SMP44-01

Room:102B

Time:May 25 09:15-09:30

Characteristics of silica solubility in the geothermal fields presented by the deep drilling
dataSAISHU, Hanae1∗ ; OKAMOTO, Atsushi2 ; TSUCHIYA, Noriyoshi2
1AIST, 2 Tohoku University

Permeability is one of the important parameters for geological events and the development of geothermal systems. Water-RockInteraction has a role for spatial and temporal change of permeability in the Earth’s crust, although geophysical properties havebeen mainly focused on. Especially silica is one of the dominant components in the Earth’s crust, thus dissolution-precipitationof silica minerals is an important geochemical reaction in the crust. The permeable-impermeable boundary is regarded to consistof the brittle-ductile transition (BPT) at around 300-450 degree C (Scholz, 2002). The depth of the boundary is typically at 10 to
20 km depth (Ingebritsen and Manning, 2010), which is too deep and too strict to be observed by drilling. Therefore, the structureof the Earth’s crust has been studied by indirect geophysical measurements and experiments, not by geological observation andgeochemical analysis.
In the geothermal field, however, some deep drilling wells are expected to reach the permeable-impermeable boundary ataround 3 km depth because of high temperature gradient. The deep drilling wells in Italy, Iceland, and U.S.A. recorded higher
pressure than hydrostatic pressure at the bottom of these wells (Fournier, 1991). The deep drilling well WD-1a at the Kakkonda
geothermal field, Japan, is the only well in the world to penetrate the boundary between the hydrothermal-convection zone andthe heat-conduction zone (permeable-impermeable boundary) at 3.1 km depth, in 24 MPa and 380 degree C (Doi et al., 1998),
which is in the supercritical conditions of water. The Kakkonda granite at >2.9 km depth is the heat source of the hydrothermal
system of the Kakkonda geothermal field. Saishu et al. (2014) calculated quartz solubility along the well WD-1a, and revealed
that the local minimum of quartz solubility consists of the permeable-impermeable boundary at 3.1 km, indicating the possibility
that a large amount of quartz precipitate induce fracture sealing, blocking the downflow to the impermeable zone, and control thedepth of the permeable-impermeable boundary.
In this study, quartz solubility is calculated to reveal the relationship between the permeability and dissolution-precipitationof silica minerals in the 4 deep drilling wells recorded overpressure at the bottom: (1)San Pompeo 2, Italy, (2)the well NJ-11,
Iceland, (3) Wilson No. 1, U.S.A., and (4) San Vito 1, Italy. In the geothermal fields, including the Kakkonda geothermal field,
the condition at the permeable-impermeable boundary is in or near the supercritical conditions of water, and quartz solubility
decreases and increases drastically in hydrostatic and lithostatic pressure, respectively. If fracturing occurs at the boundary,
downward fluid from the shallower part would dissolve large amount of silica and enhance quartz precipitation due to decreaseof quartz solubility in deeper part. In addition, upward fluid of high quartz solubility from the over pressure zone would also
trigger precipitation of quartz because of pressure decrease. Thus, in the geothermal field, the permeable-impermeable boundary
would be controlled by precipitation of silica minerals.
References) Doi, N., Kato, O., Ikeuchi, K., Komatsu, R., Miyazaki, S., Akaku, K., and Uchida, T., 1998, Geothermics, 27,663-690.; Fournier, R.O., 1991. Geophys. Res. Lett., 18, 955-958.; Ingebritsen, S.E. and Manning, C.E., 2010, Geofluids, 10,193-205.; Saishu, H., Okamoto, A., Tsuchiya, N., 2014, Terra Nova, 26, 253-259.; Scholz, C.H., 2002, 2nd edn. Cambridge
University Press, Cambridge.
Keywords: silica solubility, precipitation, deep drilling, geothermal field, permeable-impermeable boundary

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SMP44-02

Room:102B

Time:May 25 09:30-09:45

Electromagnetic imaging of fluids under the brittle crustOGAWA, Yasuo1∗
1Volcanic Fluid Research Center, Tokyo Institute of Technology

This paper presents a 3D inversion result of the magnetotelluric soundings over the caldera regions in the central part of NE
Japan arc. The 181 MT stations in total were located around the Naruko volcano with ˜3km grid covering the area of 40km (EW)x 80km (NS). We have used the full tensor components of impedance tensors at representative eight periods (0.4˜1300s). The
inversion code of WSINV3DMT was used. The initial model had a uniform earth of 100 ohmm with surrounding oceans (0.25ohmm). The final model gave rms of 2.5 with error floor of 10%. Significant features of the model are the thick resistive upper
crust in the caldera regions and sub-vertical conductors arising from the lower crust to the geothermal manifestations. The topof such sub-vertical conductor coincides with the cutoff depth of the shallow seismicity. In particular, the sub-vertical conductor
at Naruko volcano has a deep root in the mid-to-lower crust underneath the Mukaimachi Caldera, which is located 20km westof the Naruko volcano. The supply of the high salinity fluid may be originated sideways and may imply the path of the fluids,
which is presumably blocked by the impermeable consolidated volcanic rocks directly above the lower crustal conductor. The
resistivity of the mid-to-lower crustal conductors is significantly low at the Naruko volcano, compared with those at Sanzugawa
caldera to the north. The difference may be due to the salinity as well as the porosity of the fluid, because seismic tomography
result does not show such differences. A helium isotope anomaly at Naruko volcano may support that the flux from the upper
mantle is large.
Keywords: fluids, brittle crust, resistivity, electromagnetic induction, magnetotelluric method

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