Magma Mixing


Magma mixing was long thought to not be an important process in the evolution of igneous rocks (Bowen, 1956). However, it has recently become a topic of much interest and debate (Eichelberger, 1978; Eichelberger et al., 2001). Currently, the interaction between magmas in open system magma chambers has been thought to be responsible for a range of magmatic textures, mineral textures, and volcanic products. Included in these are schlieren (Weinberg et al., 2001), enclaves (Barbarin and Didier, 1992), and a host of mineral textures (Singer et al., 1995; Robinson and Miller, 1999). These are often interpreted as being the products of incomplete magma mixing, or mingling interactions.

 

Another means of determining the mixing relationships of magma is on the basis of chemistry. Generally, whole rock major and trace element chemistry is an excellent method of determining mixing relationships. Linear trends on Harker diagrams (for instance) for a range of magma compositions suggest a mixing relationship. Other mechanisms of fractionation, for example crystal fractionation, would yield trends other than linear. Isotopic data can also be utilized in a similar fashion.

 

The purpose of this study is to suggest that magma mixing can become much more efficient when a component of shearing, occurring during flow, is taken into account. The shearing of the fluid acts to increase and fold length scales, which acts to increase surface area. Increased surface area increases the efficiency of mixing and chemical and thermal diffusion.

 


 

Schlieren are anomalous concentrations of mafic minerals (biotite for example) and are commonly interpreted as forming by one of four mechanisms: 1) shearing of heterogeneities (enclaves or xenoliths), 2) crystal sorting during convective flow, 3) crystal sorting during magmatic flow, or 4) crystal settling.

 

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Enclaves are inclusions of an intruding magma in a host. The above enclave is more mafic than its host granite; however, this is not always the case. Enclaves are often found throughout their host chambers, suggesting that they are somehow distributed.

 

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All igneous minerals show some variety of zoning. This is due to the interactions between diffusion of needed components and mineral crystallization reactions, which is the supply and demand of the crystallizing mineral. The image above is a zoned alanite crystal, but zonation in plagioclase is a more simple system.

 

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