Magma transport and metasomatism in the mantle: A critical review of current geochemical models - Discussion

Nielson and Wilshire (1993) promised a review of geochemically based models for magma transport and metasomatism in the mantle. However, they focused on discrediting the chromatographic model of Navon and Stolper (1987). Nielson and Wilshire concluded that the process operating during melt percolation in the mantle "resembles ion-exchange chromatography for H₂O purification, rather than the model of chromatographic species separation proposed by Navon and Stolper (1987)." Our objective here is to show that Nielson and Wilshire (1993) did not offer any new theoretical or modeling advances. We demonstrate that the "water-purification model" (based on the finite-plate model of Helfferich 1962) is controlled by the same physics and mathematics as the frontal chromatography model discussed by Navon and Stolper (1987). Thus, the distinction made by Nielson and Wilshire between the two models has no basis. Moreover, we show that with appropriate boundary conditions, the model of Navon and Stolper (1987) produces an excellent fit to the data that Nielson et al. (1993) fitted using the "water-purification model." We emphasize that the model of Navon and Stolper can be used to model both small- and large-scale processes and that it allows the recovery of temporal information, which cannot be retrieved using the finite-plate model. The usefulness of these models for understanding the geochemistry of mantle rocks is a subject of active research and beyond the scope of this discussion.