PS6.1

The cold forearc mantle is a universal feature in global subduction zones and attributed to mechanically decoupling by the weak hydrous layer at the sub-forearc slab interface. Understanding the mechanical decoupling by the weak hydrous layer thus provides critical insight into the transition from subduction infancy to mature subduction since subduction initiation. Nevertheless, the formation and evolution of the weak hydrous layer by slab-derived fluids and its role during the transition have not been quantitatively evaluated by previous numerical models as it has been technically challenging to implement the mechanical decoupling at the slab interface without imposing ad hoc weakening mechanism. We here for the first time numerically demonstrate the formation and evolution down-dip growth of the weak hydrous layer without any ad hoc condition using the case of Southwest Japan subduction zone, the only natural laboratory on Earth where both the geological and geophysical features pertained to the transition since subduction initiation at ~17 Ma have been reported. Our model calculations show that mechanical decoupling by the spontaneous down-dip growth of the weak hydrous layer converts hot forearc mantle to cold mantle, explaining the pulsating forearc high-magnesium andesite (HMA) volcanism, scattered monogenetic forearc and arc volcanism, and Quaternary adakite volcanism. Furthermore, the weak hydrous layer providing a pathway for free-water transport toward the tip of the mantle wedge elucidates seismological observations such as large S-wave delay time and nonvolcanic seismic tremors as well as slab/mantle-originating geochemistry in the Southwest Japan forearc mantle.

How to cite: Lee, C. and Kim, Y.: Spontaneous formation and evolution of a weak hydrous layer at a slab interface: a numerical perspective, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-2121, https://doi.org/10.5194/egusphere-egu22-2121, 2022.

We examined the supra-solidus phase relations of the CaCO₃-MgCO₃ system and established trace element partition coefficient between carbonates and carbonate melt by conducting high pressure (6 and 9 GPa) and temperature (1300-1800 ^oC) experiments with a rocking multi-anvil press. It is well known that the major element composition of initial melts derived from low-degree partial melting of the carbonated mantle strongly depends on the melting relations of carbonates (e.g. 1, 2 and reference therein). Understanding the melting relations in the CaCO₃-MgCO₃ system is thus fundamental in assessing low-degree partial melting of the carbonated mantle. We show here to which extent the trace element signature of a pure carbonate melt can be used as a proxy for the trace element signature of mantle-derived CO₂-rich melts such as kimberlites.

Our results support that, in the absence of water, Ca-Mg-carbonates are thermally stable along geothermal gradients typical at subduction zones. Except for compositions close to the endmembers (~Mg_0-0.1Ca_1-0.9CO₃; Ca_0-0.1Mg_1-0.9CO₃), Ca-Mg-carbonates will partially (to completely) melt beneath mid‑ocean ridges and in plume settings. Ca-Mg-carbonates melt incongruently to dolomitic melt and periclase above 1450 ^oC and 9 GPa making the CaCO₃-MgCO₃ a (pseudo-) ternary system as the number of components increases. Further, our results show that the rare earth element signature of a dolomitic melt in equilibrium with magnesite is similar to those of Group I kimberlites, namely that HREE are depleted relative to primitive mantle signatures. This implies that dolomite-magnesite solid solutions might be useful to approximate melting relations and melt compositions of low-degree partial melting of the carbonated mantle.

References

1  Yaxley, Ghosh, Kiseeva, Mallik, Spandler, Thomson, and Walter, CO₂-Rich Melts in Earth, in Deep Carbon: Past to Present, Orcutt, Daniel, and Dasgupta, Editors. 2019, Cambridge University Press: Cambridge. p. 129-162.

2  Dasgupta and Hirschmann, The deep carbon cycle and melting in Earth's interior. Earth and Planetary Science Letters, 2010. 298 (1-2): p. 1-13.

How to cite: Sieber, M. J., Wilke, M., Oelze, M., Appelt, O., Wilke, F. D. H., and Koch-Müller, M.: Melting relations of carbonates and trace element partitioning between carbonates and carbonate liquid in the Earth's upper mantle, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3367, https://doi.org/10.5194/egusphere-egu22-3367, 2022.

Recent theoretical studies have tried to constrain internal structure and composition of Earth, Mercury and Venus using thermal evolution models. In this work, the adiabatic heat flow at the top of the core was estimated using the electronic component of thermal conductivity (k_el), a lower bound for thermal conductivity. Direct measurements of electrical resistivity (ρ) of Fe-10wt%Ni-wt%Si at core conditions can be related to k_el using the Wiedemann-Franz law. Measurements were carried out in a 3000 ton multi-anvil press using a 4-wire method. The integrity of the samples at high pressures and temperatures was confirmed with electron-microprobe analysis of quenched samples at various conditions. Measurements of ρ at melting seem to remain constant at 135 µΩcm and 141 µΩcm on the solid and liquid sides of the melting boundary. The heat flow at the top of Earth’s CMB is greatly influenced by the light element content in the core. Interpolation of the measured thermal conductivity from this study with high pressure data from the literature suggest the addition of 10-16 wt%Ni and 3-10wt%Si in Earth core results in a heat flow of 6.8 TW at the top of the core. In Mercury, the presence of a thermally stratified layer of Fe-S at the top of an Fe-rich core has been suggested, which implies a sub-adiabatic heat flow on the core side of the CMB. The calculated adiabatic heat flux at the top of Mercury’s core suggests a sub-adiabatic from 0.09-0.21 Gyr after formation, which suggest a chemically driven magnetic field after this transition. Also, the heat flow in Mercury’s interior is estimated to increase by 67% from the inner core to outer core. It has been proposed that an Earth-like core structure for Venus is only compatible with the current lack of dynamo if Venus’ core thermal conductivity is 100 Wm⁻¹K⁻¹ or more. The thermal conductivity at Venus’ core conditions is estimated to range from 44-51 Wm⁻¹K⁻¹, in agreement with scenarios of a completely solidified core.

How to cite: berrada, M., Secco, R., and Yong, W.: Heat flow in the cores of Earth, Mercury and Venus from resistivity experiments on Fe-Ni-Si, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3062, https://doi.org/10.5194/egusphere-egu22-3062, 2022.

Sulfide liquids in terrestrial environments are near mono-sulfidic and are FeS-rich with varying amounts of other chalcophile elements. At highly reducing conditions, as on Mercury, elements like Ca, Na and Mg can also form major components of sulfides and coexist with FeS [1,2,3].
Here, we re-examine the FeS-CaS and FeS-MgS binaries at 950 to 1600°C and 1100°C to 1500°C respectively, owing to the limited amount of data on these systems and the uncertainty in the eutectic point of the FeS-CaS binary [4, 5]. We use the determined phase compositions and inferred densities in the systems CaS-Fes and MgS-FeS (± additions of NaS) to assess mechanisms of sulfur accumulation on the surface of Mercury by gravitational separation of sulfides in a portential magma ocean [6].              Experiments were performed with stoichiometric mixes of pure components in graphite capsules sealed in evacuated silica tubes at ~10^-5 bar. Quenched samples were prepared under anhydrous conditions, and phase compositions determined by energy-dispersive spectroscopy. Because quenched Ca-rich sulfide liquid is labile, its composition was estimated by mass balance and image analysis. The eutectic point of the CaS-FeS system was determined by experimentally bracketing various bulk compositions.           
The solubility of FeS in oldhamite is higher than previously reported, reaching 2.5 mol% at 1065 °C. The eutectic is located at 8.5 ± 1 mol % CaS, significantly poorer in CaS than previously suggested [4], at 1070 ± 5 °C. Our data suggest that solid solution phase compositions in the MgS-FeS binary are in accord with those reported in the only other study on this system [7]. However, we find that the liquid phase in equilibrium with MgS (ss) between 1150°C and 1350°C is more FeS-rich than suggested containing <10 mol% MgS up to 1350°C. 
Our data show that Ca dissolves extensively in sulfides under graphite-saturated conditions at low pressures, which may have prevailed during crust formation on Mercury [8]. The produced solid phases of the CaS-FeS binary are sufficiently light to be able to float in a Hermean magma ocean.

[1]          Skinner + Luce (1971) AmMin

[2]          Nittler + Starr et al., (2011) Science

[3]          Barraud + Coressoundiram + Besse (2021) EPSC2021

[4]          Dilner + Kjellqvist + Selleby (2016) J Phase Equilibria Diffus

[5]          Heumann (1942) Arch Eisenhuttenwes

[6]          Malavergne et al. (2014) Earth Planet. Sci. Lett.

[7]          Andreev et al. (2006) Russ. J. Inorg. Chem.

[8]          Vander Kaaden + McCubbin (2015) J. Geophys. Res. Planets

How to cite: Pitsch, S., Sossi, P. A., Schmidt, M. W., and Liebske, C.: Experimental Phase Relations in the CaS-FeS and MgS-FeS Systems and their Bearing on the Evolution of Mercury, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12795, https://doi.org/10.5194/egusphere-egu22-12795, 2022.

The pallasite meteorites are composed of olivine crystals, Fe-Ni metal alloy and Fe-sulphide. Their formation environment was initially proposed to be at core-mantle boundaries of planetesimals (Scott et al., 1977., Geochemica et Cosmochemica Acta., p.349). However, recent studies using paleomagnetic techniques, and examining the metal concentrations across multiple pallasites, argues against the core-mantle boundary hypothesis (Nichols et al., 2021., Journal of Geophysical Research Planets., p.16). Ferrovolcanism models, which invoke Fe-FeS magma injection into mantle lithologies support paleomagnetism results, compositional trends, and olivine growth conditions (Johnson et al., 2020., Nature Astronomy., p.43). Here we present results from the recently found pallasite Sericho to further explore magmatic aspects of the ferrovolcanism hypothesis using optical microscopy together with SEM energy dispersive X-ray spectrometry (EDS) and electron backscatter diffraction (EBSD).

Sericho has a jigsaw-like texture of forsterite crystals in a troilite matrix. Crystallographic preferred orientations (CPO) of the olivine as determined by EBSD indicate a flow alignment, possibly due to the introduction of the Fe-Ni alloy resulting from upwelling within the planetesimal. Identification of a tabular inclusion within one of the olivine crystals suggests that Sericho experienced mild shock events in contrast to previously studied pallasites including Eagle Station. Our CPO results support the ferrovolcanism hypothesis and more work is underway to investigate olivine slip systems to find out type of internal misorientation is recorded within Sericho’s olivines.

How to cite: Hiramatsu, R. and Lee, M.: New insights into the formation of the pallasites from the Sericho meteorite from EBSD. , EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10678, https://doi.org/10.5194/egusphere-egu22-10678, 2022.

An essential factor for the habitability of rocky exoplanets over geologic timescales is climate regulation via the carbonate-silicate cycle. Without such regulation, uninhabitably hot or cold climates could form, even for planets lying within their host star’s habitable zone. While often associated with plate tectonics, recent work has shown that the carbonate-silicate cycle can operate on planets in a stagnant-lid regime of tectonics, as long as volcanism is active. Volcanism drives release of CO₂ to the atmosphere, without which climate could cool into a globally frozen state, and the creation of fresh rock for weathering, without which a CO₂-rich hothouse climate could form. A key factor dictating how long volcanism can last on a rocky planet is the budget of heat producing elements (U, Th, and K) it acquires during formation. While not directly measurable for exoplanets, estimates on the range of heat producing elements (HPEs) can be made from stellar composition observations. We estimate a probability distribution of HPE abundances in rocky exoplanets based on the Hypatia catalog database of stellar U, Th, and K abundances, where Eu is used as a proxy for the difficult to measure U.

We then constrain how long volcanism, and hence habitable climates, can last on rocky exoplanets in a stagnant-lid regime using a simple thermal evolution model where initial HPE abundances in the mantle are randomly drawn from the distributions constructed from the Hypatia catalog. We further explore the influence of planet size and factors such as the initial mantle temperature and mantle reference viscosity in our models. Our models are conservative, meant to estimate the earliest time that volcanism could cease on rocky exoplanets. We find volcanism lasts for ~2 Gyrs, with 95% confidence intervals of 0.6-3.8 Gyrs for an Earth-sized planet, increasing modestly to ~3.5 Gyrs (95% confidence intervals of 1.4-5.8 Gyrs) for a six Earth mass planet. The variation in volcanism lifetime is largely determined by the K abundance of the planet, as K is a potent HPE and highly variable in stars. The likelihood of acquiring high enough abundances of the long half-life HPEs, Th or ²³⁸U, to power long-lived volcanism through these heat sources is low. In most cases even Th and ²³⁸U abundances at the high end of our observationally constrained probability distributions are not sufficient to power volcanism on their own, such that planets will see volcanism cease once K concentrations have decayed. Only with a high reference viscosity can Th or ²³⁸U potentially drive long-lived volcanism, as in this case volcanism can be sustained for a lower total radiogenic heat production rate.  

How to cite: Foley, B. and Unterborn, C.: Compositional constraints on the lifetime of habitable climates on rocky exoplanets, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8661, https://doi.org/10.5194/egusphere-egu22-8661, 2022.

The fundamental drivers of Phanerozoic climate change over geological timescales (10–100s of Ma) are well recognised: degassing from the deep-earth puts carbon into the atmosphere, silicate weathering takes carbon from the atmosphere and traps it in carbonate minerals. A number of variables are purported to control or exert influence on these two mechanisms, such as the motion of tectonic plates varying the amount of degassing, the palaeogeogrpahic distribution of continents and oceans, the colonisation of land by plants and preservation of more weatherable material, such as ophiolites. We present a framework, pySCION, that integrates these drivers into a single analysis, connecting solid earth with climate and biogeochemistry. Further, our framework allows us to isolate individual drivers to determine their importance, and how it changes through time. Our model, with all drivers active, successfully reproduces the key aspects and trends of Phanerozoic temperature, to a much greater extent than previous models. We find that no single driver can explain Phanerozoic temperature with any degree of confidence, and that the most important driver varies for each geological period.

How to cite: Merdith, A., Mills, B., Maffre, P., Goddéris, Y., Donnadieu, Y., and Gernon, T.: Delineating driving mechanisms of Phanerozoic climate: building a habitable Earth, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11544, https://doi.org/10.5194/egusphere-egu22-11544, 2022.

Atmospheres are products of time-integrated mass exchange between the surface of a planet and its interior. On Earth, the most significant of these events occurred when it existed in a magma ocean state, producing its earliest atmosphere. During this stage, both steam- and carbon-rich atmospheres may have been generated in equilibrium with a magma ocean [1, 2]. However, the nature of Earth’s early atmosphere, and those around other rocky planets, remains unclear for lack of constraints on the solubility of major atmophile elements in liquids of appropriate composition.

Here we determine the solubility of water in 14 peridotite liquids synthesised in a laser-heated aerodynamic levitation furnace [2]. We explore oxygen fugacities (fO₂) between -1.5 and +6.4 log units relative to the iron-wüstite buffer at constant temperature (1900±50 °C) and total pressure (1 bar). The resulting fH₂O ranged from nominally 0 to ~0.028 bar and fH₂ from 0 to ~0.065 bar. The total H₂O contents were determined by FTIR spectroscopy of polished thick sections by examining the intensity of the absorption band at 3550 cm^-1 and applying the Beer-Lambert law.

We find that the mole fraction of dissolved water in the liquid is proportional to (fH₂O)^0.5, attesting to its dissolution as OH^-. The solubility coefficient fit to the data yields a value of ~500 ppm/bar^0.5, roughly 30 % lower than that determined for basaltic liquids at 1350 °C and 1 bar [3]. Therefore, more Mg-rich compositions and/or higher temperatures result in a significant decrease of water solubility in silicate melts. While the solubility of water remains high relative to that of CO₂, we hypothesise that steam atmospheres may form under oxidising conditions, provided sufficiently high temperatures and H/C ratios in terrestrial planets prevail.

[1] Gaillard, F. et al. (2022), Earth Planet. Sci. Lett., 577, 117255. [2] Sossi, P.A. et al. (2020), Science Adv., 6, eabd1387. [3] Newcombe, M.E. et al., (2017), Geochim. Cosmochim. Acta, 200, 330-352.

How to cite: Sossi, P., Tollan, P., Badro, J., and Bower, D.: Solubility of water in peridotite liquids and the formation of steam atmospheres on rocky planets, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11313, https://doi.org/10.5194/egusphere-egu22-11313, 2022.