THE DALY-GAP AS A MAGMATIC CATASTROPHE

Igneous rocks very commonly show a strongly bimodal distribution of co mpositions, one mode corresponding to basalt and the other to felsic m agmas(1-3). As fractional crystallization of basaltic parents produces a continuum of compositions, the paucity of rocks of intermediate com position-commonly called the Daly gap-has puzzled petrologists since t he time of Daly. Gravitational or viscous trapping(4,5), large crystal loads restraining convection(6,7), and re-melting of deep volcanic la yers(2) are among the processes that have been offered as physically m eaningful explanations of magmatic gaps. Here we propose an alternativ e interpretation, transposed from chemical reactor control theory(8): at large undercooling, thermal feedback in a continuously fed and diff erentiating magma reservoir promotes the existence of competing thermo chemical steady states. Small variations in magma residence time and c ooling rate induce a large thermal and chemical swing (magmatic bifurc ation or catastrophe), which interrupts the liquid line of descent, le ading to bimodal erupted products.