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The Role of Water in the Eclogite Phase Transition and Crustal Recycling on Venus

Introduction: Basaltic crust on Venus has apparently been vertically accreted, and may be vertically recycled by the phase transformation to eclogite. The solid state reaction to eclogite requires the presence of water to permit the phase change to occur on geologically reasonable time scales. Field evidence from Earth indicates that high concentrations of water may be required to allow the reaction to proceed past the granulite stability field. The quantity of water required to yield a certain reaction rate is unknown. The crust of Venus is probably not devoid of water because dehydration of vertically accreted crust is inefficient. Eclogite likely played a role in Venus' crustal evolution, but may not be relevant to crustal dynamics at present. One principle result of the analysis of data from the recent Magellan mission is that the evolution of the crust of Venus has not followed a uniformitarian history. Rather, the cratering record suggests that Venus has undergone a catastrophic resurfacing approximately 300 500 Myr ago, followed by a low level of activity manifested at the surface [I, 21. An important question at present is whether this catastrophic activity is episodic or monotonic [3,4, 51, and thus, what is the nature and fate of the crust before and after such an event. The need for recycling of accumulated basaltic crust on Venus is uncertain; crust is apparently not recycled by Earth-like Wilson-cycle plate tectonics. Is the thickness of the crust dynamically limited during andor after a volcanic resurfacing event? If basaltic crust has accreted vertically, it may be recycled vertically by delamination caused by the pressure-induced transformation to eclogite. Eclogite is denser than the presumably peridotitic mantle (by about 15%), and so if the phase change occurred, it could result in crustal subsidence or delamination. The thermal gradient of Venus' lithosphere is uncertain. The high surface temperature may be restricted to the near-surface, a result of the greenhouse atmosphere, and a low-temperature crust would help to explain the apparent strength of the crust. However, if crustal production and recycling are not as efficient as on Earth, due to the absence of plate tectonics, incompatible radioactive heat sources would remain at depth and contribute to longer-term internal heating [5]. It is unclear whether Venus' lithospheric temperature gradient is significantly different than Earth's. For a conductive geotherm of about 15 K km-l, which might be expected for a thickened lithosphere, the eclogite phase change will occur at about 50 km depth for a tholeiitic basalt composition [6]. The phase transformation is gradual, through the production of granulite, but may cause sufficient instability to induce gravitational instability of the crust [3]. Gravity data indicate a relatively uniform (except in the tessera highlands) crustal thickness between 20 and 50 km, which could be a product of periodic removal of eclogite from the base [7]. The Role of Water: The transformation of a basaltic mineralogy to an eclogitic one occurs through a series of solid state reactions whose kinetics are dependent on the rate of diffusion of the reacting components. The determination of reaction rate has been approached theoretically [8,9], but quantitative experimental investigation has not followed, primarily because of the complexity of the reaction mechanisms, especially considering the role of varying basalt composition, and because of the slow rate of reaction. Observations of natural terrestrial eclogite settings may provide some insight. Terrestrial eclogite samples yield information about conditions of equilibration through equilibrium mineral compositions, and information about reaction mechanisms can be gleaned if textural features indicating partial reaction can be identified, which is not common. Eclogite-forming reactions in natural rocks equilibrated at temperatures greater than 750 C usually do not show evidence for fluid infiltration [lo]. This may indicate lack of free fluid during reaction, or more likely, expulsion of fluid during the early stages of reaction. Some field evidence suggests that eclogites are formed on Earth only where kinetics are enhanced by fluids andor deformation. Terrestrial granulites in several terrestrial localities are found to persist metastably in the eclogite facies in the absence of fluid in eclogite-bearing shear zones which alternate on a scale of meters with the dry, unsheared granulites [ l l , 12, 131. Granulite production may be kinetically easier than eclogite, even on water-saturated Earth, because hydrous fluids, which facilitate rapid reaction, may be removed from the rock during metamorphism of basalt to granulite. Granulite is denser than basalt, and so may have a similar (but weaker) negative buoyancy effect on the crust as eclogite, but reaction to granulite on reasonable time scales probably also requires some amount of water. The role of deformation may also be important, because styles and depths of deformation will be different for a vertically accreting crust than for the lateral movement of Earth's lithospheric plates. The crucial question now is a familiar one: how much water is contained in Venus' crust? Venus is generally considered dry because the atmosphere is dry. This reasoning is not sufficient to constrain water content in the crust.