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Rocky planets go through at least one and likely multiple magma ocean stages, produced by the giant impacts of accretion. Planetary data and models show that giant impacts do not dehydrate either the mantle or the atmosphere of their target planets. The magma ocean liquid consists of melted target material and melted impactor, and so will be dominated by silicate melt, and also contain dissolved volatiles including water, carbon, and sulfur compounds.

As the magma ocean cools and solidifies, water and other volatiles will be incorporated into the nominally anhydrous mantle phases up to their saturation limits, and will otherwise be enriched in the remaining, evolving magma ocean liquids. The water content of the resulting cumulate mantle is therefore the sum of the traces in the mineral grains, and any water in trapped interstitial liquids. That trapped liquid fraction may in fact be by far the largest contributor to the cumulate water budget.

The water and other dissolved volatiles in the evolving liquids may quickly reach the saturation limit of magmas near the surface, where pressure is low, but degassing the magma ocean is likely more difficult than has been assumed in some of our models. To degas into the atmosphere, the gases must exsolve from the liquid and form bubbles, and those bubbles must be able to rise quickly enough to avoid being dragged down by convection and re-dissolved at higher pressures. If bubbles are buoyant enough (that is, large enough) to decouple from flow and rise, then they are also dynamically unstable and liable to be torn into smaller bubbles and re-entrained. This conundrum led to the hypothesis that volatiles do not significantly degas until a high level of supersaturation is reached, and the bubbles form a buoyant layer and rise in diapirs in a continuum dynamics sense. This late degassing would have the twin effects of increasing the water content of the cumulates, and of speeding up cooling and solidification of the planet.

Once the mantle is solidified, the timeclock until the start of plate tectonics begins. Modern plate tectonics is thought to rely on water to lower the viscosity of the asthenosphere, but plate tectonics is also thought to be the process by which water is brought into the mantle. Magma ocean solidification, however, offers two relevant processes. First, following solidification the cumulate mantle is gravitationally unstable and overturns to stability, carrying water-bearing minerals from the upper mantle through the transition zone and into the lower mantle. Upon converting to lower-mantle phases, these minerals will release their excess water, since lower mantle phases have lower saturation limits, thus fluxing the upper mantle with water. Second, the mantle will be near its solidus temperature still, and thus its viscosity will be naturally low. When fluxed with excess water, the upper mantle would be expected to form a low degree melt, which if voluminous enough with rise to help form the earliest crust, and if of very low degree, will further reduce the viscosity of the asthenosphere.

How to cite: Elkins-Tanton, L., Suckale, J., and Tikoo, S.: On the fate of water in the formation of rocky planets, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3315, https://doi.org/10.5194/egusphere-egu21-3315, 2021.

Planetary bodies with a sufficiently energetic origin are likely to begin their evolution in largely liquid state.  Cooling and crystallization at the surface of a mostly liquid magma ocean (MO) is expected to produce a sedimented partially crystallized cumulate of melt and denser mineral grains at its base.  The rate of crystallization and cumulate sedimentation are controlled by radiation through an atmosphere devolatilized from the vigorously convecting MO.  Melt retained in the cumulate is initially isolated from the overlying MO and atmosphere; but through compaction and buoyant migration in permeable cumulates, retained melt may be discharged into the overlying MO and its dissolved volatiles contributed to the growing atmosphere. The rates of cumulate compaction and radiative cooling though the atmosphere may thus play interacting and competing roles governing the time scale of MO evolution.

We explore these effects using a thermal evolution model similar to that described by Elkins-Tanton (2008; doi.org/10.1016/j.epsl.2008.03.062).  In the current study, the top of the cumulate layer is defined by a depositional melt fraction (~50%) and temperature at which a liquid of MO composition behaves like a viscous solid. Heat flux from the MO surface is limited by radiation through a gray H₂O-CO₂ rich atmosphere (Abe and Matsui, 1988; doi.org/10.1175/1520-0469(1988)045<3081:EOAIGH>2.0.CO;2).  We consider Mars and Earth-like bodies with initial bulk H₂O-CO₂ concentrations 0.5%-0.1% and 0.05%-0.01% and vary the prescribed amount of retained melt in the cumulate from 0% (instantaneous compaction) to 50% (no compaction).  For the Mars-sized body increasing retained melt fraction over this range reduces MO freezing time by nearly one order of magnitude (from ~1 Myr to <0.1 Myr) and two orders of magnitude (from ~0.1 Myr to <0.001 Myr) for the larger and smaller volatile concentrations, respectively.  The Earth-like body shows similar behavior.

The melt fraction retained in compacting cumulate deposited at constant, prescribed sedimentation rate is determined by the rate of buoyant melt migration (Shirley 1986; doi.org/10.1086/629088).  For reasonable values of cumulate grainsize (~1 mm; Solomatov and Stevenson, 1993; doi.org/10.1029/92JE02839) and interstitial melt viscosity (~0.1 Pa-s). Cumulates in a Mars-sized, 1000 km deep MO solidifying in 0.1 Myr (cumulate thickening rate ~ 10⁴ km/Myr) should retain melt fractions in the range of 10 to 30%, consistent with values the above thermal model shows are needed to produce this solidification rate.  Nearly an order of magnitude reduction in freezing time due to retained melt can be expected.

Ongoing work integrates the thermal evolution and migration of retained melt into a unified self-consistent model in which the variation of cumulate sedimentation rate with time is determined by the heat flux through the evolving atmosphere.  Our results thus far indicate that volatiles contained in melt retained within cumulates, rather than being added to a growing atmospheric mass, could significantly reduce the time scale of MO solidification.  Exploring this for small planetesimal-sized bodies will be particularly interesting since smaller gravity will reduce the rate of cumulate melt segregation while atmospheric escape may limit the mass of a growing atmosphere.

How to cite: Parmentier, E. M., Elkins-Tanton, L., and Huber, C.: On the competing roles of volatile outgassing and cumulate compaction in the solidification of magma oceans., EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9238, https://doi.org/10.5194/egusphere-egu21-9238, 2021.

Venus is our most Earth-like twin, from a geological standpoint, but lacks Earth-like plate tectonics. Its lower mean density implies a smaller core and relatively large mantle, which combined with the inhibited cooling effected by its high surface temperature, suggests that Venus today may be at an earlier evolutionary stage than Earth. Geologically, a global network of rifts and corona chains (e.g. Parga Chasma) indicate subsurface, plate tectonic-like, spreading ridges below a crustal detachment layer, but there are no obvious corresponding subduction zones. Subduction has been inferred locally at a few large corona (e.g. Artemis) but only in relation to specific plumes, not global plate tectonics. Elsewhere there is evidence for numerous large igneous provinces and perhaps an even larger Overturn Upwelling Zones (OUZO) event at Lada Terra. These features suggest a planet in transition from an Archaean-like regime dominated by instability and overturns, towards a more stable plate tectonic regime: i.e. a planet analogous to the early Proterozoic Earth.

How to cite: Ghail, R.: Is Venus an analogue for Proterozoic Earth?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12082, https://doi.org/10.5194/egusphere-egu21-12082, 2021.

The early evolution of the Earth was likely affected by a large scale magma ocean, in particular in the aftermath of the giant impact that formed the Moon. The exact structure and dynamics of the Earth following that event is unknown but several possible scenarios feature the existence of a basal magma ocean (BMO), whose last remaining drops may explain the current seismically detected ultra low velocity zones. The presence of a BMO covering the core carries many implications for the dynamics and evolution of the overlying solid mantle. The phase equilibrium between the magma and the solid mantle allows matter to flow through the boundary by melting and freezing. In practice, convective stresses in the solid create a topography of the interface which displaces the equilibrium. Heat and solute transfer in the liquid acts to erase this topography and, if this process is faster than that the producing topography, the boundary appears effectively permeable to flow. This leads to convective motions much faster than in usual mantle convection. We developed a mantle convection model coupled to a model for the thermal and compositional evolution of the BMO and the core that takes into account the phase equilibrium at the bottom of the solid mantle. It also includes the fractional crystallisation at the interface and net freezing of the magma ocean. Early in the history, convection in the mantle is very fast and dominated by down-welling currents. As fractional crystallisation proceeds, the magma ocean gets enriched in FeO which makes the cumulate to also get richer. Eventually, it becomes too dense to get entrained by mantle convection and starts to pile up at the bottom of the mantle, which inhibits direct mass flow through the phase change boundary. This allows a thermal boundary layer and hot plumes to develop.

This model therefore allows to explain the present existence of both residual partial melt and large scale compositional variations in the lower mantle, as evidenced by seismic velocity anomalies. It also predicts a regime change between early mantle convection dominated by down-welling flow to the onset of hot plumes in the more recent past.

How to cite: Labrosse, S., Morison, A., Bolrão, D., Rozel, A., Ballmer, M., Deguen, R., Alboussière, T., and Tackley, P.: The long-term evolution of the Earth mantle with a basal magma ocean, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8013, https://doi.org/10.5194/egusphere-egu21-8013, 2021.

The thermochemical structure of lithospheric and asthenospheric mantle exert primary controls on surface topography and volcanic activity. Volcanic rock compositions and mantle seismic velocities provide indirect observations of this structure. Here, we compile and analyze a global database of the distribution and composition of Neogene-Quaternary intraplate volcanic rocks. By integrating this database with seismic tomographic models, we show that intraplate volcanism is concentrated in regions characterized by slow upper mantle shear-wave velocities and by thin lithosphere (i.e. < 100 km). We observe a negative correlation between shear-wave velocities at depths of 125-175 km and melt fractions inferred from volcanic rock compositions. Furthermore, mantle temperature and lithospheric thickness estimates obtained by geochemical modeling broadly agree with values determined from tomographic models that have been converted into temperature. Intraplate volcanism often occurs in regions where uplifted (but undeformed) marine sedimentary rocks are exposed. Regional elevation of these rocks can be generated by a combination of hotter asthenosphere and lithospheric thinning. Therefore, the distribution and composition of intraplate volcanic rocks through geologic time will help to probe past mantle conditions and surface processes.

How to cite: Ball, P., White, N., Maclennan, J., and Stephenson, S.: Using Volcanic Geochemistry and Seismic Tomography to Refine Global Models of Mantle Temperature and Plate Thickness, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3572, https://doi.org/10.5194/egusphere-egu21-3572, 2021.

Numerous Mesoproterozoic mafic dyke swarms are known in Siberia. The main intrusions are concentrated in the northern part of the platform and in Sette-Daban (southeastern part of Siberia), and single intrusions are known on all the outcrops of the crystalline basement in the southern part. The largest dyke swarms are located on the Anabar shield and Sette-Daban (with ~1500 Ma and 1000-950 Ma, respectively [1,2]). In the period 1400-1300 Ma, single intrusions are known: 1382 ± 2 Ma [3] on the Anabar shield, 1385 ± 30 Ma [4] on the Udzha uplift, Listvyanka and Goloustnaya dykes in the south of the platform –  1350 ± 6 Ma [5] and 1338 ± 3 Ma [6], respectively. Also, there is the north-trending dolerite dyke at Sette-Daban, which cuts the Lower Riphean sediments of the Uchur Group. The age of this dike was estimated as 1339 ± 59 Ma employing Sm-Nd isochrone [7]. We report here a new U-Pb age on apatite, Nd isotopy, and geochemistry for this dolerite dyke.

A typical apatite grain used for the U-Pb dating. On the  Tera-Wasserburg diagram, the regression line intercepts in the lower part the concordia line at 1419 ± 15 Ma. The chemical composition of this dyke corresponds to subalkaline basalts (SiO₂ = 45.6, Na₂O+K₂O = 3.9 wt%). The rocks correspond (Mg# = 61) to the calc-alkaline series (FeO*/MgO = 1.1) with a low content of TiО₂ (1.25 wt %). A clear negative Nb-Ta anomaly on the multielement diagram suggests an IAB affinity. Incompatible element ratios such as Th/Yb, Nb/Th, Nb/Yb, Zr/Nb also suggest that these dolerites are close to arc-related basalts in composition. Eps(Nd) calculated to the initial value at 1400 Ma shows a slightly negative value -0.2, which is considered as mantle source with contribution from the enriched source.

Geochemical and Nd isotopy characteristics show the affinity of the Sette-Daban dyke with low-Ti series of the Phanerozoic flood basalt provinces (e.g. Karoo, Siberian traps, etc. [8,9]) with the suggestion that these dolerites were generated from a metasomatized subcontinental lithospheric mantle source. Assuming geochemical characteristics and new U-Pb age of the dolerite we propose flood basalt province in the southeast Siberia in Mezoproterozoic (~1400 Ma).

The research was supported by the Russian Science Foundation grant (19-77-10048).

How to cite: Malyshev, S., Khudoley, A., Ivanov, A., Kamenetsky, V., and Kamenetsky, M.: Implication of Mesoproterozoic (∼1.4 Ga) magmatism within Sette-Daban (Southeast Siberia), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13570, https://doi.org/10.5194/egusphere-egu21-13570, 2021.

Studying the early evolution of terrestrial bodies in our solar systems is challenging. In part, this is because preserved early records are poorly preserved (e.g., Hadean rocks on Earth) and/or hard to access (e.g., rocks on Mars and Venus). Another commonly underappreciated factor is that the testable predictions for the diverse proposed tectonic regimes for early terrestrial bodies are currently underexplored. A better understanding of the consequences of different tectonic regimes can enhance our ability to constrain the early evolution of terrestrial bodies, including the timing of plate tectonic initiation on Earth. In this contribution, we use the example of detrital zircon geochronology to show how first-order predictions for various tectonic modes can be made based on their basic kinematics via relatively simple tools, and how these predictions can provide 1) abundant additional interpretive probabilities for common datasets, and 2) potentially significant implications for the tectonics of early Earth. Using simple Monte Carlo methods with MATLAB codes, we simulated detrital zircon age predictions for basins predicted by heat-pipe tectonics and cold stagnant-lid tectonics based on their relevant numerical models and/or evolutionary diagrams. We show that the first-order predictions for detrital zircon age patterns can be generated by focusing on simulating key mechanisms (e.g., volcanic resurfacing) that control the detrital zircon age characteristics of these two tectonic regimes. Such simulations can be done by simple codes based on a few parameters reflecting basic kinematics of relevant tectonic regimes. We find that differences between new detrital zircon age predictions and those of plate tectonic settings permit better tectonic discrimination via a globally compiled Archean detrital zircon age dataset. The results indicate a transition from heat-pipe tectonics to plate tectonics within the ca. 3.4-3.2 Ga period. Beyond detrital zircon age patterns, we also summarized other possible categories of first-order predictions for non-plate tectonic models, including metamorphic patterns, structural patterns, and crustal thicknesses. Relevant predictions of these categories are variably explored and can potentially be easily modeled or conceptualized via geological tools.

How to cite: Zuo, J., Webb, A., McKenzie, R., Johnson, T., and Kirkland, C.: Geology of non-plate tectonic regimes: detrital zircon age records and beyond, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15374, https://doi.org/10.5194/egusphere-egu21-15374, 2021.

Peraluminous alaskites are a common phenomenon in the migmatitic domes with anatectic cores. They are geochemically unique in terms of the U-Th mineralization and present critical significance in order to better understand the orogenic crustal processes. Western Anatolia was an orogenic welt in the latest Eocene, following the continental collision between Sakarya Continent and Tauride-Anatolide platform along the Izmir-Ankara-Erzincan suture zone. Çataldağ metamorphic core complex (ÇMCC) is located on the immediate north of the Izmir-Ankara-Erzincan suture zone, in Sakarya Continent. ÇMCC consists of Eo-Oligocene peraluminous anatectic leucogranites, corresponding to the partial melts of the young orogenic crust with a thickness of ≥50 km. Some of these leucogranites can be classified as alaskitic granite due to the presence of high Th content, from 12.5 to 113 ppm and relatively high ionizing radiation dose, up to 0.35 μsv/h. These alaskitic granites made up of quartz (30-35%) + plagioclase (25-30%) + K-feldspar (20-22%) + muscovite (5%) + biotite (5-3%) + monazite (≤1%) ± garnet. Th content in the alaskitic granites increases with increasing degrees of partial melting. Th enrichment in Çataldağ alaskitic granites is possibly hosted by monazite with high saturation temperature (≥770°C). Th-rich alaskitic granites in ÇMCC were derived from the partial melting of the Tauride-Anatolide Platform (Pan-African crust) underthrusted beneath the Sakarya Continent.

How to cite: Kamacı, Ö., Ünlüer, A. T., Ünal, A., Doner, Z., Altunkaynak, Ş., and Kumral, M.: The first finding of Th-rich peraluminous alaskitic granite in Western Anatolia, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-9963, https://doi.org/10.5194/egusphere-egu21-9963, 2021.