GD1.2

Basaltic rocks generated by upwelling mantle plumes display a range of trace element and isotope compositions indicative of strong heterogeneity in deep material brought to Earth surface.  Helium isotopes are an unrivalled tracer of the deep mantle in plume-derived basalts.  It is frequently difficult to identify the composition of the deep mantle component as He isotopes rarely correlate with incompatible trace element and radiogenic isotope tracers. It is supposed that this is due to the high He concentration of the deep mantle compared to degassed/enriched mantle reservoirs dominating the He in mixtures, although this is far from widely accepted.  The modern Afar plume is natural laboratory for testing the prevailing paradigm.

The ³He/⁴He of basalt glasses from 26°N to 11°N along the Red Sea spreading axis increases systematically from 7.9 to 15 R_a. Strong along-rift relationships between ³He/⁴He and incompatible trace element ratios are consistent with a binary mixture between moderately enriched shallow asthenospheric mantle in the north and plume mantle evident in basalts from the Gulf of Tadjoura, Djibouti (the Ramad enriched component of Barrat et al. 1990).  The high-³He/⁴He basalts have trace element-isotopic compositions that are similar, but not identical, to the high ³He/⁴He (22 R_a) high Ti (HT2) flood basalts erupted during the initial phase of the Afar plume volcanism (Rogers et al. in press). This suggests that the deep mantle component in the modern Afar plume has a HIMU-like composition. From the hyperbolic ³He/⁴He-K/Th-Rb/La mixing relationships we determine that the upwelling deep mantle has 3-5 times higher He concentration than the asthenosphere mantle beneath the northern Red Sea.

Barrat et al. 1990.  Earth and Planetary Science Letters 101, 233-247.

How to cite: Stuart, F., Balci, U., and Barrat, J.-A.: Defining the composition of the deep mantle and the primordial He inventory of the Afar plume, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10891, https://doi.org/10.5194/egusphere-egu21-10891, 2021.

Quaternary Intraplate volcanoes are sparsely distributed in Northeast Asia including Northeast China and Korean Peninsula and roles of the stagnant Pacific plate in the volcanoes have been studied. Recent geochemical studies suggest that the hydrated mantle in the mantle transition zone was incorporated in the wet plumes that were generated from the hydrated layer atop the stagnant slab, and the ascending wet plumes experienced partial melting in the shallow asthenosphere. To quantitatively evaluate the incorporation of the mantle in the transition zone into the wet plumes and their partial melting in the asthenosphere, we conducted a series of two-dimensional thermochemical numerical models by including the olivine-wadsleyite phase transition at the 410km discontinuity. The buoyancy is controlled by temperature, bound-water content and mineral phase. Viscosity reduction by the bound-water is added to the temperature-dependent viscosity. Particle tracers are used to track the incorporation of the mantle in the transition zone into the wet plumes. We vary the Clapeyron slope of the phase transition and water distributions in the mantle transition zone and hydrated layer of the stagnant slab to evaluate their effects on the behavior of the wet plumes. Results show that multiple wet plumes generated from atop the stagnant slab incorporate the hydrated mantle in the transition zone. Due to the endothermic phase transition at the 410 km discontinuity, the ascending wet plumes are retarded and laterally migrated beneath the 410 km discontinuity for several million years, and enter the overlying asthenosphere as merged large wet plumes. The ascending merged wet plumes laterally spread beneath the thermal lithosphere and experience partial melting, consistent with the interpretation based on the geochemical studies. The spacing of the merged wet plumes (~440 km) caused by the phase transition at the 410 km discontinuity is consistent with the sparse volcano distribution in Northeast China and Korean Peninsula.

How to cite: Kim, H., Lee, Y., Kim, D., and Lee, C.: Behaviors of Wet Plume Controlled by Olivine-Wadsleyite Phase Transition and Water Distribution, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1944, https://doi.org/10.5194/egusphere-egu21-1944, 2021.

The global geological volatile cycle (H, C, N) plays an important role in the long term self-regulation of the Earth system. However, the complex interaction between its deep, solid Earth component (i.e. crust and mantle), Earth’s fluid envelope (i.e. atmosphere and hydrosphere) and plate tectonic processes is a subject of ongoing debate. Here, we want to draw attention to how the presence of primary, pristine melt (MI) and fluid (FI) inclusions in high grade metamorphic minerals could help constrain the crustal component of the volatile cycle. We review the distribution of pristine MI and FI throughout Earth’s history, from the onset of plate tectonics at ca. 3.0 Ga to the present day. Combined with thermodynamic modelling, our compilation indicates that periods of well-established plate tectonics regimes at 0-1.2 Ga and 1.8-2.0 Ga, might be more prone to the reworking of supracrustal lithologies and the storage of volatiles at lower crustal depths. We then argue that the lower crust might constitute an important, although temporary, volatile storage unit, capable to influence the composition of the surface envelopes through the mean of weathering, crustal thickening, partial melting and crustal assimilation during volcanic activity.

Such hypothesis has implication beyond the scope of metamorphic petrology as it potentially links geodynamic mechanisms to habitable surface conditions. MI and FI in metamorphic rocks is a rich but still relatively uncharted realm. In the near future, a concerted research effort should aim to find and characterize new instances of pristine inclusions in periods of the Earth’s history currently underrepresented in the inclusion database, e.g. the Boring Billion. The merging of the messages of thousands of minuscule droplets of fluids trapped in the deepest roots of the continental plates will then eventually provide a truly comprehensive portrait of how the Earth’s evolution proceeds through the geological timescale.

How to cite: Nicoli, G. and Ferrero, S.: Plate tectonics and volatiles: the nanorock connexion, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2066, https://doi.org/10.5194/egusphere-egu21-2066, 2021.

We evaluate the equilibrium concentration of a thermally convecting suspension that is cooled from above and in which
solid crystals are self-consistently generated in the thermal boundary layer near the top. In a previous study (Patočka et
al., 2020), we investigated the settling rate of solid particles suspended in a highly vigorous (Ra = 10⁸ , 10¹⁰, and 10¹² ),
finite Prandtl number (Pr = 10, 50) convection. In this follow-up study we additionally employ the model of crystal
generation and growth of Jarvis and Woods (1994), instead of using particles with a predefined size and density that are
uniformly injected into the carrier fluid.

We perform a series of numerical experiments of particle-laden thermal convection in 2D and 3D Cartesian geometry
using the freely available code CH4 (Calzavarini, 2019). Starting from a purely liquid phase, the solid fraction gradually
grows until an equilibrium is reached in which the generation of the solid phase balances the loss of crystals due to
sedimentation at the bottom of the fluid. For a range of predefined density contrasts of the solid phase with respect to
the density of the fluid (ρ_p /ρ_f = [0, 2]), we measure the time it takes to reach such equilibrium. Both this time and
the equilibrium concentration depend on the average settling rate of the particles and are thus non-trival to compute for
particle types that interact with the large-scale circulation of the fluid (see Patočka et al., 2020).

We apply our results to the cooling of a large volume of magma, spanning from a large magma chamber up to a
global magma ocean. Preliminary results indicate that, as long as particle re-entrainment is not a dominant process, the
separation of crystals from the fluid is an efficient process. Fractional crystallization is thus expected and the suspended
solid fraction is typically small, prohibiting phenomena in which the feedback of crystals on the fluid begins to govern the
physics of the system (e.g. Sparks et al, 1993).

References
Patočka V., Calzavarini E., and Tosi N.(2020). Settling of inertial particles in turbulent Rayleigh-Bénard convection.
Physical Review Fluids, 26(4) 883-889.

Jarvis, R. A. and Woods, A. W.(1994). The nucleation, growth and settling of crystals from a turbulently convecting
fluid. J. Fluid. Mech, 273 83-107.

Sparks, R., Huppert, H., Koyaguchi, T. et al (1993). Origin of modal and rhythmic igneous layering by sedimentation in
a convecting magma chamber. Nature, 361, 246-249.

Calzavarini, E (2019). Eulerian–Lagrangian fluid dynamics platform: The ch4-project. Software Impacts, 1, 100002.

How to cite: Patocka, V., Tosi, N., and Calzavarini, E.: A numerical study of the nucleation, growth and settling of crystals from a turbulent convecting fluid, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14371, https://doi.org/10.5194/egusphere-egu21-14371, 2021.

In the classical model, mid-ocean ridges (MOR) sit above an asthenospheric corner flow that is symmetrical about a vertical plane aligned with the ridge axis. However, geophysical observations of MORs indicate strong asymmetry in melt production and upwelling across the axis (e.g., Melt Seismic Team, 1998, Rychert et al., 2020). In order to reproduce the observed asymmetry, models of plate-driven (passive) flow require unrealistically large forcing, such as rapid asthenospheric cross-axis flow (~30 cm/yr) at high asthenospheric viscosities (~10^21 Pa.s), or temperature anomalies of >100 K beneath the MELT region in the East Pacific Rise (Toomey et al, 2002). 

Buoyancy-driven flows are known to produce symmetry-breaking behaviour in fluid systems. A small contribution from buoyancy-driven (active) flow promotes asymmetry of upwelling and melting beneath MORs (Katz, 2010). Previously, buoyancy has been modelled as a consequence of the retained melt fraction, but depletion of the residue (and heterogeneity) should be involved at a similar level. 

Here, we present new 2-D mid-ocean ridge models that incorporate density variations within the partial-melt zone due to the low density of the liquid relative to the solid (porous buoyancy), and the Fe/Mg partitioning between melt and residue (compositional buoyancy). The model is built after Katz (2010) using a new finite difference staggered grid framework for solving partial differential equations (FD-PDE) for single-/two-phase flow magma dynamics (Pusok et al., 2020). The framework uses PETSc (Balay et al., 2020) and aims to separate the user input from the discretisation of governing equations, thus allowing for extensible development and a robust framework for testing. 

Results show that compositional buoyancy beneath the ridge is negative and can partially balance porous buoyancy. Despite this, models with both chemical and porous buoyancy are susceptible to asymmetric forcing. Asymmetrical upwelling in this context is obtained for forcing that is entirely plausible. A scaling analysis is performed to determine the relative importance of the contribution of compositional and porous buoyancy to upwelling, which is followed by predictions on the crustal thickness production and asymmetry beneath the ridge axis. 

Balay et al. (2020), PETSc Users Manual, ANL-95/11-Revision 3.13.

Katz (2010), G-cubed, 11(Q0AC07), 1-29, https://doi.org/10.1029/2010GC003282

Melt Seismic Team (1998), Science, 280(5367), 1215–1218, https://doi.org/10.1126/science.280.5367.1215 

Pusok et al. (2020), EGU General Assembly 2020, EGU2020-18690 https://doi.org/10.5194/egusphere-egu2020-18690 

Rychert et al. (2020), JGR Solid Earth, 125, e2018JB016463. https://doi. org/10.1029/2018JB016463  

Toomey et al. (2002), EPSL, 200(3-4), 287-295, https://doi.org/10.1016/S0012-821X(02)00655-6

How to cite: Pusok, A. E., Katz, R. F., May, D. A., and Li, Y.: Buoyancy-driven flow beneath mid-ocean ridges: the role of chemical heterogeneity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10306, https://doi.org/10.5194/egusphere-egu21-10306, 2021.

The process of fractional crystallization within a magma body has an influence on the solubility and thus on the associated release of volatiles. Nevertheless, this mechanism is widely neglected in the literature. Due to cooling of an intrusion, nominally anhydrous minerals precipitate from the melt. These minerals mainly incorporate elements that are compatible with their crystal lattice. Since volatiles such as H₂O and CO₂ behave like incompatible elements, they accumulate in the remaining melt. At a certain point, the melt is saturated and the exsolution of the volatiles initiates. The solubility is determined by several parameters like the lithostatic and the partial pressure, the temperature and the melt composition. 
In this study, we investigate the effect of these parameters as well as the impact of fractional crystallization on the solubility and the related volatile release. We focus on the exsolution of H₂O and CO₂ from basaltic magma bodies within the lithosphere. To determine the fate of the accumulating volatiles, we compare the density of the developing liquid phase (volatiles and residual melt) with the density of the host rock. If the host rock has a higher density, the liquid phase will ascent either directly to the surface or to shallower levels of the crust. Furthermore, we take into account the possibility that hydrous minerals (e.g., amphibole) are precipitated during fractional crystallization or due to a reaction with the surrounding rock. 

How to cite: Vulpius, S. and Noack, L.: Effects on the solubility and the volatile release from magmatic intrusions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14267, https://doi.org/10.5194/egusphere-egu21-14267, 2021.

The water exchange between the Earth’s surface and the deep interior is a prime process for the geochemical evolution of our planet and its dynamics. The degassing of water from the mantle takes place through volcanism whereas mantle regassing occurs through the subduction of H₂O chemically bound to hydrous minerals. The (im)balance between degassing and regassing controls the budget of surficial liquid water over geological timescales, i.e, the long-term global sea level. Continental freeboard constraints show that the mean-sea level has remained relatively constant in the last 540 Ma (changes less than about 100 m), thus suggesting a limited imbalance. However, thermopetrological models of water fluxes at present-day subduction zones predict that regassing exceeds degassing by about 50% which, if extrapolated to the past, would have induced a drop inconsistent with the estimations of the long-term sea-level. We have made the case that these inconsistencies arise from thermodynamic predictions for the hydrated lithospheric mantle mineralogy that are poorly constrained at a high pressure (P) and temperature (T). In our study, we thus have revised the global-water flux calculations in subduction zones using petrological constraints on post-antigorite assemblages from recent laboratory experimental data on natural peridotites under high-PT conditions [e.g. Maurice et al, 2018].

We model the thermal state of all present-day mature subduction zones along with petrological modeling using the thermodynamic code Perple_X and the most updated version of the thermodynamic database of Holland and Powell [2011]. For the modeling of peridotite, we build a hybrid phase diagram that combines thermodynamic calculations at moderate PT and experimental data at high PT (> 6 GPa- 600˚C). Our updated thermopetrological model reveals that the hydrated mantle efficiently dehydrates upon the breakdown of the hydrous aluminous-phase E before reaching 250 km in all but the coldest subduction zones. Further subducting slab dehydration is expected between 300-350 km depths, regardless of its thermal state, as a result of lawsonite breakdown in the gabbroic crust. Overall, we predict that present-day global water retention in subducting plates beyond a depth of 350 km barely exceeds the estimations of mantle degassing for average thicknesses of subducting serpentinized mantle subducting at the trenches of up to 6 km. Finally, our models quantitatively support the steady-state sea level scenario over geological times.

Maurice, J., Bolfan-Casanova, N., Padrón-Navarta, J. A., Manthilake, G., Hammouda, T., Hénot, J. M., & Andrault, D. (2018). The stability of hydrous phases beyond antigorite breakdown for a magnetite-bearing natural serpentinite between 6.5 and 11 GPa. Contributions to Mineralogy and Petrology, 173(10), 86.

Holland, T. J. B., & Powell, R. (2011). An improved and extended internally consistent thermodynamic dataset for phases of petrological interest, involving a new equation of state for solids. Journal of Metamorphic Geology, 29(3), 333-383.

How to cite: Cerpa, N., Arcay, D., and Padrón-Navarta, J. A.: Limited subduction of water to mid-upper mantle depths predicted by the phase assemblages in hydrated peridotites with natural chemical composition at high-PT conditions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4015, https://doi.org/10.5194/egusphere-egu21-4015, 2021.

When partial melt occurs in the mantle, redistribution of trace elements between the solid mantle material and partial melt takes place. Partition coefficients play an important role when determining the amount of trace elements that get redistributed into the melt. Due to a lower density compared to surrounding solid rock, partial melt that was generated in the upper mantle will rise towards the surface, leaving the upper mantle depleted in incompatible trace elements and an enriched crust. Studies investigating trace element partitioning in the mantle typically rely on constant partition coefficients throughout the mantle, even though it is known that partition coefficients depend on pressure, temperature, and composition. Between the pressures of 0-15 GPa, partition coefficients vary by two orders of magnitude along both, solidus and liquidus. Since partition coefficients exhibit a parabolic relationship in an Onuma diagram, a similar variation is expected for all trace element partition coefficients that can be derived from the sodium partition coefficients.

In this study, we developed a thermodynamic model for sodium in clinopyroxene after Blundy et al. (1995). With the thermodynamic model results, we were able to deduce a P-T dependent equation for sodium partitioning that is applicable up to 12 GPa between the peridotite solidus and liquidus. Because sodium is an almost strain-free element in jadeite, it can be used as a reference to model partition coefficients for other elements, including heat producing elements like K, Th, and U. This gives us the opportunity to insert P-T dependent partition coefficient calculations of any trace element into mantle melting models, which will have a big impact on the accuracy of elemental redistribution calculations and therefore, if the partitioning of the heat producing elements is taken into account, also the evolution of the mantle and crust.

Blundy, J. et al. (1995): Sodium partitioning between clinopyroxene and silicate melts, J. Geophys. Res., 100, 15501-15515.

Schmidt, J.M. and Noack, L. (2021): Parameterizing a model of clinopyroxene/melt partition coefficients for sodium to higher upper mantle pressures (to be submitted)

How to cite: Schmidt, J. M. and Noack, L.: Modelling clinopyroxene/melt partition coefficients for higher upper mantle pressures, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12478, https://doi.org/10.5194/egusphere-egu21-12478, 2021.

At planetary interior conditions, water ice has been proved to enter a superionic phase recently since it was predicted about 30-year ago. Hydrogen in superionic water become liquid-like, and move freely within solid oxygen lattice. Under extreme pressure and temperature conditions of Earth’s deep mantle, the solid-superionic transition can also occur readily in the pyrite-type FeO₂Hx, a candidate mineral in the lower mantle and probably also in other hydrous minerals. We find that when the pressure increases beyond 73 GPa at room temperature, symmetric hydroxyl bonds are softened and the H⁺ (or proton) become diffusive within the vicinity of its crystallographic site. Increasing temperature under pressure, the diffusivity of hydrogen is extended beyond individual unit cell to cover the entire solid, and the electrical conductivity soars, indicating a transition to the superionic state which is characterized by freely-moving proton and solid FeO₂ lattice. The superionic hydrogen will dramatically change the geophysical picture of electrical conductivity and magnetism, as well as geochemical processes of hydrogen isotopic mixing and redox equilibria at local regions of Earth’s deep interiors.

How to cite: Hu, Q., Hou, M., and He, Y.: Solid to superionic transition in iron oxide-hydroxide, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-817, https://doi.org/10.5194/egusphere-egu21-817, 2021.

CO₂ emissions from geological sources have been recognized as an important input to the global carbon cycle. In regions without active volcanism, mines provide an extraordinary opportunity to observe dynamics of geogenic degassing close to its source.

High temporal resolution of stable carbon isotopes allows to outline temporal and interdependent dynamics of geogenic CO₂ contributions. We present data from an active underground salt mine in central Germany that were collected on site with a field-deployed laser isotope spectrometer.

Throughout the 34-day measurement period, total CO₂ concentrations varied between 805 ppmV (5^th percentile) and 1370 ppmV (95^th percentile). With a 400 ppm atmospheric background concentration, an isotope mixing model enabled the separation of geogenic (16–27 %) from highly dynamic contributions from anthropogenic CO₂-sources (21–54 %). The geogenic fraction was inversely correlated to established CO₂ concentrations that were driven by anthropogenic CO₂ emissions within the mine. This indicates gradient-driven diffusion along microcracks.

Read more about this work in our open access publication in Scientific Reports at: http://rdcu.be/cblTz

How to cite: Frank, A. H., van Geldern, R., Myrttinen, A., Zimmer, M., Barth, J. A. C., and Strauch, B.: Geological CO2 contributions quantified by high-temporal resolution carbon stable isotope monitoring in a salt mine, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13234, https://doi.org/10.5194/egusphere-egu21-13234, 2021.