Magmatism on Venus: Upside-down Melting in Gravitational Instabilities and a Possible Analog in the Siberian Large Igneous Province

Introduction: On Earth magmatism occurs on continents in the absence of subduction, often producing volatile-rich magmas such as those in the Leucite Hills, the Sierra Nevada, and Peru’s Altiplano [1]]. The primary hypothesis to explain this volcanism is foundering of the lower lithosphere into the mantle. The surface of Venus displays volcanic features indicating eruption of lavas with a wide range of viscosities, and its apparent one-plate structure is best compared to these continental settings on Earth. Parmentier and Hess [2] suggest that Venus has undergone cyclic catastrophic crustal recycling through gravitational instability. Both Dupeyrat and Sotin [3] and Hoogenboom and Houseman [4] suggest that eclogitization of the lower lithosphere can be a driving force for gravitational instabilities sinking from the lithosphere, the same process that has been invoked on Earth [1, 5, 6]. Instabilities may also be coupled with rising plumes [2-4, 7], which may provide both the source of the eclogitization and heat to promote ductile flow. Upside-down melting: A gravitational instability forms when a perturbation in an internal planetary boundary grows through lateral flow. The material begins to sink into the underlying mantle material as a drip, exactly analogous to but reversed in the sense of growth from an ascending plume head. The unstable material will sink more rapidly than lateral flow in the lower lithosphere can feed it, resulting in an annulus of thinned lithosphere centered on the instability. Thus no dome forms in the lower lithosphere during ductile delamintion. Traditionally magmatism associated with instabilities has been attributed to return flow of the asthenosphere into such a dome, but maintaining a dome in the lithosphere requires unusual rheological conditions not expected in such a setting [8]. This loss of the lower lithosphere is hypothesized to occur in response to a density contrast that may be caused by intruding mantle melts that freeze as eclogites. This mechanism requires no specific structural weakness beyond a dense region in the lithosphere that is gravitationally unstable with respect to the underling mantle and that possesses a rheology conducive to flow. Density contrasts of as little as 1% are fully sufficient to drive gravitational instabilities. Any volatile content in the sinking material may act in petrologically significant ways. The sinking lower lithosphere on Earth may contain 0.1 to 0.2 mass% of water even if only nominally anhydrous minerals are present. The sinking lithospheric material heats conductively as it is surrounded by hot asthenosphere. Depending upon its rate of descent and volatile content, the sinking material may (1) devolatilize (as a descending slab in a subduction zone does), (2) carry volatiles to depth, sinking in some cases faster than slabs and thus carrying volatiles to depth more efficiently, or (3) heat sufficiently quickly to cross its solidus and itself produce magma. Because this melting in instabilities would occur as they sink, we have termed this novel melting mechanism “upside-down melting” [8] (Figure 1).