Could felsic crustal plateaus have formed on a Water-Poor Venus?

Although several lines of evidence suggest that substantial water was lost from the atmosphere of Venus, it remains unclear whether surface oceans ever existed. One possible proxy for past water–basalt interactions, which are most easily accomplished within oceans, is the occurrence of abundant felsic rocks. Venus’s crustal plateaus share some characteristics with Earth’s continental crust and have been suggested to be composed of granitoids or other felsic rocks based on infrared emissivity anomalies. Future missions will continue to investigate this possibility, which is considered critical for assessing the planet’s past habitability.

The growth of Earth’s continental crust is associated with the hydrous alteration of basalts, which can either melt directly to produce felsic melts (e.g., Archean TTGs) or generate modern calc-alkaline granitoids through extensive differentiation of hydrous silicate melts at subduction zones. Anhydrous
igneous processes, on the other hand, are generally thought to produce only minute amounts of felsic material (<10%) and only under very specific conditions, such as extreme fractional crystallization.

We performed thermodynamic calculations showing that, while anhydrous processes are indeed inefficient at producing felsic melts at low pressure, abundant felsic melts (≈25%) can be generated by melting dry metabasalts at sufficiently high pressures (>1.5 GPa; depths >60 km). Such conditions were likely met on Venus, either within the roots of several crustal plateaus or during resurfacing events accompanied by crustal recycling. Although these melts are felsic (dacitic) in composition, their viscosity should be low enough to allow them to rise to the surface, implying that extensive felsic crust on Venus could be compatible with a water-poor planet that never experienced oceans.