Time Of Core Formation Research Articles

Abundance, distribution, and origin of 60Fe in the solar protoplanetary disk

Meteorites contain relict decay products of short-lived radionuclides that were present in the protoplanetary disk when asteroids and planets formed. Several studies reported a high abundance of 60Fe (t1/2=2.62±0.04Myr) in chondrites (60Fe/56Fe∼6×10−7), suggesting that planetary materials incorporated fresh products of stellar nucleosynthesis ejected by one or several massive stars that exploded in the vicinity of the newborn Sun. We measured 58Fe/54Fe and 60Ni/58Ni isotope ratios in whole rocks and constituents of differentiated achondrites (ureilites, aubrites, HEDs, and angrites), unequilibrated ordinary chondrites Semarkona (LL3.0) and NWA 5717 (ungrouped petrologic type 3.05), metal-rich carbonaceous chondrite Gujba (CBa), and several other meteorites (CV, EL H, LL chondrites; IIIAB, IVA, IVB iron meteorites). We derive from these measurements a much lower initial 60Fe/56Fe ratio of (11.5±2.6)×10−9 and conclude that 60Fe was homogeneously distributed among planetary bodies. This low ratio is consistent with derivation of 60Fe from galactic background (60Fe/56Fe≈2.8×10−7 in the interstellar medium from γ-ray observations) and can be reconciled with high 26Al/27Al∼5×10−5 in chondrites if solar material was contaminated through winds by outer layers of one or several massive stars (e.g., a Wolf–Rayet star) rich in 26Al and poor in 60Fe. We present the first chronological application of the 60Fe–60Ni decay system to establish the time of core formation on Vesta at 3.7−1.7+2.5Myr after condensation of calcium–aluminum-rich inclusions (CAIs).

Germanium chemistry and MC-ICPMS isotopic measurements of Fe–Ni, Zn alloys and silicate matrices: Insights into deep Earth processes

Chemical separation and isotopic measurement of germanium using hexapole-collision cell-MC-ICPMS were developed in various Fe–Ni, ZnS and silicates matrices in order to investigate the potentiality of Ge as an isotopic tracer of planetary differentiation and rock-forming processes. Analytical procedures are described for the critical step of silicate dissolution in HF+HNO3 medium, as well as for Ge chemical purification using a single cationic-exchange resin step for Fe–Ni and ZnS matrices, and two anionic and cationic resin steps for silicate matrices. Germanium isotopic measurements using MC-ICPMS were performed with appropriate Ar+H fluxes in the collision cell to eliminate argide interferences on Ge masses. Three methods of mass bias correction, including sample standard bracketing, external Ga mass bias correction using the exponential law, and the empirical “regression method”, give similar results and demonstrate the use of Ga as an appropriate element for mass bias correction of Ge. Results are presented as delta values with respect to JMC Ge standard, and NIST3120a Ge standard for comparison. We show a long-term 2SD reproducibility of less than 0.24‰ on the δ74Ge.These analytical methods have been applied to Fe-meteorites, sphalerite (ZnS) deposits, and geostandard silicates ranging from ultramafic to basaltic to granitic compositions, and to an iron formation composition. Fe-meteorites and terrestrial silicate samples display small variations of δ74GeJMC=+1.77±0.22‰ and +0.89±0.16‰ (2SD reproducibility), respectively. This contrasts with the large variations seen in low-temperature rocks, such as the ZnS ores (δ74GeJMC=−0.37 to −1.69‰), and banded iron formations (IF-G Isua, δ74GeJMC=+1.38‰). A slight δ74GeJMC–NBO/T negative tendency in silicate samples indicates that polymerisation of silicate melt would control the small Ge isotope fractionation among mantle silicates.A comparison of δ74Ge values of iron meteorites and Earth silicate mantle opens new perspectives in deep Earth processes. On the basis of theoretical metal-silicate isotopic equilibrium processes, the low δ74Ge of silicate Earth cannot reconcile one-stage process of core–mantle segregation. It is proposed that the δ74Ge(JMC) value of silicate earth samples of +0.89±0.16‰ (or δ74Ge(NIST3120a)=+0.53‰) represents the composition of the accessible Earth modern mantle. The δ74Ge of the silicate mantle in equilibrium with the core at time of core formation would be distinct to that of the present mantle in result of distinct thermodynamic parameters, e.g. fO2, pressure, inducing changes in coordination and valence state of Ge in the silicate crystallographic structure. In addition, the light isotopic composition of the Earth's mantle could result from reverse diffusive processes induced by an increase in oxidation state at the end of core formation. This would have some implications on core formation modelling and the use of Ge isotopes for tracing the origin of deep mantle plumes.

Disequilibrium melting and melt migration driven by impacts: Implications for rapid planetesimal core formation

The ε182W ages of magmatic iron meteorites are largely within error of the oldest solar system particles, apparently requiring a mechanism for segregation of metals to the cores of planetesimals within 1.5million years of initial condensation. Currently favoured models involve equilibrium melting and gravitational segregation in a static, quiescent environment, which requires very high early heat production in small bodies via decay of short-lived radionuclides. However, the rapid accretion needed to do this implies a violent early accretionary history, raising the question of whether attainment of equilibrium is a valid assumption. Since our use of the Hf–W isotopic system is predicated on achievement of chemical equilibrium during core formation, our understanding of the timing of this key early solar system process is dependent on our knowledge of the segregation mechanism. Here, we investigate impact-related textures and microstructures in chondritic meteorites, and show that impact-generated deformation promoted separation of liquid FeNi into enlarged sulfide-depleted accumulations, and that this happened under conditions of thermochemical disequilibrium. These observations imply that similar enlarged metal accumulations developed as the earliest planetesimals grew by rapid collisional accretion. We suggest that the nonmagmatic iron meteorites formed this way and explain why they contain chondritic fragments in a way that is consistent with their trace element characteristics. As some planetesimals grew large enough to develop partially molten silicate mantles, these enlarged metal accumulations would settle rapidly to form cores leaving sulfide and small metal particles behind, since gravitational settling rate scales with the square of metal particle size. Our model thus provides a mechanism for more rapid core formation with less radiogenic heating. In contrast to existing models of core formation, the observed rarity of sulfide-dominant meteorites is an expected consequence of our model, which promotes early and progressive separation of metal and sulfide. We suggest that the core formation models that assume attainment of equilibrium in the Hf–W system underestimate the core formation time.

Hydrous melting of the martian mantle produced both depleted and enriched shergottites

The search for water in our solar system is one of the primary driving forces for planetary science and exploration because water plays an important role in many geologic processes and is required for biologic processes as we currently understand them. Excluding Earth, Mars is the most promising destination in the inner solar system to find water, as it is undoubtedly responsible for shaping many geomorphologic features observed on the present-day martian surface; however, the water content of the martian interior is currently unresolved. Much of our information about the martian interior comes from studies of the basaltic martian meteorites (shergottites). In this study we examined the water contents of magmatic apatites from a geochemically enriched shergottite (the Shergotty meteorite) and a geochemically depleted shergottite (the Queen Alexandria Range 94201 meteorite). From these data, we determined that there was little difference in water contents between the geochemically depleted and enriched shergottite magmas. The water contents of the apatite imply that shergottite parent magmas contained 730–2870 ppm H 2 O prior to degassing. Furthermore, the martian mantle contains 73–290 ppm H 2 O and underwent hydrous melting as recently as 327 Ma. In the absence of plate tectonics, the presence of water in the interior of Mars requires planetary differentiation under hydrous conditions. This is the first evidence of significant hydrogen storage in a planetary interior at the time of core formation, and this process could support elevated H abundances in the interiors of other terrestrial bodies like the Moon, Mercury, Venus, large differentiated asteroids, and Earth.

Isotopic constraints on the age of the Earth’s core: Mutual consistency of the Hf-W and U-Pb systems

The estimation of the time of Earth’s core formation on the basis of isotopic systems with short-lived and long-lived parent nuclides gives significantly different results. Isotopic data for the 182Hf-182W system with a 182Hf half-life of approximately 9 Myr can be interpreted in such a way that the core was formed 34 Myr after the origin of the solar system assuming complete core-mantle equilibrium. Similar estimates on the basis of the U-Pb isotopic system suggest a significantly longer mean time of core formation of approximately 120 Myr. If the Earth’s core were formed instantaneously, both isotopic systems would have shown identical values corresponding to the true age. The discrepancy between the U-Pb and Hf-W systems can be resolved assuming prolonged differentiation of prototerrestrial material into silicate and metallic phases, which occurred not simultaneously and uniformly in different parts of the mantle. This resulted in the isotopic heterogeneity of the mantle, and its subsequent isotopic homogenization occurred slowly. Under such conditions, the mean isotopic compositions of W and Pb in the mantle do not correspond to the mean time of the separation of silicate and metallic phases. This is related to the fact that the exponential function of radioactive decay is strongly nonlinear at high values of the argument, and its mean value does not correspond to the mean value of the function. There are compelling reasons to believe that the early mantle was heterogeneous with respect to W isotopic composition and was subsequently homogenized by convective mixing. This follows from the fact that the lifetime of isotopic heterogeneities in the mantle is close to 1.8 Gyr for various long-lived isotopic systems. There is also no equilibrium between the mantle and the core with respect to the contents of siderophile elements. Because of this, the mean isotopic ratios of W and Pb cannot be used for the direct computation of the time of metal-silicate differentiation in the Earth. Such estimation requires more sophisticated models accounting for the duration of the differentiation process using several isotope pairs. Given the prolonged core formation, which has probably continued up to now, the question about its age becomes ambiguous, and only the most probable growth rate of the core can be estimated. The combined use of the U-Pb and Hf-W systems constrains the time of formation of 90% of the core mass between 0.12 and 2.7 billion years. These model estimates could have been realistic under the condition of complete disequilibrium between the silicate and metallic phases, which is as improbable as the suggestion of complete equilibrium between them on the whole Earth scale.

Silicon isotopes in meteorites and planetary core formation

The silicon (Si) isotope compositions of 42 meteorite and terrestrial samples have been determined using MC-ICPMS with the aim of resolving the current debate over their compositions and the implications for core formation. No systematic δ 30Si differences are resolved between chondrites (δ 30Si = −0.49 ± 0.15‰, 2σ SD) and achondrites (δ 30Si = −0.47 ± 0.11‰, 2σ SD), although enstatite chondrites are consistently lighter (δ 30Si = −0.63 ± 0.07‰, 2σ SD) in comparison to other meteorite groups. The data reported here for meteorites and terrestrial samples display an average difference Δ 30Si BSE−meteorite∗ = 0.15 ± 0.10‰, which is consistent within uncertainty with previous studies. No effect from sample heterogeneity, preparation, chemistry or mass spectrometry can be identified as responsible for the reported differences between current datasets. The heavier composition of the bulk silicate Earth is consistent with previous conclusions that Si partitioned into the metal phase during metal–silicate equilibration at the time of core formation. Fixing the temperature of core formation to the peridotite liquidus and using an appropriate metal silicate fractionation factor ( ε ∼0.89), the Δ 30Si BSE−meteorite∗ value from this study indicates that the Earth core contains at least 2.5 and possibly up to 16.8 wt% Si.

A model for osmium isotopic evolution of metallic solids at the core‐mantle boundary

Some plumes are thought to originate at the core‐mantle boundary, but geochemical evidence of core‐mantle interaction is limited to Os isotopes in samples from Hawaii, Gorgona (89 Ma), and Kostomuksha (2.7 Ga). The Os isotopes have been explained by physical entrainment of Earth's liquid outer core into mantle plumes. This model has come into conflict with geophysical estimates of the timing of core formation, high‐pressure experimental determinations of the solid metal‐liquid metal partition coefficients (D), and the absence of expected 182W anomalies. A new model is proposed where metallic liquid from the outer core is partially trapped in a compacting cumulate pile of Fe‐rich nonmetallic precipitates (FeO, FeS, Fe3Si, etc.) at the top of the core and undergoes fractional crystallization precipitating solid metal grains, followed by expulsion of the residual metallic liquid back to the outer core. The Os isotopic composition of the solids and liquids in the cumulate pile is modeled as a function of the residual liquid remaining and the emplacement age using 1 bar D values, with variable amounts of oxygen (0–10 wt %) as the light element. The precipitated solids evolve Os isotope compositions that match the trends for Hawaii (at an emplacement age of 3.5–4.5 Ga; 5%–10% oxygen) and Gorgona (emplacement age < 1.5 Ga; 0%–5% oxygen). The Fe‐rich matrix of the cumulate pile dilutes the precipitated solid metal decoupling the Fe/Mn ratio from Os and W isotopes. The advantages to using precipitated solid metal as the Os host include a lower platinum group element and Ni content to the mantle source region relative to excess iron, miniscule anomalies in 182W (<0.1 ɛ), and no effects for Pb isotopes, etc. A gradual thermomechanical erosion of the cumulate pile results in incorporation of this material into the base of the mantle, where mantle plumes subsequently entrain it. Fractional crystallization of metallic liquids within the CMB provides a consistent explanation of both Os isotope correlations, Os‐W isotope systematics, and Fe/Mn evidence for core‐mantle interaction over the entire Hawaiian source.

FORMATION OF COLLAPSING CORES IN SUBCRITICAL MAGNETIC CLOUDS: THREE-DIMENSIONAL MAGNETOHYDRODYNAMIC SIMULATIONS WITH AMBIPOLAR DIFFUSION

We employ the three-dimensional magnetohydrodynamic simulation including ambipolar diffusion to study the gravitationally-driven fragmentation of subcritical molecular clouds, in which the gravitational fragmentation is stabilized as long as magnetic flux-freezing applies. The simulations show that the cores in an initially subcritical cloud generally develop gradually over an ambipolar diffusion time, which is about a few $\times 10^7$ years in a typical molecular cloud. On the other hand, the formation of collapsing cores in subcritical clouds is accelerated by supersonic nonlinear flows. Our parameter study demonstrates that core formation occurs faster as the strength of the initial flow speed in the cloud increases. We found that the core formation time is roughly proportional to the inverse of the square root of the enhanced density created by the supersonic nonlinear flows. The density dependence is similar to that derived in quasistatically contracting magnetically supported clouds, although the core formation conditions are created by the nonlinear flows in our simulations. We have also found that the accelerated formation time is not strongly dependent on the initial strength of the magnetic field if the cloud is highly subcritical. Our simulation shows that the core formation time in our model subcritical clouds is several $\times 10^6$ years, due to the presence of large-scale supersonic flows ($\sim 3$ times sound speed). Once a collapsing core forms, the density, velocity, and magnetic field structure of the core does not strongly depend on the initial strength of the velocity fluctuation. The infall velocities of the cores are subsonic and the magnetic field lines show weak hourglass shapes.

IS PROTOSTELLAR HEATING SUFFICIENT TO HALT FRAGMENTATION? A CASE STUDY OF THE MASSIVE PROTOCLUSTER G8.68–0.37

If star formation proceeds by thermal fragmentation and the subsequent gravitational collapse of the individual fragments, how is it possible to form fragments massive enough for O and B stars in a typical star-forming molecular cloud where the Jeans mass is about 1Msun at the typical densities (10^4 cm^-3) and temperatures (10K)? We test the hypothesis that a first generation of low-mass stars may heat the gas enough that subsequent thermal fragmentation results in fragments >=10Msun, sufficient to form B stars. We combine ATCA and SMA observations of the massive star-forming region G8.68-0.37 with radiative transfer modeling to derive the present-day conditions in the region and use this to infer the conditions in the past, at the time of core formation. Assuming the current mass/separation of the observed cores equals the fragmentation Jeans mass/length and the region's average density has not changed, requires the gas temperature to have been 100K at the time of fragmentation. The postulated first-generation of low-mass stars would still be around today, but the number required to heat the cloud exceeds the limits imposed by the observations. Several lines of evidence suggest the observed cores in the region should eventually form O stars yet none have sufficient raw material. Even if feedback may have suppressed fragmentation, it was not sufficient to halt it to this extent. To develop into O stars, the cores must obtain additional mass from outside their observationally defined boundaries. The observations suggest they are currently fed via infall from the very massive reservoir (~1500Msun) of gas in the larger pc scale cloud around the star-forming cores. This suggests that massive stars do not form in the collapse of individual massive fragments, but rather in smaller fragments that themselves continue to gain mass by accretion from larger scales.

Turbulent mixing of metal and silicate during planet accretion — And interpretation of the Hf–W chronometer

In the current view of planet formation, the final assembly of the Earth involved giant collisions between proto-planets (>1000km radius), with the Moon formed as a result of one such impact. At this stage the colliding bodies had likely differentiated into a metallic core surrounded by a silicate mantle. During the Moon-forming impact, nearly all metal sank into the Earth's core. We investigate to what extent large self-gravitating iron cores can mix with surrounding silicate and how this influences the short-lived chronometer, Hf–W, used to infer the age of the Moon. We present fluid dynamical models of turbulent mixing in fully liquid systems, attempting to place constraints on the degree of mixing. Erosion of sinking cores driven by Rayleigh–Taylor instability does lead to intimate mixing and equilibration, but large blobs (>10km diameter) do not emulsify entirely. Emulsification is enhanced if most of the accreting metal cores deform into thin structures during descent through the Earth's mantle. Yet, only 1–20% of Earth's core would equilibrate with silicate during Earth's accretion. The initial speed of the impactor is of little importance. We proceed to evaluate the mixing potential for shear instabilities where silicate entrainment across vertical walls causes mixing. The turbulent structure indicates that vortices remain at the largest scale and do not mix to centimeter length scale, where diffusion operates and isotopes can equilibrate. Thus, incomplete emulsification and equilibration of accreting iron cores is likely to occur.The extent of metal–silicate equilibration provides key information for interpretation of siderophile budgets and the timing of core formation using the Hf–W chronometer. The time scale of core formation derived from the Hf–W chronometer is usually tied to the last major metal–silicate re-equilibration, believed to coincide with time of the Moon-forming impact. However, we show that large cores have limited ability to reset the Hf–W system in the silicate Earth. Excess 182W in bulk silicate Earth is more sensitive to early core formation processes than to radiogenic ingrowth after the last giant impact.

Hf–W mineral isochron for Ca,Al-rich inclusions: Age of the solar system and the timing of core formation in planetesimals

Application of 182Hf– 182W chronometry to constrain the duration of early solar system processes requires the precise knowledge of the initial Hf and W isotope compositions of the solar system. To determine these values, we investigated the Hf–W isotopic systematics of bulk samples and mineral separates from several Ca,Al-rich inclusions (CAIs) from the CV3 chondrites Allende and NWA 2364. Most of the investigated CAIs have relative proportions of 183W, 184W, and 186W that are indistinguishable from those of bulk chondrites and the terrestrial standard. In contrast, one of the investigated Allende CAIs has a lower 184W/ 183W ratio, most likely reflecting an overabundance of r-process relative to s-process isotopes of W. All other bulk CAIs have similar 180Hf/ 184W and 182W/ 184W ratios that are elevated relative to average carbonaceous chondrites, probably reflecting Hf–W fractionation in the solar nebula within the first ∼3 Myr. The limited spread in 180Hf/ 184W ratios among the bulk CAIs precludes determination of a CAI whole-rock isochron but the fassaites have high 180Hf/ 184W and radiogenic 182W/ 184W ratios up to ∼14 ε units higher than the bulk rock. This makes it possible to obtain precise internal Hf–W isochrons for CAIs. There is evidence of disturbed Hf–W systematics in one of the CAIs but all other investigated CAIs show no detectable effects of parent body processes such as alteration and thermal metamorphism. Except for two fractions from one Allende CAI, all fractions from the investigated CAIs plot on a single well-defined isochron, which defines the initial ε 182W = −3.28 ± 0.12 and 182Hf/ 180Hf = (9.72 ± 0.44) × 10 −5 at the time of CAI formation. The initial 182Hf/ 180Hf and 26Al/ 27Al ratios of the angrites D’Orbigny and Sahara 99555 are consistent with the decay from initial abundances of 182Hf and 26Al as measured in CAIs, suggesting that these two nuclides were homogeneously distributed throughout the solar system. However, the uncertainties on the initial 182Hf/ 180Hf and 26Al/ 27Al ratios are too large to exclude that some 26Al in CAIs was produced locally by particle irradiation close to an early active Sun. The initial 182Hf/ 180Hf of CAIs corresponds to an absolute age of 4568.3 ± 0.7 Ma, which may be defined as the age of the solar system. This age is 0.5–2 Myr older than the most precise 207Pb– 206Pb age of Efremovka CAI 60, which does not seem to date CAI formation. Tungsten model ages for magmatic iron meteorites, calculated relative to the newly and more precisely defined initial ε 182W of CAIs, indicate that core formation in their parent bodies occurred in less than ∼1 Myr after CAI formation. This confirms earlier conclusions that the accretion of the parent bodies of magmatic iron meteorites predated chondrule formation and that their differentiation was triggered by heating from decay of abundant 26Al. A more precise dating of core formation in iron meteorite parent bodies requires precise quantification of cosmic-ray effects on W isotopes but this has not been established yet.

Numerical simulations of the differentiation of accreting planetesimals with 26Al and 60Fe as the heat sources

Abstract— Numerical simulations have been performed for the differentiation of planetesimals undergoing linear accretion growth with 26Al and 60Fe as the heat sources. Planetesimal accretion was started at chosen times up to 3 Ma after Ca‐Al‐rich inclusions (CAIs) were formed, and was continued for periods of 0.001–1 Ma. The planetesimals were initially porous, unconsolidated bodies at 250 K, but became sintered at around 700 K, ending up as compact bodies whose final radii were 20, 50, 100, or 270 km. With further heating, the planetesimals underwent melting and igneous differentiation. Two approaches to core segregation were tried. In the first, labelled A, the core grew gradually before silicate began to melt, and in the second, labelled B, the core segregated once the silicate had become 40% molten. In A, when the silicate had become 20% molten, the basaltic melt fraction began migrating upward to the surface, carrying 26Al with it. The 60Fe partitioned between core and mantle. The results show that the rate and timing of core and crust formation depend mainly on the time after CAIs when planetesimal accretion started. They imply significant melting where accretion was complete before 2 Ma, and a little melting in the deep interiors of planetesimals that accreted as late as 3 Ma. The latest melting would have occurred at <10 Ma. The effect on core and crust formation of the planetesimal's final size, the duration of accretion, and the choice of (60Fe/56Fe)initial were also found to be important, particularly where accretion was late. The results are consistent with the isotopic ages of differentiated meteorites, and they suggest that the accretion of chondritic parent bodies began more than 2 or 3 Ma after CAIs.

Two‐dimensional stokes flow around a heated cylinder: A possible application for diapirs in the mantle

It is widely assumed that the separation of metal and silicates in a homogeneously accreted protoplanet occurred rather rapidly. The process of core formation through the descent of large, hot, iron‐rich bodies through a cold, silicate protoplanet is discussed. If iron diapirs sink toward the planet center with the Stokes velocity, they would require more than 800 Ma to form a core in the case of a planet similar to the Earth. We therefore implement a temperature‐dependent rheology for the silicate material surrounding the diapir, which leads to a reduction of the drag force exerted on the iron drop. We compare the drag force for different sinking velocities to the body force on the diapir and find that the terminal velocity in the temperature‐dependent viscosity model is a factor of 30 higher than that in an isoviscous medium. On the basis of these results, the core formation time for the Earth could be as little as 30 Ma.

How rapidly did Mars accrete? Uncertainties in the Hf–W timing of core formation

Estimates for the martian core formation timescale based on the hafnium–tungsten (Hf–W) isotopic system have varied by almost an order of magnitude, because of uncertainties in the martian mantle Hf/W ratio. Here we argue that the Hf/W ratio is ∼4 but is uncertain by ∼25%, resulting in (instantaneous) martian core formation timescales ranging from 0 to 10 Myr; accordingly, Hf–W isotope observations currently have limited utility in distinguishing between scenarios in which Mars formed as a stranded embryo and scenarios in which Mars suffered a prolonged accretion history.

Planet Formation with Migration

In the core-accretion model, gas-giant planets form solid cores which then accrete gaseous envelopes. Tidal interactions with disk gas cause a core to undergo inward type-I migration in 10^4 to 10^5 years. Cores must form faster than this to survive. Giant planets clear a gap in the disk and undergo inward type-II migration in <10^6 years if observed disk accretion rates apply to the disk as a whole. Type-II migration times exceed typical disk lifetimes if viscous accretion occurs mainly in the surface layers of disks. Low turbulent viscosities near the midplane may allow planetesimals to form by coagulation of dust grains. The radius r of such planetesimals is unknown. If r<0.5 km, the core formation time is shorter than the type-I migration timescale and cores will survive. Migration is substantial in most cases, leading to a wide range of planetary orbits, consistent with the observed variety of extrasolar systems. When r is of order 100m and midplane alpha is of order 3 times 10^-5, giant planets similar to those in the Solar System can form.

The timing of core formation in the terrestrial planets

The timing of core formation in the terrestrial planets

Large thallium isotopic variations in iron meteorites and evidence for lead-205 in the early solar system

Lead-205 decays to 205Tl with a half-life of 15 Myr and should have been present in the early solar system according to astrophysical models. However, despite numerous attempts, Tl isotopic measurements of meteorites have been unable to demonstrate convincingly its former presence. Here, we report large (∼5‰) variations in Tl isotope composition in metal and troilite fragments from a range of iron meteorites that were determined at high precision using multiple collector inductively coupled plasma mass spectrometry. The Tl isotopic compositions of seven metal samples of the IAB iron meteorites Toluca and Canyon Diablo define a correlation with 204Pb/ 203Tl. When interpreted as an isochron, this corresponds to an initial 205Pb/ 204Pb ratio of (7.4 ± 1.0) × 10 −5. Alternative explanations for the correlation, such as mixing of variably mass-fractionated meteorite components or terrestrial contamination are harder to reconcile with independent constraints. However, troilite nodules from Toluca and Canyon Diablo contain Tl that is significantly less radiogenic than co-existing metal with isotope compositions that are variable and decoupled from 204Pb/ 203Tl. These effects are similar to those recently reported by others for Fe and Ni isotopes in iron meteorite sulfides and appear to be the result of kinetic stable isotope fractionation during diffusion. Though it cannot conclusively be shown that the metal fragments are unaffected by the secondary processes that disturbed the troilites, mass balance modeling indicates that the alteration of the troilites is unlikely to have significantly affected the Tl isotope compositions of the co-existing metals. It is therefore reasonable to conclude that the IAB metal isochron is a product of the in situ decay of 205Pb. If the I–Xe ages of IAB silicate inclusions record the same event as the 205Pb– 205Tl chronometer then crystallization of the IAB metal was probably completed between 10 and 20 Myr after the condensation of the first solids. This implies an initial solar system 205Pb/ 204Pb of (1.0–2.1) × 10 −4, which is in excellent agreement with recently published astrophysical predictions. Similar calculations yield an initial solar system Tl isotope composition of ε 205Tl = −2.8 ± 1.7. The Tl isotopic composition and concentration of the silicate Earth depends critically on the timing and mechanism of core formation and Earth’s volatile element depletion history. Modeling of the Earth’s accretion and core formation using the calculated initial solar system Tl isotope composition and 205Pb/ 204Pb, however, does not yield reasonable results for the silicate Earth unless either the Earth lost Tl and Pb late in its accretion history or the core contains much higher concentrations of Pb and Tl than are found in iron meteorites.

The effect of metal composition on Fe–Ni partition behavior between olivine and FeNi-metal, FeNi-carbide, FeNi-sulfide at elevated pressure

Metal–olivine Fe–Ni exchange distribution coefficients were determined at 1500 °C over the pressure range of 1 to 9 GPa for solid and liquid alloy compositions. The metal alloy composition was varied with respect to the Fe/Ni ratio and the amount of dissolved carbon and sulfur. The Fe/Ni ratio of the metal phase exercises an important control on the abundance of Ni in the olivine. The Ni abundance in the olivine decreases as the Fe/Ni ratio of the coexisting metal increases. The presence of carbon (up to ∼ 3.5 wt.%) and sulfur (up to ∼ 7.5 wt.%) in solution in the liquid Fe–Ni-metal phase has a minor effect on the partitioning of Fe and Ni between metal and olivine phases. No pressure dependence of the Fe–Ni-metal–olivine exchange behavior in carbon- and sulfur-free and carbon- and sulfur-containing systems was found within the investigated pressure range. To match the Ni abundance in terrestrial mantle olivine, assuming an equilibrium metal–olivine distribution, a sub-chondritic Fe/Ni-metal ratio that is a factor of 17 to 27 lower than the Fe/Ni ratios in estimated Earth core compositions would be required, implying higher Fe concentrations in the core forming metal phase. A simple metal–olivine equilibrium distribution does not seem to be feasible to explain the Ni abundances in the Earth's mantle. An equilibrium between metal and olivine does not exercise a control on the problem of Ni overabundance in the Earth's mantle. The experimental results do not contradict the presence of a magma ocean at the time of terrestrial core formation, if olivine was present in only minor amounts at the time of metal segregation.

The W isotope evolution of the bulk silicate Earth: constraints on the timing and mechanisms of core formation and accretion

The W isotope composition of the bulk silicate Earth exhibits a small but resolvable excess in the abundance of 182W relative to that found in chondrites, indicating that core formation in Earth took place within the life-time of now extinct 182Hf. This 182W excess provides a firm constraint for the lower limit of the time of core formation in Earth. Separation and segregation of metal into Earth's core cannot have ceased earlier than ∼30 Myr after the start of the solar system. Determining the exact timing of core formation, however, requires knowledge of the degree of equilibration of newly accreted material with Earth's mantle. Conversely, if independent age constraints for the formation of Earth's core are available, the 182W excess of Earth's mantle relative to chondrites can be used to constrain the degree of metal-silicate equilibration during Earth's accretion. Provided that the Moon-forming event is the last large impact, the latest time core formation can have ceased in Earth is provided by the age of the oldest lunar samples and is ∼70–100 Myr after the start of the solar system. If, as seems likely, the impactor's core did not re-equilibrate extensively with the silicate proto-Earth, the Moon cannot have formed before ∼40 Myr resulting in an age of the Earth and the Moon of 40–70 Myr after the start of the solar system. The 182W excess of the bulk silicate Earth relative to chondrites of ∼2ε units is substantially smaller than the 15–20εW range expected if Earth's core formed by merging of metal cores of early differentiated planetesimals, indicating significant re-homogenization of newly accreted planetesimals with Earth's mantle. In continuous core formation models that assume growth of Earth at an exponentially decreasing rate, more than ∼70% of the newly accreted material must have equilibrated with Earth's mantle. Including the Moon-forming impact into these models such that Earth was 89% accreted at the time of the impact and 10% was added by the impactor implies more than ∼50% metal-silicate equilibration in the silicate proto-Earth. Such high degrees of metal-silicate equilibration can only be achieved if core formation occurred by the physical separation of liquid metal from mostly molten silicate providing strong support for the hypothesis of a terrestrial magma ocean. Model calculations show that formation of a magma ocean by a late Moon-forming impact is not sufficient in removing radiogenic 182W from Earth's mantle that would have accumulated as a result of early core formation. A high degree of metal-silicate equilibration of at least 50% must have been established prior to Moon formation, implying that metal segregation in the proto-Earth already occurred in a magma ocean. Therefore, the Moon-forming impact is not the only impact that led to the formation of a magma ocean, indicating multiple formations of magma oceans or a protracted life-time of the magma ocean.

Magmatic fractionation of Hf and W: constraints on the timing of core formation and differentiation in the Moon and Mars

Excesses of 182W have previously been measured in samples from the Moon and Mars, and can be derived from high Hf/W regions in their interiors during their early histories. Although planetary mantles will have superchondritic Hf/W after core formation, the extent to which high Hf/W regions could be generated by magmatic fractionation has not been evaluated. In order to address the latter possibility, we have carried out experiments from 100 MPa to 10.0 GPa, 1150 to 1850°C, at oxygen fugacities near the IW (iron-wüstite) buffer, and measured partition coefficients for W and Hf for plagioclase-liquid, olivine-liquid, orthopyroxene-liquid, clinopyroxene-liquid, garnet-liquid, and metal-liquid pairs. Clinopyroxene and garnet are both capable of fractionating Hf from W during magmatic crystallization or mantle melting, and minor variations in the measured D’s can be attributed to crystal chemical effects. Excesses of 182W and 142Nd in lunar samples can be explained by fractionation of Hf from W, and Sm from Nd (by ilmenite and clinopyroxene) during crystallization of the latest stages of a lunar magma ocean. Correlations of ε W with ε Nd in martian samples could be a result of early silicate fractionation in the martian mantle (clinopyroxene and/or garnet).