The 'Daly gap' as a magmatic catastrophe

Abstract

IGNEOUS rocks very commonly show a strongly bimodal distribution of compositions, one mode corresponding to basalt and the other to felsic magmas^1–3. As fractional crystallization of basaltic parents produces a continuum of compositions, the paucity of rocks of intermediate composition—commonly called the Daly gap—has puzzled petrologists since the time of Daly. Gravitational or viscous trapping^4,5, large crystal loads restraining convection^6,7, and re-melting of deep volcanic layers² are among the processes that have been offered as physically meaningful explanations of magmatic gaps. Here we propose an alternative interpretation, transposed from chemical reactor control theory⁸: at large undercooling, thermal feedback in a continuously fed and differentiating magma reservoir promotes the existence of competing thermochemical steady states. Small variations in magma residence time and cooling rate induce a large thermal and chemical swing (magmatic bifurcation or catastrophe), which interrupts the liquid line of descent, leading to bimodal erupted products.