The Oceanic Basalt-Trachyte Association: An Explanation of the Daly Gap

Many authors have noted the near absence of oceanic volcanic rocks intermediate in composition between basalt and trachyte and many explanations have been proposed. Examination of the frequency of chemical analyses from oceanic islands clearly demonstrates that island lavas are bimodally distributed with respect to SiO₂, CaO, and Thornton-Tuttle index. Fractionation models of a suite of samples from Reunion Island based on incompatible trace element enrichments demonstrate that neither SiO₂, CaO nor the Thornton-Tuttle index vary directly with fractionation. The transition from rocks that are basalts in terms of these three parameters to those that are trachytes represents a very small percentage of fractionation (~ 15%). The fractionation model predicts that volcanic rocks having intermediate SiO₂ (the Daly gap) should be quite rare.