Isotopic Evidence of Fluid Processes in Fault-related Rocks From TCDP Drill Cores in Taiwan

Faults and shear zones are generally thought to be major fluid conduits in crustal environment. Fluid circulation and migration may deposit or recrystallize clay or carbonate minerals in fracture within the fault zone. Isotopic signatures of such crack-fill materials will serve as a good indicator of both sources of fluid and processes of fluid-rock interaction. We here report results from carbon and oxygen isotope analyses of calcite veins retrieved by Taiwan Chelungpu Drilling Program (TCDP), which penetrated the active Chelungpu fault zone at around 1100 m depth. The calcite veins are well observed on both side of the fault zone in the drill cores and the samples between 900 and 1300 m depths have been examined. The cores are composed of the Chinshui Shale and the calcite veins mainly appear in sandstone and siltstone with several mm in width. Calcite vein samples reveal variable δ13C(PDB) values ranging from -7.0 to -2.0 permil, and δ18O(SMOW) values of 15.7 to 19.9 permil. Both of values tend to increase slightly below 1225 m in depth. The δ13C values of calcite veins are between that of marine carbonate and sedimentary rocks. Calculated δ18O values of fluids in chemical equilibrium with calcite veins are ranging from -7.9 to -2.6 permil, which are between that of seawater and meteoric water. The isotopic results indicate that the calcite veins were formed from fluids originating from meteoric water mixed with seawater and carrying bicarbonate dissolved from fossil or diagenetic carbonate.

The variation of isotopic composition is not related to the appearance of major shear zone of Chelungpu fault. Depleted oxygen isotopic signature of fluids indicates no contribution of metamorphic or mantle derived sources. Both of them infer that the Chelungpu fault may not serve as a deep fluid conduit or have no widely effect on calcite vein formation in host rocks.