Fe FeS Research Articles

The thermal regime of Venus

Models for the thermal evolution of Venus are constructed numerically. The mantle is proposed to consist of two independently convecting layers (upper and lower mantles); the floating crust keeps the temperature of the upper surface of the convecting mantle near 1200°C and the core may solidify at temperatures below the liquidus. Previously, these models were calculated with the usual parameterization of convection based mostly on investigations of constant-viscosity convection. Now this parameterization is modified to take into account new numerical investigations of convection in media with complicated rheology. The thermal evolution of Venus is assumed to have begun 4.6 billion years (b.y.) ago. It can be divided into three periods: (1) adjustment of the upper mantle to the thermal regime of the lower mantle (∼0.5 b.y.); (2) transition of the entire mantle to the asymptotic regime (∼3–4 b.y.); (3) asymptotic regime. Parameters of the convecting planet in the asymptotic regime do not depend on initial conditions (the planet “forgets” its initial state) and are found analytically. The present-day heat flux is equal to ∼50 erg cm −2s −1. The results are close to those obtained previously. Comparison of the present thermal models of Venus and Earth with the recent studies of melting of the Fe FeS system suggest that the absence of a dipole magnetic field is closely connected to the absence of solidification of the Venusian core at present. The position of the iron triple point (γ-ε-liquid) makes this conclusion very probable. The thermal regime of the Venusian crust is also discussed. Convection in the lower part of the crust is shown to play a role in certain regions with specific crustal composition. The prevailing mechanism of heat transfer to the surface is advection by magmas produced by melting the lower layers of the crust.

Ideal FeFeS, FeFeO phase relations and Earth's core

Liquid-state and solid-state model fits to melting data for Fe, FeS and FeO provide constraints for calculating ideal phase relations in FeFeS and FeFeO systems in the pressure range corresponding to the Earth's outer core. The liquid-state model fit to the Fe melting data of Williams and Jeanloz places constraints on the temperature and other properties of Fe along the liquidus beyond the range of their data. The temperature along the best-fit Fe liquidus reaches 5000 K at 136 GPa and 7250 K at 330 GPa, which is somewhat lower than that implied by the Hugoniot results (∼7800 K at 330 GPa). This discrepancy may be due to reshock in experimental targets, or some inaccuracy in the extrapolation, presuming the Hugoniot results represent the equilibrium melting behavior of Fe. Constraints on the solidi of FeS and FeO from the comparison of data and solid-state model calculations imply that FeS and FeO melt at ∼4610 and 5900 K, respectively, at 136 GPa, and ∼6150 and 8950 K, respectively, at 330 GPa. Calculations for the equilibrium thermodynamic properties of solid and liquid Fe along the coincident solidus and liquidus imply that the entropy of melting for Fe is approximately independent of pressure at a value of approximately R (where R is the gas constant), while the change in the molar heat capacity across the transition increases with pressure from ∼0.5 R to 4 R between standard pressure and 330 GPa. We use these constraints to construct ideal-mixing phase diagrams for FeFeS and FeFeO systems at outer core pressures, assuming that Fe and FeS, or Fe and FeO, respectively, are the solid phases in equilibrium with the liquid FeFeS or FeFeO mixtures, respectively. Calculated FeFeO eutectic compositions at 330 GPa (15–20 mol% O) are <25 mol% O, while calculated FeFeS eutectic compositions at 330 GPa (23–30 mol% S) are generally >25 mol% S. Combined with density considerations, these calculations imply that an O-rich outer core is more likely to lie on the FeO-rich side of the FeFeX eutectic, while an S-rich outer core is more likely to lie on the Fe-rich side of the FeFeX eutectic. In addition, eutectic temperatures in both systems are ≳5000 K at 330 GPa. Widely accepted temperature profiles for the outer core, ranging from ≲3000 K at the 136 GPa, the core-mantle boundary, to ≲4200 K at 330 GPa, the outer-inner core boundary, are ⩾800 K below this value. In the context of the outer-inner core boundary-phase boundary hypothesis, this discrepancy implies that at least one boundary layer of ⩾1000 K exists in the mantle, possibly at its base in the D″ region.

Thermodynamics of the system Fe--FeO--MgO at high pressure and temperature and a model for formation of the Earth's core

Summary Recent shock wave data of Jeanloz & Ahrens have established the presence of a phase transformation in Fe0.94 O, occurring at approximately 70 GPa, which has profound implications for the phase behaviour of the geophysically important system FeO—MgO at high pressure. We calculate the phase diagram based on the available shock wave data for FeO and MgO, and suggest that an increase of pressure in the system FeO—MgO will result in a gradual exsolution of an almost pure FeO high-pressure phase (hpp), leaving an iron-depleted (Fe, Mg)O rocksalt (Bl) phase. Using current geophysical data, exsolution of FeO (hpp) from the rocksalt phase in the present-day lower mantle is calculated to occur only in the region immediately overlying the core. Before core segregation took place, however, the increased iron content of the rocksalt phase could place the exsolution region at lesser depths, perhaps 2000 km. Extrapolation of the solubility data of oxygen in liquid iron indicates that the liquid miscibility gap in the system Fe—FeO at atmospheric pressure should close at approximately 4250K. We construct schematic phase diagrams for the system Fe—FeO at high pressure by considering the effect of pressure upon end-member melting points, upon the extent of liquid immiscibility and upon eutectic temperatures and compositions. We suggest that at high pressure (P˜ 90 GPa) the phase diagram of Fe—FeO becomes qualitatively similar to the system Fe—FeS, displaying complete liquid miscibility and a eutectic temperature well below the melting point of pure iron. On the basis of our inferences regarding behaviour in the system Fe—FeO—MgO at high pressure, we have constructed a model for core segregation. Assuming the Earth to have accreted from the primordial solar nebula as a relatively homogeneous mixture of metallic iron and silicate phases, core segregation involving oxygen as the principal light alloying element in the core would commence at a depth where pressure is sufficiently high to cause exsolution of FeO (hpp) from the rocksalt phase, and temperature is sufficiently high to allow formation of an Fe—FeO (hpp) melt without extensively melting the silicate phases. A gravitational instability arises, leading to vertical differentiation of the Earth as molten blobs of the metal phase sink downwards to form the core and the residual depleted silicate material coalesces to form large bodies which rise diapirically upwards to form the mantle.

The role of S in the evolution of the parental cores of the iron meteorites

Most iron meteorites presumably formed from the cores of parent bodies having more or less chondritic bulk compositions. Consideration of the behavior of S during condensation and core formation indicates that these cores, at least in the case of groups having high or moderate volatile contents (IIAB, IIIAB), contained a substantial amount of S. When elemental fractionations observed in these iron meteorite groups are compared to model calculations of fractional crystallization it becomes evident that at least the IIAB parent melt, and very likely the IIIAB parent melt as well, did not contain the full S complement of the parent body. We consider three possible scenarios to account for the S depletion: (1) Outgassing of S during parent body differentiation; this was probably only possible if the parent body contained organic material, which is improbable for IIIAB. (2) Liquid immiscibility. Our fractional crystallization model would predict curved log X vs. log Ni relationships in this case, which for many elements are not observed. (3) Formation of metastable liquid layers by episodic melting during core formation. This is based on the fact that the difference in melting temperature between a FeFeS eutectic and FeNi metals is ∼500 K. Two melting episodes would tend to form distinct liquid layers that maintain their identities over the crystallization lifetime of the core. Solidification of the cores parental to the main iron meteorite groups should also produce a significant number of sulfide meteorites. The scarcity of sulfide-rich meteorites can be attributed to their lower mechanical resistance to space attrition, higher ablation during atmospheric passage, and faster weathering on earth.

Phase relations of ternary compounds in the BaFeS system

The phase relations of the ternary phases at 900 and 700°C were determined by heating mixtures in the regions BaSFeSFeS 2 and FeFeSBaS and identifying the products by powder X-ray diffraction. The stable ternary phases at 900°C are BaFe 2S 3, Ba 2FeS 3, Ba 6Fe 8S 15, a solid solution BaFeS 2Ba 7Fe 6S 14, Ba 2FeS 4, the infinitely adaptive series Ba 3Fe 1+x S 5, 1 3 ≤ x ≤ 2 5 , and Ba 9Fe 16S 32 which belongs to the infinitely adaptive series Ba 1+ x Fe 2S 4. At 700°C the stable ternary phases are: BaFe 2S 3, Ba 2FeS 3, BaFeS 2Ba 7Fe 6S 14 (solid solution), Ba 6Fe 8S 15, Ba 2FeS 4, Ba 5Fe 4S 11, and two infinitely adaptive series: Ba 3Fe 1+ x S 5 and Ba 1+ x Fe 2S 4. A stable ternary phase at 700 and 900°C with probable composition Ba 2Fe 4S 5 was found in the FeFeSBaS section.

Internal constitution and evolution of the moon

The composition, structure and evolution of the moon's interior are narrowly constrained by a large assortment of physical and chemical data. Models of the thermal evolution of the moon that fit the chronology of igneous activity on the lunar surface, the stress history of the lunar lithosphere implied by the presence of mascons, and the surface concentrations of radioactive elements, involve extensive differentiation early in lunar history. This differentiation may be the result of rapid accretion and large-scale melting or of primary chemical layering during accretion; differences in present-day temperatures for these two possibilities are significant only in the inner 1000 km of the moon and may not be resolvable. If the Apollo 15 heat-flow result is representative of the moon, the average uranium concentration in the moon is 0.05–0.08 p.p.m. Density models for the moon, including the effects of temperature and pressure, can be made to satisfy the mass and moment of inertia of the moon and the presence of a low-density crust inferred from seismic refraction studies only if the lunar mantle is chemically or mineralogically inhomogeneous. The upper mantle must exceed the density of the lower mantle at similar conditions by at least 5%. The average mantle density is that of a pyroxenite or olivine pyroxenite, though the density of the upper mantle may exceed 3.5 g/cm 3. The density of the lower mantle is less than that of the combined crust and upper mantle at similar temperature and pressure, thus reinforcing arguments for early moon-wide differentiation of both major and minor elements. The suggested density inversion is gravitationally unstable and implies stresses in the mantle 2–5 times those associated with the lunar gravitational field, a difficulty that can be explained or avoided by: (1) adopting lower values for the moment of inertia and/or crustal thickness, or (2) postulating that the strength of the lower mantle increases with depth or with time, either of which is possible for certain combinations of composition and thermal evolution. A small iron-rich core in the moon cannot be excluded by the moon's mass and moment of inertia. If such a core were molten at the time lunar surface rocks acquired remanent magnetization, then thermal-history models with initially cold interiors strongly depleted in radioactive heat sources as a primary accretional feature must be excluded. Further, the presence of ∼||pre|40 K in a FeFeS core could significantly alter the thermal evolution and estimated present-day temperatures of the deep lunar interior.

Partitioning of potassium between silicates and sulphide melts: Experiments relevant to the earth's core

The partitioning of potassium between roedderite, K 2Mg 5Si 12O 30 and an FeFeS melt was investigated at temperatures about 40°C above the FeFeS eutectic. Roedderite was considered a prime candidate for one of the potassium-bearing phases in the primitive Earth because roedderite and merrihueite are the only two silicates containing essential potassium which have been identified in stony meteorites. A mean K 2S/FeS weight ratio of (3.340 ± 0.015) × 10 −3 was determined in these experiments; a K 2S FeS weight ratio of about 1.0 × 10 −2 would be sufficient to extract all potassium in a chondritic Earth into the core. Application of these results to a primitive chondritic Earth is discussed and it is concluded that extraction of most of the Earth's potassium into the FeFeS core would occur under the conditions in the early Earth.

The origin and chemical composition of the earth's core

The process of core formation in the Earth is subject to the constraints that it be nearly simultaneous with accretion and yet occur in a manner that the mantle retains radiogenic sim||pre|129 Xe produced from the extinct radioactivity of sim||pre|129 I with a half life of 17.6 × 10 6 y. From these constraints, it appears that the only feasible mechanism is the segregation of an FeFeS melt. Trace element abundances in major classes of meteorites and the silicate fractions of the Earth show that not only there is a high depletion of sulphur in the crust and the mantle, but that it is even more highly depleted than the rare gases, water and the halogens. From the nature of this depletion pattern and the fact that any model of accretion of the Earth will necessarily produce an FeFes melt, it is concluded that the light element in the core is largely sulphur with minor amounts of carbon. A consequence of this mode of core formation is found to be the availability of sim||pre|40 K radioactive heat production in the liquid core, estimated at about 10 19 erg/s at the present time. This energy is more than adequate to produce convective motions in the outer core to generate the geomagnetic field.

The chemical composition of the Earth's core: Possibility of sulphur in the core

An evaluation of the abundances of several volatile trace-elements shows that sulphur is depleted in the Earth's mantle and crust by almost two orders of magnitude more than the rare-gases, halogens and water. This deficiency does not appear to be due to a preterrestrial depletion process or volatalization during accretion. It is suggested that the missing sulphur was segregated into the core as part of the lowest melting components in the FeSO system. A feature common to most thermal history models of the Earth is that the temperature of the outermost 100–200 km never exceeds the melting point of the FeFeS eutectic. This leads to the retention of some metallic FeNi and iron sulphide in the upper mantle, which was subsequently oxidized and mixed with the lower depleted mantle. The excess abundances of some chalcophile and siderophile elements in the upper mantle can therefore be attributed to this feature. An important consequence of having sulphur as a major element in the Earth's core is that the composition of no single meteorite type satisfactorily resembles the major element composition of an Earth with a FeNiS core and a mantle similar to pyrolite.