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Internal differentiation and volatile budget of Mercury inferred from the partitioning of heat-producing elements at highly reduced conditions
... In this study, we performed 38 metal-silicate partitioning experiments over a wide range of pressures (1 bar to 26 GPa) and oxygen fugacities (IW − 6.4 to IW − 1.9) to expand the available Ni and Co D met/sil values to reducing conditions. Of the 38 experiments, 21 are published here for the first time; the other 17 were reported in Cartier et al., (2014aCartier et al., ( , 2014b, Bouhifd et al. (2015), and Pirotte et al. (2023). To construct a tool applicable to most planetary conditions, we compiled our new data with the experimental partitioning database of Fischer et al. (2015) and Huang and Badro (2018), to which we added recent studies conducted under highly reducing conditions (see Section 3.2). ...
... fO 2 is reported relative to IW, calculated assuming ideal behavior of Fe and FeO, and is estimated to have errors of ± 1 log unit (see Section 2.7). (*) indicates experiments previously published in Cartier et al., (2014aCartier et al., ( , 2014b, Bouhifd et al. (2015), and Pirotte et al. (2023). Run Device P (GPa) ...
... Metal-silicate partitioning experiments were performed on various starting chondritic compositions using multi-anvil and piston-cylinder apparatuses, internally heated pressure vessels (IHPVs), and evacuated silica tubes (ESTs) in various institutes (Table 1). Some of the 5 GPa and 26 GPa experiments were published in previous studies but the remaining experiments are published here for the first time (Cartier et al., 2014a;Bouhifd et al., 2015;Pirotte et al., 2023). Details on each method and experimental series are provided in the following subsections. ...
Moderately siderophile elements (MSEs) are potential tracers of the thermodynamic conditions prevailing during planetary core formation because their metal–silicate partition coefficients (Dmet/sil) vary as a function of P, T, and oxygen fugacity (fO2). Those properties result in the production of planetary mantles with unique MSE depletion signatures. Among the MSEs, Ni and Co are reliable barometers in magma oceans because their Dmet/sil values are strongly correlated with pressure, decreasing by almost 3 orders of magnitude between 1 bar and 100 GPa. Current pressure-dependent expressions of Dmet/sil were calibrated based on experiments performed under relatively oxidizing conditions, mostly at fO2 slightly below the iron–wüstite Fe–FeO buffer (IW), which is relevant to the mantles of Earth and Mars. However, planets and asteroids formed under a wide range of redox conditions, from Mercury, the most reduced (~IW−5.5), to the most oxidized angrite parent body (IW−1.5 to IW+1). In this study, we performed and analyzed 38 metal–silicate partitioning experiments over a wide range of pressures (1 bar to 26 GPa) and oxygen fugacities (IW−6.4 to IW−1.9) to expand the available Ni and Co Dmet/sil values to reducing conditions. We then parameterized 255 Ni and 194 Co Dmet/sil values as a function of T (1573–5700 K), P (1 bar to 100 GPa), and fO2 (IW−6.4 to IW+0.2). We also modeled the evolution of Ni and Co Dmet/sil values along the liquidus of a chondritic mantle at various P and fO2 conditions to investigate the thermodynamic conditions of various planetary bodies’ magma oceans. The P and fO2 conditions we obtained for Earth, Mars, the Moon, and Vesta are consistent with previous studies using similar methods, and the pressure during core formation is strongly correlated to planetary size. Finally, we also applied our model to several achondrite parent bodies; our results indicate a wide variety of objects, from the asteroid-sized, oxidized angrite parent body to the planet-sized, highly reduced aubrite parent body.
... Based on geodetic data, it has been suggested that a 100 km-thick FeS layer at Mercury's present day CMB may also exist 63 . The physical state (solid vs. liquid) of such a layer is unknown and experimental studies combined with geochemical data have shown that this layer, if present, should be much thinner than initially suggested from geophysics 10,64,65 . The occurrence of a diamond layer at the CMB is compatible with the presence of a FeS layer as both relate to the saturation of an element (C and S) during planetary differentiation. ...
Abundant carbon was identified on Mercury by MESSENGER, which is interpreted as the remnant of a primordial graphite flotation crust, suggesting that the magma ocean and core were saturated in carbon. We re-evaluate carbon speciation in Mercury’s interior in light of the high pressure-temperature experiments, thermodynamic models and the most recent geophysical models of the internal structure of the planet. Although a sulfur-free melt would have been in the stability field of graphite, sulfur dissolution in the melt under the unique reduced conditions depressed the sulfur-rich liquidus to temperatures spanning the graphite-diamond transition. Here we show it is possible, though statistically unlikely, that diamond was stable in the magma ocean. However, the formation of a solid inner core caused diamond to crystallize from the cooling molten core and formation of a diamond layer becoming thicker with time.
... Thus, the approach demonstrated by this study provides a pathway for future efforts to investigate the temporal evolution of the thermal profile by simulating the geodynamic environment during the lunar mantle overturn along with varying parameters like initial magma ocean depths, crystallizing phase assemblages, amounts of trapped liquid and varying bulkMoon water We demonstrate that employing partition coefficients of radioactive elements to estimate the HPE budget can enable predictions about the thermal state of the lunar interior. We note that similar approaches can also be applied to other rocky planetary bodies, (e.g.,Pirotte et al., 2023; who apply a similar approach to Mercury) and has ramifications for estimating the efficiency of mantle overturn, mantle dynamics, volcanism, and the origins of the building blocks of the Earth-Moon system.We performed high pressure and high temperature experiments to determine the mineral-melt partition coefficients of the heat-producing elements (HPEs) U, Th and K between high calcic plagioclase -, Fe rich augite -and Fe-Ti rich melt, phases expected after ~80% of the lunar magma ocean crystallized. We calculated the distribution of HPEs within the lunar interior using two different bulk silicate Moon (BSM) compositions for U and Th -from McDonough and Sun, (1995) assuming U and Th were sourced primarily from CI chondrites and Faure et al.,(2020) ...
Plain Language Summary
Mineral assemblages that constitute the mantles of reduced exoplanets, such as those formed in the inner regions of stellar nebulae, have been scarcely investigated. We propose that these exoplanets share several physical and chemical properties with planet Mercury in our Solar System, as they formed in a similar geochemical environment. In order to outline the minerals constituting these mantles, we take in account all the chemical compositions previously proposed to match Mercury's observed characteristics. These compositions are extrapolated from several classes of meteorites, among the most pristine materials currently known and best candidates as “building blocks” of planetary objects. We, then use the well‐established thermodynamic code Perple_X to predict the stable minerals at pressure and temperature ranges assumed for these exoplanets' interiors. We find that silicate mantles of exoplanets located in inner regions of nebulae are dominated by pyroxene group minerals, rather than olivine, as in the Earth's mantle. This results in different rheological (e.g., viscosity) and physical properties (e.g., melting behavior) for these reduced exoplanets, with significant implications for their mantle dynamics and evolution over time.
Piston-cylinder assemblies exhibit inhomogeneous pressure distributions and biases compared to the theoretical pressure applied to the hydraulic press because of the thermal and mechanical properties of the assembly components. Whereas these effects can partially be corrected by conventional calibration, systematic quantification of friction values remain very sparse and results vary greatly among previous studies. We performed an experimental study to investigate the behavior of the most common cell assemblies, i.e., talc (Mg3Si4O10(OH)2), NaCl, and BaCO3, during piston-cylinder experiments to estimate the effects of pressure, temperature, run duration, assembly size, and assembly materials on friction values. Our study demonstrate that friction decreases with time and also partially depends on temperature but does not depend on pressure. We determined that friction decreases from 24 to 17% as temperature increases from 900 to 1300 °C when using talc cells, indicating a friction decrease of ~2% per 100 °C increase for 24-h experiments. In contrast, friction becomes independent of pressure above 1300 °C. Moreover, at a fixed temperature of 900 °C, friction decreases from 29% in 6-h runs to 21% in 48-h runs, or by 0.2% per hour. Similar results are obtained with NaCl cell assemblies, with friction decreasing from 8% in 9-h runs to 5% in 24-h runs. At 900 °C, possible steady-state friction values are only reached after at least 48 h, indicating that friction should be considered a variable for shorter experiments. We establish that assembly materials (and their associated thermomechanical properties) influence the friction correction more than the dimensions of the assembly parts. Finally, we show that the use of polytetrafluoroethylene film instead of conventional Pb foil does not modify friction, but significantly reduces the force required for sample extraction, thus increasing the lifetime of the carbide core, which in turn enhances experimental reproducibility.
We have determined rare-earth element (REE) abundances in oldhamites (CaS) from 13 unequilibrated and equili-brated enstatite chondrites (5 EH and 8 EL) and in a few enstatites by in situ, laser ablation ICP-MS. In EH chondrites, oldhamite REE patterns vary from the most primitive petrographic types (EH3) to the most metamorphosed types (EH5). In EH3, CI-normalized REE patterns are convex downward with strong positive Eu and Yb anomalies, whereas EH5 display flat patterns with enrichments reaching about 80 times CI abundances. The positive anomalies of Eu and Yb found in oldhamites of primitive EH chondrites indicate that they represent the condensation of a residual gas fraction, in a manner similar to fine-grained CAIs of carbonaceous chondrites. The early condensate may have been preserved in the matrix of unequilibrated EH. Equilibrated EH oldhamite patterns may result from metamorphic evolution and REE redistribution on the EH parent body. On the contrary, all the oldhamites from EL chondrites (EL3 to EL6) display a single kind of patterns, which is convex upward and is about 100 times enriched relative to CI, with a negative Eu anomaly. In addition, the EL pattern is similar to that of oldhamites from aubrites (enstatite achondrites). The latter observation suggests that oldhamites of all EL metamorphic types (including primitive ones) bear the signature of a magmatic event accompanied by FeS loss as vapor, prior to the assembly of the EL parent body. Given the difficulty of obtaining precise ages on enstatite chondrites, it is not possible to discuss the chronology of the events recorded by the oldhamite REE patterns.
Recent estimates of Mercury’s rotational state yield different obliquity values, resulting in normalized polar moment of inertia values of either 0.333 or 0.346. In addition, recent measurements of Mercury’s tidal response, as expressed by its Love number k 2 , are higher than previously reported. These different measurements have implications for our understanding of Mercury’s interior structure. We perform a comprehensive analysis of models of Mercury’s interior structure using a Markov Chain Monte Carlo approach, where we explore models that satisfy the various measurements of moments of inertia and mean density. In addition, we explore models that either have Mercury’s tidal response as a measurement or predict its tidal response. We find that models that match the lower polar moment value also fit or predict the recent, higher Love number. Models that match the higher polar moments predict Love numbers even higher than current estimates. For the resulting interior structure models, we find a wide range of viscosities at the core–mantle boundary, including low values that could be consistent with the presence of partial melt, with higher viscosities also equally allowed in our models. Despite the possibility of low viscosities, our results do not show a preference for particularly high temperatures at the core–mantle boundary. Our results include predicted values for the pressure and temperature of Mercury’s core, and the displacement Love numbers.
Moderately volatile elements (MVEs) are depleted and isotopically fractionated in the Moon relative to Earth. To understand how the composition of the Moon was established, we calculate the equilibrium and kinetic isotopic fractionation factors associated with evaporation and condensation processes. We also reassess the levels of depletions of K and Rb in planetary bodies. Highly incompatible element ratios are often assumed to be minimally affected by magmatic processes, but we show that this view is not fully warranted, and we develop approaches to mitigate this issue. The K/U weight ratios of Earth and the Moon are estimated to be 9704 and 2448, respectively. The ⁸⁷ Rb/ ⁸⁶ Sr atomic ratios of Earth and the Moon are estimated to be 0.072 5 and 0.015 4, respectively. We show that the depletions and heavy isotopic compositions of most MVEs in the Moon are best explained by evaporation in 99%-saturated vapor. At 99% saturation in the protolunar disk, Na and K would have been depleted to levels like those encountered in the Moon on timescales of ∼40–400 days at 3500–4500 K, which agrees with model expectations. In contrast, at the same saturation but a temperature of 1600–1800 K relevant to hydrodynamic escape from the lunar magma ocean, Na and K depletions would have taken 0.1–103 Myr, which far exceeds the 1000 yr time span until plagioclase flotation hinders evaporation from the magma ocean. We conclude that the protolunar disk is a much more likely setting for the depletion of MVEs than the lunar magma ocean.
Mercury’s metallic core is expected to have formed under highly reducing conditions, resulting in the presence of significant quantities of silicon alloyed to iron. Here we present the phase diagram of the Fe-FeSi system, reconstructed from in situ X-ray diffraction measurements at pressure and temperature conditions spanning over those expected for Mercury’s core, and ex situ chemical analysis of recovered samples. Under high pressure, we do not observe a miscibility gap between the cubic fcc and B2 structures, but rather the formation of a re-entrant bcc phase at temperatures close to melting. Upon melting, the investigated alloys are observed to evolve towards two distinct Fe-rich and Fe-poor liquid compositions at pressures below 35-38 GPa. The evolution of the phase diagram with pressure and temperature prescribes a range of possible core crystallization regimes, with strong dependence on the Si abundance of the core.
Mercury 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.
Mercury’s early evolution is enigmatic, marked by widespread volcanism, contractional tectonics, and a magnetic field. Current models cannot reconcile an inferred gradual decrease in the rate of radial contraction beginning at ~3.9 billion years (Ga) with crustal magnetization indicating a dynamo at ~4 to 3.5 Ga and the production of extensive volcanism. Incorporating the strong cooling effects of mantle melting and effusive volcanism into an exhaustive thermal modeling study, here, we show that early, voluminous crustal production can drive a period of strong mantle cooling that both favors an ancient dynamo and explains the contractional history of the planet. We develop the first self-consistent model for Mercury’s early history and, more generally, propose an approach to assess the volcanic control over the evolution of any terrestrial planet or moon.
Piston-cylinder assemblies exhibit inhomogeneous pressure distributions and biases compared to the theoretical pressure applied to the hydraulic press because of the thermal and mechanical properties of the assembly components. Whereas these effects can partially be corrected by conventional calibration, systematic quantification of friction values remain very sparse and results vary greatly among previous studies. We performed an experimental study to investigate the behavior of the most common cell assemblies, i.e., talc (Mg3Si4O10(OH)2), NaCl, and BaCO3, during piston-cylinder experiments to estimate the effects of pressure, temperature, run duration, assembly size, and assembly materials on friction values. Our study demonstrate that friction decreases with time and also partially depends on temperature but does not depend on pressure. We determined that friction decreases from 24 to 17% as temperature increases from 900 to 1300 °C when using talc cells, indicating a friction decrease of ~2% per 100 °C increase for 24-h experiments. In contrast, friction becomes independent of pressure above 1300 °C. Moreover, at a fixed temperature of 900 °C, friction decreases from 29% in 6-h runs to 21% in 48-h runs, or by 0.2% per hour. Similar results are obtained with NaCl cell assemblies, with friction decreasing from 8% in 9-h runs to 5% in 24-h runs. At 900 °C, possible steady-state friction values are only reached after at least 48 h, indicating that friction should be considered a variable for shorter experiments. We establish that assembly materials (and their associated thermomechanical properties) influence the friction correction more than the dimensions of the assembly parts. Finally, we show that the use of polytetrafluoroethylene film instead of conventional Pb foil does not modify friction, but significantly reduces the force required for sample extraction, thus increasing the lifetime of the carbide core, which in turn enhances experimental reproducibility.
We present calculations of interior models of Mercury that are constrained to match Mercury's mean density, normalized moment of inertia factor (MoI), and 88 days libration amplitude. We show that models matching a MoI = 0.333 ± 0.005, based on a recent obliquity measurement require a new perspective on Mercury's interior. Specifically, we confirm the mandatory presence of a large inner core > 600 km in radius which, however, leads to lower mantle densities in comparison to previous models and implies a mantle with > 5wt.% C, > 10wt.% magnesium sulfide (MgS), or is partially convecting. Furthermore, we also show that the core radius is lower than previous estimates, making it inconsistent with current estimates from magnetic induction measurements. In addition, the requirement of low viscosities in the lower mantle to match recent estimates of k2 imply a significantly weaker mantle than previously believed, potentially including partial melting.
Experimental measurements of density by X‐ray absorption and of P‐wave velocity by ultrasonic techniques of liquid Fe‐(<17 wt%) Si‐(<4.5 wt%) C alloys at pressures up to 5.8 GPa are presented. These data are used to construct an Fe‐Si‐C liquid mixing model and to characterize interior structure models of Mercury with liquid outer core composed of Fe‐Si‐C. The interior structure models are constrained by geodetic measurements of the planet, such as the obliquity and libration of Mercury. The results indicate that S and/or C with concentrations at the wt% level are likely required in Mercury's core to ensure the existence of an inner core with a radius (below ∼1,200 km) that is consistent with reported dynamo simulations for Mercury's magnetic field. Interior structure models with more than 14 wt% Si in the core, estimated for Mercury by assuming an EH chondrite‐like bulk composition, are only feasible if the obliquity of Mercury is near the upper limit of observational uncertainties (2.12 arcmin) and the mantle is dense (3.43–3.68 g·cm⁻³). Interior structure models with the central obliquity value (2.04 arcmin) and less than 7.5 wt% Si in the core, consistent with estimates of Mercury's core composition from an assumed CB chondrite‐like bulk composition, are compatible with 3.15–3.35 g·cm⁻³ mantle densities and an inner core radius below 1,200 km. Interior structure models with the obliquity of Mercury near the lower observational uncertainty limit (1.96 arcmin) have a low‐density mantle (2.88–3.03 g·cm⁻³), less than 4 wt% Si in the core, and an inner core radius larger than 1,600 km.
The Mercury Imaging X-ray Spectrometer is a highly novel instrument that is designed to map Mercury’s elemental composition from orbit at two angular resolutions. By observing the fluorescence X-rays generated when solar-coronal X-rays and charged particles interact with the surface regolith, MIXS will be able to measure the atomic composition of the upper ∼10-20 μm of Mercury’s surface on the day-side. Through precipitating particles on the night-side, MIXS will also determine the dynamic interaction of the planet’s surface with the surrounding space environment.
MIXS is composed of two complementary elements: MIXS-C is a collimated instrument which will achieve global coverage at a similar spatial resolution to that achieved (in the northern hemisphere only – i.e. ∼ 50 – 100 km) by MESSENGER; MIXS-T is the first ever X-ray telescope to be sent to another planet and will, during periods of high solar activity (or intense precipitation of charged particles), reveal the X-ray flux from Mercury at better than 10 km resolution. The design, performance, scientific goals and operations plans of the instrument are discussed, including the initial results from commissioning in space.
BepiColombo has a larger and in many ways more capable suite of instruments relevant for determination of the topographic, physical, chemical and mineralogical properties of Mercury’s surface than the suite carried by NASA’s MESSENGER spacecraft. Moreover, BepiColombo’s data rate is substantially higher. This equips it to confirm, elaborate upon, and go beyond many of MESSENGER’s remarkable achievements. Furthermore, the geometry of BepiColombo’s orbital science campaign, beginning in 2026, will enable it to make uniformly resolved observations of both northern and southern hemispheres. This will offer more detailed and complete imaging and topographic mapping, element mapping with better sensitivity and improved spatial resolution, and totally new mineralogical mapping.
We discuss MESSENGER data in the context of preparing for BepiColombo, and describe the contributions that we expect BepiColombo to make towards increased knowledge and understanding of Mercury’s surface and its composition. Much current work, including analysis of analogue materials, is directed towards better preparing ourselves to understand what BepiColombo might reveal. Some of MESSENGER’s more remarkable observations were obtained under unique or extreme conditions. BepiColombo should be able to confirm the validity of these observations and reveal the extent to which they are representative of the planet as a whole. It will also make new observations to clarify geological processes governing and reflecting crustal origin and evolution.
We anticipate that the insights gained into Mercury’s geological history and its current space weathering environment will enable us to better understand the relationships of surface chemistry, morphologies and structures with the composition of crustal types, including the nature and mobility of volatile species. This will enable estimation of the composition of the mantle from which the crust was derived, and lead to tighter constraints on models for Mercury’s origin including the nature and original heliocentric distance of the material from which it formed.
The composition of a planet's core has important implications for the thermal and magmatic evolution of that planet. Here, we conducted carbon (C) solubility experiments on iron‐silicon (Fe‐Si) metal mixtures (up to 35 wt% [~52 atom%] Si) at 1 GPa and 800–1800°C to determine the carbon concentration at graphite saturation (CCGS) in metallic melt and crystalline metal with varying proportions of Fe and Si to constrain the C content of Mercury's core. Our results, combined with those in the literature, show that composition is the major controlling factor for carbon solubility in Fe‐rich metal with minimal effects from temperature and pressure. Moreover, there is a strong anticorrelation between the abundances of carbon and silicon in iron‐rich metallic systems. Based on the previous estimates of <1–25 wt% Si in Mercury's core, our results indicate that a carbon‐saturated Mercurian core has 0.5–6.4 wt% C, with 6.4 wt% C corresponding to an Si‐free, Fe core and 0.5 wt% C corresponding to an Fe‐rich core with 25 wt% Si. The upper end of estimated FeO abundances in the mantle (up to 2.2 wt%) are consistent with a core that has <1 wt% Si and up to 6.4 wt% C, which would imply that bulk Mercury has a superchondritic Fe/Si ratio. However, the lower end of estimated FeO (≤0.05 wt%) supports CB chondrite‐like bulk compositions of Mercury with core Si abundances in the range of 5–18.5 wt% and C abundances in the range of 0.8–4.0 wt%.
A striking feature of Mercury's volcanic surface is its high S and low FeO contents, which is thought to be produced by very reducing conditions compared to other terrestrial bodies. Experiments show that S solubility in silicate melts increases to % wt levels for oxygen fugacities lower than three log units below the iron‐wustite (IW) buffer. During magma ocean solidification, large amounts of sulfide could potentially precipitate. This work investigates the effects of primordial sulfide layering on the first 750 Myr of Mercury's mantle dynamics. It is proposed that sulfide layering could have been produced by fractional solidification in the highly reduced Mercury magma ocean (MMO). Such chemical layering implies mantle sources with variable sulfur contents that might have played an important role in early Mercurian magmatism. Our models investigate the production of sulfide‐rich layers and their preservation during post‐MO solid‐state mantle dynamics. An intriguing question is the role of these sulfide‐rich layers on mantle dynamics as they are expected to incorporate a substantial amount of heat‐producing elements (U, Th, and K). We use experimentally determined sulfur solubility in silicate melts to predict the depth at which sulfides precipitate in the MMO. The model produces primordial sulfide layers whose thickness and locations depend upon the oxygen fugacity (fO2) and initial sulfur content (Sinit) of the MMO. Several geodynamic regimes have been identified in the fO2‐Sinit space. This study shows that oxygen fugacity, bulk sulfur content, and sulfide segregation are key for the early thermochemical evolution of Mercury.
Geodetic analysis of radio tracking measurements of the MESSENGER spacecraft while in orbit about Mercury has yielded new estimates for the planet's gravity field, tidal Love number, and pole coordinates. The derived right ascension (α = 281.0082° ± 0.0009°; all uncertainties are 3 standard deviations) and declination (δ =61.4164° ± 0.0003°) of the spin pole place Mercury in the Cassini state. Confirmation of the equilibrium state with an estimated mean (whole-planet) obliquity ϵ of 1.968 ± 0.027 arcmin enables the confident determination of the planet's normalized polar moment of inertia (0.333 ± 0.005), which indicates a high degree of internal differentiation. Internal structure models generated by a Markov-Chain Monte Carlo process and consistent with the geodetic constraints possess a solid inner core with a radius (r
ic
) between 0.3 and 0.7 that of the outer core (r
oc
).
Extremely reducing conditions, such as those that prevailed during the accretion and differentiation of Mercury, change the “normal” pattern of behaviour of many chemical elements. Lithophile elements can become chalcophile, siderophile elements can become lithophile, and volatile elements can become refractory. In this context, unexpected elements, such as Si, are extracted to the core, while others (S, C) concentrate in the silicate portion of the planet, eventually leading to an exotic surface mineralogy. In this article, experimental, theoretical and cosmochemical arguments are applied to the understanding of how reducing conditions influenced Mercury, from the nature of its building blocks to the dynamics of its volcanism.
The depletions of potassium (K) and sodium (Na) in samples from planetary interiors have long been considered as primary evidence for their volatile behavior during planetary formation processes. Here, we use high-pressure experiments combined with laser ablation analyses to measure the sulfide-silicate and metal-silicate partitioning of K and Na at high pressure (P) – temperature (T) and find that their partitioning into metal strongly increases with temperature. Results indicate that the observed Vestan and Martian mantle K and Na depletions can reflect sequestration into their sulfur-rich cores in addition to their volatility during formation of Mars and Vesta. This suggests that alkali depletions are not affected solely by incomplete condensation or partial volatilization during planetary formation and differentiation, but additionally or even primarily reflect the thermal and chemical conditions during core formation. Core sequestration is also significant for the Moon, but lunar mantle depletions of K and Na cannot be reconciled by core formation only. This supports the hypothesis that measured isotopic fractionations of K in lunar samples represent incomplete condensation or extensive volatile loss during the Moon-forming giant impact.
Radioactive decay of potassium (K), thorium (Th), and uranium (U) power the Earth's engine, with variations in 232Th/238U recording planetary differentiation, atmospheric oxidation, and biologically mediated processes. We report several thousand $^{232}$Th/$^{238}$U ($\kappa$) and time-integrated Pb isotopic ($\kappa$$_{Pb}$) values and assess their ratios for the Earth, core, and silicate Earth. Complementary bulk silicate Earth domains (i.e., continental crust $\kappa_{Pb}^{CC}$ = 3.94 $^{+0.20}_{-0.11}$ and modern mantle $\kappa_{Pb}^{MM}$ = 3.87 $^{+0.15}_{-0.07}$, respectively) tightly bracket the solar system initial $\kappa_{Pb}^{SS}$ = 3.890 $\pm$ 0.015. These findings reveal the bulk silicate Earth's $\kappa$$_{Pb}^{BSE}$ is 3.90 $^{+0.13}_{-0.07}$ (or Th/U = 3.77 for the mass ratio), which resolves a long-standing debate regarding the Earth's Th/U value. We performed a Monte Carlo simulation to calculate the $\kappa_{Pb}$ of the BSE and bulk Earth for a range of U concentrations in the core (from 0 to 10 ng/g). Comparison of our results with $\kappa$$_{Pb}^{SS}$ constrains the available U and Th budget in the core. Negligible Th/U fractionation accompanied accretion, core formation, and crust - mantle differentiation, and trivial amounts of these elements (0.07 ppb by weight, equivalent to 0.014 TW of radiogenic power) were added to the core and do not power the geodynamo.
The chemical composition of a planetary body reflects its starting conditions modified by numerous processes during its formation and geological evolution. Measurements by X-ray, gamma-ray, and neutron spectrometers on the MESSENGER spacecraft revealed Mercury's surface to have surprisingly high abundances of the moderately volatile elements sodium, sulfur, potassium, chlorine, and thorium, and a low abundance of iron. This composition rules out some formation models for which high temperatures are expected to have strongly depleted volatiles and indicates that Mercury formed under conditions much more reducing than the other rocky planets of our Solar System. Through geochemical modeling and petrologic experiments, the planet's mantle and core compositions can be estimated from the surface composition and geophysical constraints. The bulk silicate composition of Mercury is likely similar to that of enstatite or metal-rich chondrite meteorites, and the planet's unusually large core is most likely Si rich, implying that in bulk Mercury is enriched in Fe and Si (and possibly S) relative to the other inner planets. The compositional data for Mercury acquired by MESSENGER will be crucial for quantitatively testing future models of the formation of Mercury and the Solar System as a whole, as well as for constraining the geological evolution of the innermost planet.
We have studied the thermal and magnetic field evolution of planet Mercury with a core of Fe–Si alloy to assess whether an Fe–Si core matches its present-day partially molten state, Mercury's magnetic field strength, and the observed ancient crustal magnetization. The main advantages of an Fe–Si core, opposed to a previously assumed Fe–S core, are that a Si-bearing core is consistent with the highly reduced nature of Mercury and that no compositional convection is generated upon core solidification, in agreement with magnetic field indications of a stable layer at the top of Mercury's core. This study also present the first implementation of a conductive temperature profile in the core where heat fluxes are sub-adiabatic in a global thermal evolution model.
We show that heat migrates from the deep core to the outer part of the core as soon as heat fluxes at the outer core become sub-adiabatic. As a result, the deep core cools throughout Mercury's evolution independent of the temperature evolution at the core-mantle boundary, causing an early start of inner core solidification and magnetic field generation. The conductive layer at the outer core suppresses the rate of core growth after temperature differences between the deep and shallow core are relaxed, such that a magnetic field can be generated until the present. Also, the outer core and mantle operate at higher temperatures than previously thought, which prolongs mantle melting and mantle convection. The results indicate that S is not a necessary ingredient of Mercury's core, bringing bulk compositional models of Mercury more in line with reduced meteorite analogues.
We have performed experiments at 1.5 GPa over the temperature range 1400-2100°C to determine the partitioning of lithophile elements (U, Th, Eu, Sm, Nd, Zr, La, Ce, Yb) between sulfide liquid, low-S metals and silicate melt. The data demonstrate pronounced increases in partitioning of all the lithophile elements into sulfide at very low FeO contents (<1 wt%) of the silicate melt such that exceeds 1 and may be >10 in some cases. Similarly DSm may be > 2 under the same conditions of low silicate FeO. This strong partitioning behaviour is found only be important in S-rich metals, however because the observed effect of low FeO on partitioning is uniquely confined to metallic melts close to stoichiometric FeS in composition.
This chapter examines the composition of the core and of the mantle and its domains, upper and lower, its physical and chemical attributes, and its evolution. It starts with fundamental definitions, particularly of what is the lower and upper mantle. Although a compositional model for the lower mantle that matches that of the upper mantle for major elements is most compatible with observations and constraints, uncertainties are such that competing compositional models are tenable. Based on chondritic models, more than 90% of the mass for the Earth is composed of Fe, O, Mg and Si and the addition of Ni, Ca, Al and S accounts for more than 98% by mass the composition of the Earth. Constraints on the absolute and relative abundances of volatile elements in the Earth are consistent with only ~2% by mass of sulfur and a negligible role for H, C or N in the core.
NASA’s MESSENGER spacecraft has revealed geochemical diversity across Mercury’s volcanic crust. Near-infrared to ultraviolet spectra and images have provided evidence for the Fe2+-poor nature of silicate minerals, magnesium sulfide minerals in hollows and a darkening component attributed to graphite, but existing spectral data is insufficient to build a mineralogical map for the planet. Here we investigate the mineralogical variability of silicates in Mercury’s crust using crystallization experiments on magmas with compositions and under reducing conditions expected for Mercury. We find a common crystallization sequence consisting of olivine, plagioclase, pyroxenes and tridymite for all magmas tested. Depending on the cooling rate, we suggest that lavas on Mercury are either fully crystallized or made of a glassy matrix with phenocrysts. Combining the experimental results with geochemical mapping, we can identify several mineralogical provinces: the Northern Volcanic Plains and Smooth Plains, dominated by plagioclase, the High-Mg province, strongly dominated by forsterite, and the Intermediate Plains, comprised of forsterite, plagioclase and enstatite. This implies a temporal evolution of the mineralogy from the oldest lavas, dominated by mafic minerals, to the youngest lavas, dominated by plagioclase, consistent with progressive shallowing and decreasing degree of mantle melting over time.
This paper presents an analysis of present-day interior configuration models for Mercury considering cores of Fe-S or Fe-Si alloy, the latter possibly covered by a solid FeS layer, in light of the improved limit of planetary contraction of 7 km derived from MErcury Surface, Space ENvironment, GEochemistry, and Ranging observations of surface landforms. Density profiles, generated by a Monte Carlo approach, are constrained by Mercury's mass, polar moment of inertia (C), fraction of polar moment corresponding to its outer solid shell (Cm/C), and observed planetary contraction. Results show that the outer liquid core boundary is constrained to 1985-2090 km in radius, where large radii correspond to high Si and S core contents and high mantle densities or the presence of an FeS layer at the top of the outer core. The bulk core S and Si contents are within 2.8-8.9 wt % and above 8.5 wt %, respectively, where an increase of light element core content correlates positively with mantle density and core size. The size of the inner core is constrained by the observed planetary contraction to below 1454 or 1543 km in radius for bulk cores rich in S (near 8.9 wt %) or Si (near 25 wt %), respectively. For cores poor in light elements, inner cores up to 1690 km in radius remain consistent with the observed planetary contraction. Finally, we show that solid FeS at outer core conditions, previously argued to float on liquid Fe-S, may be denser than the residual liquid. This implies that a separate mechanism may be required to maintain an FeS layer at the suggested location.
Sulfides, although modally of low abundance in most igneous rocks, have strong influences on the geochemical behavior of many elements including Pb, Cu, Ni, and the PGEs. In a recent paper, we demonstrated a simple relationship between the sulfide-silicate partition coefficients DMsulf/sil of such elements and the FeO content of the coexisting silicate melt (Kiseeva and Wood 2013).
log DMsulf/sil = A-n/2log[FeO]
where n is the valency of element M, [FeO] is the concentration of FeO in the silicate in wt%, and A is a constant that depends on temperature and pressure. Elements that closely obey the simple model are Pb, In, Sb, Cd, Co, Zn, and Cr. We show here, however, that the fitted slope n depends not only on the valency of element M, but also on how M interacts with oxygen, the dissolution of which as FeO in the sulfide increases as the FeO content of the silicate melt increases.
To take account of interactions of trace element M with FeO dissolved in the sulfide we intro- duce an additional parameter εFeOsulf (Wagner 1962), which represents the difference between the
lithophile and chalcophile properties of M and those of Fe. If εFeOsulf is positive, then element M is
more chalcophile than Fe and if negative more lithophile.
We performed experiments to investigate partitioning of lithophile Nb, Ta, Ce, and Ti between
sulfide and simplified basaltic melt and find that they all exhibit, as expected, concave upward behavior on a plot of logD vs. log[FeO]. New experiments on Cu at low FeO contents confirm that it is more chalcophile than Fe, yielding a concave-downward curve of logD vs. log[FeO]. The combined results mean that nominally lithophile elements may partition more strongly into sulfide than nominally chalcophile elements at either very low or very high FeO contents of the silicate melt. For example, as the FeO content of the silicate melt declines below about 1 wt%, the partition coefficient of Cu, DCsulf/sil declines to an unusually low value (DCsulf/sil ~80), whereas those for Nb (DNsublf/sil ~600), and rare earths (REE’s) increase strongly. Under these conditions, Nb is, therefore, substantially more “chalcophile” than Cu in that it partitions much more strongly into sulfide.
The implications of these observations for the Earth are that under a wide range of conditions one would expect significant partitioning of REE, Nb, Ta, Ti, and other lithophile elements into sulfides. Wohlers and Wood (2015) have, for example, shown that the partitioning of U and the REE into sulfides at low FeO content of the silicate is sufficiently pronounced that addition of a reduced sulfur-rich body to the accreting Earth could generate observable fractionation of Nd from Sm and, together with S, transfer sufficient U to the core to provide a significant energy source for the geodynamo.
We develop a comprehensive model to describe trace and minor element partitioning between sulphide liquids and anhydrous silicate liquids of approximately basaltic composition. We are able thereby to account completely for the effects of temperature and sulphide composition on the partitioning of Ag, Cd, Co, Cr, Cu, Ga, Ge, In, Mn, Ni, Pb, Sb, Ti, Tl, V and Zn. The model was developed from partitioning experiments performed in a piston-cylinder apparatus at 1.5 GPa and 1300 to 1700 °C with sulphide compositions covering the quaternary FeSNiSCuS0.5FeO.
This study presents improvements of internally heated pressure vessels to realize high-pressure experiments at controlled
f O2 in low-viscosity systems such as basaltic ones. The new design is a combination of two experimental techniques: a hydrogen
sensor membrane made of platinum to measure fH2, and therefore f O2, and a rapid-quench system to avoid crystallization of low-viscosity melts during quench. The experimental setup has been
tested successfully at temperatures up to 1250 °C and pressures up to 500 MPa. Basaltic melts containing up to 9.38 wt% water
can be quenched as bubble-free and crystal-free glasses. The improvements allow synthesis of hydrated glass or partly crystallized
samples with a large volume (for further studies) and to perform routine phase-equilibrium studies in basaltic systems at
geologically relevant conditions. We used the new technique to determine the effect of f O2 on water solubility in a melt with MORB composition. The results show that there is a small but significant decrease of water
solubility with decreasing fO2from MnO-Mn3O4 to QFM buffer conditions in the pressure range 50–200 MPa. Kinetic problems in crystallization experiments in basaltic systems
and the duration necessary to attain equilibrium Fe2+/Fe3+ ratio in the charge are discussed.
Enstatite‐rich meteorites, including the aubrites, formed under conditions of very low oxygen fugacity (ƒO2: iron‐wüstite buffer −2 to −6) and thus offer the ability to study reduced magmatism present on multiple bodies in our solar system. Elemental partitioning among metals, sulfides, and silicates is poorly constrained at low ƒO2; however, studies of enstatite‐rich meteorites may yield empirical evidence of the effects of low ƒO2 on elemental behavior. This work presents comprehensive petrologic and oxygen isotopic studies of 14 aubrites, including four meteorites that have not been previously investigated in detail. The aubrites exhibit a variety of textures and mineralogy, and their elemental zoning patterns point to slow cooling histories for all 14 samples. Oxygen isotope analyses suggest that the aubrite parent bodies may be more heterogeneous than originally reported or may have experienced incomplete magmatic differentiation. Contrary to the other classified aubrites and based on textural and mineralogical observations, we suggest that the Northwest Africa 8396 meteorite shows an affinity for an enstatite chondrite parentage. By measuring major elemental compositions of silicates, sulfides, and metals, we calculate new metal–silicate, sulfide–silicate, and sulfide–metal partition coefficients for aubrites that are applicable to igneous systems at low ƒO2. The geochemical behavior of elements in aubrites, as determined using partition coefficients, is similar to the geochemical behavior of elements determined experimentally for magmatic systems on Mercury. Enstatite‐rich meteorites, including aubrites, represent valuable natural petrologic analogues to Mercury and their study could further our understanding of reduced magmatism in our solar system.
Several characteristics of a planet, including its internal dynamics, hinge on the composition and crystallization regime of the core, which, in turn, depends on the phase relations, melting behaviour and thermodynamic properties of constituent materials. The Fe-Si-C ternary system can serve as a proxy for core composition and formation processes under reducing conditions. We conducted laser-heated diamond anvil cell experiments coupled with in situ X-ray diffraction and electron microscopy analysis of the recovered samples, on four different starting compositions in the Fe-Si-C ternary system. Phase relations up to 200 GPa and up to 4000 K were determined. An FeSi phase with a B2 structure and iron carbides with different stoichiometries (i.e. Fe3C and Fe7C3) are the main observed phases, along with pure C (diamond) that has an extended stability field in the subsolidus regime. Carbon is largely soluble in B2-structured FeSi, whereas Si does not partition into the carbides. The melting curve determined for the starting material containing the least amount of light elements is consistent with the one for the Fe-C system. The other starting materials display higher melting temperatures than that of Fe-C, suggesting the existence of at least two different invariant points in the Fe-Si-C system. Applied to planetary interiors, our observations highlight how a small variation in light elements content would deeply affect the solidification style of a core. Bottom-up (Fe-enriched systems) and top-down regimes (C-rich systems), as well as solidification of a crystal mush (Si-enriched systems). These three crystallization regimes influence significantly the possibility of starting and sustaining a dynamo. Our results provide new insights into the differentiation of terrestrial planets in the Solar System and beyond, contributing to the study of planetary diversity.
The NASA MESSENGER mission revealed that lavas on Mercury are enriched in sulfur (1.5-4 wt.%) compared with other terrestrial planets (<0.1 wt.%), a result of high S solubility under its very low oxygen fugacity (estimated ƒO2 between IW-3 and IW-7). Due to decreasing O availability at these low ƒO2 conditions, and an abundance of S²⁻, the latter acts as an important anion. This changes the partitioning behaviour of many elements (e.g. Fe, Mg, and Ca) and modifies the physical properties of silicate melts. To further understand S solubility and speciation in reduced magmas, we have analysed 11 high pressure experiments run at 1 GPa in a piston cylinder at temperatures of 1250 to 1475 °C and ƒO2 between IW-2.5 to IW-7.5. S K-Edge XANES is used to determine coordination chemistry and oxidation state of S species in highly reduced quenched silicate melts. As ƒO2 decreases from IW-2 to IW-7, S speciation goes through two major changes. At ∼IW-2, FeS, FeCr2S4, Na2S, and MnS species are destabilized, CaS (with minor Na2S) becomes the dominant S species. At ∼ IW-4, Na2S is destabilized, MgS becomes the dominant S species, with lesser amounts of CaS. The changes in S speciation at low ƒO2 affect the activities of SiO2, MgO and CaO in the melt, stabilizing enstatite at the expense of forsterite, and destabilizing plagioclase and clinopyroxene. These shifts cause the initial layering of Mercury’s solidified magma ocean to be enstatite-rich and plagioclase poor. Our results on S speciation at low ƒO2 are also applicable to the petrologic evolution of enstatite chondrite parent bodies and perhaps early Earth.
The Earth may have formed at very reducing conditions through
the accretion of (a) large reduced and differentiated impactor(s).
Segregation of Fe-S liquids within these bodies would have left a
geochemical mark on the mantles of reduced impactors and on the protoEarth's mantle. Here, we study the geochemical consequences of highly
reduced accretion of the Earth by large impactors. New insights into the
partitioning of trace elements between Fe-S liquid and silicate melt at
(highly) reduced conditions (ΔIW = -5 to +1) were obtained by performing
21 high pressure experiments at 1 GPa and 1683-2283 K. The observed Fe-S
liquid-silicate melt partitioning behavior is in agreement with
thermodynamic models that predict a significant role for O in Fe-S liquid
and S in the silicate melt.
The experimental results were combined with literature data to obtain new
and/or revised thermodynamic parameterizations that quantify the effects
of composition and redox state on the elemental distribution between Fe-S
liquids and highly reduced silicate melts. The results were used to
assess which elements would most likely retain the geochemical signature
of accretion of reduced impactors. Under the assumption of instantaneous
core merging, impact delivery to the proto-Earth's mantle was found to be
significant (>10% of present-day BSE concentrations) only for S, Zn, Se,
Te and Tl, whereas the abundances of the other elements remain largely
unaffected.
The results also show that present-day BSE S/Se, Se/Te, Tl/S and
potentially In/Zn as well as their absolute abundances are inconsistent
with their delivery by (a) large, highly reduced chondritic
differentiated impactor(s) during terrestrial accretion. Continued coremantle equilibration in the proto-Earth, volatility-related loss and/or
post-accretion sulfide liquid segregation in the terrestrial magma ocean
would further increase or not affect these discrepancies. We conclude
that a significant contribution of (a) large (>10% of Earth's mass)
reduced and differentiated chondritic impactor(s) during accretion of the
Earth is not reflected in the present-day S, Zn, Se, Te and Tl
systematics of the terrestrial mantle. This suggests that significant
overprinting of the primordial BSE S/Se, Se/Te and S/Tl signature could
have occurred and/or (2) that the S/Se and Se/Te ratios were set by
accretion of more oxidised CI-like materials.
In this study we investigate the likeliness of the existence of an iron sulfide layer (FeS matte) at the core-mantle boundary (CMB) of Mercury by comparing new chemical surface data obtained by the X-ray Spectrometer onboard the MESSENGER spacecraft with geochemical models supported by high-pressure experiments under reducing conditions. We present a new data set consisting of 233 Ti/Si measurements, which combined with Al/Si data show that Mercury's surface has a slightly subchondritic Ti/Al ratio of 0.035 ± 0.008. Multiphase equilibria experiments show that at the conditions of Mercury's core formation, Ti is chalcophile but not siderophile, making Ti a useful tracer of sulfide melt formation. We parameterize and use our partitioning data in a model to calculate the relative depletion of Ti in the bulk silicate fraction of Mercury as a function of a putative FeS layer thickness. By comparing the model results and surface elemental data we show that Mercury most likely does not have a FeS layer, and in case it would have one, it would only be a few kilometers thick (<13 km). We also show that Mercury's metallic Fe(Si) core cannot contain more than ∼1.5 wt.% sulfur and that the formation of this core under reducing conditions is responsible for the slightly subchondritic Ti/Al ratio of Mercury's surface.
We present the first complete dataset of partition coefficients of Rare Earth Elements (REE) between oldhamites or molten FeS and silicate melts. Values have been determined at 1300 and 1400 °C from experiments on mixtures of a natural enstatite chondrite and sulphides powders (FeS or CaS) performed in evacuated silica tubes for different fO2 conditions (from IW-6.9 to IW-4.1). Obtained REE partitioning values are between 0.5 and 5 for oldhamites and between 0.001 and 1 for FeS. In both sulphides, Eu and Yb are preferentially incorporated compared to neighbouring REE. X-ray Absorption Near Edge Structure measurements on Yb and Sm demonstrate the partial reduction to 2+ valence state for both elements, Yb reduction being more pronounced. Therefore, the Yb anomaly in the sulphides is interpreted to be an effect of the presence of Yb²⁺ in the system and the amplitude of the anomaly increases with decreasing oxygen fugacity. The obtained oldhamite/silicate melt partition coefficients patterns are unlike any of the observed data in natural oldhamites from enstatite chondrites and achondrites. In particular, the low values do not explain the observed enrichments in oldhamite crystals. However, positive Eu and Yb anomalies are observed in some oldhamites from EH chondrites and aubrites. We attribute these anomalies found in meteorites to the sole oldhamite control on REE budget. We conclude that the presence of positive Eu and Yb anomalies in oldhamites is a good indicator of their primordial character and that these oldhamites carry a condensation signature from a highly reduced nebular gas.
Chalcophile and siderophile element abundances are used to provide important constraints on the interior compositions of planetary bodies as well as the pressure (P) - temperature (T) conditions that prevailed during core formation. The oxygen fugacity (fO2) during core formation varied considerably between the various terrestrial planets and asteroidal bodies in our solar system. Mercury, the aubrite parent body (AuPB) and some terrestrial precursor bodies may have differentiated at highly reduced conditions.
At present knowledge about how the metal liquid-silicate melt and sulfide liquid-silicate melt partitioning behavior of major and trace elements are affected by high S concentrations in the silicate melt at highly reducing conditions is incomplete. Here, we experimentally study the metal-silicate and sulfide-silicate partitioning behavior of trace elements in reduced silicate melts over a wide range of S contents as a function of redox state at 1 GPa and 1833–1883 K. Silicate melt S contents ranged between ~0.5 and ~20 wt.%, with a corresponding silicate FeO range of ~0.4 to ~17.5 wt.%, in a fO_2 range between 1 and 9 log units below the iron-wüstite buffer. Our results reproduce the decrease of the S concentration at sulfide saturation (SCSS) with decreasing FeO contents down to ~3 wt.%, as well as its strong increase at <3 wt. % FeO. At S contents exceeding >6–9 wt.% S, the FeO contents increase again.
Results show that most elements (Mg, Ti, V, Cr, Mn, Cu, Zn, Se, Nb, Cd, Sb, Te, Ta, Tl, Pb and Bi) are more chalcophile than siderophile at reducing conditions, whereas Si, Co, Ni, Ga, Ge, Mo and W preferentially partition into Fe-rich melts instead of sulfide liquids. Silicon, Ti, Se, and Te preferentially partition into Fe-S over (Fe,Mg,Ca)-S liquids, whereas Mn, Zn and Cd are more compatible in the latter. As proposed by Wood and Kiseeva (2015), chalcophile elements such as Cu, Se and Te behave less chalcophile with increasing S concentrations of the silicate melt, whereas the opposite is observed for nominally lithophile elements such as Mg, Ca and Ti.
The results can be used to improve interpretations of the observed trace element systematics of aubrites and other reduced achondrites. All of the volatile elements considered here behave chalcophile at the reducing conditions inferred for differentiation of the AuPB. A significant degree of the observed volatile element depletions in aubrites may therefore reflect their preferential partitioning into sulfide liquids, rather than degassing during or after differentiation of the AuPB. These results suggest that, depending of the extent of core merging, precursor body differentiation and the efficiency of sulfide liquid segregation, reduced precursor bodies that were incorporated in the early Earth were likely more rich in volatile elements than currently assumed.
The distribution of heat-producing elements (HPE) potassium (K), uranium (U) and thorium (Th) within planetary interiors has major implications for the thermal evolution of the terrestrial planets and for the inventory of volatile elements in the inner Solar System. To investigate the abundances of HPE in Mercury's interior, we conducted experiments at high pressure and temperature (up to 5 GPa and 1900 °C) and reduced conditions (IW-1.8 to IW-6.5) to determine U, Th and K partitioning between metal, silicate and sulfide (D met/sil and D sulf/sil). Our experimental data combined with those from the literature show that partitioning into sulfide is more efficient than into metal and enhanced with decreasing FeO and increasing O contents of the silicate and sulfide melts respectively. Also, at low oxygen fugacity (log fO2 < IW-5), U and Th are more efficiently partitioned into liquid iron metal and sulfide than K. D met/sil for U, Th and K increases with decreasing oxygen fugacity, while DU met/sil and DK met/sil increase when the metal is enriched and depleted in O or Si respectively. We also used available data from the literature to constrain the concentrations of light elements (Si, S, O and C) in Fe metal and sulfide. We provided chemical compositions of Mercury's core after core segregation, for a range of fO2 conditions during its differentiation. For example, if Mercury differentiated at IW-5.5, its core would contain 49 wt% Si, 0.02 wt% S and negligible C. Also if core-mantle separation happened at a fO2 lower than IW-4, bulk Mercury Fe/Si ratio is likely to be chondritic. We calculated concentrations of U, Th and K in the Fe-rich core and possible sulfide layer of Mercury. Bulk Mercury K/U and K/Th were calculated taking all U, Th and K reservoirs into account. Without any sulfide layer or if Mercury's core segregated at a higher fO2 than IW-4, bulk K/U and K/Th would be similar to those measured on the surface, confirming more elevated volatile K concentration than previously expected for Mercury. However, Mercury could fall on an overall volatile depletion trend where K/U increases with the heliocentric distance, if core segregation Always consult and cite the final, published document. See http:/www.minsocam.org or GeoscienceWorld 5 occurred near IW-5.5 or more reduced conditions and with a sulfide layer of at least 130 km thickness. In these conditions, bulk Mercury K/Th ratio is close to Venus' and Earth's values. Since U and Th become more chalcophile with decreasing oxygen fugacity, to a higher extent than K, it is likely that at a fO2 close to or lower than IW-6, both K/U and K/Th become lower than values of the other terrestrial planets. Therefore, our results suggest that the elevated K/U and K/Th ratios of Mercury's surface should not be exclusively interpreted as the result of a volatile enrichment in Mercury, but could also indicate a sequestration of more U and Th than K in a hidden iron sulfide reservoir, possibly a layer present between the mantle and core. Hence, Mercury could be more depleted in volatiles than Mars with a K concentration similar or lower than the Earth's and Venus', suggesting a volatile depletion in the inner Solar System. In addition, we show that the presence of a sulfide layer formed between IW-4 and IW-5.5 decreases the total radioactive heat production of Mercury by up to 30%.
Unique physical and chemical characteristics of Mercury have been revealed by measurements from NASA’s MESSENGER spacecraft. The closest planet to our Sun is made up of a large metallic core that is partially liquid, a thin mantle thought to be formed by solidification of a silicate magma ocean, and a relatively thick secondary crust produced by partial melting of the mantle followed by volcanic eruptions. However, the origin of the large metal/silicate ratio of the bulk planet and the conditions of accretion remain elusive. Metal enrichment may originate from primordial processes in the solar nebula or from a giant impact that stripped most of the silicate portion of a larger planet leaving Mercury as we know it today.
Data from the Gamma-Ray Spectrometer (GRS) that flew on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft indicate that the O/Si weight ratio of Mercury's surface is 1.2 ± 0.1. This value is lower than any other celestial surface that has been measured by GRS and suggests that 12-20% of the surface materials on Mercury are composed of Si-rich, Si-Fe alloys. The origin of the metal is best explained by a combination of space weathering and graphite-induced smelting. The smelting process would have been facilitated by interaction of graphite with boninitic and komatiitic parental liquids. Graphite entrained at depth would have reacted with FeO components dissolved in silicate melt, resulting in the production of up to 0.4-0.9 wt % CO from the reduction of FeO to Fe⁰-CO production that could have facilitated explosive volcanic processes on Mercury. Once the graphite-entrained magmas erupted, the tenuous atmosphere on Mercury prevented the buildup of CO over the lavas. The partial pressure of CO would have been sufficiently low to facilitate reaction between graphite and SiO2 components in silicate melts to produce CO and metallic Si. Although exotic, Si-rich metal as a primary smelting product is hypothesized on Mercury for three primary reasons: (1) low FeO abundances of parental magmas, (2) elevated abundances of graphite in the crust and regolith, and (3) the presence of only a tenuous atmosphere at the surface of the planet within the 3.5-4.1 Ga timespan over which the planet was resurfaced through volcanic processes.
[1] A number of observations performed by the MESSENGER spacecraft can now be employed to better understand the evolution of Mercury's interior. Using recent constraints on interior structure, surface composition, volcanic and tectonic histories, we modeled the thermal and magmatic evolution of the planet. We ran a large set of Monte Carlo simulations based on one-dimensional parametrized models, spanning a wide range of parameters. We complemented these simulations with selected calculations in 2-D cylindrical and 3-D spherical geometry, which confirmed the validity of the parametrized approach and allowed us to gain additional insight into the spatiotemporal evolution of mantle convection. Core radii of 1940 km, 2040 km, and 2140 km have been considered, and while in the first two cases several models satisfy the observational constraints, no admissible models were found for a radius of 2140 km. A typical thermal evolution scenario consists of an initial phase of mantle heating accompanied by planetary expansion and the production of a substantial amount of partial melt. The evolution subsequent to 2 Gyr is characterized by secular cooling that proceeds approximately at a constant rate and implies that planetary contraction should be ongoing today. Most of the models predict mantle convection to cease after 3–4 Gyr, indicating that Mercury may be no longer dynamically active. Finally, assuming the observed surface abundance of radiogenic elements to be representative for the entire crust, we determined bulk silicate concentrations of 35–62 ppb Th, 20–36 ppb U, and 290–515 ppm K, similar to those of other terrestrial planets.
The Ca–Cr–Cu–Fe–Mg–Mn–S system has been thermodynamically assessed using all available experimental data. The thermodynamic description of the high-temperature phase Chromium and Iron Pyrrhotite is described using a two-sublattice model which allows the description of the stability range of this phase in the binary Fe–S and Cr–S systems and also the solubility of such elements as Cu, Mg and Mn in the binary systems. Particular attention was given also to the phases Oldhamite and Digenite, which exhibit very wide solubility with respect to the metal elements. In this part of the report binary and quasi-binary subsystems are presented.
Basaltic shergottites display a systematic decrease in K/Th, K/U, and K/La ratios with increasing K content. These trends are interpreted as mixing lines between relatively young martian magmas derived from highly depleted mantle sources and an ancient large-ion lithophile (LIL) element-enriched crustal component. One implication of this is that a substantial fractionation of these ratios occurs during the early crustal differentiation on Mars. Isotopic evidence from SNC meteorites and compositional data from Pathfinder and orbital gamma ray spectroscopy suggest that in excess of 50% of the LIL element complement of Mars resides in the crustal reservoir. If so, the primitive mantle of Mars is significantly more volatile-depleted (i.e., lower K/Th, K/U, K/La) than previously thought but probably (though not necessarily) still less volatile-depleted than the primitive mantle of the Earth. The La/Th ratios of virtually all SNC meteorites are subchondritic, including those with the most severe LREE-depletion. Extrapolation of the basaltic shergottite trend suggests that both the depleted mantle end member and the enriched crustal end member have subchondritic La/Th ratios. This is in contrast with the Earth where basalts from:LIL element-depleted. sources such as MORB have superchondritic La/Th ratios, complementary to the subchondritic ratios of the continental crust. Accordingly, assuming that the refractory elements are in chondritic proportions for the Mars primitive mantle, an additional major geochemical reservoir must exist on Mars that may not yet have been sampled.
Phase equilibrium experiments were conducted on a synthetic rock composition matching that of the northern volcanic plains of Mercury as measured by the MErcury Surface, Space ENvironment, GEochemistry and Ranging spacecraft (MESSENGER). The northern volcanic plains are smooth plains of suspected volcanic origin that cover more than 6% of the surface area of Mercury. The northern volcanic plains are less cratered than their surroundings and reported to be the product of flood volcanism, making them a prime candidate for experimental study. The bulk composition of the northern volcanic plains is that of an alkali-rich boninite and represents the first silica-enriched crustal terrane identified on an extraterrestrial planet from orbital data. Phase equilibrium experiments were conducted over the pressure range of the mercurian mantle (0.5-5 GPa) at very low oxygen fugacity (~δIW0 to -7) using a piston-cylinder apparatus ( P 0.5-1.7 GPa) and a Walker-style multi-anvil device ( P≥ 2.5 GPa). Our results indicate the origin of the northern volcanic plains lavas (boninites) are best explained by high degrees of partial melting of an olivine-dominant, pyroxene- and plagioclase-bearing mantle source at low pressure (≤1.4 GPa) and does not require hydrous melting to achieve the silica-enriched melt composition. The formation mechanism for boninites on Mercury contrasts substantially with terrestrial boninites, which typically occur in oxidized and hydrous arc environments associated with subduction zones. Instead, mercurian boninites form at exceptionally low oxygen fugacity and do not require melting of hydrated source materials. The NVP lavas represent a novel mechanism by which planetary bodies can form silica-enriched secondary crusts without the aid of water.
Orbital measurements obtained by the MESSENGER Gamma-Ray Spectrometer have been analyzed to determine the surface abundance of chlorine in Mercury’s northern hemisphere. The derived Cl/Si mass ratio is 0.0057 ± 0.001, which for an assumed Si abundance of 24.6 wt% corresponds to 0.14 ± 0.03 wt% Cl. The abundance of Cl is a factor of 2.9 ± 1.3 higher in the north polar region (>80°N) than at latitudes 0−60°N, a latitudinal variation similar to that observed for Na. Our reported Cl abundances are consistent with measured bulk concentrations of neutron-absorbing elements on Mercury, particularly those observed at high northern latitudes. The Cl/K ratio on Mercury is chondritic, indicating a limited impact history akin to that of Mars, which accreted rapidly. Hypotheses for the origin of Mercury’s high metal-to-silicate ratio must be able to reproduce Mercury’s observed elemental abundances, including Cl. Chlorine is also an important magmatic volatile, and its elevated abundance in the northern polar region of Mercury indicates that it could have played a role in the production, ascent, and eruption of flood volcanic material in this region. We have identified several candidate primary mineralogical hosts for Cl on Mercury, including the halide minerals lawrencite (FeCl2), sylvite (KCl), and halite (NaCl), as well as Cl-bearing alkali sulfides. Amphiboles, micas, apatite, and aqueously deposited halides, in contrast, may be ruled out as mineralogical hosts of Cl on Mercury.
The first map of variations in the abundances of thermal-neutron-absorbing elements across Mercury’s surface has been derived from measurements made with the anti-coincidence shield on MESSENGER’s Gamma-Ray Spectrometer (GRS). The results, which are limited to Mercury’s northern hemisphere, permit the identification of four major geochemical terranes at the 1000-km horizontal scale. The chemical properties of these regions are characterized from knowledge of neutron production physics coupled with elemental abundance measurements acquired by MESSENGER’s X-Ray Spectrometer (XRS) and GRS. The results indicate that the smooth plains interior to the Caloris basin have an elemental composition that is distinct from those of other volcanic plains units, suggesting that the parental magmas were partial melts from a chemically distinct portion of Mercury’s mantle. Mercury’s high-magnesium region, first recognized from XRS measurements, also contains high concentrations of unidentified neutron-absorbing elements. At latitudes north of ∼65°N, there is a region of high neutron absorption that corresponds closely to areas known to be enhanced in the moderately volatile lithophile elements Na, K, and Cl, and which has distinctly low Mg/Si ratios. The boundaries of this terrane differ from those of the northern volcanic plains, which constitute the largest geological unit in this region.