Dyke widths and ascent rates of silicic magmas on Venus

Abstract

The ascent of silicic magmas in dykes and diapirs on Venus is investigated using magma transport models for granitic melts on Earth. For fixed planetary thermal and melt properties, differences in critical minimum dyke widths, and hence magma ascent rates, are controlled by gravitational strength alone. For density contrasts of 200-600 kg/m3 and a solidus temperature of 1023 K, minimum critical dyke widths (wc) on Venus range from c. < 1-1200 m for a transport distance of 20 km. Dyke widths are especially sensitive to small changes in the far-field lithospheric temperature at values close to a critical Stefan number (S∞crit) of 0-83 where dyke magma temperatures are equal to the mean surface temperature. Typical magma ascent rates range from 0-02m/s (ηm = 105 Pa s) to 10 -9m/s (ηm = 1017 Pa s) giving transport times of between 12 days and c. 105 years. Dyke ascent velocities for highly viscous melts are compared with diapiric rise of a hot Stokes body of radius comparable with the pancake dome average (c. 12km), and require dyke widths of the order of 100 times the average width of low viscosity flows to prevent freezing. In both cases, magma flow is characterised by Peclet numbers between 1 and 4, although even at high viscosities (> 1014 Pa s), dyke ascent is still 100 to 1000 times faster than diapiric rise. At a melt viscosity of 1017 Pa s, critical dyke widths are between c. 1% and 5% the diameter of an average width pancake dome on Venus, indicating that even for extreme melt viscosities, domes can easily be fed by dykes. Given the abundance of dome structures and associated surface features related to hyperbasal magmatism, batholithic volumes of silicic rocks may be present on Venus. Intermediate to high silica melts formed by partial melting of the Venusian crust should be compositionally more akin to Na-rich terrestrial adakites and trondhjemites than calc-alkaline dacites or rhyolites.