Felsic Magmatism on Venus Generated by Crustal Recycling and Melting

Kurzfassung

The observation of low viscosity lava flows and shield volcanoes on radar images, combined with X-ray fluorescence analyses performed by Soviet landers, strongly suggests that Venus's crust is primarily basaltic. However, near-infrared emissivity data from the Galileo and Venus Express missions indicate that crustal plateaus may be compositionally distinct and contain higher proportions of felsic minerals than the surrounding plains. The hypothesis that these highlands are felsic has led to suggestions of past oceans on Venus, as the generation of such rocks on Earth typically requires significant amounts of water. We use thermodynamic modeling to explore the conditions under which silica-rich melts can form without water, through deep melting of basaltic crust in thickened crustal roots or during crustal recycling in the mantle. Felsic melts containing 60–65 wt.% can form at depths 60 km. In regions of thickened crust, such melts would be produced along a thermal gradient of 10 K/km, whereas for higher gradients (15 K/km), melting occurs at depths where quartz is not yet present (50 km), resulting in silica-poor melts. The presence of small amounts of water allows similar felsic melts (in composition and volume) to form at moderately shallower depths (40 km). Our results indicate that large volumes of felsic melt could form even under anhydrous conditions on Venus, via crustal thickening or recycling mechanisms such as subduction or lithospheric dripping. This supports the idea that part of Venus's highlands could be the product of remelted basaltic crust, regardless of whether surface water existed in the past.