Formation of intermediate to felsic crustal plateaus on Venus: Is water required?

Kurzfassung

The observation of low viscosity lava flows and shield volcanoes on radar maps, combined with in-situ X-ray fluorescence analyses performed by three Soviet landers, strongly suggests that Venus' crust is primarily basaltic. Still, some of the most intriguing features of Venus are its crustal plateaus, characterized by heavily deformed terrains, which have long been suggested to bear a superficial resemblance to Earth’s continental crust and mountain ranges. Infra-red emissivity spectra from the Galileo and Venus Express missions tend to support the presence of a larger fraction of felsic minerals in the plateaus compared to the surrounding basaltic plains [1-2]. On Earth, flux melting of the mantle wedge at subduction zones, followed by fractional crystallization or partial melting of hydrous basalts, are believed to be the two primary mechanisms generating the large volumes of intermediate to felsic rocks that make up the continental crust. By contrast, igneous differentiation of water-poor basaltic melts typically yields negligible amounts of felsic melts. The possibility that highland plateaus are dominated by intermediate to felsic rocks will be evaluated by the EnVision and Veritas missions, in the hope of providing evidence for the presence of water oceans and, therefore, habitable conditions in Venus' distant past. In this work, we show, using thermodynamic calculations (Perple_X), that the melting of dry eclogite is another viable mechanism that can produce large volume of intermediate to felsic melts, in the absence of water. An average basaltic crust, of composition identical to the ones analyzed at the Venera 14 and Vega 2 landing sites, would transform into a quartz eclogite at a depth of 50 to 60 km. Partial melting of this material can produce 15-25 % of dacitic melts. The crust of Venus could have reached this depth under crustal plateaus, according to gravity and topography investigations [3]. It has also been suggested that the current young surface of Venus could indicate that abundant basaltic material was recycled to the mantle [4]. Remelting of this material could fuel occasional but large-scale magmatic events and account for the formation of felsic crustal plateaus in the absence of water. Confirmation that crustal plateaus are dominantly felsic by future missions might therefore not necessarily indicate that wetter and more hospitable conditions prevailed on early Venus.