Abundances Of Volatile Elements Research Articles

Quantification of evaporative loss of volatile metals from planetary cores and metal-rich planetesimals

The processes responsible for the isotopic compositions and abundances of volatile elements in the early solar system remain highly debated. Orders of magnitude variation of (highly) volatile elements exist between different magmatic iron meteorite groups, but it is unclear to what extent their depletions can be explained by evaporation from metal melts during parent body accretion and/or subsequent break up. To this end, we present 86 new evaporation experiments with the aim of constraining the volatility of most volatile metals from metallic melts. The results confirm the previously proposed important effects of S in metal melt on the volatility of the elements of interest governed by their S-loving or S-phobic behavior. Nominally S-loving elements In, Sn, Te, Pb and Bi are significantly more volatile in Fe melt relative to FeS liquid, whereas nominally S-avoiding elements Ga and Sb are more volatile in FeS liquid relative to Fe melt, at a given pressure and temperature. The newly derived volatility sequences for S-free/poor and S-rich metallic melts were also compared with commonly used volatility models based on condensation temperatures. The results indicate significant differences between the latter, including the much more volatile behavior of Te, relative to Se, in both explored bulk compositions, which are traditionally assumed to be equally volatile. The (minimum) degree of volatile element depletion due to evaporation was quantified using the new experimental results and models. A comparison between the volatile element depletions in magmatic iron meteorites and the predicted depletions appropriate for evaporation from Fe melts shows that the latter depletions can be easily reconciled with (an) evaporation event(s). Altogether, the new data and models will provide an important framework when more accurate and precise estimates of magmatic iron meteorite bulk volatile element contents are available.

The origin of 4-Vesta's volatile depletion revealed by the zinc isotopic composition of diogenites.

Volatile element abundances vary substantially among terrestrial planetary bodies like Earth, Mars, Moon, and differentiated asteroids, leading to intense debate about the governing processes. howardites-eucrites-diogenites (HED) meteorites, most likely from asteroid 4-Vesta, represent highly volatile-depleted Solar System samples, offering critical insights into these processes. Zinc is a moderately volatile element and its isotopic composition reveals sources of volatiles in planetary bodies. Our study finds Zn isotopic anomalies in diogenites overlapping with noncarbonaceous reservoirs, indicating negligible contributions of outer solar system materials to 4-Vesta's volatiles. Besides, zinc isotopic composition of 4-Vesta is lighter than that of chondrites, contrary to the expected signature of evaporation-based volatile depletion. This suggests that after 4-Vesta lost all its volatiles through evaporation during the magma ocean stage, partial kinetic recondensation occurred that produced the observed isotopically light composition. These insights, combined with previous data, underscore the process of global evaporation followed by partial condensation as a key factor influencing the final volatile budget of planetary bodies.

Evaporation of moderately volatile elements from metal and sulfide melts: Implications for volatile element abundances in magmatic iron meteorites

Volatile element abundances in magmatic iron meteorites provide fundamental insights into the processing of volatile elements in the early solar system. Although Cu, Ge and Ag concentrations of magmatic iron meteorites deviate up to 4 orders of magnitude between different magmatic iron meteorite groups, the role of evaporation on these volatile abundances is poorly constrained. Here, we experimentally assess the volatility of Cu, Ge, Ag, S, Cr, Co, Ni, Mo, Ru, Pd, W, Re and Ir from metal and sulfide melts as a function of pressure (10−4 and 1 bar), temperature (1573–1823 K) and time (5–120 min) for two end-member compositions (Fe versus FeS). These novel experiments demonstrate that the presence of S is a major parameter in establishing the volatility of Cu, Ge, Mo, Ag, Ru, W, Re and Ir. At constant P-T and time, the volatility of Ge, Mo, Ru, W, Re and Ir is greatly increased in the presence of S, whereas Cu and Ag are less volatile in the presence of S. At 1773 K and ∼0.001 bar, the volatility of S is sufficiently high that the degassed FeS liquid showed immiscibility of a S-rich sulfide and a S-poor Fe melt. Empirical equations were derived that predict the evaporative loss of Cu, Ge, Mo, Ag from Fe and/or FeS liquid as a function of temperature and time. A comparison of the newly derived volatility sequences with commonly applied 50% condensation temperature models shows that the condensation temperature models cannot be applied to sulfur-bearing Fe liquids and therefore to magmatic iron meteorites. Application of the new models on previously derived elemental depletions in the IVB parent body shows that evaporation, if it occurred, cannot have taken place under S-rich conditions. The latter would result in a depletion of Mo, which is not observed for the IVB irons. However, evaporation of a S-free or S-poor Fe liquid reproduces the observed volatility depletion trend for IVB irons under a wider range of temperature and evaporation times, demonstrating the potential importance of evaporative loss on the IVB parent body.

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.

Origin of moderately volatile element depletion on differentiated bodies: Insights from the evaporation of indium from silicate melts

In comparison with the Sun and CI chondrites, moderately volatile elements (MVEs) are depleted in terrestrial planets and other small, rocky differentiated bodies in the inner solar system. The abundances of most MVEs in the bulk silicate Earth (BSE) fall on a trend that defines a near log-linear decrease with their 50% nebular condensation temperature (Tc50). This temperature scale has traditionally been used to infer elemental volatility during planetary formation and accretion, however, indium (In) deviates from this correlation. Despite being a siderophile element that could have been depleted by core formation, In is overabundant for its calculated Tc50 in the BSE, as well as in the silicate portions of other small bodies (e.g., Moon and Vesta). This overabundance of In indicates that Tc50, calculated under nebular conditions, may not be applicable to planetary evaporation that occurs at much higher oxygen fugacity (fO2) and pressure than nominal nebular conditions. Here, we conduct a series of evaporation experiments for basaltic melts to quantify the volatility of In under conditions relevant to planetary evaporation. Our results show that, when using the evaporation temperature (Te1, refers to the temperature at which 1% of element i has evaporated from liquid to gas phase under equilibrium) as the volatility scale, the abundances of volatile elements, including In, of the Moon and Vesta display a progressive depletion with increasing volatility (decreasing Te1). This smooth depletion pattern contrasts with the overabundance of In shown on the Tc50 scale, suggesting that volatile depletion on small bodies occurred under non-nebular environment instead of during nebular condensation. On the other hand, the volatile element composition of the BSE (including In) could be explained by integrating (i) early accreted precursor materials of the proto-Earth that underwent volatile loss under conditions more oxidizing than those of the solar nebula with (ii) late added volatile-rich materials.

Spinel Harzburgite-Derived Silicate Melts Forming Sulfide-Bearing Orthopyroxenite in the Lithosphere. Part 1: Partition Coefficients and Volatile Evolution Accompanying Fluid- and Redox-Induced Sulfide Formation

We report abundances of major trace and volatile elements in an orthopyroxenite vein cutting a sub-arc, mantle-derived, spinel harzburgite xenolith from Kamchatka. The orthopyroxenite contains abundant sulfides and is characterized by the presence of glass (formerly melt) both interstitially and as inclusions in minerals, comparable with similar veins from the West Bismarck arc. The glass formed by quenching of residual melts following crystallization of abundant orthopyroxene, amphibole, and minor olivine and spinel. The interstitial glass has a low-Ti, high-Mg# andesite composition, with a wide range of H2O and S contents but more limited F and Cl variations. We calculate trace element partition coefficients using mineral and glass data, including those for halogens in amphibole, which agree with experimental results from the literature. Despite having a similar, high-Mg# andesite composition, the orthopyroxene-hosted glass inclusions usually contain much more H2O and S than the interstitial glass (4–7 wt% and ∼2,600 ppm, respectively). The initial vein-forming melts were oxidized, recording oxygen fugacity conditions up to ∼1.5 log units above the fayalite–magnetite–quartz oxygen buffer. They intruded the sub-arc mantle lithosphere at ≥1,300°C, where they partially crystallized to form high-Mg# andesitic derivative melts at ca. 1,050–1,100°C. Comparison with literature data on glass-free orthopyroxenite veins from Kamchatka and the glass-bearing ones from West Bismarck reveals fundamental similarities indicating common parental melts, which were originally produced by low-degree melting (≤5%) of spinel harzburgite at ≥1,360°C and ≤1.5 GPa. This harzburgite source likely contained ≤0.05 wt% H2O and a few ppm of halogens. Volatile evolution inferred from glass compositions shows that (i) redox exchange between S6+ in the original melt and Fe2+ in the host mantle minerals, together with (ii) the formation of an S-bearing, (H2O, Cl)-rich hydrothermal fluid from the original melt, provides the conditions for the formation of abundant sulfides in the orthopyroxenites during cooling. During this process, up to 85% of the original melt S content (∼2,600 ppm) is locally precipitated as magmatic and hydrothermal sulfides. As such, melts derived from spinel harzburgite sources can concentrate chalcophile and highly siderophile metals in orthopyroxenite dykes and sills in the lithosphere.

Trace element volatility and the conditions of liquid-vapor separation in the proto-lunar disk

The Moon is thought to have formed from material ejected by a giant impact that took place at the end of Earth's accretion. The material ejected to space generated a large hot structure where material beyond the Roche limit accreted to form the Moon. It has long been known that the Moon is characterized by abundances in moderately volatile elements (MVE) lower than that of the Earth, while more recent studies have established that the concentrations in refractory elements are similar to the bulk Silicate Earth. The thermodynamic conditions that prevailed after this impact are poorly known and understanding the origin of the Moon-Earth differences in MVE requires a knowledge of the volatility of elements under these conditions. In this study, we reexamine the volatility of a large set of geochemically relevant elements and attempt to determine the P-T conditions under which volatiles were putatively separated from the liquid material. Our model predicts very different condensation temperatures due to higher pressures, compared with the conditions of the Solar Nebula and we extend the values of these temperatures to a wide number of trace elements (Se, Ag, Pt, Mo, W, Zn, Sn, Sb, Rb, Cs, U, Th, Cr, Ni, Co, Ga, Ge, Cu, and P). Our modeling shows that the observed lunar compositions cannot be explained by a single set of P and T conditions. Rather, it is best explained by a mixture between high-temperature condensates (~4000 K) and low temperature condensates (2000-2500 K). An important constraint is that for the low temperature condensates, liquid metal must have been stable and this is crucial for matching the abundance of volatile siderophile elements in the bulk Moon.

The mantle source of basalts from Reunion Island is not more oxidized than the MORB source mantle

Glasses quenched from relatively undegassed ocean island magmas erupted from volcanoes at Iceland, Hawaii, the Canary Islands, and Erebus have elevated Fe3+/∑Fe ratios compared to glasses quenched from mid-ocean ridge basalts. This has been ascribed to elevated fO2 of their mantle sources, plausibly due to subducted, oxidized near-surface-derived components in their mantle sources. The basaltic magmas from Reunion Island in the Indian ocean have Sr–Nd-Hf-Pb-Os isotopic compositions suggesting that their mantle sources contain little or no subducted near-surface materials and contain the C/FOZO/PREMA mantle component. To constrain the fO2 of the C/FOZO/PREMA mantle component and test the link between oxidized OIB and recycled surface-derived materials in their sources, we measured major and volatile element abundances and Fe3+/∑Fe ratios of naturally glassy, olivine-hosted melt inclusions from Piton de La Fournaise volcano, La Reunion. We conclude that the fO2 of the mantle source of these Reunion lavas is lower than of the mantle sources of primitive, undegassed magmas from Hawaii, Iceland, the Canary Islands, and Mt. Erebus, and indistinguishable from that of the Indian-ocean upper mantle. This finding is consistent with previous suggestions that the source of Reunion lavas (and the C/FOZO/PREMA mantle component) contains little or no recycled materials and with the suggestion that recycled oxidized materials contribute to the high fO2 of some other OIBs, especially those from incompatible-element-enriched mantle sources. Simple mixing models between oxidized melts of EM1 and HIMU components and relatively reduced melts of DMM can explain the isotopic compositions and Fe3+/∑Fe ratios of lavas from Hawaii, Iceland, the Canary Islands, and Mount Erebus; this model can be tested by study of additional OIB magmas, including those rich in the EM2 component.

Mars’ atmospheric neon suggests volatile-rich primitive mantle

Martian atmospheric neon (Ne) has been detected by Viking and also found as trapped gas in Martian meteorites, though its abundance and isotopic composition have not been well determined. Because the timescale of Ne loss via atmospheric escape estimated from recent measurements with MAVEN is short (0.6–1 × 108 years), the abundance and isotope composition of Martian atmospheric Ne reflect recent atmospheric gas supply mostly from volcanic degassing. Thus, it can serve as a probe for the volatile content of the interior. Here we show that the tentatively-informed atmospheric Ne abundance suggests recent active volcanism and the mantle being richer in Ne than Earth’s mantle today by more than a factor of 5–80. The estimated mantle Ne abundance requires efficient solar nebular gas capture or accretion of Ne-rich materials such as solar-wind-implanted dust in the planet formation stage, both of which provide important constraints on the abundance of other volatile elements in the interior and the accretion history of Mars. More precise determination of atmospheric Ne abundance and isotopic composition by in situ analysis or Mars sample return is crucial for distinguishing the possible origins of Ne.

Covariation of Slab Tracers, Volatiles, and Oxidation During Subduction Initiation

AbstractSubduction‐related lavas have higher Fe3+/∑Fe than midocean ridge basalts (MORB). Hypotheses for this offset include imprint from subducting slabs and differentiation in thickened crust. These ideas are readily tested through examination of the time‐dependent evolution of slab‐derived signatures, thickening crust of the overriding plate, and evolving redox during subduction initiation. Here, we present Fe3+/ΣFe and volatile element abundances of volcanic glasses recovered from International Ocean Discovery Program (IODP) Expedition 352 to the Izu‐Bonin‐Mariana (IBM) forearc. The samples include forearc basalts (FAB) that are stratigraphically overlain by low‐ and high‐silica boninite lavas. The FAB glasses have 0.18–0.85 wt% H2O, 75–233 ppm CO2, S contents controlled by saturation with a sulfide phase (602–1,386 ppm), Ba/La from 3.9‐10, and Fe3+/ΣFe ratios from 0.136 to 0.177. These compositions are similar to MORB and suggest that decompression melting of dry and reduced mantle dominates the earliest stages of subduction initiation. Low‐ and high‐silica boninite glasses have 1.51–3.19 wt% H2O, CO2 below detection, S contents below those required for sulfide saturation (5–235 ppm), Ba/La from 11 to 29, and Fe3+/∑Fe from 0.181 to 0.225. The compositions are broadly similar to modern arc lavas in the IBM arc. These data demonstrate that the establishment of fluid‐fluxed melting of the mantle, which occurs in just 0.6–1.2 my after subduction initiation, is synchronous with the production of oxidized, mantle‐derived magmas. The coherence of high Fe3+/∑Fe and Ba/La ratios with high H2O contents in Expedition 352 glasses and the modern IBM arc rocks strongly links the production of oxidized arc magmas to signatures of slab dehydration.

Exploring relationships between shock-induced microstructures and H2O and Cl in apatite grains from eucrite meteorites

The abundance and isotopic composition of volatile elements in meteorites is critical for understanding planetary evolution, given their importance in a variety of geochemical processes. There has been significant interest in the mineral apatite, which occurs as a minor phase in most meteorites and is known to contain appreciable amounts of volatiles (up to wt. % F, Cl, and OH). Impact-driven shock metamorphism, pervasive within many meteorites, can potentially modify the original signatures of volatiles through processes such as devolatilization and diffusion. In this study, we investigate the microstructures of apatite grains from six eucrites across a broad range of shock stages (S1–S5) using electron backscatter diffraction (EBSD) to explore shock-induced crystallographic features in apatite. New Cl and H abundance and isotopic composition data were collected on moderate to highly shocked samples (S3-S5) by Nano Secondary Ion Mass Spectrometry (NanoSIMS). Previously reported volatile data for S1 and S2 eucrites were integrated with EBSD findings in this study. Our findings indicate that apatite microstructures become increasingly more complex at higher shock stages. At low shock stages (S1–S2) samples display brecciation and fracturing of apatite. Samples in S3 and S4 display increasing crystal plastic deformation indicated by increasing spread in pole figures. At the higher shock stages (S4/S5) there is potential recrystallisation demonstrated by an increased density of subgrain boundaries. The Cl content and δ37Cl values of highly-shocked apatite grains range from ∼940 to 1410 ppm and −3.38 to +7.70‰, respectively, within the range observed in less-shocked eucrites. In contrast, H2O abundances are more variable (from 186 to ∼4010 ppm), however, the measured water content still falls within the range previously reported for low-shock eucrites. The measured δD values range from −157 to +163‰, also within the range of values from known low-shock basaltic eucrites. Weighted averages for both isotopic systems (δD −122 ± 20‰, δ37Cl + 1.76 ± 0.66‰) are consistent with the range displayed in other inner Solar System bodies. NanoSIMS isotope images of apatite grains display heterogeneity in their Cl abundance at the nanoscale which increases in complexity with shock stage. This increasing complexity, however, does not correlate with deformation microstructures observed in EBSD or with the Cl isotopic composition at either an inter-grain or intra-grain scale. These findings are similar to analyses of variably shocked lunar apatite and, therefore, apatite appears to be a robust recorder of Cl and H (at least at spatial resolution and precision currently achievable by NanoSIMS) on airless bodies, despite intensive shock.

Highly volatile element (H, C, F, Cl, S) abundances and H isotopic compositions in chondrules from carbonaceous and ordinary chondrites

The partial pressures and isotopic compositions of volatiles present during chondrule formation can be constrained by the highly volatile element or HVE (H, C, F, Cl, and S) abundances and isotopic compositions in chondrules. Here we present the results of high spatial resolution and low background secondary ion mass spectroscopy (SIMS) analyses of the HVE concentrations and H isotopic compositions in type I and II chondrules in primitive ordinary chondrites Semarkona (LL3.00) and Queen Alexandra Range (QUE) 97008 (L3.05), and the primitive carbonaceous chondrite Dominion Range (DOM) 08006 (CO3.00). The HVEs in the chondrules primarily reside in the mesostases, in which the HVE contents and H isotopic compositions vary significantly (H2O: 8–10,200 ppm, CO2: 2.4–1170 ppm, F: 0.3–30 ppm, Cl: 0.07–175 ppm, S: 0.38–4400 ppm, δD: 77–15,000‰). To dissolve such HVE contents in a silicate melt requires significantly higher total pressures (up to 1900 bars), and in some cases requires anomalous gas compositions (CO dominated), compared to those expected from canonical conditions of chondrule formation (∼10−3 bars, H2 + H2O dominated). Rather, the enrichments of H2O, CO2, Cl, and F in the mesostases at the edges of some chondrules suggest that there were secondary influxes of HVEs into the chondrule mesostases from the surrounding matrix during parent body processes. Consistent with this, melt inclusions sealed in olivine phenocrysts have significantly lower HVE contents than the mesostases in contact with the surrounding matrix material. Further, the calculated diffusion distances of H2O in silicate glasses under the relevant conditions are comparable to the radii of the chondrules. The high δD values in the mesostases could have been generated through isotopic Rayleigh fractionation as a result of the loss of very D-poor H2 generated from Fe metal oxidation by H2O in the parent bodies. Based on these results, we hypothesize that the bulk of the HVEs in the chondrules are secondary in origin. However, a small portion of the HVEs in chondrules could be primary, as there are low but measurable amounts of HVEs in the melt inclusions that are sealed in phenocrysts. Further, measured S contents in some chondrule mesostases agree with those predicted in a sulfide saturated silicate melt based on an experimentally calibrated thermodynamic model. We constrain the upper limits of primary HVEs in the chondrules based on the lowest measured HVE contents to minimize the effects of the secondary influx of HVEs (type I H2O: 7–11 ppm, CO2: 0.3–0.6 ppm, F: 0.1–0.2 ppm, Cl: 0.01–0.03 ppm, S: 0.3–60 ppm, and type II H2O: 50–85 ppm, CO2: 0.4–3 ppm, F: 0.04–2 ppm, Cl: 0.04–2 ppm, S: 190–260 ppm).

Collisions and compositional variability in chondrule-forming events

Compound chondrules, i.e. chondrules fused together, make a powerful probe of the density and compositional diversity in chondrule-forming environments, but their abundance among the dominating porphyritic textures may have been drastically underestimated. I report herein microscopic observations and LA-ICP-MS analyses of lobate chondrules in the CO3 chondrites Miller Range 07193 and 07342. Lobes in a given chondrule show correlated volatile and moderately volatile element abundances but refractory element concentrations are essentially independent. This indicates that they formed by the collision of preexisting droplets whose refractory elements behaved in closed system, while their more volatile elements were buffered by the same gaseous medium. The presence of lobes would otherwise be difficult to explain, as surface tension should have rapidly imposed a spherical shape at the temperature peak. In fact, since most chondrules across chondrite groups are nonspherical, a majority are probably compounds variously relaxed toward sphericity. The lack of correlation of refractory elements between conjoined compound chondrule components is inconsistent with derivation of chondrules from the disruption of homogenized melt bodies as in impact scenarios and evokes rather the melting of independent mm-size nebular aggregates. Yet a “nebular” setting for chondrule formation would need to involve not only increased solid concentration, e.g. by settling to the midplane, but also a boost in relative velocities between droplets during chondrule-forming events to account for observed compound chondrule frequencies.

Terrestrial modification of the Ivuna meteorite and a reassessment of the chemical composition of the CI type specimen

The rare CI carbonaceous chondrites are the most aqueously altered and chemically primitive meteorites but due to their porous nature and high abundance of volatile elements are susceptible to terrestrial weathering. The Ivuna meteorite, type specimen for the CI chondrites, is the largest twentieth-century CI fall and probably the CI chondrite least affected by terrestrial alteration that is available for study. The main mass of Ivuna (BM2008 M1) has been stored in a nitrogen atmosphere at least since its arrival at the Natural History Museum (NHM), London, in 2008 (70 years after its fall) and could be considered the most pristine CI chondrite stone. We report the mineralogy, petrography and bulk elemental composition of BM2008 M1 and a second Ivuna stone (BM1996 M4) stored in air within wooden cabinets. We find that both Ivuna stones are breccias consisting of multiple rounded, phyllosilicate-rich clasts that formed through aqueous alteration followed by impact processing. A polished thin section of BM2008 M1 analysed immediately after preparation was found to contain sulphate-bearing veins that formed when primary sulphides reacted with oxygen and atmospheric water. A section of BM1996 M4 lacked veins but had sulphate grains on the surface that formed in ≤6 years, ∼3 times faster than previous reports for CI chondrite sections. Differences in the extent of terrestrial alteration recorded by BM2008 M1 and BM1996 M4 probably reflect variations in the post-recovery curation history of the stones prior to entering the NHM collection, and indicate that where possible pristine samples of hydrated carbonaceous should be kept out of the terrestrial environment in a stable atmosphere to avoid modification. The bulk elemental composition of the two Ivuna stones show some variability due to their heterogeneous nature but in general are similar to previous analyses of CI chondrites. We combine our elemental abundances with literature values to calculate a new average composition for the Ivuna meteorite, which we find is in good agreement with existing compilations of element compositions in the CI chondrites and the most recent solar photospheric abundances.

Tracking the volatile and magmatic history of Vesta from chromium stable isotope variations in eucrite and diogenite meteorites

Although Solar System bodies exhibit large variations in their volatile element abundances, the mechanisms and conditions that lead to these variations remain ambiguous. The howardite-eucrite-diogenite (HED) meteorites that likely sample the asteroid 4 Vesta, provide evidence for extensive volatile depletion on their parent body. Isotopic variations in moderately volatile elements, such as Zn, have been used to track the origin of such volatile loss. Although not nominally volatile, Cr is useful because it has several oxidized gas species that render it volatile under the oxidizing conditions that characterize planetary accretion. As such, volatile loss of Cr has the potential to produce an isotopically light evaporation residue under an equilibrium regime. This contrasts with other moderately volatile elements that show heavy isotope enrichments in the residue following both kinetic or equilibrium fractionation. Here, we report the Cr stable isotope composition of 11 eucrites and four diogenites. The eucrites possess systematically lighter Cr isotope compositions than diogenites, which is onset by the accumulation of isotopically heavy Cr3+-rich orthopyroxene and spinel in diogenites during their magmatic evolution. We estimate for the primary eucrite melt with Mg# ≈ 50, a δ53Cr (53Cr/52Cr deviation relative to NIST SRM 979 in per mile) of −0.22 ± 0.03‰ (2SD), lighter than any chondritic meteorite group by ∼0.1‰. This deficit may result from either partial melting with residual Cr3+-bearing phases (e.g. chromite) that retain heavy isotopes, or from vapor loss that occurred at equilibrium with a magma ocean on Vesta. Isotopic fractionation during partial melting would necessitate implausibly high Cr contents in the Vestan mantle, and oxygen fugacities high enough to stabilize chromite in the mantle source. Isotopic fractionation during evaporation would require an oxidized vapor and a reduced residue, as predicted by thermodynamic constraints on the composition of the vapor phase above a silicate magma ocean. Therefore, this Cr isotopic deficit between Vesta and chondrites may be caused by Cr loss at relatively high oxygen fugacity in a gas phase at equilibrium with the liquid from which it evolved. Temperatures of volatile loss are estimated to be lower than 2300 K, consistent with loss from a large-scale magma ocean model for formation of Vesta, which may be a common evolutionary stage in accreting planetesimals.

Fluorine and chlorine fractionation during magma ocean crystallization: Constraints on the origin of the non-chondritic F/Cl ratio of the Earth

Previous studies have reported that the relative abundances of volatile elements in the silicate Earth are non-chondritic. The abundance and distribution of volatile elements in terrestrial planets would have been predominantly controlled by planetary formation processes, including core-mantle separation, magma ocean crystallization, and volatility-dependent high-temperature fractionation. Thus, the current abundance patterns of volatile elements in the silicate fraction of terrestrial planets are the key to understanding the accretional history of terrestrial volatiles and the chemical differentiation of terrestrial planets. Although the origin of the non-chondritic ratios of volatile elements in terrestrial planets has been previously studied, it is still a matter of debate. In this study, we focused on the super-chondritic F/Cl ratio of the bulk silicate Earth and experimentally investigated the silicate mineral-melt partitioning of fluorine and chlorine at pressures from 18 GPa to 25 GPa. Our experimental results show that fluorine is moderately compatible with mantle minerals, whereas chlorine is highly incompatible. These results support the formation of a solid mantle with high F/Cl ratios, and a residual magma ocean and steam atmosphere with low F/Cl ratios during magma ocean crystallization. Thus, the F/Cl ratio in the residual solid parts of terrestrial planets would have become relatively enriched following escape of volatile elements from the planetary surface into outer space. This model is consistent with the collisional erosion hypothesis of primordial crusts and atmospheres, and current observations on the abundance and distribution of terrestrial fluorine and chlorine.

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.

HD 219666 b: a hot-Neptune from TESS Sector 1

We report on the confirmation and mass determination of a transiting planet orbiting the old and inactive G7 dwarf star HD 219666 (M⋆ = 0.92 ± 0.03 M⊙, R⋆ = 1.03 ± 0.03 R⊙, τ⋆ = 10 ± 2 Gyr). With a mass of Mb = 16.6 ± 1.3 M⊕, a radius of Rb = 4.71 ± 0.17 R⊕, and an orbital period of Porb ≃ 6 days, HD 219666 b is a new member of a rare class of exoplanets: the hot-Neptunes. The Transiting Exoplanet Survey Satellite (TESS) observed HD 219666 (also known as TOI-118) in its Sector 1 and the light curve shows four transit-like events, equally spaced in time. We confirmed the planetary nature of the candidate by gathering precise radial-velocity measurements with the High Accuracy Radial velocity Planet Searcher (HARPS) at ESO 3.6 m. We used the co-added HARPS spectrum to derive the host star fundamental parameters (Teff = 5527 ± 65 K, log g⋆ = 4.40 ± 0.11 (cgs), [Fe/H]= 0.04 ± 0.04 dex, log R′HK = −5.07 ± 0.03), as well as the abundances of many volatile and refractory elements. The host star brightness (V = 9.9) makes it suitable for further characterisation by means of in-transit spectroscopy. The determination of the planet orbital obliquity, along with the atmosphericmetal-to-hydrogen content and thermal structure could provide us with important clues on the formation mechanisms of this class of objects.

Significant depletion of volatile elements in the mantle of asteroid Vesta due to core formation

Vesta, the second-largest body in the asteroid belt, is believed to be a prime example of a rocky protoplanet. Studies of the HED (Howardite–Eucrite–Diogenite) meteorite suite, considered to be sourced from the mantle of Vesta, provide insight into protoplanet differentiation processes. Eucrites and diogenites are depleted in many volatile elements, but the depletions of many volatile siderophile elements are not well constrained. Although previous work indicated that the Vestan core equilibrated with its mantle in a global melting event, the possible contribution of core formation to the volatile budget of HED meteorites has not been assessed to date. This prohibits an assessment of the overall volatile depletion systematics of Vesta.Here, we use a compilation of published volatile siderophile element (VSE; C, S, Zn, As, Se, Cd, In, Sb, Te, Tl, Pb, Bi) abundances in diogenites and eucrites to constrain their depletions in the Vestan mantle. We assess to what extent these depletions may be volatility-related, caused by partitioning into sulfides in the mantle, and/or a result of their preferential partitioning into the Vestan core during core-mantle equilibration. Our new estimates for VSE depletions in the Vestan mantle show no correlation with condensation temperature, suggesting the depletions are difficult to reconcile with partial condensation or degassing during accretionary processes only. Sulfide saturation of the diogenite and eucrite source region is also unlikely because of low indigenous S abundances in these samples compared to calculated S contents at sulfide saturation. Consideration of the metal-silicate partitioning behaviour of the VSE shows that most of these elements are sufficiently siderophile to explain a significant part of their observed depletions in the Vestan mantle by their preferential partitioning into a S-rich core.Moderately volatile elements As and Sb both appear more abundant in the Vestan mantle than expected from their highly siderophile behaviour in systems with basaltic melts. These apparent overabundances may be related to strong silicate melt compositional effects on their metal-silicate partitioning behaviour. On the other hand, Zn, and to a lesser extent Te and Tl are significantly more depleted than expected from metal-silicate partitioning alone. This could be evidence for volatile loss of these elements from Vesta through kinetic loss in a hard vacuum. Alternatively, the perceived loss of Zn may be the result of a Zn-poor Vestan bulk composition and/or partly by preferential partitioning of Zn into spinel. The additional depletion of Tl and by extension other highly chalcophile elements such as Se and Te may be explained by segregation of sulfides in the eucritic melt source region(s), if significant amounts of S degassed from eucritic melts after sulfide segregation.Our results underline the feasibility of planetary cores being a major reservoir for many volatile elements that are depleted in planetary mantles, and that are currently assumed to have suffered incomplete condensation during accretion or (partial) degassing after accretion.

Water and Volatile Inventories of Mercury, Venus, the Moon, and Mars

We review the geochemical observations of water, $\mbox{D}/\mbox{H}$ and volatile element abundances of the inner Solar System bodies, Mercury, Venus, the Moon, and Mars. We focus primarily on the inventories of water in these bodies, but also consider other volatiles when they can inform us about water. For Mercury, we have no data for internal water, but the reducing nature of the surface of Mercury would suggest that some hydrogen may be retained in its core. We evaluate the current knowledge and understanding of venusian water and volatiles and conclude that the venusian mantle was likely endowed with as much water as Earth of which it retains a small but non-negligible fraction. Estimates of the abundance of the Moon’s internal water vary from Earth-like to one to two orders of magnitude more depleted. Cl, K, and Zn isotope anomalies for lunar samples argue that the giant impact left a unique geochemical fingerprint on the Moon, but not the Earth. For Mars, an early magma ocean likely generated a thick crust; this combined with a lack of crustal recycling mechanisms would have led to early isolation of the Martian mantle from later delivery of water and volatiles from surface reservoirs or late accretion. The abundance estimates of Martian mantle water are similar to those of the terrestrial mantle, suggesting some similarities in the water and volatile inventories for the terrestrial planets and the Moon.