TWO-STAGE MELTING AND THE GEOCHEMICAL EVOLUTION OF THE MANTLE: A RECIPE FOR MANTLE PLUM-PUDDING

Abstract

We explore a geochemical model for mantle evolution where a sequence of hotspot and ridge upwelling has melted the mantle to make hotspot and mid-ocean ridge basalts and their residues, and plate subduction has re-cycled and stirred all of these differentiation products back into the mantle. After billions of years this process has mixed various `plums' of incompatible-element rich veins within a matrix made from the residues of melting that have been depleted in incompatible elements. We propose that the mantle flows upward and melts in a two-stage process. During the first stage, plume upwelling and melting creates an enriched ocean island basalt by extracting a low degree melt (~1-4%) from the rising mantle mixture. The plums are easier to melt, so proportionally more of the incompatible elements are extracted from these components. After melt extraction, the mixture of leftovers is depleted in composition, even though it still contains ~96-99% of the mass of the original plume upwelling. These depleted leftovers are hot and buoyant so they pond beneath the lithosphere as an asthenosphere layer. When they rise and melt a second time beneath a mid-ocean ridge, a depleted mid-ocean ridge basalt is extracted. The now extremely depleted leftovers, ~85% of the mass of the original plume upwelling, accrete to oceanic lithosphere which eventually subducts to recycle leftovers, eroded continental crust, and basaltic plums back into the mantle. Observed trace element, rare gas, and isotopic contrasts between oceanic island and mid-ocean ridge basalts can be produced by a recipe which assumes that throughout Earth history these two sequential stages of deep plume and shallower ridge melting have both created and reprocessed the plums and residues that make up the present-day mantle. In this recipe the two-stage melting process does not change through time, but the rate of mantle overturn slows over time in proportion to the decrease in radioactive heat production.