Formation of steep-sided domes on Venus via eruption of high crystallinity magmas

Steep-sided domes are distinctive volcanic landforms on Venus inferred to form by the eruption of highly viscous magma. However, mechanisms responsible for the formation of steep-sided domes remain unclear and the subject of debate. Here we use the rhyolite-MELTS algorithm to constrain the range of magma compositions produced by fractional crystallisation and remelting of Venusian crust, based on rock compositions derived from Venera 13 (alkaline basalt) and Venera 14 (sub-alkaline basalt) lander data. We then calculate liquid-only and liquid plus crystal magma viscosities and compare results to physical models which propose critical minimum viscosities required to form Venusian steep-sided domes. Extensive (>85–90 %) fractionation of Venera 13-based compositions results in the highest viscosity liquids in our models of 9.3 × 108 Pa·s. However, fractional crystallisation and crustal remelting alone is unable to produce liquids with viscosities required to account for formation of steep-sided domes. The intrinsic effect of H2O by acting as a network modifier in reducing liquid viscosity is more significant than the indirect increases in liquid viscosity caused by modifying phase relations and magma compositions, such as the increase in SiO2 content. Regardless, even if complete degassing of initially H2O-saturated magmas is invoked, modelled liquid-only viscosities are still insufficient to account for formation of steep-sided domes. Instead, we find that magmas with high crystal contents, typically >60 vol%, are required to produce sufficiently viscous magmas, a value relatively independent of liquid composition and viscosity parameterisation. Rare terrestrial examples of similarly crystal-rich lava flows and volcanic domes require processes such as rapid degassing-induced crystallisation or fault-controlled mechanisms to accumulate such high crystallinity.