IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

4W_1D-P20

Date/Time: July 24 Poster

The role of bubble ascent in magma mixing
Sebastian Wiesmaier1 , Daniele Morgavi1 , Diego Perugini2 , Cristina De Campos1 , Kai Uwe Hess1 , Yan
Lavallee3 , Donald B Dingwell1
1Ludwig-Maximilians-University Munich, Earth and Environmental Sciences, Germany, 2 University ofPerugia, Department of Earth Sciences, Italy, 3 University of Liverpool, School of EnvironmentalSciences, United Kingdom
E-mail: sebastian.wiesmaier@min.uni-muenchen.deUnderstanding the processes that affect the rate of liquid state homogenization provides fundamental clues on
the otherwise inaccessible subsurface dynamics of magmatic plumbing systems. Compositional heterogeneities
detected in the matrix of magmatic rocks represent the arrested state of a chemical equilibration. Magmatic
homogenization is divided into a) the mechanical interaction of magma batches (mingling) and b) the diffusive
equilibration of compositional gradients, where diffusive equilibration is exponentially enhanced by progressive
mechanical interaction (1). The mechanical interaction between two distinct batches of magma has commonly
been attributed to shear and folding movements between two distinct liquids. A mode of mechanical interaction
scarcely invoked is the advection of mafic material into a felsic one through bubble motion. Yet, experiments withanalogue materials demonstrated that bubble ascent has the potential to enhance the fluid mechanical componentof magma mixing (2).
Here, we present preliminary results from bubble-advection experiments. For the first time, experiments of this kindwere performed using natural materials at magmatic temperatures. Cylinders of Snake River Plain (SRP) material
were prepared for a static layered set-up, with basalt glass placed underneath rhyolite glass. Upon heating, airtrapped in the interstices between glass cylinders and the crucible expanded, forming bubbles in the now molten
basalt. The bubbles rose, thus entraining a portion of basaltic material into the rhyolite.
The plume-like structure that the advected basalt formed within the rhyolite was characterized by microCT andsubsequent high-resolution EMP analyses. The diffusional gradient around the plume tail showed a progressive
diffusional equilibration from top to bottom, consistent with increasing time of interaction towards the bottom endof the plume tail. Furthermore, single protruding filaments at the bottom end of the plume tail indicate that theplumous structure is a composite of many smaller plume tails. Possibly, the first bubble rising created a preferential
pathway for bubble ascent thereafter. The normalised variance, which serves as a proxy of cation diffusion rate
at the interface of rhyolite and basalt, is unsystematic. This is most likely a result of the many small plume tails
combined in the hybridised region. In turn, stretching and folding experiments produce very systematic normalisedvariances. The normalised variance measured in natural magma mixing structures may thus provide characteristic
evidence to distinguish between mixing induced by bubble action or scenarios of stretching and folding.
(1) De Campos et al. (2011) Enhancement of magma mixing efficiency by chaotic dynamics: an experimental
study, Contrib. Mineral. Petrol., 161(6), 863-881.
(2) Thomas et al. (1993) Mixing of stratified liquids by the motion of gas bubbles: application to magma mixing,
Earth Planet. Sci. Lett., 115(1-4), 161-175.

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