Role of fractional crystallization in the descent: Basalt ? trachyte

Abstract

The role of fractional crystallization in the descent: basalt → trachyte is critically examined. For each “simple basalt” magma type — alkaline, critically undersaturated, and oversaturated — there is a possible trachyte derived by way of fractional crystallization. Olivine removal is the main physical control that may interfere with trachytic trends at low pressure. Higher pressures widen the field of the initial basic compositions from which alkaline trachytes may fractionate. Bowen's hypothesis of trachyte formation from an undercooled basic magma is not supported by recent experimental data. Inefficiency and lack of scope of the fractional crystallization process, in the late stage of differentiation, may lead to an excess of trachytes over trachyandesites. In general, more time, taken in the basic and the salic stages of the descent, due to steeper liquidus and solidus, would result in a greater abundance of the basic and the salic rock types, compared to the intermediate varieties.