Thomas Ruedas

Iceland: The current picture of a ridge-centred mantle plume

Currently the North Atlantic ridge is overriding the Iceland plume. Due to several ridge jumps the plume has been virtually ridge-centred since 20–25 Ma giving rise to extensive melting and crust formation. This review gives an overview over the results of the geophysical and, to minor extent, the geochemical research on the general structure of the Icelandic crust and the mantle beneath Iceland. In the first part, results mostly from topography/bathymetry, gravity, seismics/seismology, magnetotellurics, and geodynamical numerical modelling are summarised. They support the main conclusion that the Icelandic crust is up to ca. 40 km thick, whereby the lower crust and the uppermost mantle have an anomalously small density contrast and a gradual transition rather than a well-defined Moho. The interpretation of a good electrical conductor at 10–15 km depth as a molten layer is irreconcilable with a thick crust, so that alternative explanations have to be sought for this still enigmatic feature. In the second part, results from different branches of seismology, geochemistry, and numerical modelling on the Iceland plume arc reviewed and discussed. For the upper mantle, combining seismological models, geodynamical models and crustal thickness data suggests that the plume has a radius of 100–120 km and an excess temperature of 150–200 K, while the structure of the plume head is less well known. The volume flux is likely to be 5–6 km3/a, and numerical modelling indicates that water and its loss upon melting have a substantial impact on melt production and on the dynamics and distribution of segregating melt. Geochemical studies indicate that the plume source is quite heterogeneous and very probably contains material from the lower mantle. An origin of the plume somewhere in the lower mantle is also supported by several seismological findings, but evidence is not unambiguous yet and has still to be improved.

Magnetic Fields in Earth-like Exoplanets and Implications for Habitability around M-dwarfs

We present estimations of dipolar magnetic moments for terrestrial exoplanets using the Olson & Christiansen (EPS Lett 250:561–571, 2006) scaling law and assuming their interior structure is similar to Earth. We find that the dipolar moment of fast rotating planets (where the Coriolis force dominates convection in the core), may amount up to ~80 times the magnetic moment of Earth, M⊕, for at least part of the planets’ lifetime. For slow rotating planets (where the force of inertia dominates), the dipolar magnetic moment only reaches up to ~1.5 M⊕. Applying our calculations to confirmed rocky exoplanets, we find that CoRoT-7b, Kepler-10b and 55 Cnc e can sustain dynamos up to ~18, 15 and 13 M⊕, respectively. Our results also indicate that the magnetic moment of rocky exoplanets not only depends on rotation rate, but also on their formation history, thermal state, age, composition, and the geometry of the field. These results apply to all rocky planets, but have important implications for the particular case of planets in the Habitable Zone of M-dwarfs.

Impact-induced changes in source depth and volume of magmatism on Mercury and their observational signatures

Mercury’s crust is mostly the result of partial melting in the mantle associated with solid-state convection. Large impacts induce additional melting by generating subsurface thermal anomalies. By numerically investigating the geodynamical effects of impacts, here we show that impact-generated thermal anomalies interact with the underlying convection modifying the source depth of melt and inducing volcanism that can significantly postdate the impact depending on the impact time and location with respect to the underlying convection pattern. We can reproduce the volume and time of emplacement of the melt sheets in the interior of Caloris and Rembrandt if at about 3.7–3.8 Ga convection in the mantle of Mercury was weak, an inference corroborated by the dating of the youngest large volcanic provinces. The source depth of the melt sheets is located in the stagnant lid, a volume of the mantle that never participated in convection and may contain pristine mantle material.Mantle partial melting produced the volcanic crust of Mercury. Here, the authors numerically model the formation of post-impact melt sheets and find that mantle convection was weak at around 3.7–3.8 Ga and that the melt sheets of Caloris and Rembrandt may contain partial melting of pristine mantle material.

On the relative importance of thermal and chemical buoyancy in regular and impact-induced melting in a Mars-like planet†

We ran several series of two-dimensional numerical mantle convection simulations representing in idealized form the thermochemical evolution of a Mars-like planet. In order to study the importance of compositional buoyancy of melting mantle, the models were set up in pairs of one including all thermal and compositional contributions to buoyancy and one accounting only for the thermal contributions. In several of the model pairs, single large impacts were introduced as causes of additional strong local anomalies, and their evolution in the framework of the convecting mantle was tracked. The models confirm that the additional buoyancy provided by the depletion of the mantle by regular melting can establish a global stable stratification of the convecting mantle and throttle crust production. Furthermore, the compositional buoyancy is essential in the stabilization and preservation of local compositional anomalies directly beneath the lithosphere and offers a possible explanation for the existence of distinct, long-lived reservoirs in the martian mantle. The detection of such anomalies by geophysical means is probably difficult, however; they are expected to be detected by gravimetry rather than by seismic or heat flow measurements. The results further suggest that the crustal thickness can be locally overestimated by up to ∼20 km if impact-induced density anomalies in the mantle are neglected.

Globally smooth approximations for shock pressure decay in impacts

New forms of empirical formulae that provide an approximate description of the decay of shock pressure with distance in hypervelocity impacts are proposed. These forms, which are intended for use in applications such as large-scale mantle convection models, are continuous and smooth from the point of impact to arbitrarily large distances, thereby avoiding the need to divide the domain into different decay regimes and yielding the maximum pressure in a self-consistent way without resorting to the impedance-match solution. Individual fits for different impact velocities as well as a tentative general fitting formula are given, especially for the case of dunite-on-dunite impacts. The temperature effects resulting from the shock are estimated for different decay models, and the differences between them are found to be substantial in some cases, potentially leading to over- or underestimates of impact heating and melt production in modeling contexts like mantle convection, where such parameterizations are commonly used to represent giant impacts.

Radioactive heat production of six geologically important nuclides

Heat production rates for the geologically important nuclides

Isocrater impacts: Conditions and mantle dynamical responses for different impactor types

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Temperature and melting of a ridge-centred plume with application to Iceland. Part I: Dynamics and crust production

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Temperature and melting of a ridge-centred plume with application to Iceland. Part II: Predictions for electromagnetic and seismic observables

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Pressure- and temperature-dependent thermal expansivity and the effect on mantle convection and surface observables

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