Magma Ocean Stage Research Articles

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.

Rubidium isotopic compositions of angrites controlled by extensive evaporation and partial recondensation.

The planetesimals in the solar system exhibit varying degrees of moderately volatile elements (MVEs) depletion compared to the protosolar composition. Revealing the relevant mechanisms is crucial for exploring early solar system evolution. Most volatile-depleted materials in the solar system exhibit enrichments in the heavier isotopes of MVEs, which have traditionally been attributed to the loss of volatiles through partial evaporation. Angrites are so far an exception as they are enriched in the lighter isotopes of K. This has been interpreted as reflecting condensation processes. Here, we present Rb isotopic data of angrites and find that they have lighter Rb isotopic compositions than Vesta, Mars, and the Moon. The δ87Rb value of the angrite parent body (APB) is estimated to range between -1.19‰ and -0.67‰. The extremely light Rb isotopic composition of the APB is likely a result of the kinetic recondensation of Rb after near-complete evaporation during the magma ocean stage. This finding provides further support for the partial recondensation model to explain the light Rb and K isotopic compositions of the APB. In addition, the APB, alongside other terrestrial planetary bodies (e.g., Earth, Mars, Moon, and Vesta), exhibit a strong correlation between their Rb and K isotopic compositions. This coupling of Rb and K isotopes is indicative of a volatility-driven isotopic fractionation rather than nucleosynthetic anomalies. The extremely light Rb-K isotopic signatures of the APB suggest that beyond evaporation, condensation plays an equally significant role in shaping the planetary-scale distributions of volatile elements.

Mixing time of heterogeneities in a buoyancy-dominated magma ocean

SUMMARY During the accretion stage, large impacts provided sufficient energy to melt the entire mantle into a terrestrial magma ocean. Processes occurring in the magma ocean may have led to the formation of heterogeneities still found in modern ocean island basalts. So far, no definitive mechanism exists to explain the survival of early heterogeneities for approximately 4.5 Ga. Addressing this question requires understanding the efficiency of convective mixing during both the early molten and the solid-state stages experienced by the Earth’s mantle. While mixing in the solid mantle and in an essentially crystallized magma ocean has been relatively well documented, the efficiency of convective mixing in a liquid magma ocean has received less attention. In this paper we characterized the mixing efficiency of a convecting fluid in a rotating spherical shell, accounting for inertial effects, by computing finite-time Lyapunov exponents (i.e. the Lagrangian strain rate). We conducted a series of numerical experiments for a regime where the influence of the buoyancy force dominates that of rotation and we derived scaling laws to predict the mixing efficiency. We found that for a terrestrial magma ocean, in its fully liquid state, mixing time is of the order of a few minutes or less, even for initially large (∼1000 km) heterogeneities. Therefore, passive early mantle heterogeneities cannot survive a fully molten magma ocean stage. This suggests that short-lived heterogeneities (e.g. 182Hf−182W) were either created at the end of the accretional stage, or were stored in deeper regions of the Earth.

Mars volatile inventory and outgassing history

The timing of formation of the Mars atmosphere controlled the early climate and its subsequent evolution. We examine this timing through the role played in atmospheric formation by outgassing from the mantle resulting from volcanism through time. To do this, we compare the amount of gas released by volcanism, derived based on measurements of the Martian meteorites, with the amounts present at one time in the atmosphere. Results indicate that volcanic outgassing of H2O and 36Ar are insufficient to explain their current atmospheric abundances; 40Ar data also require a very low outgassing efficiency. The bulk of these components of the atmosphere must have been degassed earlier, during accretion/core formation, an early magma-ocean stage, and/or early-Noachian crust formation. These components of the Mars atmosphere appear to have been essentially fully formed at the time of onset of the observable geologic record in the middle and late Noachian, with subsequent evolution dominated by a draw-down of gas via loss to the crust and to space.

Anatomy of rocky planets formed by rapid pebble accretion

We explore the heating and differentiation of rocky planets that grow by rapid pebble accretion. Our terrestrial planets grow outside of the ice line and initially accrete 28% water ice by mass. The accretion of water stops after the protoplanet reaches a mass of 0.01MEwhere the gas envelope becomes hot enough to sublimate the ice and transport the vapour back to the protoplanetary disc by recycling flows. The energy released by the decay of26Al melts the accreted ice to form clay (phyllosilicates), oxidized iron (FeO), and a water surface layer with ten times the mass of Earth’s modern oceans. The ocean–atmosphere system undergoes a run-away greenhouse effect after the effective accretion temperature crosses a threshold of around 300 K. The run-away greenhouse process vaporizes the water layer, thereby trapping the accretion heat and heating the surface to more than 6000 K. This causes the upper part of the mantle to melt and form a global magma ocean. Metal melt separates from silicate melt and sediments towards the bottom of the magma ocean; the gravitational energy released by the sedimentation leads to positive feedback where the beginning differentiation of the planet causes the whole mantle to melt and differentiate. All rocky planets thus naturally experience a magma ocean stage. We demonstrate that Earth’s small excess of182W (the decay product of182Hf) relative to the chondrites is consistent with such rapid core formation within 5 Myr followed by equilibration of the W reservoir in Earth’s mantle with182W-poor material from the core of a planetary-mass impactor, provided that the equilibration degree is at least 25–50%, depending on the initial Hf/W ratio. The planetary collision must have occurred at least 35 Myr after the main accretion phase of the terrestrial planets.

Potassium isotope heterogeneity in the early Solar System controlled by extensive evaporation and partial recondensation

Volatiles are vital ingredients for a habitable planet. Angrite meteorites sample the most volatile-depleted planetesimal in the Solar System, particularly for the alkali elements. They are prime targets for investigating the formation of volatile-poor rocky planets, yet their exceptionally low volatile content presents a major analytical challenge. Here, we leverage improved sensitivity and precision of K isotopic analysis to constrain the mechanism of extreme K depletion (>99.8%) in angrites. In contrast with the isotopically heavy Moon and Vesta, we find that angrites are strikingly depleted in the heavier K isotopes, which is best explained by partial recondensation of vaporized K following extensive evaporation on the angrite parent body (APB) during magma-ocean stage. Therefore, the APB may provide a rare example of isotope fractionation controlled by condensation, rather than evaporation, at a planetary scale. Furthermore, nebula-wide K isotopic variations primarily reflect volatility-driven fractionations instead of presolar nucleosynthetic heterogeneity proposed previously.

Hadean isotopic fractionation of xenon retained in deep silicates.

Our understanding of atmosphere formation essentially relies on noble gases and their isotopes, with xenon (Xe) being a key tracer of the early planetary stages. A long-standing issue, however, is the origin of atmospheric depletion in Xe1 and its light isotopes for the Earth2 and Mars3. Here we report that feldspar and olivine samples confined at high pressures and high temperature with diluted Xe and krypton (Kr) in air or nitrogen are enriched in heavy Xe isotopes by +0.8 to +2.3‰ per AMU, and strongly enriched in Xe over Kr. The upper +2.3‰ per AMU value is a minimum because quantitative trapping of unreacted Xe, either in bubbles or adsorbed on the samples, is likely. In light of these results, we propose a scenario solving the missing Xe problem that involves multiple magma ocean stage events at the proto-planetary stage, combined with atmospheric loss. Each of these events results in trapping of Xe at depth and preferential retention of its heavy isotopes. In the case of the Earth, the heavy Xe fraction was later added to the secondary CI chondritic atmosphere through continental erosion and/or recycling of a Hadean felsic crust.

Magma oceans, iron and chromium redox, and the origin of comparatively oxidized planetary mantles

The mantles of both Earth and Mars are more oxidized than would be expected based on low pressure equilibration of molten silicate and alloy during their magma ocean stages. High pressure silicate-alloy equilibration in a magma ocean can produce appreciable ferric iron in the silicate, leading to comparatively oxidized near surface conditions and overlying atmospheres. Upon crystallization, this may feasibly be sufficient to account for oxygen fugacities prevailing in basalt source regions of Earth and Mars. Experiments and first principles studies affirm that Fe3+ is stabilized at high pressure, but to date there has been no model that accounts accurately for the combined effects of melt composition, temperature, pressure, and oxygen fugacity on magma ocean Fe3+/FeT. We calibrate a new model for Fe3+/FeT as a function of temperature, pressure, melt composition, and fO2 which reproduces Fe3+/FeT for experimental peridotite liquids and which incorporates differences in FeO and Fe2O3 liquid heat capacities into a potentially realistic temperature function. For the effects of pressure, two versions of the model are implemented based on recent equations of state (EOS), though only the EOS of Deng et al. (2020) is applicable to pressures relevant to metal-silicate equilibration in a deep terrestrial magma ocean. For Earth, metal-silicate equilibration at 28–53 GPa, 2300–4100 K, and fO2 set by plausible mantle and core compositions produces Fe3+/FeT between 0.034 and 0.10, with variation mostly owing to differences in assumed temperatures. For Mars, different proposed mantle compositions produce Fe3+/FeT ratios that range from 0.026 for FeO* of 13.5 wt.% up to 0.038 for FeO* of 18.1 wt.%.Although significant Fe3+ may be present in magma oceans owing to high pressure equilibration with alloy, the budget of Fe2O3 in crystallized mantles is expected to be modified from that in the molten state. An important additional factor is the influence of Cr, which is Cr2+ in molten silicate equilibrated with alloy and Cr3+ in terrestrial upper mantles. Production of Cr3+ and Fe2+ by reaction with Cr2+ and Fe3+ during crystallization can destroy much of the Fe2O3 present during the magma ocean stage. Considering the stability of Cr2+ in olivine and the temperature-dependent partitioning of Cr3+ between mantle silicates, we construct an empirical model for the fraction of Cr that is Cr2O3 in solid spinel peridotite as a function of temperature and fO2. For Earth, at least 0.35 wt.% Fe2O3 is destroyed by oxidation of magma ocean CrO and for Mars, more than 0.55 wt.% Fe2O3 should be destroyed. Consequently, either the terrestrial and martian magma oceans were significantly more enriched in Fe2O3 than their present-day upper mantles or other processes contributed to oxidation of the latter. Over-enrichment of Fe2O3 in the magma oceans is plausible only if terrestrial metal-silicate equilibration occurred above 3300 K and if the martian mantle contains >17 wt.% FeO*. Subsolidus disproportionation of ferrous iron may have contributed to the present-day redox state of the Earth’s mantle, and late accretion of chondrite-like material and hydrogen degassing also likely affected the solidified mantles of both Earth and Mars.

The Origin of Earth's Mantle Nitrogen: Primordial or Early Biogeochemical Cycling?

AbstractEarth's mantle nitrogen (N) content is comparable to that found in its N‐rich atmosphere. Mantle N has been proposed to be primordial or sourced by later subduction, yet its origin has not been elucidated. Here we model N partitioning during the magma ocean stage following planet formation and the subsequent cycling between the surface and mantle over Earth history using argon (Ar) and N isotopes as tracers. The partitioning model, constrained by Ar, shows that only about 10% of the total N content can be trapped in the solidified mantle due to N's low solubility in magma and low partitioning coefficients in minerals in oxidized conditions supported from geophysical and geochemical studies. A possible solution for the primordial origin is that Earth had about 10 times more N at the time of magma ocean solidification. We show that the excess N could be removed by impact erosion during late accretion. The cycling model, constrained by N isotopes, shows that mantle N can originate from efficient N subduction, if the sedimentary N burial rate on early Earth is comparable to that of modern Earth. Such a high N burial rate requires biotic processing. Finally, our model provides a methodology to distinguish the two possible origins with future analysis of the surface and mantle N isotope record.

Retention of Water in Terrestrial Magma Oceans and Carbon-rich Early Atmospheres

Massive steam and CO2 atmospheres have been proposed for magma ocean outgassing of Earth and terrestrial planets. Yet formation of such atmospheres depends on volatile exchange with the molten interior, governed by volatile solubilities and redox reactions. We determine the evolution of magma ocean–atmosphere systems for a range of oxygen fugacities, C/H ratios, and hydrogen budgets that include redox reactions for hydrogen (H2–H2O), carbon (CO–CO2), methane (CH4), and solubility laws for H2O and CO2. We find that small initial budgets of hydrogen, high C/H ratios, and oxidizing conditions suppress outgassing of hydrogen until the late stage of magma ocean crystallization. Hence, early atmospheres in equilibrium with magma oceans are dominantly carbon-rich, and specifically CO-rich except at the most oxidizing conditions. The high solubility of H2O limits its outgassing to melt fractions below ∼30%, the fraction at which the mantle transitions from vigorous to sluggish convection with melt percolation. Sluggish melt percolation could enable a surface lid to form, trapping water in the interior and thereby maintaining a carbon-rich atmosphere (equilibrium crystallization). Alternatively, efficient crystal settling could maintain a molten surface, promoting a transition to a water-rich atmosphere (fractional crystallization). However, additional processes, including melt trapping and H dissolution in crystallizing minerals, further conspire to limit the extent of H outgassing, even for fractional crystallization. Hence, much of the water delivered to planets during their accretion can be safely harbored in their interiors during the magma ocean stage, particularly at oxidizing conditions.

Inefficient Water Degassing Inhibits Ocean Formation on Rocky Planets: An Insight from Self-Consistent Mantle Degassing Models.

A sufficient amount of water is required at the surface to develop water oceans. A significant fraction of water, however, remains in the mantle during magma ocean solidification, and thus the existence of water oceans is not guaranteed even for exoplanets located in the habitable zone. To discuss the likelihood of ocean formation, we built two models to predict the rate of mantle degassing during the magma ocean stage and the subsequent solid-state convection stage. We find that planets with low H2O/CO2 ratios would not have a sufficient amount of surface water to develop water oceans immediately after magma ocean solidification, and the majority of the water inventory would be retained in the mantle during their subsequent evolution regardless of planetary size. This is because oceanless planets are likely to operate under stagnant lid convection, and for such planets, dehydration stiffening of the depleted lithospheric mantle would limit the rate of mantle degassing. In contrast, a significant fraction of CO2 would already be degassed during magma ocean solidification. With a strong greenhouse effect, all surface water would exist as vapor, and water oceans may be absent throughout planetary evolution. Volatile concentrations in the bulk silicate Earth are close to the threshold amount for ocean formation, so if Venus shared similar concentrations, small differences in solar radiation may explain the divergent evolutionary paths of Earth and Venus.

Evolution of Mercury’s Earliest Atmosphere

Abstract MESSENGER observations suggest a magma ocean formed on proto-Mercury, during which evaporation of metals and outgassing of C- and H-bearing volatiles produced an early atmosphere. Atmospheric escape subsequently occurred by plasma heating, photoevaporation, Jeans escape, and photoionization. To quantify atmospheric loss, we combine constraints on the lifetime of surficial melt, melt composition, and atmospheric composition. Consideration of two initial Mercury sizes and four magma ocean compositions determines the atmospheric speciation at a given surface temperature. A coupled interior–atmosphere model determines the cooling rate and therefore the lifetime of surficial melt. Combining the melt lifetime and escape flux calculations provides estimates for the total mass loss from early Mercury. Loss rates by Jeans escape are negligible. Plasma heating and photoionization are limited by homopause diffusion rates of ∼106 kg s−1. Loss by photoevaporation depends on the timing of Mercury formation and assumed heating efficiency and ranges from ∼106.6 to ∼109.6 kg s−1. The material for photoevaporation is sourced from below the homopause and is therefore energy limited rather than diffusion limited. The timescale for efficient interior–atmosphere chemical exchange is less than 10,000 yr. Therefore, escape processes only account for an equivalent loss of less than 2.3 km of crust (0.3% of Mercury’s mass). Accordingly, ≤0.02% of the total mass of H2O and Na is lost. Therefore, cumulative loss cannot significantly modify Mercury’s bulk mantle composition during the magma ocean stage. Mercury’s high core:mantle ratio and volatile-rich surface may instead reflect chemical variations in its building blocks resulting from its solar-proximal accretion environment.

Experimental Investigation of Mercury's Magma Ocean Viscosity: Implications for the Formation of Mercury's Cumulate Mantle, Its Subsequent Dynamic Evolution, and Crustal Petrogenesis

AbstractMercury has a compositionally diverse surface that was produced by different periods of igneous activity suggesting heterogeneous mantle sources. Understanding the structure of Mercury's mantle formed during the planet's magma ocean stage could help in developing a petrologic model for Mercury, and thus, understanding its dynamic history in the context of crustal petrogenesis. We present results of falling sphere viscometry experiments on late‐stage Mercurian magma ocean analogue compositions conducted at the Advanced Photon Source, beamline 16‐BM‐B, Argonne National Laboratory. Owing to the presence of sulfur on the surface of Mercury, two compositions were tested, one with sulfur and one without. The liquids have viscosities of 0.6–3.9 (sulfur‐bearing; 2.6–6.2 GPa) and 0.6–10.9 Pa·s (sulfur‐free; 3.2–4.5 GPa) at temperatures of 1600–2000°C. We present new viscosity models that enable extrapolation beyond the experimental conditions and evaluate grain growth and the potential for crystal entrainment in a cooling, convecting magma ocean. We consider scenarios with and without a graphite flotation crust, suggesting endmember outcomes for Mercury's mantle structure. With a graphite flotation crust, crystallization of the mantle would be fractional with negatively buoyant minerals sinking to form a stratified cumulate pile according to the crystallization sequence. Without a flotation crust, crystals may remain entrained in the convecting liquid during solidification, producing a homogeneous mantle. In the context of these endmember models, the surface could result from dynamical stirring or mixing of a mantle that was initially heterogeneous, or potentially from different extents of melting of a homogeneous mantle.

The Thermo‐Chemical Evolution of Mars With a Strongly Stratified Mantle

AbstractThe Martian mantle probably experienced an early global magma ocean stage. The crystallization and the fractionation and overturn of such a magma ocean likely led to the formation of a compositionally distinct layer at the bottom of the mantle. This layer would have been heavily enriched in iron and in heat‐producing elements (HPE). The significant iron enrichment can lead to long‐term stability with little mixing between the layer and the overlying mantle. We studied the influence of such an enriched basal layer on the thermal and chemical evolution of the Martian mantle using both 2‐D finite‐volume modeling at mantle scale, and a parameterized convection approach at the entire planetary scale. The basal layer is most likely stably stratified because of its moderate thickness and/or its gradual enrichment in iron with depth that prevents the development of convection in this region. We explored a wide parameter space in our parameterized models, including the layer thickness and the mantle rheology. We show that the presence of an enriched basal layer has a dramatic influence on the thermo‐chemical evolution of Mars, strongly delaying deep cooling, and significantly affecting nearly all present‐day characteristics of the planet (heat flux, thermal state, crustal and lithospheric thickness, Love number and tidal dissipation). In particular, the enrichment of the layer in iron and HPE generates large volumes of stable melt near the core‐mantle boundary. Due to their intrinsic low viscosity and seismic velocities, these regions of silicate melt could be erroneously interpreted as core material.

Evidence for post-nebula volatilisation in an exo-planetary body

The loss and gain of volatile elements during planet formation is key for setting their subsequent climate, geodynamics, and habitability. Two broad regimes of volatile element transport in and out of planetary building blocks have been identified: that occurring when the nebula is still present, and that occurring after it has dissipated. Evidence for volatile element loss in planetary bodies after the dissipation of the solar nebula is found in the high Mn to Na abundance ratio of Mars, the Moon, and many of the solar system's minor bodies. This volatile loss is expected to occur when the bodies are heated by planetary collisions and short-lived radionuclides, and enter a global magma ocean stage early in their history. The bulk composition of exo-planetary bodies can be determined by observing white dwarfs which have accreted planetary material. The abundances of Na, Mn, and Mg have been measured for the accreting material in four polluted white dwarf systems. Whilst the Mn/Na abundances of three white dwarf systems are consistent with the fractionations expected during nebula condensation, the high Mn/Na abundance ratio of GD362 means that it is not (>3σ). We find that heating of the planetary system orbiting GD362 during the star's giant branch evolution is insufficient to produce such a high Mn/Na. We, therefore, propose that volatile loss occurred in a manner analogous to that of the solar system bodies, either due to impacts shortly after their formation or from heating by short-lived radionuclides. We present potential evidence for a magma ocean stage on the exo-planetary body which currently pollutes the atmosphere of GD362.

Two-stage formation of pallasites and the evolution of their parent bodies revealed by deformation experiments

Pallasites, stony-iron meteorites predominantly composed of olivine crystals and Fe-Ni metal, are samples of the interior of early solar system bodies and can thus provide valuable insights into the formation of terrestrial planets. However, pallasite origin is controversial, either sampling the core-mantle boundary or the shallower mantle of planetesimals that suffered an impact. We present high strain-rate deformation experiments with the model system olivine + FeS melt ± gold melt to investigate pallasite formation and the evolution of their parent bodies and compare the resulting microstructures to two samples of Seymchan pallasite. Our experiments reproduced the major textural features of pallasites including the different olivine shapes, olivine aggregates, and the distribution of the metal and sulfide phases. These results indicate that pallasites preserve evidence for a two-stage formation process including inefficient core-mantle differentiation and an impact causing disruption, metal melt injection, and fast cooling within months to years. Olivine aggregates, important constituents of angular pallasites, are reinterpreted as samples of a partially differentiated mantle containing primordial metallic melt not stemming from the impactor. The long-term retention of more than 10 vol% of metal melt in a silicate mantle sampled by olivine aggregates indicates high effective percolation thresholds and inefficient metal-silicate differentiation in planetesimals not experiencing a magma ocean stage.

A mushy Earth's mantle for more than 500 Myr after the magma ocean solidification

SUMMARYIn its early evolution, the Earth mantle likely experienced several episodes of complete melting enhanced by giant impact heating, short-lived radionuclides heating and viscous dissipation during the metal/silicate separation. After a first stage of rapid and significant crystallization (Magma Ocean stage), the mantle cooling is slowed down due to the rheological transition, which occurs at a critical melt fraction of 40–50%. This transition first occurs in the lowermost mantle, before the mushy zone migrates toward the Earth's surface with further mantle cooling. Thick thermal boundary layers form above and below this reservoir. We have developed numerical models to monitor the thermal evolution of a cooling and crystallizing deep mushy mantle. For this purpose, we use a 1-D approach in spherical geometry accounting for turbulent convective heat transfer and integrating recent and solid experimental constraints from mineral physics. Our results show that the last stages of the mushy mantle solidification occur in two separate mantle layers. The lifetime and depth of each layer are strongly dependent on the considered viscosity model and in particular on the viscosity contrast between the solid upper and lower mantle. In any case, the full solidification should occur at the Hadean–Eoarchean boundary 500–800 Myr after Earth's formation. The persistence of molten reservoirs during the Hadean may favor the absence of early reliefs at that time and maintain isolation of the early crust from the underlying mantle dynamics.

Experimental constraints on the solidification of a hydrous lunar magma ocean

AbstractThe identification of hydrogen in a range of lunar samples and the similarity of its abundance and isotopic composition with terrestrial values suggest that water could have been present in the Moon since its formation. To quantify the effect of water on early lunar differentiation, we present new analyses of a high‐pressure, high‐temperature experimental study designed to model the mineralogical and geochemical evolution of the solidification material equivalent to 700 km deep lunar magma oceans first reported in Lin et al. (2017a). We also performed additional experiments to better quantify water contents in the run products. Water contents in the melt phases in hydrous run products spanning a range of crystallization steps were quantified directly using a secondary ion mass spectrometry (SIMS). Results suggest that a significant but constant proportion (68 ± 5%) of the hydrogen originally added to the experiments was lost from the starting material independent of run conditions and run duration. The volume of plagioclase formed during our crystallization experiments can be combined with the measured water contents and the observed crustal thickness on the Moon to provide an updated lunar interior hygrometer. Our data suggest that at least 45–354 ppm H2O equivalent was present in the Moon at the time of crust formation. These estimates confirm the inference of Lin et al. (2017a) that the Moon was wet during its magma ocean stage, with corrected absolute water contents now comparable to estimates derived from the water content in a range of lunar samples.

Syneruptive incorporation of martian surface sulphur in the nakhlite lava flows revealed by S and Os isotopes and highly siderophile elements: implication for mantle sources in Mars

Martian lava flows likely acquired S-rich material from the regolith during their emplacement on the planet’s surface. We investigated five of the twenty known nakhlites (Nakhla, Lafayette, Miller Range (MIL) 090032, Yamato 000593, and Yamato 000749) to determine whether these lavas show evidence of regolith assimilation, and to constrain the potential implications that this process has on chemical tracing of martian mantle source(s). To establish the proportionate influence of atmospheric, hydrothermal, and volcanic processes on nakhlite isotopic systematics we obtained in situ sulphur isotope data (Δ33S and δ34S) for sulphide grains (pyrrhotite and pyrite) in all five nakhlite samples. For Nakhla, Lafayette, and MIL 090032, these data are integrated with highly siderophile element (HSE) abundances and Os-isotope compositions, as well as textural information constrained prior to isotopic analysis. This work thereby provides the first Re-Os isotope systematics for two different nakhlites, and also the first Re-Os isotope data for martian sample for which detailed petrographic information was constrained prior to digestion.We report the largest variation in δ34S yet found in martian meteorites (−13.20‰ to +15.16‰). The relatively positive Δ33S and δ34S values of MIL 090032 (δ34S = +10.54 ± 0.09‰; Δ33S = −0.67 ± 0.10‰) indicate this meteorite assimilated sulphur affected by UV-photochemistry. In contrast, the strongly negative values of Lafayette (δ34S = −10.76 ± 0.14‰; Δ33S = −0.09 ± 0.12‰) are indicative of hydrothermal processes on Mars. Nakhla, Yamato 000593, and Yamato 000749 sulphides have a narrower range of sulphur isotope compositions (Δ33S and δ34S ∼ 0) that is consistent with no assimilation of martian surface materials during lava flow emplacement. Consequently we used this second group of Δ33S values to approximate the Δ33S of the nakhlite source, yielding a Δ33S value of −0.1‰.Nakhlite HSE patterns result from a sulphide-saturated melt where Ru-Os-Ir alloys/sulphide were likely crystallized during earlier phases of magmatic processing in Mars to result in the fractionated HSE patterns of the nakhlites. Our data, alongside a synthesis of previously published data, suggest assimilation of an enriched component to the primary nakhlite melt, potentially a late-stage crystallization cumulate from the martian magma ocean stage. In the context of this model, and within large uncertainties, our data hint at perturbation and potential decoupling of nakhlite Re-Os isotope systematics from other isotopic systems as a result of small degrees of assimilation of a regolith component with highly radiogenic 187Os/188Os.

Theoretical models and experimental determination methods for equations of state of silicate melts: A review

Silicate melts are very active in the interior of the Earth and other terrestrial planets, and are important carriers for the transport of material and energy. The determination of the equation of state (EOS) for silicate melts and the acquisition of a precise quantitative relationship between molar volume (or density) and temperature, pressure, and composition is essential for simulating the generation, migration, and eruption processes of magmas and the evolution of the magma ocean stage during the early formation of the Earth and other terrestrial planets, for calculating and modeling the phase equilibria involving silicate melts, and for revealing the variation of the microstructure of silicate melts with pressure. However, it is experimentally challenging to determine the volumetric properties of silicate melts and the accumulated density data at high pressure are still very limited due to a series of problems such as: the high liquidus temperature of silicate rocks; proneness for silicate melts to react with sample capsules to change the melt composition; and proneness for melts to flow and leak during the high pressure and high temperature experiments. In recent years, there is rapid progress in the high pressure and high temperature experimental techniques, in terms of not only the extension of temperature and pressure ranges but also the improvement on the accuracy of measurements, and the emergence of new methods for in-situ measurements. Here, we review the widely-used theoretical models of ambient-pressure and high-pressure EOS for silicate melts, and illustrate some problems that need to be solved urgently: (1) the room pressure EOS for iron- and titanium-bearing silicate melts needs to be improved; (2) the partial molar properties of the H2O and CO2 components in silicate melts containing volatile components may vary markedly with the melt composition, which need to be addressed in high-pressure EOS; (3) how the formulation and applicable range of EOS correspond to changes in melt structure and compression mechanism requires further study. We highlight the basic principle and applicable range of various methods for determining the EOS for silicate melts, and compare the advantages and disadvantages of double-bob Archimedes method, fusion curve analysis, shock compression experiments, sink-float method, X-ray absorption, X-ray diffraction and ultrasonic interferometry. Future trends in this field are to develop experimental techniques for in situ measurements on melt density or sound velocity at high temperature and high pressure and to accumulate more experimental data, and on the other hand, to improve the theoretical models of the EOS for silicate melts by a combination of research on the microstructure and compression mechanisms of silicate melts.