Geology

""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.""

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.

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.

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 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 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%.

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 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 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.