Iron speciation in fault gouge from the Ushikubi fault zone central Japan

Chemical species of iron and sulfur were measured using ⁵⁷Fe Mössbauer spectroscopy and X-ray near edge structure, respectively, for the fault gouge samples collected from two sites along the ENE-WSW trending Ushikubi fault zone in central Japan. These gouge samples have distinguishable variations in their physical properties such as surface color and structure and these features are also reflected by the chemical speciation of iron and sulfur. Newly formed minerals, including calcite, dolomite, siderite, iron sulfide and pyrite, have close relation to the colors of fault gouge and respective to the geochemical environment within the fault zone. In addition, the variations in iron and sulfur species may have significance to evaluate the redox conditions in the fractures and furthermore to estimate the history and activity of the faults. Generally there is observacious enrichment of reducing species of iron and sulfur as well as chlorite in the relatively younger fracture, indicating favorable connection pathway with deep position and the fault zone is active. On the other hand, the stable fracture with a longer history is relatively enriched in ferric iron species and almost no sulfur in the gouge. These results from iron and sulfur speciation have a good agreement with evidence indicated by ¹⁴C dating from this fault zone.