17:15 〜 18:30
[MIS14-P03] Redox change by coseismic water-rock reactions in the interior of planets
キーワード：地震、酸化還元状態変化、岩石水反応
An earthquake releases elastic energy stored in the planets. The stored energy is dissipated mostly as a heat, which facilitates coseismic water-rock reactions along a fault and eventually changes in redox conditions in the planets. In this study we have reproduced earthquake faulting by laboratory friction experiments and determined redox change within a simulated fault by X-ray absorption near edge structure (XANES) analysis, in order to explore a hypothesis that the coseismic water-rock reactions could provide a potential metabolic energy for life on the planets.
Friction experiments have performed on San Carlos olivine powder with grain sizes of 100~180 μm and on clay-rich décollement materials from the Nankai Trough plate boundary using a rotary shear apparatus at Kochi/JAMSTEC. Earthquake faulting was reproduced at slip velocity of 1.0 m/s with different displacements of 2.6~18 m under normal stress and fluid pressure conditions of 30 and 25 MPa, respectively. After the experiments, the specimens were thin-sectioned to observe microstructures under FE-SEM and to determine how the (Fe2+/ΣFe) and (S0/ΣS) ratios change with input shear energy by the XANES analyses using the BL27SU at SPring-8.
On the olivine specimen the mean value of Fe2+/ΣFe in the fault zone decreases from 0.68 to 0.59 (i.e. oxidization) as increasing shear energy from 0 to ~34 MJ/m2. In contrast, on the décollement specimen the Fe2+/ΣFe ratio tends to increase with shear energy from 0.48 at 0 MJ/m2 (starting material) to 0.51 at ~3.2 MJ/m2 (i.e. reduction). However, the décollement specimen contains sulfide minerals (S-2) in which the S0/ΣS increases from ~0.3 to 0.09 as increasing shear energy to ~3.2 MJ/m2 (i.e. oxidization). Thus, both specimens oxidized during rapid coseismic faulting in several seconds. This is likely attributed to the reactions between minerals and water, that is enhanced by the formation of tens-nanometer-size minerals with fresh reactive surfaces by rapid faulting. Particularly in the olivine experiments with high shear energy inputs, frictional heat transforms liquid water into supercritical state, promoting the oxidization reactions. If seismicity in the planets could be monitored, the proposed correlation between the shear energy (equivalent to earthquake magnitude) and the oxidation progress enables us to estimate the quake-induced redox change in the interior of planets.
Friction experiments have performed on San Carlos olivine powder with grain sizes of 100~180 μm and on clay-rich décollement materials from the Nankai Trough plate boundary using a rotary shear apparatus at Kochi/JAMSTEC. Earthquake faulting was reproduced at slip velocity of 1.0 m/s with different displacements of 2.6~18 m under normal stress and fluid pressure conditions of 30 and 25 MPa, respectively. After the experiments, the specimens were thin-sectioned to observe microstructures under FE-SEM and to determine how the (Fe2+/ΣFe) and (S0/ΣS) ratios change with input shear energy by the XANES analyses using the BL27SU at SPring-8.
On the olivine specimen the mean value of Fe2+/ΣFe in the fault zone decreases from 0.68 to 0.59 (i.e. oxidization) as increasing shear energy from 0 to ~34 MJ/m2. In contrast, on the décollement specimen the Fe2+/ΣFe ratio tends to increase with shear energy from 0.48 at 0 MJ/m2 (starting material) to 0.51 at ~3.2 MJ/m2 (i.e. reduction). However, the décollement specimen contains sulfide minerals (S-2) in which the S0/ΣS increases from ~0.3 to 0.09 as increasing shear energy to ~3.2 MJ/m2 (i.e. oxidization). Thus, both specimens oxidized during rapid coseismic faulting in several seconds. This is likely attributed to the reactions between minerals and water, that is enhanced by the formation of tens-nanometer-size minerals with fresh reactive surfaces by rapid faulting. Particularly in the olivine experiments with high shear energy inputs, frictional heat transforms liquid water into supercritical state, promoting the oxidization reactions. If seismicity in the planets could be monitored, the proposed correlation between the shear energy (equivalent to earthquake magnitude) and the oxidation progress enables us to estimate the quake-induced redox change in the interior of planets.