Differentiation of magma composition: Reactivation of mush and melt reaction in a magma chamber

Abstract

Understanding the storage and evolution of magma in the Earth's crust is of great significance for the study of igneous rocks. In recent years, the low melt state is considered to be the normal state in a magma chamber for long periods of time, while the high melt state is relatively short-lived and closely related to magma eruption. Under this condition, the effect of fractional crystallization at a low melt state has been questioned. The separation of minerals is seriously restricted under the condition of low temperature and low melt. It is necessary to recognize the differentiation and evolution of magma. Therefore, based on the whole rock geochemistry, trace element composition of zircons, and thermodynamic simulation of Mesozoic trachytic volcanic rock samples from a typical volcanic basin in the Suolun area, we evaluated the reaction process in the magma chamber. The results showed that the trachytic volcanic rock samples exhibited obvious characteristics of K-feldspar accumulation, which could not be explained by simple fractional crystallization. The results of the thermodynamic simulation indicated that the differentiation of magma in the study area is mainly the result of a crystal-melt reaction in the magma chamber as oppose to the traditional view of separation crystallization. This process may be universal in low temperature magma chambers dominated by low melt. This conclusion provides a new perspective for us to understand the formation of magma, and it can help us understand the formation of deposits which related to magmatic evolution, such as rare metal granite deposits.