11:30 AM - 11:45 AM
[SCG55-21] Alteration of oceanic crust inferred from physical properties measurements of dredge samples from the Marie Celeste transform fault
Keywords:oceanic crust, alteration, seismic velocity, electrical resistivity
Since water in the Earth's deep interior is supplied by hydrous oceanic plates, understanding the process of plate hydration is essential to understanding the Earth's hydrological cycle. Plate hydration is characterized by a decrease in seismic velocity, whereas crustal hydration is difficult to estimate from geophysical observations because seismic wave velocity does not change. In addition, it is difficult to distinguish the effects of alteration and porosity from seismic velocity because porosity changes as the rock is altered. In this study, we investigate how physical properties respond to hydrous processes in the oceanic crust by measuring seismic velocity and electrical resistivity.
Dredge samples collected during the KH-24-4 cruise on the Marie Celeste transform fault in the Indian Ocean Central Ridge from October to November 2024 were used in this study. Seismic velocity and electrical resistivity were measured for each sample under dry and wet conditions. Mass and solid volume were also measured to calculate density and porosity. The rocks used for the measurements were basalts, dolerites, gabbros, and rocks strongly altered by these rocks, which were collected by dredging on the Marie Celeste transform fault. These rocks show large variations in porosity and alteration. These rocks were shaped into cm-sized cores or cubes and used for physical property measurements. Seismic velocities were calculated by the pulse transmission method using a function generator and an oscilloscope, and electrical resistivity was calculated by the two-electrode method using an LCR meter.
Seismic velocities did not differ significantly among lithologies, with a slight trend toward higher velocities in wet condition compared to dry condition. P- and S-wave velocities in the wet condition were 4.0 to 6.0 km/s and 2.0 to 3.5 km/s, respectively, and showed variation in the same lithology. The seismic velocity tended to decrease with increasing porosity. The Vp/Vs ratio did not vary with lithology, and the Vp/Vs ratio tended to be higher in wet conditions than in dry conditions. The Vp/Vs ratio in wet condition was about 2.0. Electrical resistivity did not vary with lithology, and the wet condition tended to have lower electrical resistivity than the dry condition. In particular, the high porosity samples followed Archie’s law, which is a power-law relationship between porosity and resistivity. These physical properties include both the effect of porosity in the rock and the effect of rock alteration. Regarding the effect of porosity, the pore shape of basalt tends to have a higher pore aspect ratio than other lithology based on the effective medium theory. These high aspect ratio pores are the result of the primary formation of the rock rather than the effect of alteration. On the other hand, the effect of alteration on the density of rocks is difficult to evaluate because the density of epidote and hornblende formed by high temperature alteration is like that of pyroxene. Therefore, it is necessary to determine the degree of alteration by measuring the bulk chemical composition of the rock. At present, we don’t have data on the bulk chemical compositions of the rocks, but we plan to measure the bulk chemical compositions of the samples in the future and evaluate the effects of alteration separately from the effects of oceanic crustal porosity using principal component analysis.
Dredge samples collected during the KH-24-4 cruise on the Marie Celeste transform fault in the Indian Ocean Central Ridge from October to November 2024 were used in this study. Seismic velocity and electrical resistivity were measured for each sample under dry and wet conditions. Mass and solid volume were also measured to calculate density and porosity. The rocks used for the measurements were basalts, dolerites, gabbros, and rocks strongly altered by these rocks, which were collected by dredging on the Marie Celeste transform fault. These rocks show large variations in porosity and alteration. These rocks were shaped into cm-sized cores or cubes and used for physical property measurements. Seismic velocities were calculated by the pulse transmission method using a function generator and an oscilloscope, and electrical resistivity was calculated by the two-electrode method using an LCR meter.
Seismic velocities did not differ significantly among lithologies, with a slight trend toward higher velocities in wet condition compared to dry condition. P- and S-wave velocities in the wet condition were 4.0 to 6.0 km/s and 2.0 to 3.5 km/s, respectively, and showed variation in the same lithology. The seismic velocity tended to decrease with increasing porosity. The Vp/Vs ratio did not vary with lithology, and the Vp/Vs ratio tended to be higher in wet conditions than in dry conditions. The Vp/Vs ratio in wet condition was about 2.0. Electrical resistivity did not vary with lithology, and the wet condition tended to have lower electrical resistivity than the dry condition. In particular, the high porosity samples followed Archie’s law, which is a power-law relationship between porosity and resistivity. These physical properties include both the effect of porosity in the rock and the effect of rock alteration. Regarding the effect of porosity, the pore shape of basalt tends to have a higher pore aspect ratio than other lithology based on the effective medium theory. These high aspect ratio pores are the result of the primary formation of the rock rather than the effect of alteration. On the other hand, the effect of alteration on the density of rocks is difficult to evaluate because the density of epidote and hornblende formed by high temperature alteration is like that of pyroxene. Therefore, it is necessary to determine the degree of alteration by measuring the bulk chemical composition of the rock. At present, we don’t have data on the bulk chemical compositions of the rocks, but we plan to measure the bulk chemical compositions of the samples in the future and evaluate the effects of alteration separately from the effects of oceanic crustal porosity using principal component analysis.