Shallow Magma Ocean Research Articles

A heterogeneous mantle and crustal structure formed during the early differentiation of Mars.

Highly siderophile element abundances and Os isotopes of nakhlite and chassignite meteorites demonstrate that they represent a comagmatic suite from Mars. Nakhlites experienced variable assimilation of >2-billion-year-old altered high Re/Os basaltic crust. This basaltic crust is distinct from the ancient crust represented by meteorites Allan Hills 84001 or impact-contaminated Northwest Africa 7034/7533. Nakhlites and chassignites that did not experience crustal assimilation reveal that they were extracted from a depleted lithospheric mantle distinct from the deep plume source of depleted shergottites. The comagmatic origin for nakhlites and chassignites demonstrates a layered martian interior comprising ancient enriched basaltic crust derived from trace element-rich shallow magma ocean cumulates, a variably metasomatized mantle lithosphere, and a trace element-depleted deep mantle sampled by plume magmatism.

Tidal Heating in a Subsurface Magma Ocean on Io Revisited

AbstractWe investigate the tidal dissipation in Io's hypothetical fluid magma ocean using a new approach based on the solution of the 3D Navier‐Stokes equations. Our results indicate that the presence of a shallow magma ocean on top of a solid, partially molten layer leads to an order of magnitude increase in dissipation at low latitudes. Tidal heating in Io's magma ocean does not correlate with the distribution of hot spots, and is maximum for an ocean thickness of about 1 km and a viscosity of less than 104 Pa s. Due to the Coriolis effect, the k2 Love number can depend on the harmonic order. We show that the analysis of k2 may not reveal the presence of a fluid magma ocean if the ocean thickness is less than 2 km. If the fluid layer is thicker than 2 km, k20 ≈ k22/2 ≈ 0.7.

Experimental determination of N2 solubility in silicate melts and implications for N2–Ar–CO2 fractionation in magmas

The solubility of nitrogen in magmatic melts is key for understanding the degassing of Earth’s interior and formation of Earth’s N-rich atmosphere. We performed high-pressure experiments to determine the solubility of nitrogen at N2-saturation in basaltic to granitic melts at pressures of 0.3–8 GPa, temperatures of 1200–1600 °C, and oxygen fugacities between the Fe–FeO and Ni–NiO oxygen buffers. The solubility and speciation of nitrogen in the quench silicate melts were quantified by Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), electron probe microanalyzer (EPMA), and Nano-scale secondary ion mass spectrometry (NanoSIMS). The results show that the predominant N-species in our silicate melts is N2, which dissolves physically by filling the interstitial sites of the silicate melt network. The measured N-solubilities in silicate melts (SN2melt) by using different approaches agree with each other, and they vary from 200 to 17,000 ppm by weight. The SN2melt increases with increasing pressure and/or decreasing silicate melt NBO/T (the ratio of non-bridging oxygens to tetrahedrally coordinated cations), but it is temperature-independent. Using our newly obtained SN2melt and previous SN2melt and SArmelt data, we develop an empirical model that describes the physical dissolution of N2 and Ar in silicate melts as a function of gas partial pressure and silicate melt composition, and this model can be used to predict SN2melt and SArmelt in magmatic melts of Earth’s mantle and crust. The comparison of our SN2melt data with previous SArmelt and SCO2melt data shows that the solubility in granitic melts is in the order of SArmelt >SCO2melt >SN2melt, but in basaltic melts it is in the order of SCO2melt >SArmelt > SN2melt. The lower SN2melt than SArmelt at Earth’s mantle and crustal conditions suggests N2–Ar fractionation during degassing of mid-ocean ridge basalts and arc magmas. Therefore, the N2/Ar ratios in degassed magmas may not fully represent those in their mantle source regions. The solubility-controlled, preferential degassing of N2 relative to CO2 in Earth’s oxidized shallow magma ocean, in conjunction with collision-induced atmospheric loss, can explain the superchondritic C/N ratio in the bulk silicate Earth. However, the two orders of magnitude higher N2/36Ar ratio in Earth’s mantle than in atmosphere could have resulted from preferential retention of reduced N-species in Earth’s reduced silicate magma ocean or preferential deep subduction of nitrogen.

A speciation model linking the fate of carbon and hydrogen during core – magma ocean equilibration

The core – mantle differentiation in the magma ocean constitutes a major planetary event that involved two elements that are essential to life: carbon (C) and hydrogen (H). These two elements are conventionally classified as volatiles (i.e. atmophile), but they can also evolve into being siderophile and lithophile at the extreme conditions found in planetary magma oceans. We report here a model for H and C species dissolved in silicate melts in equilibrium with iron-rich alloys under variable pressure, temperature and redox conditions. This speciation model is able to reconcile and reproduce a large body of experimental data on metal–silicate partitioning for H and C at carbon-saturation and in C-undersaturated systems. At low pressure, we conclude that the prevailing species in a silicate magma ocean are CO2, CO, H2O, H2, whereas CH4 appears to dominate at high pressure. These speciation changes explain recent experimental observations that (i) C evolves from being strongly siderophile at low pressure to moderately siderophile at high pressure, and (ii) H is not siderophile at low pressure but becomes increasingly so as pressure rises. Moreover, it shows that H becomes increasingly siderophile as the total H content of the silicate melt and C-activity are lowered. Despite it offers a promising reconciliation of a large set of experimental and molecular dynamics observations, this model still suffers from large uncertainties when extrapolated to high pressure. In particular, endmember and mixing properties in both the silicate melt and the molten metal must be independently deciphered.The enhanced CH4 stability in the silicate melt at high pressure couples the fate of C and H in deep magma oceans. In such cases, the solubility of C in the basal ocean depends on the H-content and is higher than C-solubility at low pressure. This implies an increase in C activity as the ascending convective cells of the magma ocean, which may cause C saturation as graphite or diamond unless Fe-metal droplets, having a great C-solubility, are present in the shallow magma ocean. Under certain conditions, enhanced siderophile behaviour for H can lead via a runaway process to the desiccation of the magma ocean.

A model for evolving crust on 4 Vesta through combined compositional and thermal modelling

Combined phase equilibrium and thermal modelling has been used to investigate the evolution of asteroid 4 Vesta. Orthopyroxene compositions of 200 natural diogenite meteorites are used as a basis for constructing a staged mantle melting model for Vesta, which is then used to develop a staged thermal evolution model. Our pMELTS models find that removal of 15–20% of a mean eucrite component from an initial Vestan mantle composition allows a second stage of melting that crystallises low-calcium orthopyroxenes that match the observed compositions of those in natural diogenites, whereas single stage melting produces orthopyroxenes that are too calcic. Using the compositions generated by the pMELTS modelling, THERMOCALC models were created for an initial Vestan mantle composition and an evolved composition generated by a melt extraction stage. These models suggest that melt production for second-stage diogenite generation required considerably hotter temperatures (>1340°C) than for eucrites (<1240°C). Staged and layered thermal evolution models developed using these composition and temperature constraints, based on the decay of 26Al and 60Fe, suggest that Vesta accreted 1.50 to 1.75 Myr after calcium-aluminium inclusion (CAI) formation. Earlier accretion results in conditions that are inconsistent with the petrology of the HED meteorites, whereas later accretion predicts temperatures that are insufficient to produce diogenites. We suggest that upward migration of 26Al-rich melt initially created a convecting shallow magma ocean of <20 km depth that rapidly crystallised to form a 26Al-rich eucritic crust that acted as a hot insulating lid. The second stage of crust formation began once the depleted mantle residue reached high enough temperatures to produce diogenite-forming magmas. These results further support the view that diogenites likely formed as crustal intrusions rather than as magma ocean cumulates.

Onset of magma ocean solidification on Mars inferred from Mn-Cr chronometry

The mantle of Mars probably differentiated through the crystallization of a magma ocean during the first tens of million years (Ma) of Solar System evolution. However, the exact timescale of large-scale silicate differentiation of the martian mantle is debated, and in particular, it remains unclear when differentiation commenced. Here we applied the short-lived 53Mn-53Cr system to martian meteorites in order to date the onset of large-scale mantle differentiation on Mars. The new Cr isotope data demonstrate that martian meteorites exhibit no resolvable radiogenic 53Cr variations, and instead have a uniform +20.2±1.2 (95% conf.) parts-per-million excess in 53Cr/52Cr relative to the terrestrial mantle. The investigated groups of martian meteorites are lithologically varied and derive from diverse mantle sources that probably had variable Mn/Cr. Hence, the lack of 53Cr variability among martian meteorites demonstrates that silicate differentiation on Mars occurred after the extinction of 53Mn. Provided that the sources of the martian meteorites have Mn/Cr variations that are typical of the terrestrial planets, this result implies that the onset of large-scale silicate differentiation must have occurred later than 20±5 Ma after Solar System formation. The onset of silicate differentiation on Mars inferred here is significantly later than time estimates for segregation of the martian core which conservatively occurred within <10 Ma after Solar System formation. Thus, the new Mn-Cr data imply that there was a small, but resolvable, time gap of at least 5 Ma between core formation and magma ocean solidification on Mars. If the age of core segregation is taken at face value, our results imply that the martian magma ocean remained mostly molten over several Ma. This inferred longevity of the magma ocean is inconsistent with thermal models predicting rapid (<1 Ma) solidification of the martian magma ocean. Although there is currently no unique solution to this conundrum, our results can potentially be explained by a protracted history of impact bombardment that delayed differentiation in a shallow magma ocean on Mars, or perhaps more readily, by the presence of an early and dense atmosphere that acted as an insulator and prevented the magma ocean from cooling quickly.

Escape of rock-forming volatile elements and noble gases from planetary embryos

Large planetesimals and planetary embryos ranging from several hundred to a few thousand kilometers can develop magma oceans through mutual collisions, gravitational energy, and the heating of short-lived radioactive elements. During their solidification after the dissipation of the disk a steam atmosphere will be catastrophically outgassed and may be lost efficiently via hydrodynamic escape, as long as it does not condense. The escaping H-atoms that originate from the dissociation of H2O and H2 will drag heavier trace elements like noble gases such as Ne and Ar and outgassed moderately volatile rock-forming elements such as K, Na, Si, Mg, etc. into space. Under consideration of various EUV flux evolution scenarios of young solar-type stars, we apply an upper atmosphere hydrodynamic escape model that includes the dragging of heavier species by escaping H-atoms. We investigate the atmospheric/elemental escape and fractionation from planetary embryos with masses of 1 MMoon, 0.5 MMars, 1 MMars, and 1.5 MMars at different orbital distances between the orbits of Venus and Mars. Our results indicate that the steam atmospheres and the embedded trace elements will be lost efficiently before they condense for masses ≤0.5 MMars and orbital distances up to 1 AU. For heavier embryos of up to 1.5 MMars almost all of the considered steam atmospheres can be lost within ≈12 Myr, which lies within the time frame of the formation of the first Martian protocrust after ≈20 Myr, i.e. for such steam atmospheres to be lost completely a shallow magma ocean must remain below the atmosphere, which might be achieved through frequent impacts onto the planetary embryo. The considered outgassed noble gases and rock-forming elements will be completely dragged away together with the steam atmospheres under the assumption that the trace elements will reach the thermosphere. For embryos with masses ≤MMoon the gravity is too weak for a dense atmosphere to build up for the high magma ocean related surface temperatures and all outgassed elements will escape immediately to space. For all considered planetary masses and orbits the loss rates of Ar and Ne are so high that there will be no fractionation of their isotopes. The studied planetary embryos, even though not isotopically fractionated, will therefore be severely depleted in noble gases and moderately volatile elements. Hydrodynamic escape might then also affect the final composition of terrestrial planets that accrete out of such planetary embryos, such as the volatile content and the Fe/Mg ratio of a planet.

Towards 3D modelling of convection in planetesimals and meteorite parent bodies

ABSTRACT Observations of asteroid belt members, investigations of meteorites and thermal evolution models converge on the paradigm of the ubiquity of melting processes in the planetesimals of the early Solar system. At least partial melting of planetesimals that fulfilled size and accretion time requirements to surpass the solidus temperatures of metal and silicates led to the weakening of the rock due to the interstitial melt. A decrease of the viscosity relative to melt-free material facilitates solid-state convection on partially molten bodies. Additional melting can produce liquid-like layers with suspended particles, i.e. magma oceans. Thermal evolution models indicate that partially molten layers can occur in the interior of undifferentiated bodies and in silicate mantles of differentiated ones. They can exist before a magma ocean forms or after it solidifies and above a whole-mantle magma ocean or below a shallow magma ocean. Thus, convection is likely. Attempts to model and to quantify the effects of convection in planetesimals remain rare. This study discusses the possibility of solid-state convection in partially molten planetesimals, presents a first-order comparison of a 3D mantle convection model with a conduction model taking a Vesta-sized body as an example and illustrates the importance of convection for meteorite parent bodies.

The Effects of Carbon Concentration and Silicate Composition on the Metal‐Silicate Partitioning of Carbon in a Shallow Magma Ocean

AbstractThe partitioning of carbon between the core and mantle during the formation of terrestrial planets may have controlled the distribution of carbon in terrestrial planets. However, the abundance of carbon in the Earth's mantle is higher than a prediction based on previous metal‐silicate partitioning experiments of carbon at carbon‐saturated conditions by more than an order of magnitude. Here, we report new metal‐silicate partitioning experiments of carbon at carbon contents of 0.25–0.5 wt%. We show that the metal‐silicate partition coefficient of carbon ( ) strongly correlates with nonbridging oxygen per tetrahedral cations (nbo/t) of silicate melts at conditions where C‐H species are stable. Moreover, the results suggest that at carbon‐undersaturated conditions may be lower than that at carbon‐saturated condition. Thus, at low carbon concentrations is essentially important to investigate the distribution of carbon in the Earth during the core formation.

Effect of silicon on activity coefficients of siderophile elements (Au, Pd, Pt, P, Ga, Cu, Zn, and Pb) in liquid Fe: Roles of core formation, late sulfide matte, and late veneer in shaping terrestrial mantle geochemistry

Earth’s core contains ∼10% of a light element that may be a combination of Si, S, C, O or H, with Si potentially being the major light element. Metal-silicate partitioning of siderophile elements can place important constraints on the P-T-fO2 and composition of the early Earth, but the effect of Si alloyed in Fe liquids is unknown for many of these elements. In particular, the effect of Si on the partitioning of highly siderophile elements (Au, Re and PGE) is virtually unknown. To address this gap in understanding, we have undertaken a systematic study of the highly siderophile elements Au, Pd, and Pt, and the volatile siderophile elements P, Ga, Cu, Zn, and Pb at variable Si content of metal, and 1600 °C and 1 GPa. From our experiments we derive epsilon interaction parameters between these elements and Si in Fe metallic liquids. The new parameters are used to update an activity model for trace siderophile elements in Fe alloys; Si causes large variation in the magnitude of activity coefficients of these elements in FeSi liquids. Because the interaction parameters are all positive, Si causes a decrease in their metal/silicate partition coefficients. We combine these new activity results with experimental studies of Au, Pd, Pt, P, Ga, Cu, Zn and Pb, to derive predictive expressions for metal/silicate partition coefficients which can then be applied to Earth. The expressions are applied to two scenarios for continuous accretion of Earth; specifically for constant and increasing fO2 during accretion. The results indicate that mantle concentrations of P, Ga, Cu, Zn, and Pb can be explained by metal-silicate equilibrium during accretion of the Earth where Earth’s early magma ocean deepens to pressures of 40–60 GPa. Au, Pd, and Pt, on the other hand become too high in the mantle in such a scenario, and require a later removal mechanism, rather than an addition as traditionally argued. A late reduction event that removes 0.5% metal from a shallow magma ocean can lower the Au, Pd, and Pt contents to values near the current day BSE. On the other hand, removal of 0.2–1.0% of a late sulfide-rich matte to the core would lower the Au, Pd, and Pt concentrations in the mantle, but not to chondritic relative concentrations observed in the BSE. If sulfide matte is called upon to remove HSEs, they must be later added via a late veneer to re-establish the high and chondritic relative PUM concentrations. These results suggest that although accretion and core formation (involving a Si, S, and C-bearing metallic liquid) were the primary processes establishing many of Earth’s mantle volatile elements and HSE, a secondary removal process is required to establish HSEs at their current and near-chondritic relative BSE levels. Mn and P - two siderophile elements that are central to biochemical processes (photosynthesis and triphosphates, respectively) - have significant and opposite interactions with FeSi liquids, and their mantle concentrations would be notably different if Earth had a Si-free core.

Multistage Core Formation in Planetesimals Revealed by Numerical Modeling and Hf‐W Chronometry of Iron Meteorites

AbstractIron meteorites provide some of the most direct insights into the processes and timescales of core formation in planetesimals. Of these, group IVB irons stand out by having one of the youngest 182Hf‐182W model ages for metal segregation (2.9 ± 0.6 Ma after solar system formation), as well as the lowest bulk sulfur content and hence highest liquidus temperature. Here, using a new model for the internal evolution of the IVB parent body, we show that a single stage of metal‐silicate separation cannot account for the complete melting of pure Fe metal at the relatively late time given by the Hf‐W model age. Instead, a complex metal‐silicate separation scenario is required that includes migration of partial silicate melts, formation of a shallow magma ocean, and core formation in two distinct stages of metal segregation. In the first stage, a protocore formed at ≈1.5 Ma via settling of metal particles in a mantle magma ocean, followed by metal segregation from a shallow magma ocean at ≈5.4 Ma. As these stages of metal segregation occurred at different times, the two metal fractions had different 182W compositions. Consequently, the final 182W composition of the IVB core does not correspond to a single differentiation event, but represents the average composition of early‐ and late‐segregated core fractions. Our best fit model indicates an ≈100 km radius for the IVB parent body and provides an accretion age of ≈0.1–0.5 Ma after solar system formation. The computed solidification time is, furthermore, consistent with the Re‐Os age for crystallization of the IVB core.

Mg‐rich harzburgites from Vesta: Mantle residua or cumulates from planetary differentiation?

AbstractWe describe petrographic, electron microprobe, and laser ablation ICP‐MS analyses of Mg‐rich harzburgite clasts in the Dominion Range 2010 howardites, and conclude that they are xenolithic samples of the vestan mantle. Key chemical and petrologic characteristics of these rocks provide tests for differentiation models. Our results indicate the mantle of Vesta formed through variable degrees of partial melting, which left behind a harzburgite and possibly dunite residuum. The Mg‐rich clasts are composed of orthopyroxene and olivine, with minor clinopyroxene, FeNi metal, and distinctive pyroxene–chromite symplectites. We use mineral chemistry to demonstrate the absence of a genetic link between diogenites and the Mg‐rich harzburgites. We propose a secondary origin for the formation of symplectites: interaction of silicate and metallic melts during primordial differentiation and core formation. The occurrence of FeNi metal containing ~1.5 wt% Cr within the assemblage indicates a very reducing environment during mantle differentiation (≪IW). Our study suggests that Vesta did not experience complete melting early in its history, and instead supports the formation of a shallow magma ocean.

Modeling phase separation and phase change for magma ocean solidification dynamics

AbstractJust after accretion, the Earth's mantle was significantly molten by the heat dissipation due to large impacts and to the segregation of the core. The mineralogical observations and thermodynamics models of solid‐liquid equilibrium of silicates show that several types of crystallization may have happened at different depths in the mantle. Solids were probably formed first at the bottom of the lower mantle or at midmantle leaving two possible magma oceans, a shallow one and an abyssal one. Near the bottom of the mantle, the liquid phase might become denser than solids due to iron enrichment. In the shallow magma ocean, the crystallizing solid phase was denser and sank through the magma to settle and compact at depth. To understand these complex dynamics, we develop a two‐phase numerical code that can handle simultaneously convection in each phase and in the slurry, and the compaction or decompaction of the two phases. Although our code can only run in a parameter range (Rayleigh number, viscosity contrast between phases, Prandlt number) far from what would be realistic, we think it already provides a rich dynamics that illustrates what could have happened. We show situations in which the crystallization front is gravitationally stable and situations where the newly formed solids are gravitationally unstable and can snow across the magma. Our study suggests that the location of a density contrast between solid and magma must be considered of equal importance with that of the intersection between liquidus and isentrope for what concerns mantle solidification.

Experimental determination of CO2 content at graphite saturation along a natural basalt-peridotite melt join: Implications for the fate of carbon in terrestrial magma oceans

Knowledge of the carbon carrying capacity of peridotite melt at reducing conditions is critical to constrain the mantle budget and planet-scale distribution of carbon set at early stage of differentiation. Yet, neither measurements of CO2 content in reduced peridotite melt nor a reliable model to extrapolate the known solubility of CO2 in basaltic (mafic) melt to solubility in peridotitic (ultramafic) melt exist. There are several reasons for this gap; one reason is due to the unknown relative contributions of individual network modifying cations, such as Ca2+ versus Mg2+, on carbonate dissolution particularly at reducing conditions. Here we conducted high pressure, temperature experiments to estimate the CO2 contents in silicate melts at graphite saturation over a compositional range from natural basalts toward peridotite at a fixed pressure (P) of 1.0 GPa, temperature (T) of 1600 °C, and oxygen fugacity (log⁡fO2∼IW+1.6). We also conducted experiments to determine the relative effects of variable Ca and Mg contents in mafic compositions on the dissolution of carbonate. Carbon in quenched glasses was measured and characterized using Fourier transform infrared spectroscopy (FTIR) and Raman Spectroscopy and was found to be dissolved as carbonate (CO32−). The FTIR spectra showed CO32− doublets that shifted systematically with the MgO and CaO content of silicate melts. Using our data and previous work we constructed a new composition-based model to determine the CO2 content of ultramafic (peridotitic) melt representative of an early Earth, magma ocean composition at graphite saturation. Our data and model suggest that the dissolved CO2 content of reduced, peridotite melt is significantly higher than that of basaltic melt at shallow magma ocean conditions; however, the difference in C content between the basaltic and peridotitic melts may diminish with depth as the more depolymerized peridotite melt is more compressible. Using our model of CO2 content at graphite saturation as a function of P–T–fO2-melt composition, we predict that a superliquidus shallow magma ocean should degas CO2. Whereas if the increase of fO2 with depth is weak, a magma ocean may ingas a modest amount of carbon during crystallization. Further, using the carbon content of peridotite melt at log⁡fO2 of IW and the knowledge of C content of Fe-rich alloy melt, we also consider the core–mantle partitioning of carbon, showing that DCmetal/peridotite of a shallow magma ocean is generally higher than previously estimated.

Constraints on core formation in Vesta from metal–silicate partitioning of siderophile elements

It is now widely accepted that the asteroid 4-Vesta has an Fe-rich metallic core, but the composition of the core and the conditions prevailing during core–mantle differentiation are poorly constrained. In light of new constraints on Vesta’s geophysical and geochemical properties obtained by the DAWN mission, we have re-examined the conditions at which core–mantle differentiation in Vesta may have occurred by linking the estimated mantle depletions of siderophile elements P, Co, Ni, Cu, Ga, Ge, Mo and W in the vestan mantle to newly derived predictive equations for core–mantle partitioning of these elements. We extend the number of elements previously considered in geochemical modeling of vestan core formation and use published metal–silicate partitioning data obtained at low pressures to characterize the dependence of metal/silicate partition coefficients (D) on pressure, temperature, oxygen fugacity and composition of the silicate and metallic melt. In our modeling we implement newly derived mantle depletions of P, Co, Ni and Ga through analysis of published HED meteorite analyses and assess two contrasting bulk compositional models for Vesta.Modeling results using Monte Carlo simulations constrain vestan core formation to have occurred at mildly reducing conditions of approximately 2log units below the iron–wüstite (IW) buffer (ΔIW=−2.05±0.20) if the two most likely bulk compositions (binary mixtures of H+CM or H+CV chondritic meteorites) are considered, assuming a temperature range between 1725 and 1850K and a sulfur-free pure Fe core. If the core is assumed to be sulfur-rich (15wt.% S) as predicted by the latter bulk compositional models, observed depletions for all eight siderophile elements can be simultaneously satisfied at ΔIW=−2.35±0.10 and 1725–1850K for the H+CV bulk composition and ΔIW=−2.30±0.15 and 1725–1850K for the H+CM bulk composition. More reducing conditions are not consistent with the observed siderophile element depletions in the vestan mantle, independent of the sulfur content of the vestan core and of the bulk compositional model chosen.Our analysis shows that a previously proposed shallow magma ocean on Vesta during core formation is not consistent with the observed mantle depletion of Ga for the two considered bulk compositions irrespective of core composition. Instead, our results are consistent with the existence of a deep magma ocean during core formation on Vesta, requiring >50–100% mantle melting.

Impact induced surface heating by planetesimals on early Mars

We investigate the influence of impacts of large planetesimals and small planetary embryos on the early Martian surface on the hydrodynamic escape of an early steam atmosphere that is exposed to the high soft X-ray and EUV flux of the young Sun. Impact statistics in terms of number, masses, velocities, and angles of asteroid impacts onto the early Mars are determined via n-body integrations. Based on these statistics, smoothed particle hydrodynamics (SPH) simulations result in estimates of energy transfer into the planetary surface material and according surface heating. For the estimation of the atmospheric escape rates we applied a soft X-ray and EUV absorption model and a 1-D upper atmosphere hydrodynamic model to a magma ocean-related catastrophically outgassed steam atmosphere with surface pressure values of 52 bar H2O and 11 bar CO2. The estimated impact rates and energy deposition onto an early Martian surface can account for substantial heating. The energy influx and conversion rate into internal energy is most likely sufficient to keep a shallow magma ocean liquid for an extended period of time. Higher surface temperatures keep the outgassed steam atmosphere longer in vapor form and therefore enhance its escape to space within about 0.6 Myr after its formation.

Can a fractionally crystallized magma ocean explain the thermo-chemical evolution of Mars?

The impact heat accumulated during the late stage of planetary accretion can melt a significant part or even the entire mantle of a terrestrial body, giving rise to a global magma ocean. The subsequent cooling of the interior causes the magma ocean to freeze from the core–mantle boundary (CMB) to the surface due to the steeper slope of the mantle adiabat compared to the slope of the solidus.Assuming fractional crystallization of the magma ocean, dense cumulates are produced close to the surface, largely due to iron enrichment in the evolving magma ocean liquid. A gravitationally unstable mantle thus forms, which is prone to overturn. We investigate the cumulate overturn and its influence on the thermal evolution of Mars using mantle convection simulations in 2D cylindrical geometry. We present a suite of simulations using different initial conditions and a strongly temperature-dependent viscosity. We assume that all radiogenic heat sources have been enriched during the freezing-phase of the magma ocean in the uppermost 50 km and that the initial steam-atmosphere created by the degassing of the freezing magma ocean was rapidly lost, implying that the surface temperature is set to present-day values. In this case, a stagnant lid quickly forms on top of the convective interior preventing the uppermost dense cumulates to sink, even when allowing for a plastic yielding mechanism. Below this dense stagnant lid, the mantle chemical gradient settles to a stable configuration. The convection pattern is dominated by small-scale structures, which are difficult to reconcile with the large-scale volcanic features observed over Mars' surface and partial melting ceases in less than 900 Ma. Assuming that the stagnant lid can break because of additional mechanisms and allowing the uppermost dense layer to overturn, a stable density gradient is obtained, with the densest material and the entire amount of heat sources lying above the CMB. This stratification leads to a strong overheating of the lowermost mantle, whose temperature increases to values that exceed the liquidus. The iron-rich melt would most likely remain trapped in the lower part of the mantle. The upper mantle in that scenario cools rapidly and only shows partial melting during the first billion year of evolution. Therefore a fractionated global and deep magma ocean is difficult to reconcile with observations. Different scenarios assuming, for instance, a hemispherical or shallow magma ocean, or a crystallization sequence resulting in a lower density gradient than that implied by pure fractional crystallization will have to be considered.

Differentiation of Vesta: Implications for a shallow magma ocean

The Dawn mission confirms earlier predictions that the asteroid 4 Vesta is differentiated with an iron-rich core, a silicate mantle and a basaltic crust, and supports the conjecture of Vesta being the parent body of the HED meteorites. To better understand its early evolution, we perform numerical calculations of the thermo-chemical evolution adopting new data obtained by the Dawn mission such as mass, bulk density and size of the asteroid.We have expanded the thermo-chemical evolution model of Neumann et al. (2012) that includes accretion, compaction, melting and the associated changes of the material properties and the partitioning of incompatible elements such as the radioactive heat sources, advective heat transport, and differentiation by porous flow, to further consider convection and the associated effective cooling in a potential magma ocean. Depending on the melt fraction, the heat transport by melt segregation is modelled either by assuming melt flow in a porous medium or by simulating vigorous convection and heat flux of a magma ocean with a high effective thermal conductivity.Our results show that partitioning of 26Al and its transport with the silicate melt is crucial for the formation of a global and deep magma ocean. Due to the enrichment of 26Al in the liquid phase and its accumulation in the sub-surface (for formation times t0<1.5 Ma), a thin shallow magma ocean with a thickness of 1 to a few tens of km forms – its thickness depends on the viscosity of silicate melt. The lifetime of the shallow magma ocean is O(104)–O(106) years and convection in this layer is accompanied by the extrusion of 26Al at the surface, resulting in the formation of a basaltic crust. The interior differentiates from the outside inwards with a mantle that is depleted in 26Al and core formation is completed within ∼0.3 Ma. The lower mantle experiences a maximal melt fraction of 45% suggesting a harzburgitic to dunitic composition. Our results support the formation of non-cumulate eucrites by the extrusion of early partial melt while cumulate eucrites and diogenites may form from the crystallising shallow magma ocean. Silicate melt is present in the mantle for up to 150 Ma, and convection in a crystallising core proceeds for approximately 100 Ma, supporting the idea of an early magnetic field to explain the remnant magnetisation observed in some HED meteorites.

Carbon solution and partitioning between metallic and silicate melts in a shallow magma ocean: Implications for the origin and distribution of terrestrial carbon

The origin of bulk silicate Earth carbon inventory is unknown and the fate of carbon during the early Earth differentiation and core formation is a missing link in the evolution of the terrestrial carbon cycle. Here we present high pressure (P)–temperature (T) experiments that offer new constraints upon the partitioning of carbon between metallic and silicate melt in a shallow magma ocean. Experiments were performed at 1–5GPa, 1600–2100°C on mixtures of synthetic or natural silicates (tholeiitic basalt/alkali basalt/komatiite/fertile peridotite) and Fe–Ni–C±Co±S contained in graphite or MgO capsules. All the experiments produced immiscible Fe-rich metallic and silicate melts at oxygen fugacity (fO2) between ∼IW-1.5 and IW-1.9. Carbon and hydrogen concentrations of basaltic glasses and non-glassy quenched silicate melts were determined using secondary ionization mass spectrometry (SIMS) and speciation of dissolved C–O–H volatiles in silicate glasses was studied using Raman spectroscopy. Carbon contents of metallic melts were determined using both electron microprobe and SIMS. Our experiments indicate that at core-forming, reduced conditions, carbon in deep mafic–ultramafic magmas may dissolve primarily as various hydrogenated species but the total carbon storage capacity, although is significantly higher than solubility of CO2 under similar conditions, remains low (<500ppm). The total carbon content in our reduced melts at graphite saturation increases with increasing melt depolymerization (NBO/T), consistent with recent spectroscopic studies, and modestly with increasing hydration. Carbon behaves as a metal-loving element during core-mantle separation and our experimental DCmetal/silicate varies between ∼4750 and ⩾150 and increases with increasing pressure and decreases with increasing temperature and melt NBO/T.Our data suggest that if only a trace amount of carbon (∼730ppm C) was available during early Earth differentiation, most of it was partitioned to the core (with 0.20–0.25wt.% C) and no more than ∼10–30% of the present-day mantle carbon budget (50–200ppm CO2) could be derived from a magma ocean residual to core formation. With equilibrium core formation removing most of the carbon initially retained in the terrestrial magma ocean, explanation of the modern bulk silicate Earth carbon inventory requires a later replenishment mechanism. Partial entrapment of metal melt in solid silicate matrix, carbon ingassing by magma ocean–atmosphere interaction, and carbon outgassing from the core aided by reaction of core metal and deeply subducted water are some of the viable mechanisms.

Carbon solubility in core melts in a shallow magma ocean environment and distribution of carbon between the Earth’s core and the mantle

The solubility of carbon in Fe and Fe–5.2 wt.% Ni melts, saturated with graphite, determined by electron microprobe analysis of quenched metal melts was 5.8 ± 0.1 wt.% at 2000 °C, 6.7 ± 0.2 wt.% at 2200 °C, and 7.4 ± 0.2 wt.% at 2410 °C at 2 GPa, conditions relevant for core/mantle differentiation in a shallow magma ocean. These solubilities are slightly lower than low-pressure literature values and significantly beneath calculated values for even higher pressures [e.g., Wood B. J. (1993) Carbon in the core. Earth Planet. Sci. Lett. 117, 593–607]. The trend of C solubility versus temperature for Fe–5.2 wt.% Ni melt, within analytical uncertainties, is similar to or slightly lower (∼0.2–0.4 wt.%) than that of pure Fe. Carbon content of core melts and residual mantle silicates derived from equilibrium batch or fractional segregation of core liquids and their comparison with our solubility data and carbon content estimate of the present day mantle, respectively, constrain the partition coefficient of carbon between silicate and metallic melts, D C silicate/metal in a magma ocean. For the entire range of possible bulk Earth carbon content from chondritic to subchondritic values, D C silicate/metal of 10 −4 to 1 is derived. But for ∼1000 ppm bulk Earth carbon, D C silicate/metal is between 10 −2 and 1. Using the complete range of possible D C silicate/metal for a magma ocean at ∼2200 °C, we predict maximum carbon content of the Earth’s core to be ∼6–7 wt.% and a preferred value of 0.25 ± 0.15 wt.% for a bulk Earth carbon concentration of ∼1000 ppm.