The Basalt-Eclogite Phase Transition on Venus: Implications for Crustal Recycling and Partial Melting

Abstract

On Venus, the transition to eclogite, characterised by the disappearance of plagioclase in favour of denser pyroxene and garnet, can play an important role for crustal delamination. This transition occurs when crustal material reaches pressures higher than 1.5 GPa, and since it is commonly associated with a negative buoyancy, it may limit the maximum crustal thickness. On Venus, crustal plateaus, whose composition might be similar to the continental crust on Earth [1] present some of the largest crustal thicknesses on the planet [2,3] and thus might undergo this transition. High surface heat flow estimations in these regions, however, could have led to localised melting before the eclogite transition occurred [3]. Here we use petrological models to calculate the depth of the eclogite transition for a variety of crustal compositions ranging from basalt to granodiorite. We use recent thermal gradient estimations of Venus’s lithosphere [4,5,6] and calculate the mineral phase proportions and associated density profiles (Fig. 1). We focus on the density contrast between the eclogitic material and the underlying mantle to evaluate whether the conditions for crustal delamination are met. Furthermore, we test if melting occurs prior to the eclogite transition. Future measurements of the composition of crustal materials from VERITAS and EnVision combined with our investigation of the depth of the eclogite transition and its associated density will provide essential information to understand crustal delamination and melting on Venus