Differentiation of eruptive magma in the late Miocene Shirasawa Caldera and present geothermal reservoir

Abstract

In order to evaluate the present geothermal resources in fossil volcanic caldera systems, petrological structures under calderas were investigated and compared with geophysical structures for Shirasawa Caldera (10-8 Ma) and overlying Jogi Caldera (7 Ma), Sendai, NE Japan. The petrological composition, depth, and evolution of the caldera-forming magmas were constrained by the analysis of melt inclusions in quartz crystals from volcanic ejecta of the calderas. The major and trace element compositions of the melt inclusions in the Shirasawa and Jogi Calderas are similar, and classified as low-alkaline tholeiitic dacite-rhyolite, with noticeable variations in Na₂O and K₂O contents among the formations. The SiO₂ and K₂O contents of the melt inclusions suggest the depth of generation of the magma is approximately 10-20 km. The compositions of melt inclusions on the Qtz-Ab-Or system form a fractional trend of plagioclase, with some samples showing fractional trends of both plagioclase and quartz. The trace element compositions of the Shirasawa and Jogi Calderas are depleted in Sr and Eu, suggesting the fractionation of plagioclase. The melt entrapment pressure is estimated on the basis of the pressure dependency of the eutectic composition between Qtz-Ab, and it concentrates on 30-300 MPa, suggesting that the depth of entrapment is 1-11 km. These ranges of entrapment pressure indicate that magma chambers had existed 1-11 km under the calderas. The H₂O contents in the melt inclusions are 4 wt% on average, showing that the most of the melt in the magma chambers was saturated with H₂O.

These results suggest that the caldera-forming magma was generated at a depth of 10-20 km, and magma chambers were formed at 1-11 km due to gravitational equilibria. Subsequent fractionation of plagioclase and quartz led the release of volatiles, which promoted eruptions and formed the Shirasawa Caldera. It is expected that the fluids in the melt were liberated during the ascent and cooling of the magma chambers, and were trapped at depth, which can be detected by the present geophysical observations.

The locations and depth of the ancient magma chambers (1-11 km) coincides with that of the fluid distributions estimated from present seismic observations (i.e., low velocity zones; 0-10 km), suggesting that the remnants of the magma chambers act as fluid reservoir. Seismic clusters at a depth of 8-13 km under the Shirasawa Caldera suggest that the fluids are still active. The depth of 2-5 km — which is the peak of the entrapment depth of the melt inclusions — coincides with seismic reflectors, and abundant fluid is expected at 2-5 km depth under the Shirasawa Caldera.