Earth's volatile contents established by melting and vaporization

Early planetesimal differentiation and late accretion shaped Earth's nitrogen budget

The relative roles of protoplanetary differentiation versus late accretion in establishing Earth's life-essential volatile element inventory are being hotly debated. To address this issue, we employ first-principles calculations to investigate nitrogen (N) isotope fractionation during Earth's accretion and differentiation. We find that segregation of an iron core would enrich heavy N isotopes in the residual silicate, while evaporation within a H2-dominated nebular gas produces an enrichment of light N isotope in the planetesimals. The combined effect of early planetesimal evaporation followed by core formation enriches the bulk silicate Earth in light N isotopes. If Earth is comprised primarily of enstatite-chondrite-like material, as indicated by other isotope systems, then late accretion of carbonaceous-chondrite-like material must contribute ~ 30-100% of the N budget in present-day bulk silicate Earth. However, mass balance using N isotope constraints shows that the late veneer contributes only a limited amount of other volatile elements (e.g., H, S, and C) to Earth.

Chalcogen isotopes reveal limited volatile contribution from late veneer to Earth

The origin of Earth's volatile elements is highly debated. Comparing the chalcogen isotope ratios in the bulk silicate Earth (BSE) to those of its possible building blocks, chondritic meteorites, allows constraints on the origin of Earth's volatiles; however, these comparisons are complicated by potential isotopic fractionation during protoplanetary differentiation, which largely remains poorly understood. Using first-principles calculations, we find that core-mantle differentiation does not notably fractionate selenium and tellurium isotopes, while equilibrium evaporation from early planetesimals would enrich selenium and tellurium in heavy isotopes in the BSE. The sulfur, selenium, and tellurium isotopic signatures of the BSE reveal that protoplanetary differentiation plays a key role in establishing most of Earth's volatile elements, and a late veneer does not substantially contribute to the BSE's volatile inventory.

Thermodynamic determination of condensation behavior for the precursory elements of radioxenon following an underground nuclear explosion

The measurement of radioactive xenon isotopes (radioxenon) in the atmosphere is a tool used to detect underground nuclear explosions, provided that some radioxenon escaped containment and that fractionation leading to the alteration of the relative proportions of these isotopes, is accounted for. After the explosion, volatilization followed by melting of the surrounding rocks produces a magma where the more refractory radioactive species get dissolved while the more volatile ones contribute to the gas phase that might escape. Indium, tin, antimony, tellurium and iodine are the main fission products involved in the decay chains leading to radioxenon. In this study, condensation as a function of temperature for these precursors of radioxenon were determined using thermodynamic calculations for systems with complex chemical composition corresponding to major environments of known underground nuclear explosions and for a range of pressure values representative of the cavity evolution. Our results illustrate a large difference between the relevant condensation temperatures for the radioxenon precursors and the tabulated boiling temperatures of the pure compounds often used as indicators of their volatility. For some precursory elements such as tin, the often-considered Heaviside function represents an oversimplification of the concept of condensation temperature, as condensation occurs over a temperature range as large as 2000 K. This results from the speciation of the elements in the gas phase mainly driven by the formation of oxides. Condensation also strongly depends on pressure while it moderately depends on the bulk chemical composition of the system. This study shows the importance and complexity of the condensation process following underground nuclear explosions. It also shows how thermodynamic computations allow the prediction of the quantity and the relative proportions of radioactive xenon isotopes in the gas phase in the presence of magma, before their potential emission to the atmosphere. Better detection, discrimination and understanding of underground nuclear explosions should arise by taking into account the fractionation resulting from the condensation of the radionuclides producing radioxenon in nuclear cavities.

Earth's volatile depletion trend is consistent with a high-energy Moon-forming impact

The abundance of volatile elements in the silicate Earth relative to primitive chondrites provides an important constraint on the thermochemical evolution of the planet. However, an overabundance of indium relative to elements with similar nebular condensation temperatures is a source of debate. Here we use ab initio molecular dynamics simulations to explore the vaporization behavior of indium from pyrolite melt at conditions of the early magma ocean just after the Moon-forming impact. We then compare this to the vaporization behavior of other minor elements. When considering the volatility of the elements from the magma ocean in the absence of the solar nebula gas, we find that there is no overabundance of indium. On the contrary, there is a slight deficit in the abundance of indium, which is consistent with its moderately siderophile nature. Thus, we propose that a high-energy Moon-forming impact may have had a more significant contribution to volatile depletion than previously believed.

Stochastic accretion of the Earth

Chondritic meteorites are thought to be representative of the material that formed the Earth. However, the Earth is depleted in volatile elements in a manner unlike that in any chondrite, and yet these elements retain chondritic isotope ratios. Here we use N-body simulations to show that the Earth did not form from chondrites, but rather by stochastic accretion of many precursor bodies whose variable compositions reflect the temperatures at which they formed. Earth's composition is reproduced when initial temperatures of planetesimal- to embryo-sized bodies are set by disk accretion rates of (1.08±0.17)×10^-7 solar masses/yr, although they may be perturbed by ²⁶Al heating on bodies formed at different times. Our model implies that a heliocentric gradient in composition was present in the protoplanetary disc and that planetesimals formed rapidly within ~1 Myr, in accord with radiometric volatile depletion ages of the Earth.

A revised trapped melt model for iron meteorites applied to the IIIAB group

As the largest magmatic iron meteorite group, the IIIAB group is often used to investigate the process of core crystallization in asteroid-sized bodies. However, previous IIIAB crystallization models have not succeeded in both explaining the scatter among IIIAB irons around the main crystallization trends and using elemental partitioning behavior consistent with experimental determinations. This study outlines a revised approach for modeling the crystallization of irons that uses experimentally determined partition coefficients and can reproduce the IIIAB trends and their associated scatter for 12 siderophile elements simultaneously. A key advancement of this revised trapped melt model is the inclusion of an effect on the resulting solid metal composition due to the formation of troilite. The revised trapped melt model supports the previous conclusion that trapped melt played an important role in the genesis of IIIAB irons and matches the trace element fractionation trends observed in the Cape York suite as due to different amounts of trapped melt. Applying the revised trapped melt model to 16 elements as well as S and Fe, the bulk composition of the IIIAB core is found to have a composition consistent with that expected from a chondritic precursor for refractory siderophile elements but with evidence for depletions of more volatile elements. The bulk S composition of the IIIAB core is estimated as 9 ± 1 wt%, implying that a substantial amount of S-rich material from the IIIAB core is underrepresented in our meteorite collections. Future applications of the revised trapped melt model to other magmatic iron meteorite groups can enable comparisons between the core compositions and crystallization processes across the early solar system.

High Temperature Evaporation and Isotopic Fractionation of K and Cu

The chemical and isotopic signatures of moderately volatile elements are useful for understanding processes of volatile depletion in planetary formation and differentiation. However, the fractionation factors between gas and melt phases during evaporation that are required to model these planetary volatile depletion processes are still sparse. In this study, twenty heating experiments were conducted in 1 atm gas-mixing furnaces to constrain the behavior of K, Cu, and Zn evaporation and isotopic fractionation from basaltic melts at high temperatures. The temperatures range from 1300 °C to 1400 °C, and durations are from 2 to 8 days. Oxygen fugacities (fO2) range from one log unit below to ten log units above that of the iron-wüstite buffer (IW-1 to IW+10, corresponding to logfO2 of -10.7 to -0.68 at 1400 °C). The conditions were selected to achieve an evaporation-dominated regime (where timescales of diffusion << evaporation for trace elements) in order to avoid diffusion-limited evaporation. Our results show during evaporation Zn behaved as the most volatile, followed by Cu and then K, regardless of temperature and oxygen fugacity. Partitioning of Zn into spinel layers within experimental capsules, however, has been observed, which has substantial effects on the Zn isotope fractionation factor. Therefore, Zn results are presented but further discussion is excluded. Element loss depends on both temperature and oxygen fugacity, where higher temperatures and lower oxygen fugacities promote evaporation. However, with varying temperature and oxygen fugacity, the kinetic isotopic fractionation factors, α (where,R R 0 = f α - 1 ), for K and Cu remain constant, thus these factors can be applied to a wider range of conditions than those in this study. The experimentally determined fractionation factors for K, and Cu during evaporation from basaltic melts are 0.9944, and 0.9961, respectively. The fractionation factors for these elements with varying volatilities are all significantly larger than the "apparent observed fractionation factors," which approach one and are inferred from lunar basalts relative to the Bulk Silicate Earth. This observation suggests near-equilibrium conditions during volatile-element loss from the Moon as the "apparent observed fractionation factors" of lunar basalts are similar for all three elements.

Early volatile depletion on planetesimals inferred from C-S systematics of iron meteorite parent bodies

During the formation of terrestrial planets, volatile loss may occur through nebular processing, planetesimal differentiation, and planetary accretion. We investigate iron meteorites as an archive of volatile loss during planetesimal processing. The carbon contents of the parent bodies of magmatic iron meteorites are reconstructed by thermodynamic modeling. Calculated solid/molten alloy partitioning of C increases greatly with liquid S concentration, and inferred parent body C concentrations range from 0.0004 to 0.11 wt%. Parent bodies fall into two compositional clusters characterized by cores with medium and low C/S. Both of these require significant planetesimal degassing, as metamorphic devolatilization on chondrite-like precursors is insufficient to account for their C depletions. Planetesimal core formation models, ranging from closed-system extraction to degassing of a wholly molten body, show that significant open-system silicate melting and volatile loss are required to match medium and low C/S parent body core compositions. Greater depletion in C relative to S is the hallmark of silicate degassing, indicating that parent body core compositions record processes that affect composite silicate/iron planetesimals. Degassing of bare cores stripped of their silicate mantles would deplete S with negligible C loss and could not account for inferred parent body core compositions. Devolatilization during small-body differentiation is thus a key process in shaping the volatile inventory of terrestrial planets derived from planetesimals and planetary embryos.

Conditions and extent of volatile loss from the Moon during formation of the Procellarum basin

Rocks from the lunar interior are depleted in moderately volatile elements (MVEs) compared to terrestrial rocks. Most MVEs are also enriched in their heavier isotopes compared to those in terrestrial rocks. Such elemental depletion and heavy isotope enrichments have been attributed to liquid-vapor exchange and vapor loss from the protolunar disk, incomplete accretion of MVEs during condensation of the Moon, and degassing of MVEs during lunar magma ocean crystallization. New Monte Carlo simulation results suggest that the lunar MVE depletion is consistent with evaporative loss at 1,670 ± 129 K and an oxygen fugacity +2.3 ± 2.1 log units above the fayalite-magnetite-quartz buffer. Here, we propose that these chemical and isotopic features could have resulted from the formation of the putative Procellarum basin early in the Moon's history, during which nearside magma ocean melts would have been exposed at the surface, allowing equilibration with any primitive atmosphere together with MVE loss and isotopic fractionation.

Bifurcation of planetary building blocks during Solar System formation

Geochemical and astronomical evidence demonstrates that planet formation occurred in two spatially and temporally separated reservoirs. The origin of this dichotomy is unknown. We use numerical models to investigate how the evolution of the solar protoplanetary disk influenced the timing of protoplanet formation and their internal evolution. Migration of the water snow line can generate two distinct bursts of planetesimal formation that sample different source regions. These reservoirs evolve in divergent geophysical modes and develop distinct volatile contents, consistent with constraints from accretion chronology, thermochemistry, and the mass divergence of inner and outer Solar System. Our simulations suggest that the compositional fractionation and isotopic dichotomy of the Solar System was initiated by the interplay between disk dynamics, heterogeneous accretion, and internal evolution of forming protoplanets.

Vitrification treatment of municipal solid waste bottom ash

This study involves the chemical characterization of municipal solid waste (MSW) bottom ash (BA) produced at a combustor facility in Rimini (Italy), and vitrification experiments, performed at lab scale under atmospheric conditions, maximum temperature of 1100 °C, for different durations (2-16 h). LA-ICP-MS analyses of the glasses obtained revealed that the overall volatility of metal elements increases with the time but it cannot be simply predicted by element boiling point. Elements have been here categorized into three different groups depending on their volatility comparing the glass product with the BA starting sample- high, medium or low, respectively- (1) Cu, W, Cl; (2) Pb, Zn, Sr; (3) Mo, Cr, V, Ni. The behavior of Cs, Rb, Ag, Ba, Ga, Co, U, Zr, Hf and Ta in the glassy slag is not defined because we did not observe statistically significant changes in their volatility behavior. Vitrification allows us to produce chemically stable glassy materials and immobilize potentially harmful elements, thus producing from waste new vitreous materials that are relatively inert and suitable for potential re-utilization in new products and/or applications for building and construction industries. Moreover, the samples show REE chondrite-normalized patterns indicating relative enrichments in Light-Rare Earth Elements (LREE), in particular La and Nd which may be interesting from an economic point of view in terms of waste recovery. Thus, the results obtained show how to treat bottom ashes from incinerator in order to provide more chemically inert and economically useful resources for recycle and reuse of solid waste BA.

Earth's volatile element depletion pattern inherited from a carbonaceous chondrite-like source

Earth's volatile element abundances (e.g., sulfur, zinc, indium and lead) provide constraints on fundamental processes such as planetary accretion, differentiation, and the delivery of volatile species, like water, which contributed to Earth becoming a habitable planet. The composition of the silicate Earth suggests chemical affinity but isotopic disparity to carbonaceous chondrites, meteorites that record the earliest element fractionations in the protoplanetary disk. However, the volatile element depletion pattern of the silicate Earth is obscured by core formation. Another key problem is the overabundance of indium, which could not be reconciled with any known chondrite group. Here we complement recently published volatile element abundances for carbonaceous chondrites with high precision sulfur, selenium, and tellurium data. We show that both Earth and carbonaceous chondrites exhibit a unique hockey stick volatile element depletion pattern where volatile elements with low condensation temperatures (750 - 500 K) are unfractionated from each other. This abundance plateau accounts for the apparent overabundance of indium in the silicate Earth without the need of exotic building materials or vaporization from precursors or during the Moon-forming impact and suggests the accretion of 10-15 % CI-like material before core formation ceased. Finally, more accurate estimates of volatile element abundances in the core and bulk Earth can now be provided.

Near-equilibrium isotope fractionation during planetesimal evaporation

Silicon and Mg in differentiated rocky bodies exhibit heavy isotope enrichments that have been attributed to evaporation of partially or entirely molten planetesimals. We evaluate the mechanisms of planetesimal evaporation in the early solar system and the conditions that controled attendant isotope fractionations. Energy balance at the surface of a body accreted within ~1 Myr of CAI formation and heated from within by ²⁶Al decay results in internal temperatures exceeding the silicate solidus, producing a transient magma ocean with a thin surface boundary layer of order < 1 meter that would be subject to foundering. Bodies that are massive enough to form magma oceans by radioisotope decay (≥ 0.1% M _⊕) can retain hot rock vapor even in the absence of ambient nebular gas. We find that a steady-state rock vapor forms within minutes to hours and results from a balance between rates of magma evaporation and atmospheric escape. Vapor pressure buildup adjacent to the surfaces of the evaporating magmas would have inevitably led to an approach to equilibrium isotope partitioning between the vapor phase and the silicate melt. Numerical simulations of this near-equilibrium evaporation process for a body with a radius of ~ 700 km yield a steady-state far-field vapor pressure of 10^-8 bar and a vapor pressure at the surface of 10^-4 bar, corresponding to 95% saturation. Approaches to equilibrium isotope fractionation between vapor and melt should have been the norm during planet formation due to the formation of steady-state rock vapor atmospheres and/or the presence of protostellar gas. We model the Si and Mg isotopic composition of bulk Earth as a consequence of accretion of planetesimals that evaporated subject to the conditions described above. The results show that the best fit to bulk Earth is for a carbonaceous chondrite-like source material with about 12% loss of Mg and 15% loss of Si resulting from near-equilibrium evaporation into the solar protostellar disk of H₂ on timescales of 10⁴ to 10⁵ years.

Volatile element depletion of the Moon-The roles of precursors, post-impact disk dynamics, and core formation

The compositional and isotopic similarity of Earth's primitive upper mantle (PUM) and the Moon supports the derivation of the Moon from proto-Earth, but the Moon's inventory of volatile lithophile elements-Na, K, Rb, and Cs-is lower than Earth's PUM by factors of 4 to 5. The abundances of 14 other volatile elements exhibit siderophile behavior [volatile siderophile elements (VSEs); i.e., P, As, Cu, Ag, Sb, Ga, Ge, Bi, Pb, Zn, Sn, Cd, In, and Tl] that can be used to evaluate whether the Moon was derived from proto-Earth and if core formation or volatility controlled their depletion. At lunar core formation conditions, As, Sb, Ag, Ge, Bi, and Sn are siderophile, whereas P, Cu, Ga, Pb, Zn, Cd, In, and Tl are weakly siderophile or lithophile. VSEs may help to discriminate between physical and chemical processes that formed the Moon such as low- versus high-energy impacts and gas-melt interactions.

Depletion of potassium and sodium in mantles of Mars, Moon and Vesta by core formation

The depletions of potassium (K) and sodium (Na) in samples from planetary interiors have long been considered as primary evidence for their volatile behavior during planetary formation processes. Here, we use high-pressure experiments combined with laser ablation analyses to measure the sulfide-silicate and metal-silicate partitioning of K and Na at high pressure (P) – temperature (T) and find that their partitioning into metal strongly increases with temperature. Results indicate that the observed Vestan and Martian mantle K and Na depletions can reflect sequestration into their sulfur-rich cores in addition to their volatility during formation of Mars and Vesta. This suggests that alkali depletions are not affected solely by incomplete condensation or partial volatilization during planetary formation and differentiation, but additionally or even primarily reflect the thermal and chemical conditions during core formation. Core sequestration is also significant for the Moon, but lunar mantle depletions of K and Na cannot be reconciled by core formation only. This supports the hypothesis that measured isotopic fractionations of K in lunar samples represent incomplete condensation or extensive volatile loss during the Moon-forming giant impact.