Australia: The Land Where Time Began

A biography of the Australian continent 

Open-System Dynamics and Mixing in Magma Mushes

Magma is present dominantly in a crystal-rich or mushy state that is cooling slowly (1-3). Yet, when crystals (4) that are zoned complexly are observed it is found that some had formed in 1-10 years (5-9), as well as crystal fabrics that are time-transgressive (10) which implies that magmas mix and transition rapidly from a crystal mush that is locked to a mobile, eruptible fluid (5,6). In this study Bergantz et al. resolved the crystal-scale granular interactions by simulating the open-system dynamics of a magma mush with a discrete-element numerical model. They found that the existing magma responds as a viscoplastic material when new magma is injected into a reservoir from below: around the edges of the new injection fault-like surfaces form that act as a mixing bowl of magma that can be unlocked and fluidised, which allows complex mixing. Bergantz et al. identified 3 distinct dynamic regimes which depended on the rate of magma injection. The intruded magma spreads by porous media flow through the crystal mush if the injection rate of magma is slow. The entire mixing bowl becomes fluidised at higher velocities. Crystals from the walls are entrained by circulation within the mixing bowl which brings together crystals from different parts of the reservoir that may have experienced different physiochemical environments with the result that there is little melt that is unmixed. Bergantz et al. conclude that when both granular and fluid dynamics are considered simultaneously observations of complex crystal fabrics and zoning that have been observed in many magmatic systems can be explained.

Sources & Further reading

  1. Bergantz, G. W., J. M. Schleicher and A. Burgisser (2015). "Open-system dynamics and mixing in magma mushes." Nature Geosci 8(10): 793-796.


Author: M. H. Monroe
Last Updated 31/10/2015
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