for understanding its melting point and therefore the temperature at the inner-core boundary and the temperature profile of the core and mantle. In addition, the partitioning of light elements between solid and liquid, as the outer core freezes at the inner-core boundary, is believed to drive compositional convection4, which in turn generates the Earth's magnetic field. It is generally accepted that the liquid outer core and the solid inner core consist mainly of iron1. The outer core, however, is also thought to contain a significant fraction of light elements, because its density—as deduced from seismological data and other measurements—is 6–10 per cent less than that estimated for pure liquid iron1,2,3. Similar evidence indicates a smaller but still appreciable fraction of light elements in the inner core5,6. The leading candidates for the light elements present in the core are sulphur, oxygen and silicon3. Here we obtain a constraint on core composition derived from ab initio calculation of the chemical potentials of light elements dissolved in solid and liquid iron. We present results for the case of sulphur, which provide strong evidence against the proposal that the outer core is close to being a binary iron–sulphur mixture7.
13. The melting curve of iron at the pressures of the Earth's core from ab initio calculations (Nature, 1999)
The solid inner core of the Earth and the liquid outer core consist mainly of iron1 so that knowledge of the high-pressure thermodynamic properties of iron is important for understanding the Earth's deep interior. An accurate knowledge of the melting properties of iron is particularly important, as the temperature distribution in the core is relatively uncertain2,3,4 and a reliable estimate of the melting temperature of iron at the pressure of the inner-core boundary would put a much-needed constraint on core temperatures. Here we used ab initio methods to compute the free energies of both solid and liquid iron, and we argue that the resulting theoretical melting curve competes in accuracy with those obtained from high-pressure experiments. Our results give a melting temperature of iron of ∼6,700 ± 600 K at the pressure of the inner-core boundary, consistent with some of the experimental measurements. Our entirely ab initio methods should also be applicable to many other materials and problems.
14. Prediction of crystal–melt partition coefficients from elastic moduli (Nature, 1994)
MANY geochemical processes, such as crystallization of silicate magmas or planetary differentiation, require a knowledge of the way in which elements become partitioned between coexisting crystal and liquid phases1,2. But quantitative prediction of crystal/melt partition coefficients from thermodynamic principles has not previously been possible. By studying the partitioning of 15 elements between silicate minerals and their coexisting melts, we show here that the partitioning behaviour of any series of isovalent cations can be rationalized in terms of a simple model in which the size and elasticity of the crystal lattice sites play a critical role. We find that elasticity varies linearly with the formal charge of the cation. This model allows us to predict element partitioning behav-iour solely from the physical characteristics of the cation sites in the crystal.
15. Element Partitioning: The Role of Melt Structure and Composition (Science, 2006)
We segregated coexisting gabbroic and granitic melts by centrifuging them at high pressures and temperatures and measured the trace element compositions of the melts by laser ablation inductively coupled plasma mass spectrometry. Our results demonstrate that the effect of melt structure contributes about one order of magnitude to crystal/melt partition coefficients. Partitioning of alkali and alkaline earth elements strongly depends on field strength: Amphoteric and lone pair electron elements partition into the polymerized granitic melt; and rare earth, transition, and high field strength elements coordinated by nonbridging oxygens partition remarkably similar into the gabbroic melt. A regular solution model predicts these effects.