Chemical dieferentiation of a convecting planetary interior: Consequences for a one plate planet such as Venus

Abstract

Partial melting to generate the crust of a planet creates compostionally buoyant residual mantle. In the absence of mantle flow associated with plate tectonics, this buoyant, refractory layer may collect at the top of the mantle with important implications for the evolution of the interior and surface. In this study models of the thermal and chemical evolution of a planetary interior demonstrate the possible consequences of a chemically buoyant depleted mantle layer. As the depleted layer thickens the melting temperature at the top of the underlying convecting mantle also increases and the degree of partial melting of mantle added to the depleted layer decreases. As less depleted mantle with less positive compositional buoyancy is added, negative thermal buoyancy of the layer eventually exceeds its positive compositional buoyancy. The depleted layer then sinks into and mixes with the convecting interior. The top of the convecting mantle then moves to a shallower depth, larger degrees of melting resume, and a new depleted layer accumulates. This accumulation and instability of the depleted layer occurs repeatedly over a substantial portion of the planet's evolution with a period of 300–500 Myr. On Venus the population of impacts craters is indistinguishable from a random distribution over the surface and give a surface age of about 500 Myr. We speculate that the mechanism described above may explain this episodic global resurfacing of Venus.

Resurfacing on Venus

“…The global resurfacing event calls for a geodynamical explanation (Parmentier and Hess, 1992;Head et al, 1994;Moresi and Solomatov, 1998;Reese et al,1999). The hypothesis of global episodic subduction events as a mechanism for cooling Venus proposed by Turcotte (1993Turcotte ( , 1995Turcotte ( , 1996 and Turcotte et al (1999) provides a simple explanation.…”

Geologic units on Venus: evidence for their global correlation

“…Among these are gravitational instabilities causing mantle overturn Parmentier and Hess, 1992), an oscillatory convective regime of the mantle (ArkaniHamed and Toksoz, 1984;Arkani-Hamed et al, 1993), episodic plate tectonics (Turcotte, 1993), a`catastrophic' convective episode caused by a phase transition in the mantle (Herrick and Parmentier, 1994;Steinbach and Yuen, 1992;Weinstein, 1993), a di erent convective regime (Phillips and Hansen, 1994;Solomatov and Moresi, 1996), thermal evolution during stagnant lid convection (Reese et al, 1999), or something else. This intensive tectonism was accompanied by volcanic activity .…”

Preliminary analysis of an expanded corona database for Venus

“…In the absence of plate recycling, it is hypothesized that a low density, depleted mantle layer can form the lower part of the lithosphere [Parmentier and Hess, 1992]. Smrekar and Stofan [1997] modeled the formation of coronae from an upwelling with and without the presence of a depleted mantle layer.…”