[S-SS30_29PM2] New perspective of great earthquakes along subduction zones

[SSS30-31] Fluid inclusion as fossil fluid in seismogenic zone, trap mechanism and interpretations for fault science

*Arito SAKAGUCHI¹, Kazumichi YANAGISAWA² (1.Yamaguchi Univ./JAMSTEC, 2.Kochi Univ.)

Keywords:Fluid inclusion, trap mechanism, artificial calcite

A fluid inclusion, fluid-fill capsule within rigid crystal, preserves density and chemical composition of fluid in deep crust. This records pressure, temperature and other information of the fluid when the fluid trapped. Pore fluid pressure drop due to rapid fluid ejection along the fault was discussed in Kodiak accretionary complex (Vrolijk et al., 1988). CH4-H2O fluid inclusions are reported at pseudotachylyte bearing Nobeoka Thrust, Shimanto accretionary complex (Kondo et al., 2005). Thermal stretching of fluid inclusions due to seismic frictional heating were found at seismogenic Mugi Melange, Shimanto accretionary complex (Ujiie et al., 2008). Though fluid inclusion tells us fluid condition in deep crust, trapping mechanism within crystal is still uncertain. The fluid inclusion is one of crystallographic defect, but general size from sub-micron meter to several mm is much greater than crystal lattice. A crystal tend to growth without large defect, and it seems irregular process to be formed a fluid inclusion. The trap mechanism is significant to interpret the fluid inclusion data.We succeeded to make artificial fluid inclusion in calcite during hydrothermal experiment. A calcite crystals are nucleated and grown with temperature decrease in autoclave. Fluid inclusions were never formed in simple cooling procedure, but many large fluid inclusions were found at the overgrowth zone formed by re-heating process. Surface condition of artificial calcite of re-heating and overgrowth process were observed using SEM. Etched pattern covers the surface of re-heated calcite crystal. Some depressions are wide shallow and others are small deep. Many growth steps were found on surface of over-growth calcite. The calcite surface may have been advanced with lateral motion of growth steps. This growth step covers most of the etched depressions except small deep one. These small deep depressions are surrounded by new grown surface and became increasingly deep. Some depressions may make large pore within overgrowth zone in this process. This observation shows that the fluid inclusion were made during overgrowth after surface etching, and this requires temporary solubility change in crystal growth process. The fluid inclusions may record pore-fluid condition after the event of pressure, temperature and/or chemical change in deep crust.