""Partitioning of sulphur between liquid Fe-rich metals and silicates (Dmet/silS) was investigated at temperatures from 1800◦C to 2400◦C, pressures from 2 to 23 GPa and oxygen fugacities from 3.5 to 1.5 log units below the iron–wüstite... more
""Partitioning of sulphur between liquid Fe-rich metals and silicates (Dmet/silS) was investigated at temperatures from 1800◦C to 2400◦C, pressures from 2 to 23 GPa and oxygen fugacities from 3.5 to 1.5 log units below the iron–wüstite buffer, using multi-anvil apparatus. The results are combined with previous experimental works to refine a multi-variable thermodynamic model of Dmet/silS. Sulphur appears to become more siderophile with increasing pressure and FeO content of the silicate melt, and less siderophile with increasing temperature and with Si, C, O, Fe and Ni contents of the metal. We then modelled the behaviour of sulphur in the course of planetary accretion, using different possible scenarios of mantle dynamics and evolution with time of oxygen fugacity. We investigated three end-member models for metal–silicate segregation of the incoming impactors:(i) the planetary mantle does not mix and is kept chemically stratified,(ii) the magma ocean is continuously mixed chemically, and (iii) both the magma ocean and the solid lower mantle are well mixed.
We show that if S is accreted along the accretion, whatever the oxidation path, its distribution between core and mantle can lead to the observed S concentration of the mantle (200±80ppm) and to the estimations of S content of the core (from its depletion in the mantle relative to the other elements with the same volatility). In the case of an Earth built with reduced material, to explain the present-day 200 (±80) ppm S found in the mantle, it is necessary that both the magma ocean and the solid lower mantle mix at each major step of the planetary accretion. S could also be accreted in the last 10 to 20% of Earth’s growth and reach its observed present terrestrial abundances if the magma ocean is chemically mixed along the accretion. Consequently, our models show that the S terrestrial abundances do not formally require an S accretion in a late veneer but can be explained by a core–mantle equilibration alone.""
We show that if S is accreted along the accretion, whatever the oxidation path, its distribution between core and mantle can lead to the observed S concentration of the mantle (200±80ppm) and to the estimations of S content of the core (from its depletion in the mantle relative to the other elements with the same volatility). In the case of an Earth built with reduced material, to explain the present-day 200 (±80) ppm S found in the mantle, it is necessary that both the magma ocean and the solid lower mantle mix at each major step of the planetary accretion. S could also be accreted in the last 10 to 20% of Earth’s growth and reach its observed present terrestrial abundances if the magma ocean is chemically mixed along the accretion. Consequently, our models show that the S terrestrial abundances do not formally require an S accretion in a late veneer but can be explained by a core–mantle equilibration alone.""
The growth rate of ringwoodite reaction rims between MgSiO3 perovskite and periclase was investigated at 22.5 GPa and 1,800 °C for 1–24 h using the Kawai-type high-pressure apparatus. The reaction was likely to proceed by a... more
The growth rate of ringwoodite reaction rims between MgSiO3 perovskite and periclase was investigated at 22.5 GPa and 1,800 °C for 1–24 h using the Kawai-type high-pressure apparatus. The reaction was likely to proceed by a diffusion-controlled mechanism in which the dominant diffusion mechanism was grain-boundary diffusion. The reaction constant (the width of the ringwoodite reaction rim squared divided by time) determined from these experiments was between 1.3 × 10−15 and 5.6 × 10−15 m2/s. A Pt inert marker experiment indicated that the MgO component migrated faster than the SiO2 component in ringwoodite. Thus, either Mg or O having the slower diffusion rate controlled the reaction. Because previous diffusion studies have shown that diffusion rates of O are slower than those of Mg, O would be a rate-controlling element for ringwoodite formation from MgSiO3 perovskite and periclase. The growth rate appeared to be too fast to explain the observed topographic rise (~10 km) inside mantle plumes at the 660-km discontinuity.
- by Asmaa Boujibar and +1
- •
- Kinetics, Mantle Dynamics, Perovskites, Earth's mantle
Early in the Solar System’s history, energetic collisions of differentiated bodies affected the final composition of the terrestrial planets through partial destruction. Enstatite chondrites (EC) are the best candidates to represent the... more
Early in the Solar System’s history, energetic collisions of differentiated bodies affected the final composition of the terrestrial planets through partial destruction. Enstatite chondrites (EC) are the best candidates to represent the primordial terrestrial precursors as they present the most similar isotopic compositions to Earth. Here we report that collisional erosion of >15% of the early Earth’s mass can reconcile the remaining compositional differences between EC and the Earth. We base our demonstration on experimental melting of an EC composition at pressures between 1 bar and 25 GPa. At low pressures, the first silicate melts are highly enriched in incompatible elements Si, Al and Na, and depleted in Mg. Loss of proto-crusts through impacts raises the Earth’s Mg/Si ratio to its present value. To match all major element compositions, our model implies preferential loss of volatile lithophile elements and re-condensation of refractory lithophile elements after the impacts.
- by Asmaa Boujibar and +1
- •
- Earth Sciences, Petrology, Planetary Science, Cosmochemistry
Using large volume press, samples of bridgmanites (Bg) in equilibrium with both silicate melt and liquid Fe-alloy were synthesized to replicate the early period of core-mantle segregation and magma ocean crystallization. We observe that... more
Using large volume press, samples of bridgmanites (Bg) in equilibrium with both silicate melt and liquid Fe-alloy were synthesized to replicate the early period of core-mantle segregation and magma ocean crystallization. We observe that the Fe partition coefficient between Bg and silicate melt Embedded Image varies strongly with the degree of partial melting (F). It is close to 1 at very low F and adopts a constant value of ~0.3 for F values above 10 wt%. In the context of a partially molten mantle, a larger F (closer to liquidus) should yield Fe-depleted Bg grains floating in the liquid mantle. In contrast, a low F (closer to solidus) should yield buoyant pockets of silicate melt in the dominantly solid mantle.
We also determined the valence state of Fe in these Bg phases using X-ray absorption near-edge spectroscopy (XANES). Combining our results with all available data sets, we show a redox state of Fe in Bg more complex than generally accepted. Under the reducing oxygen fugacities Embedded Image of this study ranging from IW-1.5 and IW-2, the measured Fe3+ content of Bg is found moderate (Fe3+/ΣFe = 21 ± 4%) and weakly correlated with Al content. When Embedded Image is comprised between IW-1 and IW, this ratio is correlated with both Al content and oxygen fugacity. When Embedded Image remains between IW and Re/ReO2 buffers, Fe3+/ΣFe ratio becomes independent of Embedded Image and exclusively correlated with Al content.
Due to the incompatibility of Fe in Bg and the variability of its partition coefficient with the degree of melting, fractional crystallization of the magma ocean can lead to important chemical heterogeneities that will be attenuated ultimately with mantle stirring. In addition, the relatively low-Fe3+ contents found in Bg (21%) at the reducing conditions (IW-2) prevailing during core segregation seem contradictory with the 50% previously suggested for the actual Earth’s lower mantle. This suggests the presence of 1.7 wt% Fe3+ in the lower mantle, which reduces the difference with the value observed in the upper mantle (0.3 wt%). Reaching higher concentrations of trivalent Fe requires additional oxidation processes such as the late arrival of relatively oxidized material during the Earth accretion or interaction with oxidized subducting slabs.
We also determined the valence state of Fe in these Bg phases using X-ray absorption near-edge spectroscopy (XANES). Combining our results with all available data sets, we show a redox state of Fe in Bg more complex than generally accepted. Under the reducing oxygen fugacities Embedded Image of this study ranging from IW-1.5 and IW-2, the measured Fe3+ content of Bg is found moderate (Fe3+/ΣFe = 21 ± 4%) and weakly correlated with Al content. When Embedded Image is comprised between IW-1 and IW, this ratio is correlated with both Al content and oxygen fugacity. When Embedded Image remains between IW and Re/ReO2 buffers, Fe3+/ΣFe ratio becomes independent of Embedded Image and exclusively correlated with Al content.
Due to the incompatibility of Fe in Bg and the variability of its partition coefficient with the degree of melting, fractional crystallization of the magma ocean can lead to important chemical heterogeneities that will be attenuated ultimately with mantle stirring. In addition, the relatively low-Fe3+ contents found in Bg (21%) at the reducing conditions (IW-2) prevailing during core segregation seem contradictory with the 50% previously suggested for the actual Earth’s lower mantle. This suggests the presence of 1.7 wt% Fe3+ in the lower mantle, which reduces the difference with the value observed in the upper mantle (0.3 wt%). Reaching higher concentrations of trivalent Fe requires additional oxidation processes such as the late arrival of relatively oxidized material during the Earth accretion or interaction with oxidized subducting slabs.
Knowledge of melting properties is critical to predict the nature and the fate of melts produced in the deep mantle. Early in the Earth’s history, melting properties controlled the magma ocean crystallization, which potentially induced... more
Knowledge of melting properties is critical to predict the nature and the fate of melts produced in the
deep mantle. Early in the Earth’s history, melting properties controlled the magma ocean crystallization,
which potentially induced chemical segregation in distinct reservoirs. Today, partial melting most probably
occurs in the lowermost mantle as well as at mid upper-mantle depths, which control important
aspects of mantle dynamics, including some types of volcanism. Unfortunately, despite major experimental
and theoretical efforts, major controversies remain about several aspects of mantle melting. For example,
the liquidus of the mantle was reported (for peridotitic or chondritic-type composition) with a
temperature difference of 1000 K at high mantle depths. Also, the Fe partitioning coefficient (DFe
Bg/melt)
between bridgmanite (Bg, the major lower mantle mineral) and a melt was reported between 0.1
and 0.5, for a mantle depth of 2000 km. Until now, these uncertainties had prevented the construction
of a coherent picture of the melting behavior of the deep mantle.
In this article, we perform a critical review of previous works and develop a coherent, semiquantitative,
model. We first address the melting curve of Bg with the help of original experimental measurements,
which yields a constraint on the volume change upon melting (DVm). Secondly, we apply a
basic thermodynamical approach to discuss the melting behavior of mineralogical assemblages made
of fractions of Bg, CaSiO3-perovskite and (Mg,Fe)O-ferropericlase. Our analysis yields quantitative constraints
on the SiO2-content in the pseudo-eutectic melt and the degree of partial melting (F) as a function
of pressure, temperature and mantle composition; For examples, we find that F could be more than 40%
at the solidus temperature, except if the presence of volatile elements induces incipient melting. We then
discuss the melt buoyancy in a partial molten lower mantle as a function of pressure, F and DFe
Bg/melt. In the
lower mantle, density inversions (i.e. sinking melts) appear to be restricted to low F values and highest
mantle pressures.
The coherent melting model has direct geophysical implications: (i) in the early Earth, the magma
ocean crystallization could not occur for a core temperature higher than 5400 K at the core-mantle
boundary (CMB). This temperature corresponds to the melting of pure Bg at 135 GPa. For a mantle composition
more realistic than pure Bg, the right CMB temperature for magma ocean crystallization could
have been as low as 4400 K. (ii) There are converging arguments for the formation of a relatively homogeneous
mantle after magma ocean crystallization. In particular, we predict the bulk crystallization of a
relatively large mantle fraction, when the temperature becomes lower than the pseudo-eutectic temperature.
Some chemical segregation could still be possible as a result of some Bg segregation in the lowermost
mantle during the first stage of the magma ocean crystallization, and due to a much later descent of
very low F, Fe-enriched, melts toward the CMB. (iii) The descent of such melts could still take place today.
There formation should to be related to incipient mantle melting due to the presence of volatile elements.
Even though, these melts can only be denser than the mantle (at high mantle depths) if the controversial
value of DFe
Bg/melt is indeed as low as suggested by some experimental studies. This type of melts could
contribute to produce ultra-low seismic velocity anomalies in the lowermost mantle.
deep mantle. Early in the Earth’s history, melting properties controlled the magma ocean crystallization,
which potentially induced chemical segregation in distinct reservoirs. Today, partial melting most probably
occurs in the lowermost mantle as well as at mid upper-mantle depths, which control important
aspects of mantle dynamics, including some types of volcanism. Unfortunately, despite major experimental
and theoretical efforts, major controversies remain about several aspects of mantle melting. For example,
the liquidus of the mantle was reported (for peridotitic or chondritic-type composition) with a
temperature difference of 1000 K at high mantle depths. Also, the Fe partitioning coefficient (DFe
Bg/melt)
between bridgmanite (Bg, the major lower mantle mineral) and a melt was reported between 0.1
and 0.5, for a mantle depth of 2000 km. Until now, these uncertainties had prevented the construction
of a coherent picture of the melting behavior of the deep mantle.
In this article, we perform a critical review of previous works and develop a coherent, semiquantitative,
model. We first address the melting curve of Bg with the help of original experimental measurements,
which yields a constraint on the volume change upon melting (DVm). Secondly, we apply a
basic thermodynamical approach to discuss the melting behavior of mineralogical assemblages made
of fractions of Bg, CaSiO3-perovskite and (Mg,Fe)O-ferropericlase. Our analysis yields quantitative constraints
on the SiO2-content in the pseudo-eutectic melt and the degree of partial melting (F) as a function
of pressure, temperature and mantle composition; For examples, we find that F could be more than 40%
at the solidus temperature, except if the presence of volatile elements induces incipient melting. We then
discuss the melt buoyancy in a partial molten lower mantle as a function of pressure, F and DFe
Bg/melt. In the
lower mantle, density inversions (i.e. sinking melts) appear to be restricted to low F values and highest
mantle pressures.
The coherent melting model has direct geophysical implications: (i) in the early Earth, the magma
ocean crystallization could not occur for a core temperature higher than 5400 K at the core-mantle
boundary (CMB). This temperature corresponds to the melting of pure Bg at 135 GPa. For a mantle composition
more realistic than pure Bg, the right CMB temperature for magma ocean crystallization could
have been as low as 4400 K. (ii) There are converging arguments for the formation of a relatively homogeneous
mantle after magma ocean crystallization. In particular, we predict the bulk crystallization of a
relatively large mantle fraction, when the temperature becomes lower than the pseudo-eutectic temperature.
Some chemical segregation could still be possible as a result of some Bg segregation in the lowermost
mantle during the first stage of the magma ocean crystallization, and due to a much later descent of
very low F, Fe-enriched, melts toward the CMB. (iii) The descent of such melts could still take place today.
There formation should to be related to incipient mantle melting due to the presence of volatile elements.
Even though, these melts can only be denser than the mantle (at high mantle depths) if the controversial
value of DFe
Bg/melt is indeed as low as suggested by some experimental studies. This type of melts could
contribute to produce ultra-low seismic velocity anomalies in the lowermost mantle.
A large number of siderophile (iron-loving) elements are also volatile, thus offering constraints on the origin of volatile elements in differentiated bodies such as Earth, Moon, Mars and Vesta. Metal–silicate partitioning data for many... more
A large number of siderophile (iron-loving) elements are also volatile, thus offering constraints on the origin of volatile
elements in differentiated bodies such as Earth, Moon, Mars and Vesta. Metal–silicate partitioning data for many of these
elements is lacking, making their overall mantle concentrations in these bodies difficult to model and origin difficult to distinguish
between core formation and volatile depletion. To address this gap in understanding, we have undertaken systematic
studies of four volatile siderophile elements – Sb, As, Ge and In – at variable temperature and variable Si content of metal.
Several series were carried out at 1 GPa, and between 1500 and 1900 C, for both C saturated and C-free conditions. The
results show that temperature causes a decrease in the metal/silicate partition coefficient for all four elements. In addition,
activity coefficients for each element have been determined and show a very strong dependence on Si content of Fe alloy.
Si dissolved in metal significantly decreases the metal/silicate partition coefficients, at both 1600 and 1800 C. The combination
of temperature and Si content of the metal causes reduction of the metal–silicate partition coefficient to values that are
close to those required for an origin of mantle As, Sb, Ge, and In concentrations by metal–silicate equilibrium processes.
Combining these new results with previous studies on As, Sb, Ge, and In, allowed derivation of predictive expressions for
metal/silicate partition coefficients for these elements 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 As, Sb, Ge, and In can be explained by metal–silicate equilibrium during an accretion scenario.
The modeling is not especially sensitive to either scenario, although all element concentrations are explained better by a
model with variable fO2. The specific effect of Si is important and calculations that include only S and C (and no Si) cannot
reproduce the mantle As, Sb, Ge, and In concentrations. The final core composition in the variable fO2 model is 10.2% Si, 2%
S, and 1.1% C (or XSi = 0.18, XS = 0.03, and XC = 0.04. These results suggest that core formation (involving a Si, S, and Cbearing
metallic liquid) and accretion were the most important processes establishing many of Earth’s mantle volatile elements
(indigenous), while post-core formation addition or re-equilibration (exogenous) was of secondary or minor importance.
elements in differentiated bodies such as Earth, Moon, Mars and Vesta. Metal–silicate partitioning data for many of these
elements is lacking, making their overall mantle concentrations in these bodies difficult to model and origin difficult to distinguish
between core formation and volatile depletion. To address this gap in understanding, we have undertaken systematic
studies of four volatile siderophile elements – Sb, As, Ge and In – at variable temperature and variable Si content of metal.
Several series were carried out at 1 GPa, and between 1500 and 1900 C, for both C saturated and C-free conditions. The
results show that temperature causes a decrease in the metal/silicate partition coefficient for all four elements. In addition,
activity coefficients for each element have been determined and show a very strong dependence on Si content of Fe alloy.
Si dissolved in metal significantly decreases the metal/silicate partition coefficients, at both 1600 and 1800 C. The combination
of temperature and Si content of the metal causes reduction of the metal–silicate partition coefficient to values that are
close to those required for an origin of mantle As, Sb, Ge, and In concentrations by metal–silicate equilibrium processes.
Combining these new results with previous studies on As, Sb, Ge, and In, allowed derivation of predictive expressions for
metal/silicate partition coefficients for these elements 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 As, Sb, Ge, and In can be explained by metal–silicate equilibrium during an accretion scenario.
The modeling is not especially sensitive to either scenario, although all element concentrations are explained better by a
model with variable fO2. The specific effect of Si is important and calculations that include only S and C (and no Si) cannot
reproduce the mantle As, Sb, Ge, and In concentrations. The final core composition in the variable fO2 model is 10.2% Si, 2%
S, and 1.1% C (or XSi = 0.18, XS = 0.03, and XC = 0.04. These results suggest that core formation (involving a Si, S, and Cbearing
metallic liquid) and accretion were the most important processes establishing many of Earth’s mantle volatile elements
(indigenous), while post-core formation addition or re-equilibration (exogenous) was of secondary or minor importance.
Accretion from primordial material and its subsequent differentiation into a planet with core and mantle are fundamental problems in terrestrial and solar system. Many of the questions about the processes, although well developed as model... more
Accretion from primordial material and its subsequent differentiation into a planet with core and mantle are fundamental problems in terrestrial and solar system. Many of the questions about the processes, although well developed as model scenarios over the last few decades, are still open and much debated. In the early Earth, during its formation and differentiation into rocky mantle and iron-rich core, it is likely that silicate melts played an important part in shaping the Earth's main reservoirs as we know them today.
Here, we review several recent results in a deep magma ocean scenario that give tight constraints on the early evolution of our planet. These results include the behaviour of some siderophile elements (Ni and Fe), lithophile elements (Nb and Ta) and one volatile element (Helium) during Earth's core formation. We will also discuss the melting and crystallization of an early magma ocean, and the implications on the general feature of core-mantle separation and the depth of the magma ocean. The incorporation of Fe2 + and Fe3 + in bridgmanite during magma ocean crystallization is also discussed. All the examples presented here highlight the importance of the prevailing conditions during the earliest time of Earth's history in determining the composition and dynamic history of our planet.
Here, we review several recent results in a deep magma ocean scenario that give tight constraints on the early evolution of our planet. These results include the behaviour of some siderophile elements (Ni and Fe), lithophile elements (Nb and Ta) and one volatile element (Helium) during Earth's core formation. We will also discuss the melting and crystallization of an early magma ocean, and the implications on the general feature of core-mantle separation and the depth of the magma ocean. The incorporation of Fe2 + and Fe3 + in bridgmanite during magma ocean crystallization is also discussed. All the examples presented here highlight the importance of the prevailing conditions during the earliest time of Earth's history in determining the composition and dynamic history of our planet.
- by Asmaa Boujibar and +2
- •
- Helium, Perovskite, Magma Ocean, Earth's Core
MgO exsolution has been proposed to drive an early geodynamo. Experimental studies, however, have drawn different conclusions regarding the applicability of MgO exsolution. While many studies suggest that significant Mg can dissolve into... more
MgO exsolution has been proposed to drive an early geodynamo. Experimental studies, however, have drawn different conclusions regarding the applicability of MgO exsolution. While many studies suggest that significant Mg can dissolve into the Earth's core, the amount of MgO exsolved out of the core, which hinges on the temperature dependence of MgO solubility, remains unclear. Here we present new high‐temperature experiments to better constrain the temperature and compositional dependence of Mg partitioning between Fe alloys and silicate liquids. Our experiments show that Mg partitioning is weakly dependent on temperature, while confirming its strong dependence on oxygen content in Fe alloys. This implies that MgO exolution is limited as the core cools but can help drive an early geodyanamo if the core heat loss is slightly subadiabatic. If an exosolution‐driven geodynamo did occur, it was likely over a limited time span that depends on the core thermal history and conductivity.
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... more
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.
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... more
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 (Dmet/sil and Dsulf/sil). Our experimental data combined with those from the literature show that partitioning into sulfide is more efficient than into metal and that partitioning is 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. Dmet/sil for U, Th, and K increases with decreasing oxygen fugacity, while Dmet/silU and Dmet/silK 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 calculated 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, the 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 occurred near IW-5.5 or more reduced conditions, and with a sulfide layer of at least 130 km thickness. At these conditions, the bulk Mercury K/Th ratio is close to Venus’s 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 an 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 to or lower than the Earth’s and Venus’s, suggesting 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%.
Alkali metals Na, K, Rb and Cs are depleted in planetary mantles and their depletion is commonly attributed to the effect of volatility during the condensation of the first solids in the solar nebula or the high temperatures involved... more
Alkali metals Na, K, Rb and Cs are depleted in planetary mantles and their depletion is commonly attributed to the effect of volatility during the condensation of the first solids in the solar nebula or the high temperatures involved during planetary growth. Most models of planetary differentiation assume that alkalis behave entirely as lithophile elements and do not participate in core segregation. Here, we tested this hypothesis by determining experimentally the partitioning of Na, Cs and Rb between iron sulfide and silicate (Dsulf/sil) and combining it with available data from the literature on K, Na and Cs partitioning. Our experiments were conducted at 1–3.5 GPa, with an additional one at 8 GPa, 1600–1900 °C, and varying FeO contents, which lead to a relatively large range of O content in the sulfide phases (up to 13 wt%). We found maximum Dsulf/sil of 0.8, 0.4, and 0.36 for Na, Cs and Rb respectively. In addition, Dsulf/sil for Na, K, Cs and Rb increases with temperature and O content in the sulfide and decreases with FeO content in the silicate. The degree of polymerization of the silicate melt and the S content of the sulfide additionally increase Dsulf/sil for Na, K and Cs. Since the solubility of O in sulfides is correlated with the FeO content of the silicate and both have opposite effects on Dsulf/sil, varying the oxidation state of equilibrating material does not significantly affect Dsulf/sil, which is more controlled by the temperature of equilibration. We modeled core formation for Earth, Mars and asteroid Vesta, assuming that some of the accreted embryos contained immiscible sulfides, that segregated into planetary cores. Our results show that with such a scenario, significant amounts of Na, K, Cs and Rb were sequestered in planetary cores, leading to core/mantle distribution of alkalis between 4·10−5 and 0.15. The depletion of alkalis in the mantles of Earth, Mars and Vesta could have resulted from combined effects of volatility and core segregation, but are largely due to volatile depletion in the accreting materials.
The essential data for interior and thermal evolution models of the Earth and super-Earths are the density and melting of mantle silicate under extreme conditions. Here, we report an unprecedently high melting temperature of MgSiO3 at... more
The essential data for interior and thermal evolution models of the Earth and super-Earths are the density and melting of mantle silicate under extreme conditions. Here, we report an unprecedently high melting temperature of MgSiO3 at 500 GPa by direct shockwave loading of pre-synthesized dense MgSiO3 (bridgmanite) using the Z Pulsed Power Facility. We also present the first high-precision density data of crystalline MgSiO3 to 422 GPa and 7200 K and of silicate melt to 1254 GPa. The experimental density measurements support our density functional theory based molecular dynamics calculations, providing benchmarks for theoretical calculations under extreme conditions. The excellent agreement between experiment and theory provides a reliable reference density profile for super-Earth mantles. Furthermore, the observed upper bound of melting temperature, 9430 K at 500 GPa, provides a critical constraint on the accretion energy required to melt the mantle and the prospect of driving a dyn...
Les abondances en elements siderophiles du manteau terrestre indiquent une segregation du noyau dans un ocean magmatique profond. Il est neanmoins difficile de contraindre les conditions d’oxydation prevalant lors de l’accretion... more
Les abondances en elements siderophiles du manteau terrestre indiquent une segregation du noyau dans un ocean magmatique profond. Il est neanmoins difficile de contraindre les conditions d’oxydation prevalant lors de l’accretion planetaire, en se basant sur les traceurs geochimiques, en raison du nombre important de parametres qui affectent leurs partages entre metal et silicate. D’autre part, l’etat d’oxydation des planetes peut evoluer au cours de l’accretion. Par consequent, la nature des materiaux accretes lors de la formation des planetes reste incertaine. Afin d’apporter de nouveaux elements de reponses a cette problematique, nous avons modelise les equilibres chimiques ayant lieu dans la Terre primitive. Ces equilibres peuvent evoluer (i) en augmentant les conditions de pression et de temperature de la segregation du noyau lors de la croissance de la planete, (ii) en raison de la cristallisation de l’ocean magmatique et (iii) a travers l’accretion de materiaux heterogenes de ...
The Earth is a unique rocky planet with liquid water at the surface and an oxygen-rich atmosphere, consequences of its particular accretion history. The earliest accreting bodies were small and could be either differentiated and... more
The Earth is a unique rocky planet with liquid water at the surface and an oxygen-rich atmosphere, consequences of its particular accretion history. The earliest accreting bodies were small and could be either differentiated and undifferentiated; later larger bodies had formed cores and mantles with distinct properties. In addition, there may have been an overall trend of early reduced and later oxidized material accreting to form the Earth. This paper provides an overview—based on natural materials in our Earthbound sample collections, experimental studies on those samples, and calculations and numerical simulations of differentiation processes—of planetary accretion, core–mantle equilibration, mantle redox processes, and redox variations in Earth, Mars, and other terrestrial bodies.
Introduction: The abundances of volatile elements in the Earth’s mantle are correlated with their temperatures of condensation. This depletion can be due to either incomplete condensation of the elements during the nebula condensation or... more
Introduction: The abundances of volatile elements in the Earth’s mantle are correlated with their temperatures of condensation. This depletion can be due to either incomplete condensation of the elements during the nebula condensation or evaporation processes during planetary growth. Elements that have affinities with metals (siderophile) and sulfides (chalcophile) are additionally depleted due to their segregation into the core. Therefore, study of lithophile elements could be useful to isolate processes of volatilization and their effect on the abundance of the elements in the Earth’s mantle. However, the correlation of these lithophile elements including alkali elements, with their temperatures of condensation shows a significant scatter, which is difficult to reconcile with a depletion by vaporization or incomplete condensation alone. On the other hand, previous studies showed that alkali elements K and Na have affinities with sulfides with elevated O-contents (e.g. [1]). Severa...
- by Asmaa Boujibar and +1
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