Mixing Mechanism of Mafic and Felsic Magmas : Origin of Heterogeneous Magmas

Abstract

Evidence for mixing of felsic and mafic magmas can be recognized from various disequilibrium or heterogeneous features in volcanic rocks. Some pyroclastic ejecta (e.g., those of the 1707 eruption of Fuji Volcano) change from initial felsic ejecta into mafic ejecta with a narrow transitional zone containing banded pumice. These ejecta are the examples showing the lowest degree of mixing. On the other hand, some mixed lava flows have nearly homogeneous groundmass with incompatible phenocryst assemblage (e.g., forsteritic olivine and quartz) or disequilibrium textures in phenocrysts (e.g., dissolved plagioclase). These are the examples showing the highest degree of mixing. There are various types of heterogeneous magmas between these extreme cases. The origin of diversity in heterogeneous magmas is discussed, based on the reviews of recent theoretical and experimental studies on the fluid dynamic aspects of magma mixing. Injection of a dense mafic magma into a felsic magma does not lead to the formation of a homogeneous mixed magma chamber, but results in vertically stratified one. A chemically stratified magma chamber can also be formed from a initially homogeneous magma chamber by liquid fractionation in which differentiated liquids formed along the wall segregate gravitationally. On the other hand, the stratification can turn over during the ascent through a conduit due to the contrasting mobilities of the less viscous mafic magma compared with the viscous felsic magma and mixed magma can be formed. The mode of magma mixing in a conduit greatly depends on whether the conduit is connected to the magma chamber or it is separated from the magma chamber (“a rootless conduit”). Two magmas with viscosity contrast can circulate in a chaotic way and efficiently mix with each other in the rootless conduit. The diversity of heterogeneous magmas may result from the diversity of ascending mechanisms through a conduit. Petrographic features indicate that magma mixing occurred shortly before the eruption. This fact also suggests the significance of magma mixing in a conduit. Previously, based on the contemporaneity of mixing and eruption, it was proposed that the injection of mafic magma into the felsic magma resulting in magma mixing could be a trigger of an eruption. However, the conduit mixing model gives another interpretation for the contemporaneity, that is, magma mixing is trig-gered by magma ascent accompanied with eruption.