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

4P1_1D-O19

Room B1

Date/Time: July 24 16:00-16:15

Relationships between mafic replenishment and eruption triggering at silicic systems:
Insights from time scales of magma interactions
Caroline Bouvet de Maisonneuve1 , Fidel Costa1 , Christian Huber2
1

Earth Observatory of Singapore, Nanyang Technological University, Singapore, 2 School of Earth andAtmospheric Sciences, Georgia Tech., USA
E-mail: carolinebouvet@ntu.edu.sgIn silicic systems (dacites to rhyolites), evidences of mixing/mingling with a mafic component (basalts tobasaltic-andesites) are frequently observed. The time scales for mixing prior to eruption have been inferredusing the presence of disequilibrium mineral assemblages and applying reaction and diffusion-based models.
Interestingly, these time scales are often on the order of a few hours to months, and the usual interpretation isthat the intrusion of mafic magma triggered the silicic eruption. However, given the complexity of fluid dynamic
interactions between magmas of contrasting physical properties it is unclear whether these time scales accurately
record the arrival of mafic magma in the system, or if there is a delay between replenishment and mixing that is notregistered by the crystals and their reactions.
The efficiency with which two magmas interact and mix depends on several factors, including the mafic injection
rate, the Ra number (i.e. the ability to convect), and the viscosities of both magmas. In the case of large injection
rates, low magma viscosities, and the presence of a large reservoir (i.e. large Ra), silicic and mafic magmas will
mix rapidly. Therefore the time scales recorded by reaction and zoning of mafic minerals will accurately report thetime between mafic injection and eruption. On the other hand, if the injection rates are low and the viscosity of theambient silicic magma is high (i.e. low Ra), the mafic magma can pond at the base of the reservoir rather than mix.
In this case, the zoned mafic minerals record a time scale that can be significantly shorter than that of injection,and thus lead to erroneous interpretation of eruption triggers.
We investigate the likelihood and implications of each scenario by combining theoretical arguments and thermal
modelling with petrological investigations. We consider the case of the Rabaul caldera where above backgroundseismic and deformation signals started in 1971, and peaked between 1983 and 1985, but the onset of eruptive
activity only occurred in 1994.

Evidence for basaltic injection into a dacitic reservoir is preserved in the1994-to-present eruptive products. A textural maturation from fine, acicular to large, blocky mafic crystal clots
implies different relative ages of formation. Modelling the trace element zoning in plagioclase yields two contrastingtime scales of mixing. One event occurred a couple of decades prior to the 2006 eruption and coincides with thepeak of unrest. The other occurred a few days to weeks prior to the 2006 eruption, which could imply that mafic
injections were the trigger for this eruption. We explore the range of mechanical parameters pertinent to the Rabaulsystem to address whether (i) all the basaltic replenishment occurred in 1983-85 and mixing occurred subsequently
as a number of discrete events, or (ii) multiple replenishment events are required to produce the observed sample
textures and characteristics.

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