Geochemical Consequences of Melt Percolation: The Upper Mantle as a Chromatographic Column

Abstract

As magmas rise toward the surface, they traverse regions of the mantle and crust with which they are not in equilibrium; to the extent that time and the intimacy of their physical contact permit, the melts and country rocks will interact chemically. We have modeled aspects of these chemical interactions in terms of ion-exchange processes similar to those operating in simple chromatographic columns. The implications for trace element systematics are straightforward: the composition of melt emerging from the top of the column evolves from close to that of the incipient melt of the column matrix toward that of the melt introduced into the base of the column. The rate of evolution is faster in the incompatible than the compatible elements and, as a result, the abundance ratios of elements of different compatibilities can vary considerably with time. If diffusion and other dispersive processes in the melt are negligible and if exchange between melt and solid rock is rapid, extreme fractionations may occur, and the change from initial to final concentration for each element can be through an abrupt concentration front. Integration and mixing of the column output in a magma chamber or dispersive processes within the column, in particular the incomplete equilibration between matrix and fluid due to the slow diffusion in the solid phases, may lead to diffuse fronts and smooth trace element abundance patterns in the column output. If the matrix material is not replenished, the chromatographic process is a transient phenomenon. In some geological situations (e.g., under island arcs and oceanic islands), fresh matrix may be fed continuously into the column, leading to the evolution of a steady state. Aspects of the geochemistry of ultramafic rocks, island arc lavas, and comagmatic alkaline and tholeiitic magmas may be explained by the operation of chromatographic columns.

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