Effects of intrusive magmatism on the thermal evolution and present-day state of Venus

ABSTRACTRecent analysis of NASAs Magellan radar data revealed that magmatic activity is ongoing on Venus. The geodynamic regime is, however, poorly constrained. Many scenarios have been proposed to explain Venus evolution being one of them the plutonic-squishy lid regime, which involves the presence of magmatic intrusions. We investigate the effects of magmatic intrusions on mantles thermal evolution and the present-day thermal state of the interior. We vary the percentage of extrusive magmatism and compare our results to available geophysical constraints to select successful models. Our findings support recent observations of a magmatically active planet, with Venus being primarily governed by highly intrusive magmatic processes.INTRODUCTIONA recent study examined synthetic aperture radar data from NASAs Magellan mission and identified a volcanic vent that changed shape in the 8-month interval between two radar images, which was interpreted as a sign of ongoing volcanic activity on Venus 1. Moreover, the large number of volcanoes recently compiled in a global catalog 2 indicate that the planets evolution and present-day state has been dominated by volcanic processes.Venuss geodynamic regime and surface tectonics are, however, poorly constrained and several scenarios including catastrophic resurfacing, episodic plate tectonics, and the plutonic-squishy lid regime were proposed 3. This last one, which has been suggested to be representative for the tectonic regime on early Earth 4, considers that part of the melt that formed in the interior rises to the surface but a significant part remains trapped in the crust and lithosphere as magmatic intrusions.Here, we investigate effects of intrusive magmatism on the evolution and present-day state of Venus. We evaluate our models by comparing our present-day constraints based on studies of the elastic lithosphere thickness 5,6,7 and interior viscosity of Venus 8.METHODSWe use the geodynamic code GAIA in a 2D spherical annulus geometry 9,10. The viscosity is temperature- and depth-dependent, following an Arrhenius law 11. The thermal conductivity and thermal expansivity in our models are also pressure- and temperature-dependent 12. We assume a homogeneous distribution of the heat sources and account for the decay in time of radioactive elements (i.e. 238U, 235U, 232Th, and 40K) and for core cooling.Melting occurs when the temperature of the mantle rocks is higher than the solidus, for which we assume similar values as proposed for the Earths interior 13. We model that the melt can reach the surface (extrusive magmatism), but also that part of this melt can remain trapped in the crust and lithosphere (intrusive magmatism). Fig.1 shows the melt treatment in our models for fully extrusive and fully intrusive cases. We vary the extrusive melt between 0% (fully intrusive) and 100% (fully extrusive) in steps of 20%, and we vary it between 10km and 90km in steps of 20km.RESULTSOur results show that magmatic intrusions affect the temperature, viscosity, and velocity across the mantle. The intrusive cases have more vigorous convection than the fully extrusive one and larger temperature variations in the convecting mantle with clearly distinguishable mantle plumes (Fig.1). Moreover, the intrusive melt depth seems to control the growth of the stagnant lid: the more intrusive magmatism, the thinner the lid is (Fig.3d).On average, Venus models with highly intrusive magmatism are characterized by a cooler interior, produce more melt at shallower depths and colder temperatures, and have higher thermal gradients, and consequently higher surface heat fluxes during their entire evolution (Fig.2).At present day, our models show that depending on the percentage of extrusive melt and the depth of magmatic intrusions, the maximum thermal gradient varies from a few K/km up to almost 40K/km, with higher values obtained for higher percentages of intrusive melt and shallower magmatic intrusions (Fig.3a). Models in which the extrusive magmatism is higher than 60% and the depth of magmatic intrusions lies deeper than 50km cannot explain high local thermal gradients as suggested by studies of elastic lithosphere thickness 5,6,7.In a recent study 8, the presence of a low viscosity layer (LVL) in the shallow Venusian mantle has been suggested to be related to the presence of partial melt. The LVL starts beneath the lithosphere at depths shallower than 200km. This places constraints on the depth of melting that we can use to select successful models. Models that are compatible with partial melting starting at a depth of 200km or less beneath the surface require