Internal circulation in a buoyant two-fluid Newtonian sphere: implications for composed magmatic diapirs

Abstract

Igneous plutons frequently show chemical zoning. The most commonly documented zoning is with the lighter, more silicic, rocks in the centre of the body and the denser, more basic, rocks in the external zone (normal zoning). Less commonly, some plutons show reverse zoning so that the more basic rocks occupy the centre. Widespread evidence shows that zoning in many plutons is the result of interaction between basic and silicic melts. This work studies, by means of finite difference numerical models, pluton zoning which is due to internal circulation in diapirs comprising two magmas of different composition. Diapirs are modelled here as buoyant isothermal spheres composed of two Newtonian fluids rising through a Newtonian ambient fluid. Ratios of viscosities and densities of the two fluids were varied and the results demonstrated two different styles of internal circulation in rising spheres. The first style, termed “coupled circulation”, is characterised by continuous overturning of both the fluids in a single cell, evolving through both normal and reverse compositional zoning. The overturns stir the fluids and enhance both magma mingling and mixing. Coupled circulation develops in spheres comprising fluids of similar densities and viscosities. As these properties become increasingly different the internal circulation tends to decouple. “Decoupled circulation”, is characterised by circulation of the fluids in two separate cells. Decoupling stops the overturns between the two magmas so that the diapir preserves a reverse zoning throughout its rise, with the denser fluid occupying the central zone. There is less possibility of magma mingling in diapirs undergoing decoupled circulation. Thus, pairs of magmas of similar properties, such as andesite and rhyolite, are most likely to develop coupled circulation leading to both normal and reverse zoning in diapirs; whereas magmas of very different properties, such as basalt and rhyolite, are most likely to decouple resulting in reverse zonation. The models indicate that reverse zoning would be the most common internal pluton geometry if zoning were controlled by internal circulation alone. Model diapirs which rise along channels of warm, low viscosity wall-rock (hot Stokes' models) or low viscosity shear zones show an increased tendency towards coupled circulation and more intense mechanical stirring of the magmas.