Time Of Core Formation Research Articles

Role of 26Al and impact-generated atmosphere in the early thermal evolution, differentiation, and volatile-rich core of Mars

The InSight mission's high-quality seismic data and cosmochemical models have demonstrated that Mars has a volatile-rich core. It is proposed that Mars formed early, before the solar nebular gas dispersed, and/or it accreted volatile-rich material to account for its volatile-rich interior. The early accreted Mars must have gravitationally attracted proto-atmosphere from the surrounding solar nebula. If its building blocks were rich in volatiles, it might have acquired impact-generated atmosphere during accretion. By considering the decay energy of the short-lived radionuclide (SLR) 26Al and the blanketing effect of the primordial and impact-generated atmosphere during the initial 10 Ma of the formation of the solar system, we present the results of numerical simulations carried out for the early thermal evolution and core formation of Mars. To explain the formation of a large iron core of Mars as suggested by the seismometer of InSight mission, we considered the 60% EH- and 40% CI-chondrite composition for the building blocks of Mars. The blanketing effect of impact generated H2O + CO2+CO + H2 atmosphere caused the melting of surface silicates to form magma ocean at surface. We thoroughly examined the effects of several variables on the timing of complete core formation, including the initial water content in the accreted material, timescale of onset of accretion after the formation of CAIs, duration of accretion, absorption coefficient of CO2, efficiency of degassing of volatiles, and melting temperature. The results of present study show that the decay energy of SLR 26Al caused the widespread heating and melting of interior of Mars. For complete core formation, Mars must accrete within the first ∼2 Ma in order to explain the volatile-rich core. Further, the timescales of core formation were found to be strongly dependent on the assumed melting curve. For longer timescales of accretion, the interior of Mars experienced incomplete differentiation even by assuming high water content in the accreted material and larger absorption coefficient for CO2. However, Mars does not have to accrete within 2 Ma for the core to fully develop if the Rayleigh- Taylor (RT) instability is present in the dense metallic layer on top of the undifferentiated interior. A short accretion time is only valid if RT-instability is neglected. Hence, we expect that the volatiles in the magma ocean could also be transferred to core by Rayleigh-Taylor instability. We also parameterized the thermal influence of gravitational heat produced during the differentiation of Mars to study its influence on the thermal evolution of the planet. The outcomes of the present work have implications to explain the timing of partition of volatiles in the Martian interior and formation of an early atmosphere on Mars.

Open-system 182W[sbnd]142Nd isotope evolution of the Earth

We present a five-reservoir open-system model for the 182Hf182W and 146Sm142Nd isotope evolution of the Earth to constrain early mantle-crust exchange and late accretion processes. In the presented model, core formation is complete within 30–100 Myr after solar system formation. The complementary bulk silicate Earth (BSE) differentiates to form a continental crust (CC), an incompatible element depleted upper mantle (UM), an initially primitive but continuously evolving lower mantle (LM), and an isolated reservoir (IR) where the recycled crust is stored before mixing into the LM. In our default model, late accretion adds bulk Earth-like material to the mantle after core formation, concurrent to progressive silicate differentiation. The 182W and 142Nd isotope evolution in each reservoir is calculated with a series of differential equations that compute the changing abundance of each isotope from the start of solar system evolution (t = 4.56 Ga) to the present (t = 0 Ga). Core formation, by far, has the greatest influence on the 182Hf182W isotopic evolution of the bulk silicate Earth (BSE). It determines the ∼200 ppm offset between BSE's μ182W (μ182WBSE ∼ 0) and chondrites (μ182Wchondrite ∼ −200 ppm), whereas late accretion adds ∼10% of BSE's W which changes μ182WBSE by only ∼several 10s ppm. Most compatible with the modeled 182W evolution and W content of the silicate Earth are core formation processes lasting until ∼45–60 Myr after solar system initial and subsequent addition of about 1.0–0.5% late accreted material to Earth's mantle. However, both the pre-late accretion μ182W and W content of the BSE depend strongly on the rate and duration of core formation and ultimately determine the amount and type of late accreted material necessary to evolve to the present μ182WBSE ∼ 0, or any other value slightly higher or lower. Because tight constraints on the timing and rate of core formation during accretion are still lacking, the trade-off between core formation processes and the effects of late accretion on BSE's 182W evolution remains grossly under-constrained, making the μ182W evolution of Earth's mantle-crust system similarly under-determined. Nevertheless, robust constraints are that heterogeneous distribution of the late accreted material to Earth's mantle in combination with Hadean crust formation and recycling is required to form a mantle reservoir with a small range of 182W excesses between 10 and 15 ppm throughout the Archean, as observed in most Archean rocks. Reproducing the observed 142Nd and 182W signatures in Hadean-Eoarchean mantle-derived rocks with our model further requires that continental crust formation starts within the first ∼50 Myr after core formation. Continuous exchange between the crust and the different mantle reservoirs in our model leads to post-Archean homogenization of early-formed 182W and 142Nd heterogeneities and makes it difficult to preserve ancient μ182W and μ142Nd deviations from BSE values. Nevertheless, μ182W < 0 inherited from the combined effects of early crust-mantle differentiation and late accretion may under certain conditions be preserved in parts of Earth's mantle throughout the Archean, and perhaps even until today. The μ182W < 0 observed in recent oceanic basalts may therefore be inherited from early silicate differentiation or originate from potential core-mantle interaction at any time during Earth's history.

Early differentiation of magmatic iron meteorite parent bodies from Mn–Cr chronometry

Magmatic iron meteorite groups such as IIAB, IIIAB and IVA, represent the largest sampling of extraterrestrial core material from the earliest accreted distinct planetary bodies in the solar system. Chromium isotope compositions of chromite/daubreelite from seven samples, translated into 53Cr/52Cr model ages, provide robust time information on planetary core formation. These ages range within 1.5 Ma after formation of calcium-aluminium-rich inclusions (CAIs) and define the time of metal core formation in the respective parent bodies, assuming metal-silicate separation was an instantaneous event that induced strong chemical fractionation of Mn from the more siderophile Cr. The early core formation ages support accretion and differentiation of the magmatic iron meteorite parent bodies to have occurred prior to the chondrule formation interval. The calibration of Mn-Cr ages with established Hf-W ages of samples from the same magmatic iron meteorite groups constrains the initial E53Cr of the solar system to -0.30, and thus lower than previously estimated.

Timing of Martian core formation from models of Hf–W evolution coupled with N-body simulations

Determining how and when Mars formed has been a long-standing challenge for planetary scientists. The size and orbit of Mars are difficult to reproduce in classical simulations of planetary accretion, and this has inspired models of inner solar system evolution that are tuned to produce Mars-like planets. However, such models are not always coupled to geochemical constraints. Analyses of Martian meteorites using the extinct hafnium–tungsten (Hf–W) radioisotopic system, which is sensitive to the timing of core formation, have indicated that the Martian core formed within a few million years of the start of the solar system itself. This has been interpreted to suggest that, unlike Earth’s protracted accretion, Mars grew to its modern size very rapidly. These arguments, however, generally rely on simplified growth histories for Mars. Here, we combine likely accretionary histories from a large number of N-body simulations with calculations of metal–silicate partitioning and Hf–W isotopic evolution during core formation to constrain the range of conditions that could have produced Mars.We find that there is no strong correlation between the final masses or orbits of simulated Martian analogs and their 182W anomalies, and that it is readily possible to produce Mars-like Hf–W isotopic compositions for a variety of accretionary conditions. The Hf–W signature of Mars is very sensitive to the oxygen fugacity (fO2) of accreted material because the metal–silicate partitioning behavior of W is strongly dependent on redox conditions. The average fO2 of Martian building blocks must fall in the range of 1.3–1.6 log units below the iron–wüstite buffer to produce a Martian mantle with the observed Hf/W ratio. Other geochemical properties (such as sulfur content) also influence Martian 182W signatures, but the timing of accretion is a more important control. We find that while Mars must have accreted most of its mass within ∼5 million years of solar system formation to satisfy the Hf–W isotopic constraints, it may have continued growing afterwards for over 50 million years. There is a high probability of simultaneously matching the orbit, mass, and Hf–W signature of Mars even in cases of prolonged accretion if giant impactor cores were poorly equilibrated and merged directly with the proto-Martian core.

Iron and Nickel Isotopes in IID and IVB Iron Meteorites: Evidence for Admixture of an SN II Component and Implications for the Initial Abundance of 60Fe

Abstract We measured Fe and Ni isotopes in 13 iron meteorites from magmatic groups IID and IVB. Resolvable deficits (≈−0.14) are observed in ε 60Ni in both groups. Small deficits (≈−0.08) are also evident in ε 56Fe but not in the most neutron-rich isotope of Fe (58Fe). Relative to terrestrial material, the observed ε iFe values are consistent with the presence of a small excesses of material in the parent bodies of IID and IVB irons that was produced in a type II supernova. The ε 60Ni and ε 56Fe values are uncorrelated in both groups. This is consistent with a nucleosynthetic origin of the ε 56Fe deficits, whereas the ε 60Ni deficits are best explained as radiogenic in nature due to the former presence of live 60Fe (t 1/2 = 2.62 Ma) in the early solar system. The 60Ni deficits correspond to 60Fe/56Fe ratios of ≈3 × 10−7 at the time of core formation on the IID and IVB parent bodies. These data, in conjunction with previously published 182Hf–182W core formation ages, are used to estimate a solar system initial 60Fe/56Fe = (6.4 ± 2.0) × 10−7 for the formation region of carbonaceous chondrites.

Constraints on Martian Chronology from Meteorites

Martian meteorites provide the only direct constraints on the timing of Martian accretion, core formation, magmatic differentiation, and ongoing volcanism. While many radiogenic isotope chronometers have been applied to a wide variety of Martian samples, few, if any, techniques are immune to secondary effects from alteration and terrestrial weathering. This short review focuses on the most robust geochronometers that have been used to date Martian meteorites and geochemically model the differentiation of the planet, including 147Sm/143Nd, 146Sm/142Nd, 176Lu/176Hf, 182Hf/182W, and U-Th-Pb systematics.

Ag isotopic and chalcophile element evolution of the terrestrial and martian mantles during accretion: New constraints from Bi and Ag metal-silicate partitioning

The Earth's timing of accretion and acquisition of moderately volatile compounds is uncertain. Hafnium-W and Mn-Cr isotopic data can bracket the timing of early planetary differentiation and core formation. The Ag-Pd system has also been utilized but its application has been limited by a lack of high pressure and temperature metal-silicate partitioning for Pd and Ag. Because Ag (and Bi) are volatile chalcophile siderophile elements, understanding their early distribution can constrain the origin of volatile elements in differentiated bodies and planets. Unfortunately, neither Ag or Bi have been studied across the wide range of pressure and temperature conditions that are relevant to accretion and core-mantle differentiation. Here, new high-pressure and temperature multi-anvil metal-silicate equilibrium experiments for Bi and Ag have been carried out at conditions relevant to planetary accretion and metal silicate differentiation that allow a more refined and complete understanding of element partitioning during core formation. The new metal-silicate partitioning data are combined with previously reported data, and utilized to predict the distributions of Bi, Pd, and Ag at conditions of accretion relevant for Earth and Mars. Application of the new partitioning results to Earth shows that D(Bi) and D(Ag) (D = metal/silicate concentration ratio) are lowered due to the effect of pressure and Si alloyed in the metallic liquid, resulting in higher predicted mantle Bi and Ag abundances than in the bulk silicate Earth (BSE), as well as high and variable Pd/Ag. The unradiogenic Ag isotopic composition of the BSE could have been generated by early accretion of volatile-poor (high Pd/Ag) pre-cursors, followed by later accretion of volatile–rich (low Pd/Ag) material, in agreement with earlier studies of Pd-Ag and Mn-Cr (Schönbächler et al., 2010). However, these main accretion phases would have to be followed by segregation of a sulfide liquid (at least 1.5% of magma ocean) at high pressures (>30 GPa), to explain the primitive upper mantle (PUM) Bi, Pd, and Ag, as well as Au, Pt, Cu and Ni concentrations as proposed previously. If the early accreted bulk Earth was volatile depleted with high Pd/Ag ratios, portions of the mantle may contain ancient domains that developed positive 107Ag isotopic anomalies (as also argued by noble gases, Nd, W, and Os isotopes). In comparison, Bi, Pd, and Ag concentrations in the martian mantle could have been set by simple metal-silicate equilibrium. Mars accreted and differentiated relatively rapidly, while also developing a deep magma ocean with a high Pd/Ag ratio that could have evolved positive 107Ag anomalies, in contrast to Earth. Measurements on shergottites may reveal these predicted Ag isotopic anomalies.

Neural network modeling and simulation of the synthesis of CuInS2/ZnS quantum dots

AbstractThe development of recipes for synthesis of quantum dots (QDs), a novel semiconductor material for application in optoelectronic devices, is currently purely based on experiments. Since the quality of QDs represented by quantum yield (QY) and emission peak strongly depends on a number of different parameters (route, precursors, conditions, etc), a large number of experiments is necessary. In this article, we show that data‐driven modeling can be used as a supporting tool for optimization and a better understanding of the synthesis process. By using the results collected during the development of CuInS2/ZnS (CIS/ZnS) QDs, a neural network model has been established. The model is able to predict the optical properties (QY and emission peak) of CIS/ZnS QDs as a function of the most important synthesis parameters, such as reaction temperature, time of CIS core formation and ZnS shell growth, feed molar ratio of Cu/In and Zn/Cu, various starting precursors, and types of ligands. Finally, a model analysis under various effects influencing the quality of QDs is performed.

Multi-element isotope study of natrocarbonatites (1993 lava flows) from Oldoinyo Lengai volcano, Tanzania: Implications for core-mantle interactions

We contribute to the debate of core-mantle interactions by providing tungsten (W) isotope (ε182W) data from natrocarbonatite samples collected from the 1993 lava flows at Oldoinyo Lengai (OL) volcano, Tanzania. The 182Hf-182W system (half-life = 8.9 Ma) is well suited to constraining the timing of core formation and monitoring incursions of the core into the mantle. Most mantle-derived rocks are difficult to analyze because of their ultra-low W contents. We consider natrocarbonatites as suitable proxy for quantifying the core-mantle interactions given their unusually high W concentrations (published values range from ~40 to 115 ppm). The ε182W values of 7 natrocarbonatites measured by multi-collector inductively coupled plasma mass spectrometry (MC-ICPMS) represent the first dataset obtained for the OL lavas (OLLs). The ε182W values calculated from W isotope ratios range from −0.01 to +0.16 and are indistinguishable within limits of uncertainty, from those of the ambient modern mantle sources (i.e., EM1, HIMU, and HIMU-like). The relatively uniform W isotope compositions associated with the lack of any ε182W anomaly from the natrocarbonatites imply that there may have been no contribution from the core (i.e., ε182W < 0) into the magma source of the OLLs. Alternately, the core signature could have been obscured either by the addition of mantle components that survived since the Earth's early accretion, or by an unknown region formed by an inefficient percolation of metal during core-mantle segregation that left parts of the mantle enriched in W. New C–O isotope data from the same samples (δ13C = −5.9 to −6.9‰; δ18O = 9.5–10.9‰) rule out any involvement of the continental crust into the OL magma and are consistent with other studies that show a mantle signature for most natrocarbonatites. The heavier δ18O values can be attributed to sub-solidus processes. The potential oxygen isotopic shift would not, however, affect the W isotopic compositions of the OLLs.

β-sheet elasticity of peptide self-assembly mimic, PSAM, with a grafted sequence characterized by comprehensive analyses of isomorphous crystals

Peptides and proteins are self-assembled into β-rich self-assemblies such as amyloids in liquid water environments. Despite its importance, detailed structural analysis of this class is hindered by their insoluble and heterogeneous nature. For circumventing this problem, we designed a peptide self-assembly mimic (PSAM) that consists of a central single-layer β-sheet (SLB) capped by terminal domains. In a previous study, we grafted part of an amyloid forming chameleon sequence into a PSAM (termed PSAM-VLGDV1) and found a β-sheet bending at SLB induced by the grafted sequence using x-ray crystallography (Hori et al., 2019 [1]). Later, we found that another crystal structure of PSAM-VLGDV1 obtained from an isomorphous crystal had a different conformation and we gained an interest in the structural differences of PSAMs within isomorphous crystals. Herein, we report the structural variations trapped within isomorphous crystals by comprehensive structure determinations. Additionally, we demonstrate the structural plasticity of PSAM-VLGDV1 via molecular dynamics simulations. Hydration structure analysis revealed that water molecule locates on the β-sheet surface is important for its plasticity. Our findings suggest that a marginal structural difference can be trapped at the time of crystal core formation, which propagates during crystal growth. Our results suggest that structural plasticity of β-sheet would play an important role in the macroscopic shape formation of peptide self-assemblies.

Effects of core formation on the Hf–W isotopic composition of the Earth and dating of the Moon-forming impact

Earth's core formation set the initial compositions of the core and mantle. Various aspects of core formation, such as the degree of metal–silicate equilibration, oxygen fugacity, and depth of equilibration, have significant consequences for the resulting compositions, yet are poorly constrained. The Hf–W isotopic system can provide unique constraints on these aspects relative to other geochemical or geophysical methods. Here we model the Hf–W isotopic evolution of the Earth, improving over previous studies by combining a large number of N-body simulations of planetary accretion with a core formation model that includes self-consistent evolution of oxygen fugacity and a partition coefficient of tungsten that evolves with changing pressure, temperature, composition, and oxygen fugacity. The effective average fraction of equilibrating metal is constrained to be k>0.2 for a range of equilibrating silicate masses (for canonical accretion scenarios), and is likely <0.55 if the Moon formed later than 65 Ma. These values of k typically correspond to an effective equilibration depth of ∼0.5–0.7× the evolving core–mantle boundary pressure as the planet grows. The average mass of equilibrating silicate was likely at least 3× the impactor's silicate mass. Equilibration temperature, initial fO2, initial differentiation time, semimajor axis, and planetary mass (above ∼0.9 M⊕) have no systematic effect on the 182W anomaly, or on fHf/W (except for fO2), when applying the constraint that the model must reproduce Earth's mantle W abundance. There are strong tradeoffs between the effects of k, equilibrating silicate mass, depth of equilibration, and timing of core formation, so the terrestrial Hf–W isotopic system should be interpreted with caution when used as a chronometer of Earth's core formation. Because of these strong tradeoffs, the Earth's tungsten anomaly can be reproduced for Moon-forming impact timescales spanning at least 10–175 Ma. Early Moon formation ages require a higher degree of metal–silicate equilibration to produce Earth's 182W anomaly.

New geochemical models of core formation in the Moon from metal–silicate partitioning of 15 siderophile elements

We re-examine the conditions at which core formation in the Moon may have occurred by linking the observed lunar mantle depletions of 15 siderophile elements, including volatile siderophile elements (VSE) to predictive equations derived from a database compilation of metal–silicate partition coefficients obtained at lunar-relevant pressure–temperature–oxygen fugacity (P–T–fO2P–T–fO2) conditions. Our results suggest that at mantle temperatures between the solidus and liquidus the depletions for all elements considered can be satisfied, but only if the Moon was essentially fully molten at the time of core formation while assuming a S-rich (>8 wt%) core comprising 2.5 wt% of the mass of the Moon. However, we observe that at temperatures exceeding the mantle liquidus, with increasing temperature the core S content required to satisfy the element depletions is reduced. As a S-poor core is likely from recent lunar mantle estimates of S abundance, this suggests much higher temperatures during lunar core formation than previously proposed. We conclude that the VSE depletions in the lunar mantle can be solely explained by core formation depletion, suggesting that no significant devolatilization has occurred in later periods of lunar evolution. This is in agreement with the discovery of significant amounts of other volatiles in the lunar interior, but hard to reconcile with current lunar formation models.

High-Precision Tungsten Isotopic Analysis by Multicollection Negative Thermal Ionization Mass Spectrometry Based on Simultaneous Measurement of W and 18O/16O Isotope Ratios for Accurate Fractionation Correction

Determination of the (182)W/(184)W ratio to a precision of ± 5 ppm (2σ) is desirable for constraining the timing of core formation and other early planetary differentiation processes. However, WO3(-) analysis by negative thermal ionization mass spectrometry normally results in a residual correlation between the instrumental-mass-fractionation-corrected (182)W/(184)W and (183)W/(184)W ratios that is attributed to mass-dependent variability of O isotopes over the course of an analysis and between different analyses. A second-order correction using the (183)W/(184)W ratio relies on the assumption that this ratio is constant in nature. This may prove invalid, as has already been realized for other isotope systems. The present study utilizes simultaneous monitoring of the (18)O/(16)O and W isotope ratios to correct oxide interferences on a per-integration basis and thus avoid the need for a double normalization of W isotopes. After normalization of W isotope ratios to a pair of W isotopes, following the exponential law, no residual W-O isotope correlation is observed. However, there is a nonideal mass bias residual correlation between (182)W/(i)W and (183)W/(i)W with time. Without double normalization of W isotopes and on the basis of three or four duplicate analyses, the external reproducibility per session of (182)W/(184)W and (183)W/(184)W normalized to (186)W/(183)W is 5-6 ppm (2σ, 1-3 μg loads). The combined uncertainty per session is less than 4 ppm for (183)W/(184)W and less than 6 ppm for (182)W/(184)W (2σm) for loads between 3000 and 50 ng.

Hf–W chronology of the eucrite parent body

The 182Hf–182W systematics of 12 whole-rock eucrites, including basaltic and cumulate samples, have been investigated. The Hf–W isotope systematics of both basaltic and cumulate eucrites are consistent with derivation from a single mantle source characterized by a strongly suprachondritic Hf/W ratio (180Hf/184W of ∼19). The elevated Hf/W of this mantle source was established by core formation within ∼1Ma after CAI formation or, alternatively, represents that of the residual melt of a magma ocean from which the eucrites ultimately formed. In the latter case the time of core formation is more uncertain and core formation may have occurred slightly later than ∼1Ma. The investigated basaltic eucrites fall into three distinct age groups with Hf–W ages of ∼4Ma (Stannern), ∼11Ma (Bereba, Bouvante) and ∼22Ma (Camel Donga, Juvinas) after CAI formation and provide evidence for a protracted history of magmatism and crustal metamorphism on the eucrite parent body, lasting for at least ∼20Ma. Evidence for even later activity is provided by the cumulate eucrites, which exhibit only small if any variations in 182W/184W in spite of variable 180Hf/184W, indicating cooling below the Hf–W closure temperature when 182Hf was nearly extinct. On the basis of three cumulate eucrites, a Hf–W age of 38±21Ma is inferred, but even younger ages would also be consistent with the Hf–W data.

Estimation of trace element concentrations in the lunar magma ocean using mineral‐ and metal‐silicate melt partition coefficients

AbstractThis study uses experimentally determined plagioclase‐melt D values to estimate the trace element concentrations of Sr, Hf, Ga, W, Mo, Ru, Pd, Au, Ni, and Co in a crystallizing lunar magma ocean at the point of plagioclase flotation. Similarly, experimentally determined metal‐silicate partition experiments combined with a composition model for the Moon are used to constrain the concentrations of W, Mo, Ru, Pd, Au, Ni, and Co in the lunar magma ocean at the time of core formation. The metal‐silicate derived lunar mantle estimates are generally consistent with previous estimates for the concentration of these elements in the lunar mantle. Plagioclase‐melt derived concentrations for Sr, Ga, Ru, Pd, Au, Ni, and Co are also consistent with prior estimates. Estimates for Hf, W, and Mo, however, are higher. These elements may be concentrated in the residual liquid during fractional crystallization due to their incompatibility. Alternatively, the apparent enrichment could reflect the inappropriate use of bulk anorthosite data, rather than data for plagioclase separates.

INDUCED CORE FORMATION TIME IN SUBCRITICAL MAGNETIC CLOUDS BY LARGE-SCALE TRANS-ALFVÉNIC FLOWS

We clarify the mechanism of accelerated core formation by large-scale nonlinear flows in subcritical magnetic clouds by finding a semi-analytical formula for the core formation time and describing the physical processes that lead to them. Recent numerical simulations show that nonlinear flows induce rapid ambipolar diffusion that leads to localized supercritical regions that can collapse. Here, we employ non-ideal magnetohydrodynamic simulations including ambipolar diffusion for gravitationally stratified sheets threaded by vertical magnetic fields. One of the horizontal dimensions is eliminated, resulting in a simpler two-dimensional simulation that can clarify the basic process of accelerated core formation. A parameter study of simulations shows that the core formation time is inversely proportional to the square of the flow speed when the flow speed is greater than the Alfv\'en speed. We find a semi-analytical formula that explains this numerical result. The formula also predicts that the core formation time is about three times shorter than that with no turbulence, when the turbulent speed is comparable to the Alfv\'en speed.

Evolution of prolate molecular clouds at H舁ii boundaries – I. Formation of fragment-core structures

The evolution of a prolate cloud at an Hii boundary is investigated using Smoothed Particle Hydrodynamics (SPH). The prolate molecular clouds in our investigation are set with their semi-major axis perpendicular to the radiative direction of a plane parallel ionising Extreme Ultraviolet (EUV) flux. Simulations on three high mass prolate clouds reveal that EUV radiation can trigger distinctive high density core formation embedded in a final linear structure. This contrasts with results of the previous work in which only an isotropic Far Ultraviolet (FUV) interstellar background flux was applied. A systematic investigation on a group of prolate clouds of equal mass but different initial densities and geometric shapes finds that the distribution of the cores over the final linear structure changes with the initial conditions of the prolate cloud and the strength of the EUV radiation flux. These highly condensed cores may either scatter over the full length of the final linear structure or form two groups of high density cores at two foci, depending on the value of the ionising radiation penetration depth d_EUV, the ratio of the physical ionising radiation penetration depth to the minor axis of the cloud. Data anlysis on the total mass of the high density cores and the core formation time finds that the potential for EUV radiation triggered star formation efficiency is higher in prolate clouds with shallow ionisation penetration depth and intermediate major to minor axial ratio, for the physical environments investigated. Finally, it is suggested that the various fragment-core structures observed at Hii boundaries may result from the interaction between ionising radiation and pre-existing prolate clouds of different initial geometrical and physical conditions.

Redox controls on tungsten and uranium crystal/silicate melt partitioning and implications for the U/W and Th/W ratio of the lunar mantle

The timing of core formation is essential for understanding the early differentiation history of the Earth and the Moon. Because Hf is lithophile and W is siderophile during metal–silicate segregation, the decay of 182Hf to 182W (half-life of 9 Ma) has proven to be a useful chronometer of core–mantle differentiation events. A key parameter for the interpretation of 182Hf/182W data is the Hf/W ratio of the primitive (i.e. undepleted) mantle. Since W is incompatible during mantle melting, its ratio relative to U and other similarly incompatible elements in basalts (e.g. Th, La) may be used as proxies for their mantle sources. However, the assumption that W and U are equally incompatible may be flawed for petrological systems that equilibrated over a large range of oxygen fugacity (fO2). Although W is typically perceived as being homovalent, evidence suggests that U is heterovalent over the range of fO2 inferred for the silicate mantles of the Earth and the Moon.Here we report new partitioning data for W, U, high-field-strength elements (HFSE), and Th between clinopyroxene, orthopyroxene, olivine, plagioclase and silicate melt. In agreement with previous studies, we show that these elements behave as homovalent elements at fO2 characteristic of Earth's upper mantle. However, both W and U become more compatible at low fO2, indicating a change in their redox state, with W becoming more compatible at progressively lower fO2. This result for W is particularly unexpected, because this element was thought to be hexavalent even at very low fO2. The much higher compatibility of W4+ (the species inferred here at low fO2) relative to W6+ means that even a small fraction of W4+ will have a significant effect on the overall compatibility of W. Our results imply that over the range of reducing conditions in which lunar differentiation is thought to have taken place (i.e. ∼IW-2 to IW-0.5), W is likely to become fractionated from U. When our partitioning data are applied to model the fractional crystallization of a lunar magma ocean, lunar trends for U/W, Hf/W and Th/W are well reproduced. The result of this model carries with it the implication that the Hf/W of the bulk silicate fractions that comprise the Earth and the Moon are virtually identical.

Protracted core formation and rapid accretion of protoplanets

Understanding core formation in meteorite parent bodies is critical for constraining the fundamental processes of protoplanet accretion and differentiation within the solar protoplanetary disk. We report variations of 5 to 20 parts per million in (182)W, resulting from the decay of now-extinct (182)Hf, among five magmatic iron meteorite groups. These (182)W variations indicate that core formation occurred over an interval of ~1 million years and may have involved an early segregation of Fe-FeS and a later segregation of Fe melts. Despite this protracted interval of core formation, the iron meteorite parent bodies probably accreted concurrently ~0.1 to 0.3 million years after the formation of Ca-Al-rich inclusions. Variations in volatile contents among these bodies, therefore, did not result from accretion at different times from an incompletely condensed solar nebula but must reflect local processes within the nebula.

The chronology of planetary embryos

Planetary Formation Protoplanets, or early planetary embryos such as iron meteorite parent bodies, formed in the early protoplanetary disk from dust, debris, and planetesimals. Defining the precise chronology of accretion and differentiation—including core formation—of these planetary embryos will aid in a richer understanding of the chemical evolution of the solar system. Through high-precision tungsten isotope measurements, Kruijer et al. show that the timing of accretion and core formation for iron meteorite groups falls within 0.6 to 2 million years of the age of the solar system (see the Perspective by Elliott). Differences of timing within this group are probably a function of volatile contents of the parent bodies or spatial and chemical heterogeneity within the protoplanetary disk. Science , this issue p. [1150][1]; see also p. [1086][2] [1]: /lookup/volpage/344/1150?iss=6188 [2]: /lookup/volpage/344/1086?iss=6188