GD3.1

The internal magnetic field of Mercury is best described by a northward offset dipole with almost zero obliquity. Its offset, weakness, axisymmetry and lack of secular variation still poses a challenge to dynamo theory. After NASA’s Mariner 10 flybys in the 1970’s and MESSENGER’s orbital mission in 2011-2015, BepiColombo performed a flyby at Mercury in October 2021. For the first time, magnetic field measurements are obtained from the southern hemisphere by the fluxgate magnetometer MPO-MAG. We will present an overview of the flyby data and compare the new in-situ data to magnetospheric models obtained from the previous missions to the innermost terrestrial planet. Does the flyby data reveal any secular variation? Has the dipole offset changed? These are some of the questions we will discuss with this unprecedented magnetometer data. We will close with a discussion on what is to be expected from the orbital phase of BepiColombo. 

How to cite: Heyner, D., Carr, C., Auster, U., Richter, I., Kolhey, P., Exner, W., Mieth, J., Plaschke, F., Pump, K., Wicht, J., Langlais, B., Berghofer, G., Schmid, D., Baumjohann, W., Fischer, D., Horbury, T., Magnes, W., Masters, A., Slavin, J., and Glassmeier, K.-H. and the MPO-MAG Team: BepiColombo at Mercury: First close-in magnetic field measurements from the southern hemisphere, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8509, https://doi.org/10.5194/egusphere-egu22-8509, 2022.

Since the discovery of Mercury’s peculiar magnetic field it has raised questions about the underlying dynamo process in its fluid core. The global magnetic field at the surface is rather weak compared to other planetary magnetic fields, strongly aligned to the planet's rotation axis and its magnetic equator is shifted towards north. Especially the latter characteristic is difficult to explain using common dynamo model setups. One promising model suggests that a thermal stably stratified layer right underneath the core-mantle boundary is present. As a consequence the magnetic field deep inside the core is efficiently damped by passing through the stably stratified layer due to the skin effect. Additionally, the non-axisymmetric parts of the magnetic field are vanishing, too, such that a dipole dominated magnetic is left at the planet’s surface. In this study we present new direct numerical simulations of the magnetohydrodynamical dynamo problem which include a stably stratified layer on top of the outer core, which can also reproduce the shift of the magnetic equator towards north. We revisit a model configuration for Mercury’s dynamo action, which successfully reproduced the magnetic field features, in which core convection is driven by thermal buoyancy as well as compositional buoyancy (double-diffusive convection). While we find that this model configuration produces Mercury-like magnetic field only in a limited parameter range (Rayleigh and Ekman number), we show that also a simple codensity model is sufficient over a wide parameter range to produce Mercury-like magnetic fields.

How to cite: Kolhey, P., Heyner, D., Wicht, J., Gastine, T., and Plaschke, F.: Dynamo models reproducing the offset dipole of Mercury’s magnetic field, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8532, https://doi.org/10.5194/egusphere-egu22-8532, 2022.

Seismic anisotropy observations indicate the presence of an innermost and outermost inner core, but the origin of this structure is unknown. Records of the past geomagnetic field provide a means to probe inner core evolution by establishing when growth started. The Ediacaran (~565 million-year-old) geodynamo was near collapse, with a strength 10 times weaker than that of the present-day consistent with model predictions for the field before the onset of inner core nucleation. But the timing of the key transition to stronger intensities typical of the Phanerozoic Eon, needed for establishing an exact onset age, has been unclear. We present single crystal paleointensity results from anorthosites of the early Cambrian (~532 million-year-old) Glen Mountains Layered Mafic Complex (Oklahoma). Data from single plagioclase crystals bearing single domain magnetite and titanomagnetite inclusions yield a time-averaged dipole moment of 3.5 +/- 0.9 x 10²² A m², 5 times greater than that recorded in the Ediacaran Period. This rapid field recovery is the expectation at the start of inner core growth, as new thermal and compositional sources of buoyancy to power the geodynamo become available. We will discuss thermal models, which together with our new paleointensity results, allow us to constrain growth of the inner core and when its structure may have changed.

How to cite: Zhou, T., Tarduno, J., Cottrell, R., and Nimmo, F.: Early Cambrian renewal of the geodynamo and the origin of inner core structure, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10532, https://doi.org/10.5194/egusphere-egu22-10532, 2022.

The geodynamo inside the liquid core is part of the Earth’s rotation. We discovered that electric currents in the heat exchanging liquid core need to follow the handedness of the spiraling liquids given by Coriolis force. Coriolis force splits the buoyant heat exchanging liquid into the two, north and south hemispheres, each with its unique handedness of spiraling convection systems. Convection spiraling model of the core fluid revealed that any planetary dynamo with a liquid conducitng core must have a two-component bimodal structure magnetic contribution, where, for Earth, the southern hemisphere is always associated with a dominating normal polarity component and northern hemisphere with a dominating component of reverse magnetic polarity. We show that the geodynamo would have a non-random distribution of the probability of generation of dynamo’s magnetic polarity, depending on a difference in a degree of buoyancy vigorousness between the two hemispheres.  In this work, the individual treatment of normal and reversed polarity durations revealed that while before 80 Ma geodynamo was generating predominantly normal polarity durations, after the Tertiary transition at ~ 60 Ma, the geodynamo produced predominantly reverse polarity durations. This observation of predominance of magnetic polarity durations is constrained by the existing temperature models near the core/mantle boundary (CMB) and we show a novel connection how a lower mantle temperature distribution may reorganize its convection pattern in the core and change the stability of the dipolar field in favor of a specific polarity.

How to cite: Kletetschka, G.: Chirality of the Geodynamo from the Core’s buoyancy and Sense of Spinning, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9908, https://doi.org/10.5194/egusphere-egu22-9908, 2022.

The Earth’s rotation is not perfectly steady: both its rotation rate (its spin rate) and its orientation in space change in time due to the gravitational pull of the Sun and Moon. The precession-nutation response of the Earth to this external tidal forcing depends strongly on the planet’s deep interior structure. 
In particular, the existence of the Earth’s liquid outer core is known to produce a resonance in the nutation signal at a near-diurnal frequency (as measured in the Earth-bound rotating frame). Physically, this resonance corresponds to the excitation of free mode whereby the liquid core experiences a global rotation of uniform vorticity, hence its name: Free Core Nutation (FCN). 

In parallel, experimental and theoretical studies of fluid dynamics have since long demonstrated that rotating fluids can support oscillatory motions known as inertial waves, which are due to the restoring effect of the Coriolis force. In planetary situations where the fluid domain is bounded by solid boundaries, these oscillations become global, so that they are sometimes referred to as inertial modes. The Spin-Over Mode (SOM), is the simplest of these inertial mode, with uniform vorticity. Because of this and the fact that the SOM, like the FCN, has a near-diurnal frequency, the two modes have often been identified as one and the same. In a former study, we showed that the FCN is in fact a generalization of the SOM to the case of a (non-steadily) freely rotating planet (Rekier et al 2020). 

In the present work, we analyse the relation between the SOM and the FCN in more details by showing how the two modes can, in fact, coexist together in a planet subjected to external gravitational forcing. We also show that the proximity between the frequencies of the SOM and the FCN can have a significant effect on the shape and the intensity of the FCN resonance – represented by the transfer function for nutations – when viscous and/or electromagnetic coupling is introduced at the planet’s Core-Mantle Boundary (CMB). In particular, we estimate that this can cause an increase of ∼1 day in the (retrograde) period of the resonance as measured in the inertial frame. 

We conclude with a discussion on some of the implications of our findings for the nutations of other planetary objects like Mars and the Moon.

Reference:

• Rekier, J., Trinh, A., Triana, S. A., & Dehant, V. (2020). Inertial modes of a freely rotating ellipsoidal planet and their relation to nutations. The Planetary Science Journal, 1(1), 20

How to cite: Rekier, J.: The Spin-Over Mode of freely rotating planets and its relation to their Free Core Nutation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3972, https://doi.org/10.5194/egusphere-egu22-3972, 2022.

Various types of waves exist in the Earth’s core. Waves associated with the magnetic field can leave a signature in the observed geomagnetic field, which may allow one to infer properties of the core. Among those, a balance of magnetic, Coriolis and pressure forces forms a type of waves known as Magneto-Coriolis (MC) waves. Previous studies of MC wave have mostly been focused on the ideal limit (without magnetic diffusion and viscous dissipation) with a columnar ansatz for the flow field. In this study, we investigate this problem by retaining the magnetic diffusion and three-dimensional flows in a full sphere. With several choices of axisymmetric background magnetic field, we analyse various branches of normal modes. The dependence of the normal mode's structure on the background field is clearly seen. A westward propagating branch with perfect columnar flows is found for some background B. We have also found eastward propagating modes constituted by flows with weaker columnarity. With the choice of Elsasser number Λ=1 (Coriolis and magnetic forces of similar magnitude), for axisymmetric background fields we find most of the MC modes have decay rates comparable or larger than their frequencies.

How to cite: Luo, J., Jackson, A., and Marti, P.: Waves in the Earth’s core. 2: Diffusive Magneto-Coriolis waves., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2363, https://doi.org/10.5194/egusphere-egu22-2363, 2022.

An ever-expanding catalogue of satellite data has laid the foundations for new studies of Earth’s secular variation and acceleration. Studies that encode a-priori the axial rigidity conferred to core flows by the Earth’s rapid rotation have revealed novel fast dynamics and improved estimates for the magnetic field strength inside the core. Within this context, a new formalism christened “plesio-geostrophy” (PG) was developed by Jackson and Maffei (Proc. Roy. Soc. A, 476(2243), 2020) with the purpose of describing core dynamics in a regime closer to Earth's conditions. This model makes use of axial integration of the equations of fluid motion and magnetic induction to collapse all three-dimensional quantities into two-dimensional scalars. We report on new results within the PG formalism.

We consider the dynamics of a conducting, inviscid fluid in a full sphere subject to various background magnetic fields. The eigenmodes sustained by the Coriolis and Lorentz forces split into two branches: a fast and a slow one. We characterise these eigenmodes and compare their structure and frequency to fully three-dimensional results. Previous studies are extended by incorporating the effects of horizontal magnetic diffusion.

How to cite: Holdenried-Chernoff, D., Jackson, A., and Maffei, S.: A comparison between the magnetohydrodynamical modes of plesio-geostrophy and fully 3D calculations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6447, https://doi.org/10.5194/egusphere-egu22-6447, 2022.

We bring together two important features of planetary cores: 1) wave propagation in the fluid and 2) topography of the fluid-solid interface. On one hand, inertial waves contribute to the maintenance of quasi geostrophic motions or to the formation of elongated structures in rotating turbulence. On the other hand, topography of the core-mantle boundary has been prososed in various seismological and geodynamical studies and can modify the fluid flow in the core, for example, by altering global fluid modes. Here, we focus on inertial waves excited by topography.

We present results from a combined numerical and experimental investigation of inertial wave motion which is forced by an oscillating topography. To allow comparison with the theory of linear inertial waves, we use a complex topography characterised by a single wavenumber in the spectral domain. Both, the wavenumber and the frequency of the oscillations are varied, allowing us to characterise the transport of kinetic energy at different length scales as well as the interactions of direct and reflected inertial waves. 

How to cite: Burmann, F. and Noir, J.: Inertial waves excited by topography, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8071, https://doi.org/10.5194/egusphere-egu22-8071, 2022.

Generation and reversal of the Earth’s magnetic field have remained one of the most controversial topics.  It is well known that the Earth’s magnetic field is generated by dynamo action in the liquid iron outer core. This mechanism explains how a rotating, convecting, and electrically conducting fluid sustains a magnetic field.

In this study, we investigate the kinematic dynamo action associated with the well-known ABC-flow (see Dombre et al. [1986]). We focus on the “A = B = C = 1. Its dynamo properties have been assessed in 1981 by Arnold et al. [1981]. It belongs to fast dynamo action: a flow which achieves exponential magnetic field amplification over a typical time related to the advective timescale and not the ohmic diffusive timescale (in which case it is referred to as a “slow dynamo”).

We use DNS method for solving the kinematic dynamo problem, for which a solenoidal magnetic field evolution is governed under a prescribed flow by the induction equation.

In this work, we propose a deep learning method to solve the inverse dynamo problem by estimating the velocity field from the magnetic field. We train our deep learning algorithm from the velocity field and the magnetic field values obtained from the above flow model. Once the algorithm parameters are trained, the optimized algorithm is tested for the velocity field estimation from magnetic field. 

How to cite: Mouhali, W., Jun, J.-Y., and Lehner, T.: Velocity field reconstruction by Machine Learning during kinematic dynamo process, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13478, https://doi.org/10.5194/egusphere-egu22-13478, 2022.

We present pymagglobal, a simple to use python interface for global geomagnetic field models. Pymagglobal was developed to provide easy access to global, spherical harmonics based magnetic main field models over historical and paleomagnetic times. The software readily handles cubic-spline based geomagnetic field models stored in the same file format as gufm1 or the CALSxk model series out of the box. Models in other file formats can be incorporated with minimal effort using the python backend. The python interface can, e.g., give model curves for any location, time series of dipole moment or spherical harmonic coefficients or grids and maps of magnetic field components. 

Pymagglobal can be installed by a single command and comes with a command line interface and a GUI, that allows easy extraction and visualization of information from the models. Additionally, the python backend can be used to access the models, for example to generate synthetic data or refer to them in your own analysis. Emphasis is put on documentation and accessibility. The package is available via a git repository  and a custom website at https://git.gfz-potsdam.de/sec23/korte/pymagglobal.

How to cite: Schanner, M. A., Mauerberger, S., and Korte, M.: A python interface for global geomagnetic field models: pymagglobal, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4857, https://doi.org/10.5194/egusphere-egu22-4857, 2022.

Taking advantage of the Swarm three-satellite magnetic field mission by ESA, launched on 22 November 2013 and still orbiting, and ground observatory magnetic data, we determine a spatiotemporal regional model for the geomagnetic field using the R-SCHA technique over the area comprising the South Atlantic Anomaly (SAA). The SAA is the region above the South Atlantic and South America where the geomagnetic field intensity is much lower than expected by a simple dipolar field. Its origin is deep in the outer core and is likely due to a reverse magnetic flux area that has been increasing in the last four centuries. On the basis of this model, we observe 1) the recent evolution of the anomaly from 2014 up to date, with a focus on its “tails” towards South Africa and West Pacific, 2) some features that can be related to important properties of the main geomagnetic field, such as its secular variation and the occurrence of geomagnetic jerks.

How to cite: Campuzano, S. A., De Santis, A., and Pavón-Carrasco, F. J.: Regional geomagnetic field model over the area comprising the South Atlantic Anomaly, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11293, https://doi.org/10.5194/egusphere-egu22-11293, 2022.

The recent Kalmag and archaeomagnetic ArchKalmag14K models together represent the global geomagnetic field model evolution over the past 14000 years and resolve temporal scales of the order of a month over the last 122 years. They are obtained through the sequential assimilation of archeomagnetic and volcanic data, and survey, observatory and satellite data, respectively. Both these models provide full posterior information about the core field, and in the case of Kalmag also about other magnetic sources such as the lithospheric or some tidal fields. These models are made accessible online through different physical and statistical quantities associated with them. In this presentation, we will detail our modeling strategy, the type of results we are getting with it, and how the community can access and use our models by an online interface at https://ionocovar.agnld.uni-potsdam.de/Kalmag/ and https://ionocovar.agnld.uni-potsdam.de/Kalmag/Archeo/.

How to cite: Baerenzung, J., Schanner, M. A., Korte, M., Saynisch, J., and Holschneider, M.: The Kalmag and ArchKalmag14K geomagnetic field models: their derivation principle, properties and availability, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7290, https://doi.org/10.5194/egusphere-egu22-7290, 2022.

The South Atlantic Anomaly (SAA) is a region at Earth’s surface where the intensity of the magnetic field is particularly low. Accurate characterization of the SAA is important for both fundamental understanding of core dynamics and the geodynamo as well as societal issues such as the erosion of instruments at surface observatories and onboard spacecrafts. Here, we propose new measures to better characterize the SAA area and center, accounting for surface intensity changes outside the SAA region and shape anisotropy. Applying our characterization to a geomagnetic field model covering the historical era, we find that the SAA area and center are more time dependent, including episodes of steady area, eastward drift and rapid southward drift. We interpret these special events in terms of the secular vari‑ation of relevant large‑scale geomagnetic flux patches on the core–mantle boundary. Our characterization may be used as a constraint on Earth‑like numerical dynamo models.

How to cite: Amit, H., Terra-Nova, F., Lézin, M., and Trindade, R.: Non-monotonic growth and motion of the South Atlantic Anomaly, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4635, https://doi.org/10.5194/egusphere-egu22-4635, 2022.

Top-down solidification has been suggested in the liquid cores of small planets, moons, and large asteroids. An iron snow is then thought to exist, consisting of the crystallization of free iron crystals at the top of these cores and of their settling in a stably stratified ambient, until they remelt in a hotter, deeper region. This inward crystallization and associated buoyancy flux may sustain dynamo action by convection below the remelting depth. However, thermal evolution models are up-to-now oversimplified, assuming a constant-in-time and homogeneous-in-space buoyancy flux at the bottom of the snow zone. We have shown from analog experiments that the buoyancy flux is heterogeneous in time and space, with intense snow events, corresponding to an explosion of frazil-ice,  separated by quiescent periods where the snow zone supercools. We found that a wide range of crystal sizes exists, with large crystals overshooting the convection region and challenging the thermodynamic equilibrium hypothesis underlying the evolution models. The spatio-temporal variability of the energy source obviously impacts the shape and intensity of the generated magnetic field, which may provide alternative explanations for the observed and surprising features of Mercury's and Ganymede's magnetic fields.

How to cite: Huguet, L. and Le Bars, M.: A laboratory model for iron snow in planetary cores, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3740, https://doi.org/10.5194/egusphere-egu22-3740, 2022.

Given their small sizes and low central pressures, the cores of most asteroids are expected to have started crystallising at the core mantle boundary (CMB) instead of at their centre, as is the case for the Earth. This so-called top-down crystallisation is thermally unstable but compositionally stable, making the conditions for dynamo generation more difficult to achieve. Nevertheless, modern observations of Ganymede show an active magnetic field, where it has been suggested that solidification occurs away from the CMB as an iron snow. This model proposes that iron crystals grow in a snow zone and subsequently sink into the interior and melt, releasing dense fluid that drives convection and a magnetic field. However, whether this process could have occurred in asteroid cores is uncertain due to the significantly smaller adiabatic temperature difference between the CMB and the centre of their cores. This weak temperature gradient may also prevent crystallisation away from the CMB. Therefore, the power for a compositional dynamo may result from an increase in convective velocities caused by the formation of dense crystals at the CMB or turbulence caused by the settling of the crystals themselves.

To investigate these possibilities, we employ analogue tank experiments to explore the possible mechanisms driving convection during inward asteroid core crystallisation. An ammonium chloride solution is cooled from above with a layer of buoyant propanol separating the solution from the cold plate to prevent the growth of crystals on this boundary. Instead, the crystals form below the buoyant layer in a ‘snow zone’. We vary the temperature difference across this buoyant layer to investigate the different regimes that may exist. At each driving temperature difference, we measure the velocity fields of any fluid flow within the ammonium chloride solution using particle imaging velocimetry. This enables us to compare the convective velocities with and without crystallisation as well as develop scaling laws to apply the results of these experiments to models of core thermal evolution.

We find that the mean convective speeds increase by over an order of magnitude when the fluid is crystallising. This increase in speed is driven by an increase in the bulk density of the fluid in the snow zone due to the presence of a small crystal fraction. While the motion of crystals themselves do not induce any turbulence in the fluid due to their small size, they act to locally increase the density of the fluid, causing dense, crystal-rich plumes to emanate from the snow zone, which drive faster convective speeds throughout the fluid. This result provides a new mechanism for dynamo generation in inwardly crystallising cores, especially if remelting of falling iron crystals is delayed until deep within the core’s interior, as has recently been proposed for Mars, or if there is a nucleation barrier that causes significant undercooling before the onset of crystallisation. We also measure the temperature and composition as a function of depth within the tank, from which we may assess whether thermal equilibrium can be assumed when modelling snow zones in cores.

How to cite: Dodds, K., Bryson, J., Neufeld, J., and Harrison, R.: Exploring the dynamics of inward core solidification using analogue tank experiments, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11668, https://doi.org/10.5194/egusphere-egu22-11668, 2022.

Light elements play a key role in the chemical and physical processes of planetary Fe-rich metallic cores [1].  H and Si are believed important candidates in planetary cores and previous estimates indicate as much as 0.6 wt% H and 13 wt% Si in the Earth’s core [2, 3]. However, existing studies are on Fe-H or Fe-Si binary systems and knowledge on Fe-Si-H ternary at high pressure and temperature is still limited [4, 5]. We conducted a series of experiments to understand the impact of hydrogen on Fe-Si alloy system. Fe-Si alloys with three compositions, Fe-9Si (9 wt% Si), Fe-16Si (16 wt% Si), and FeSi (33.3 wt% Si), reacted with H separately up to 125 GPa and 3700 K in diamond-anvil cells by combining pulsed laser heating with high-energy synchrotron X-ray diffraction. Results show little H solubility in B20 and B2 phases of FeSi (0.3 wt% and <0.1 wt% H, respectively) up to 62 GPa, which is significantly smaller than H solubility in Fe metal (1.8 wt% H) [6]. The low H solubility in these phases is likely because of their highly distorted interstitial sites which are not favorable for H incorporation. We found that the low-Si alloys (Fe-9Si and Fe-16Si) convert into FeH_x (fcc or dhcp), FeSi (B20 or B2), and Fe-Si-H ternary phases up to 125 GPa and 3700 K. Particularly, a Fe₅Si₃H_x phase is stable below 43 GPa and the cubic FeH₃ can appear after reactions above 100 GPa. These results indicate that H alters the behavior of the Fe-Si system severely. Considering the various sizes and masses of planets in the solar and exoplanetary systems, the planetary cores can have a wide range of Si contents. If Fe-droplets in early magma ocean contain much Si, Si could limit the amount of H incorporated in the core. On the other hand, for cores with low Si, crystallization at the solid-liquid core boundary may result in formation of separate H-rich and Si-rich crystals in the solid core, potentially inducing heterogeneities in the region [7]. 

References:

1. Shahar, A., et al., What makes a planet habitable? Science, 2019. 364(6439): p. 434-435.

2. Tagawa, S., et al., Experimental evidence for hydrogen incorporation into Earth’s core. Nature Communications, 2021. 12(1): p. 2588.

3. Hirose, K., B. Wood, and L. Vočadlo, Light elements in the Earth’s core. Nature Reviews Earth & Environment, 2021. 2(9): p. 645-658.

4. Terasaki, H., et al., Hydrogenation of FeSi under high pressure. American Mineralogist, 2011. 96(1): p. 93-99.

5. Tagawa, S., et al., Compression of Fe–Si–H alloys to core pressures. Geophysical Research Letters, 2016. 43(8): p. 3686-3692.

6. Pépin, C.M., et al., New iron hydrides under high pressure. Physical review letters, 2014. 113(26): p. 265504.

7. Deuss, A., Heterogeneity and anisotropy of Earth's inner core. Annual Review of Earth Planetary Sciences, 2014. 42: p. 103-126.

How to cite: Fu, S., Chariton, S., Prakapenka, V., Chizmeshya, A., and Shim, S.-H.: Phase Relations in the Fe-Si-H Ternary up to 125 GPa and 3700K: Implications for the Structure and Chemistry of Planetary Cores, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3229, https://doi.org/10.5194/egusphere-egu22-3229, 2022.

Many studies have suggested silicon as a candidate light element for the cores of Earth and Mercury. However, the effect of silicon on the melting temperatures of core materials and thermal profiles of cores is poorly understood, due to disagreements among melt detection techniques, uncertainties in sample pressure evolution during heating, and sparsity of studies investigating the combined effects of nickel and silicon on the phase diagram of iron. In this work (Dobrosavljevic et al. 2022), we develop a multi-technique approach for measuring the high-pressure melting and solid phase relations of iron alloys and apply it to Fe_0.8Ni_0.1Si_0.1 (Fe-11wt%Ni-5.3wt%Si), a composition compatible with recent estimates for the cores of Earth and Mercury.

This approach combines results (20-83 GPa) from two in-situ techniques: synchrotron Mössbauer spectroscopy (SMS) and synchrotron x-ray diffraction (XRD). Melting is independently detected by the loss of the Mössbauer signal, produced exclusively by solid-bound iron nuclei, and the onset of a liquid diffuse x-ray scattering signal. The use of a burst heating and background updating method for quantifying changes in the reference background during heating facilitates the determination of liquid diffuse signal onsets and leads to strong reproducibility and excellent agreement in melting temperatures determined separately by the two techniques. XRD measurements additionally constrain the hcp-fcc phase boundary and in-situ pressure evolution of the samples during heating.

We apply our updated thermal pressure model to published SMS melting data on fcc-Fe and fcc-Fe_0.9Ni_0.1 to precisely evaluate the effect of silicon on melting temperatures. We find that the addition of 10mol% Si to Fe_0.9Ni_0.1 reduces melting temperatures by ~250 K at low pressures (<60 GPa) and flattens the hcp-fcc phase boundary. Extrapolating our results, we constrain the location of the hcp-fcc-liquid quasi-triple point at 147±14 GPa and 3140±90 K, which implies a melting temperature reduction of 500 K compared with Fe_0.9Ni_0.1. The results demonstrate the advantages of combining complementary experimental techniques in investigations of melting under extreme conditions.

Reference:

Dobrosavljevic, V. V., Zhang, D., Sturhahn, W., Zhao, J., Toellner, T. S., Chariton, S., Prakapenka, V. B., Pardo, O. S., Jackson, J. M. (2022). Earth and Planetary Science Letters (in press).

How to cite: Dobrosavljevic, V., Zhang, D., Sturhahn, W., Zhao, J., Toellner, T., Chariton, S., Prakapenka, V., Pardo, O., and Jackson, J.: Melting and phase relations of Fe-Ni-Si determined by a multi-technique approach, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8916, https://doi.org/10.5194/egusphere-egu22-8916, 2022.