Dyke widths and ascent rates of silicic magmas on Venus

The ascent of silicic magmas in dykes and diapirs on Venus is investigated using magma transport models for granitic melts on Earth. For fixed planeta ry thermal and melt properties, differences in critical minimum dyke widths , and hence magma ascent rates, are controlled by gravitational strength al one. For density contrasts of 200-600kg/m(3) and a solidus temperature of 1 023 K, minimum critical dyke widths (av,) 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-infinity crit) of 0.83 where dyke magma temper atures are equal to the mean surface temperature. Typical magma ascent rate s range from 0.02m/s (eta (m) = 10(5) Pa s) to 10(-9) m/s (eta (m) = 10(17) Pa s) giving transport times of between 12 days and c. 105 years. Dyke asc ent velocities for highly viscous melts are compared with diapiric rise of a hot Stokes body of radius comparable with the pancake dome average (c. 12 km), and require dyke widths of the order of 100 times the average width o f low viscosity flows to prevent freezing. In both cases, magma flow is cha racterised by Peclet numbers between 1 and 4, although even at high viscosi ties (> 10(14)Pa s), dyke ascent is still 100 to 1000 times faster than dia piric rise. At a melt viscosity of 10(17)Pa s, critical dyke widths are bet ween c. 1% and 5% the diameter of an average width pancake dome on Venus, i ndicating that even for extreme melt viscosities, domes can easily be fed b y dykes. Given the abundance of dome structures and associated surface feat ures related to hyperbasal magmatism, batholithic volumes of silicic rocks may be present on Venus. Intermediate to high silica melts formed by partia l melting of the Venusian crust should be compositionally more akin to Na-r ich terrestrial adakites and trondhjemites than calc-alkaline dacites or rh yolites.