Magmatic processes leading to compositional diversity in igneous rocks: Bowen (1928) revisited

Abstract

Here we present a perspective on the evolution of thought on the origin of compositional diversity in igneous rocks, starting with the pioneer Norman Levi Bowen. In pursing this question of diversity, which was first clearly identified by Daly (1914), Bowen established the utility of experimentally determined phase equilibria as an aid to understanding geologic processes. His work ultimately led him to attribute igneous rock diversity to a singular path of fractional crystallization. We summarize the evolution of understanding acquired by petrologists during and after Bowen's time. Experimentalists beyond Bowen were crucial in furthering the understanding of the origin of the diversity of igneous rocks by discovering that more than one fractional crystallization path can occur in nature: at a minimum, differentiation can either be dry (tholeiitic) or hydrous (calc-alkaline). We also reassess the five alternative igneous processes that may give rise to compositional diversity that Bowen considered, but found to be wanting. These are magma mixing, liquid immiscibility, Soret diffusion, compositional gradients in liquids, and contamination of magma by foreign material (assimilation). These processes play important roles in igneous petrogenesis, that is, roles larger than Bowen envisioned, yet fractional crystallization remains fundamentally important.