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

3A2_3C-O12

Room A5

Date/Time: July 23 11:45-12:00

Investigating the state of the reservoir below the Rabaul caldera (Papua New Guinea)
Caroline Bouvet de Maisonneuve1 , Fidel Costa1 , Christian Huber2 , Herman Patia3
1Earth Observatory of Singapore, Nanyang Technological University, Singapore, 2 School of Earth andAtmospheric Sciences, Georgia Tech., USA, 3 Rabaul Volcano Observatory, Papua New Guinea
E-mail: carolinebouvet@ntu.edu.sg

The area of Rabaul (Papua New Guinea) consists of at least seven - possibly nine - nested-calderas that haveformed over the past 200 ky. The last caldera-forming eruption occurred 1400 y BP, and produced about 10
km3 of crystal-poor, two-pyroxene dacite. The most recent major explosive activity in the area was preceded bydecade-long unrest (starting in 1971) until the simultaneous eruption of Vulcan and Tavurvur, two vents on opposite
sides of the caldera (September 19th, 1994). Ten years prior to the eruption, a peak of seismic activity anddeformation occurred, which promoted part of the local population to evacuate spontaneously. The recent history
at Rabaul is therefore a perfect example of the complexity and problems associated with volcanic unrest and activity
in a caldera setting. In addition, it is possible to study the deposits of the eruptions that ensued from the prolonged
period of seismic and deformational activity, and therefore gain insights into the processes responsible for theunrest. The 1994-to-present deposits show clear signs of mixing/mingling between basaltic and dacitic magmas,
in the form of banded pumices, quenched mafic enclaves, and hybrid bulk rock compositions. We estimate that atleast 20-35 wt% basalt has mixed with the resident silicic magma. The time scales of mixing indicated by kinetic
modeling of plagioclase zoning suggest that replenishment events coincide with the main period of unrest (1971to 1985). We use a petrological, geochemical, and numerical modeling approach to investigate the current stateof the main reservoir and its evolution since the last caldera-forming eruption. Our working model is that basaltic
melts ascend to shallow crustal levels before intruding a main silicic reservoir beneath the Rabaul caldera. Storage
depths and temperatures estimated from mineral-melt equilibria and rock densities suggest that basalts ascendfrom ca. 20 km (600 MPa) to ca.7 km (200 MPa) and cool from ca. 1150-1100 C before intruding a dacitic magma
reservoir at ca.950 C. Depending on the state of the reservoir and the volumes of basalt injected, the replenishingmagma may either trigger an eruption or cool and crystallize. Preliminary geochemical modeling suggests that thecaldera-forming dacitic magma could be generated by fractional crystallization of the basaltic magma at shallow
depths (ca.7 km, 200 MPa) and under relatively dry conditions (less than 3 wt% H2O). We use evidence from major
and trace element geochemistry, volatile contents, and the comparison of successive eruptions since 1400 y BP toaddress the question of whether another potentially caldera-forming magma is presently brewing beneath Rabaul.

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