[S-IT40_1AM1] 地殻流体：その分布と変動現象への役割

[SIT40-06] 花崗岩中の空隙の連結度

*渡辺 了¹、樋口 明良¹、米田 明² (1.富山大学大学院理工学研究部、2.岡山大学地球物質科学研究センター)

キーワード：空隙, クラック, 連結度, 花崗岩, 電気伝導度

Seismic velocity and electrical conductivity are used to map the fluid distribution in the crust. Seismic velocity reflects the contiguity of solid phases, while electrical conductivity the connectivity of fluid phases. The combination of velocity and conductivity could provide us a strong constraint on the fluid distribution. However, mapping of the fluid distribution has not been successful. The connectivity of fluid phases in rocks is poorly understood. In order to understand the connectivity of fluid phases in rocks, we have made conductivity measurements on a fluid-bearing granitic rock under various confining pressures.Fine grained (100-500μm) biotite granite (Aji, Kagawa pref., Japan) was used as a rock sample. The sample is composed of 52.8% plagioclase, 36.0% Quartz, 3.0% K-feldspar, 8.2% biotite. The density is 2.66(1) g/cm³, and the porosity 0.8(1) %. The porosity was estimated from the mass of the dry and wet samples. Cylindrical samples have dimensions of 25 mm in diameter and 30 mm in length, and saturated with 0.01 mol/l KCl aqueous solution. Simultaneous measurements of elastic wave velocity and electrical conductivity were made using a 200 MPa hydrostatic pressure vessel. The pore-fluid is electrically insulated from the metal work by using plastic devices (Watanabe and Higuchi, 2013). The confining pressure was progressively increased up to 125 MPa, while the pore-fluid pressure was kept at 0.1 MPa. It took five days or longer for the electrical conductivity to become stationary after increasing the confining pressure.Elastic wave velocities and electrical conductivity showed reproducibly contrasting changes for a small increase in the confining pressure. Elastic wave velocities increased only by 5% as the confining pressure increased from 0.1 MPa to 25 MPa, while electrical conductivity decreased by an order of magnitude. The increase in velocities is caused by the closure of cracks. Most (〜80%) of the decrease in electrical conductivity occurred below the confining pressure of 5 MPa. The decrease in electrical conductivity must also be caused by the closure of cracks. The decrease in porosity was only 0.07(1) %. Such a small change in porosity caused a large change in electrical conductivity. The connectivity of fluid was maintained at least up to the confining pressure of 125 MPa. A calculation with the effective medium theory (Kirkpatrick, 1973) suggests that the fluid forms a network with small coordination number (average coordination number=2.3), and that the connectivity at higher pressures is maintained by stiff pores. More cracks are open at lower pressures to link pores, drastically increasing electrical conductivity.