Compositional Convection Trumps Silicate Liquid Immisciblity in Layered Intrusions: a Discussion of 'Liquid immiscibility and the evolution of basaltic magma' by Veksler et al., Journal of Petrology 48, 2187-2210

Abstract

I NTRODUCTION The argument byVeksler et al. (2007a) is that silicate liquid immiscibility began in the Skaergaard intrusion near the Magnetiteþ event in the upper LZc. However, the experimental design of their study is flawed by their choice of bulk compositions unlike those of the rocks and liquids of the real intrusion, a lack of mineral^melt equilibrium and a failure of most compositions to achieve unmixing where they supposedly should. Beyond that, they have not shown that megascopic layers of immiscible liquids can grow on any time scale from the emulsions achieved in the laboratory. Because of these shortcomings alone, the conclusions of the study are not supported by the evidence. Of more serious concern, the two-liquid hypothesis cannot account for the presence of cumulus plagioclase in the rocks if a dense, Fe-rich liquid layer of the proposed composition existed, and it cannot compete with the reality of strong compositional convection of light solute rejected from mafic cumulates, the streaming of which would overcome any tendency to generate conjugate liquids on a megascopic scale. These are radical statements, intended as counterhypotheses, potentially falsifiable, but here considered viable and worthy of further clarification. What this discussion is not about is the subliquidus unmixing of Skaergaard liquids, among crystals, in the laboratory and in the intrusion, as will be discussed. The work of Veksler et al. (2007a) is an admirable account of both failed and successful attempts to achieve liquid immiscibility under a variety of experimental conditions including centrifugation. The main problems arise in the choice of excessively quartz-normative and otherwise strange bulk compositions, the unnoticed failure to achieve mineral^melt equilibrium, and the lack of attention to relevant petrological phase equilibria.We may well begin with the last.