Geomagnetic field variations in the past: an introduction

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Thermodynamic Limits on Magnetodynamos in Rocky Exoplanets

To ascertain whether magnetic dynamos operate in rocky exoplanets more massive or hotter than the Earth, we developed a parametric model of a differentiated rocky planet and its thermal evolution. Our model reproduces the established properties of Earth's interior and magnetic field at the present time. When applied to Venus, assuming that planet lacks plate tectonics and has a dehydrated mantle with an elevated viscosity, the model shows that the dynamo shuts down or never operated. Our model predicts that at a fixed planet mass, dynamo history is sensitive to core size, but not to the initial inventory of long-lived, heat-producing radionuclides. It predicts that rocky planets larger than 2.5 Earth masses will not develop inner cores because the temperature-pressure slope of the iron solidus becomes flatter than that of the core adiabat. Instead, iron "snow" will condense near or at the top of these cores, and the net transfer of latent heat upwards will suppress convection and a dynamo. More massive planets can have anemic dynamos due to core cooling, but only if they have mobile lids (plate tectonics). The lifetime of these dynamos is shorter with increasing planet mass but longer with higher surface temperature. Massive Venus-like planets with stagnant lids and more viscous mantles will lack dynamos altogether. We identify two alternative sources of magnetic fields on rocky planets: eddy currents induced in the hot or molten upper layers of planets on very short period orbits, and dynamos in the ionic conducting layers of "ocean" planets with ~10% mass in an upper mantle of water (ice).Comment: Accepted to The Astrophysical Journa

A magnetic fabric study of the Aigoual–Saint Guiral–Liron granite pluton (French Massif Central) and relationships with its associated dikes.

International audienceIn the southeastern French Massif Central, the Aigoual–Saint Guiral–Liron pluton consists of porphyritic and microgranitic types. The latter is encountered within dikes forming the northern end of the pluton. Both types show prefull crystallization microstructures indicating weak subsolidus deformations. An anisotropy of magnetic susceptibility (AMS) study has been carried out to determine the granite fabric. Biotite, local hornblende, and small grains of magnetite are the main carriers of AMS in both types. Porphyritic granite and dikes display different AMS patterns related to magma dynamics and regional deformation recorded during crystallization. In the porphyritic type, the AMS lineation is mainly consistent with the regional, NW-SE to E-W trending, extensional event coeval with emplacement and crystallization of the pluton indicating an influence of regional tectonics on the linear fabric development. The dome shaped foliation pattern of the Saint Guiral massif likely corresponds to internal deformation-related processes within the pluton. In the microgranite dike swarm, the NNE-SSW trending lineations with shallow plunges record magmatic flow processes within dikes, i.e., magma injection and filling of dikes from a probable source located southwestward. Regional tectonics played a significant role in the pluton geometry and fabric development. For example, the NE-SW trend of the dikes suggests that extensional fractures took place in the same extensional strain field as elsewhere in the pluton. Different fabric development modes were therefore responsible for the contrasted fabric patterns between the microgranite and the porphyritic granite