A Modeling Experiment In The Laboratory Of Magma Mixing Between Basaltic And Silicic Magmas And Its Application To The Tianchi Volcano, Changbai Mountain

When the hot basic magma injects into a subsurface reservoir containing cooler and less dense silicic magma , it begins to cool, crystallise and convect.Typically the intruded magma is dense and ponds below the evolved, viscous magma already in the chamber.As the lower layer crystallises, the magma becomes saturated and volatile bubbles are exsolved. If all the bubbles remain in the lower convecting layer, then the density of this layer may fall below that of the overlying magma and large cale overturn of the two magma bodies may ensue.However, if the bubbles rise out of the basalt, they can form a foam at the interface. Since the foam is bubble-rich, it will be less dense than the overlying liquid and a series of plumes will rise from the foam into the upper layer, mixing small inclusions of the juvenile magma into the more evolved, overlying magma although there is no large-scale overturn of the system.Geological evidence suggests that each of these mixing processes can occur in magma reservoirs. The former may be manifested by the eruption of mixed and co-mingled magmas, while the latter is manifested by the eruption of evolved magma containing inclusions of the newly intruded magma.The calculations have identified that if the basalt is of sufficiently low viscosity, then bubbles can separate from the melt on a time-scale shorter than the cooling and hence bubble production time. Under such conditions, the model predicts that large-scale overturn may be suppressed. However, for faster cooling rates or bubbles with slower rise speed, large-scale overturn can occur. In order to test this model, this work has conducted a series of analogue experiments in the laboratory to test the model. In the experiments, small bubbles were produced by electrolysis as an analogue to bubble production by cooling and crystallisation of magma. The experiment used a glass tank with the size of 500 mm×200 mm×300 mm (deep) as the electrolysis cell, small bubbles of diameter 30~50 micron were produced on a sheet of fine nickel gauze cathode at the base of the tank.A series of experiments were conducted using different viscosity and density contrasts over which the initial bubble size remained approximately constant, with the upper layer being always more viscous.For the higher values of viscosity it was observed that large-scale overturn and mixing of the two layers of fluid occurred. In contrast, for smaller values of viscosity, there was no large-scale overturn, but bubbles accumulated at the interface to form a foamy layer, which periodically shed plumes of bubbles into the upper layerBy the model calculations and analogue experiments, field observations of magma mixing following intrusion of a dense basic magma into a reservoir charged with more evolved cooler magma may be interpreted. The conclusions of the experimental and theoretical analysis, are as follows1. Large-scale overturn is likely with high cooling rates and intrusions of relatively viscous juvenile magma into a chamber of volatile saturated silicic magma;2. If the silicic magma is volatile unsaturated, then the magma is much less compressible and so, prior to any overturn event, the pressurization associated with the expansion of the basalt might be sufficient to trigger an eruption;3. Magma-volatile separation is likely to be very significant in either a low viscosity lower juvenile layer or with a low cooling rate, and this may suppress large-scale magma overturn. Instead, magma mixing now occurs across the interface, through localized bubble-rich plumes.These general conclusions have been established using a simplified model for the evolution of two cooling and crystallising magmas. The impact of bubble separation in suppressing overturn of two viscous liquid layers is shown in laboratory analogue experiments. It is noted that while these experiments do not simulate all aspects of the behavior of magmatic systems and show the behavior more complex than that parameterized in the model, the general principles mentioned above still hold. For example, in experiments with a very viscous upper layer, a foam can form at the interface between the liquid layers due to bubble separation from the lower layer. In magmatic systems, this foam layer may not form but bubbles can be absorbed at the interface by the silicic magma. However, the purpose of this study is to explore conditions under which overturn may occur, and regardless the bubbles form a distinct foam layer, pass into the silicic layer, or are resorbed, the outcome is the same: the lower layer density remains unchanged, and so the bubble separation suppresses overturn.Although it is difficult to ascertain all the key quantitative parameters for field examples of the Tianchi volcano, Changbaishan, there are a number of field observations in which prove magma mixing has occurred and the principles outlined above can be used to do some further studies.