Oxygen and Carbon Isotopic Studies of Carbonate Minerals from TCDP Hole-A 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 carbonate minerals in fractures 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.
This study analyzed carbon and oxygen isotopic compositions of calcite veins, fillings of slickensides, cementations and fossils retrieved by Taiwan Chelungpu Drilling Program (TCDP) Hole-A Drill cores. The core penetrated the active Chelungpu fault zone at around 1100 m depth and was composed of the Cholan, Chinshui and Keichulin formation. The calcite veins mainly appeared in sandstone and siltstone with several mm in width.
Calcite veins revealed variable δ18OVSMOW values ranging from 10 to 20‰, and δ13CVPDB values of -13 to -2‰. Filling of slickensides shown δ18OVSMOW values from 14 to 20‰ and δ13CVPDB values from -10 to -2‰. The δ18O VSMOW and δ13CVPDB values of cementation were between 16 to 20‰ and -11 to -3‰, respectively, and the valves for fossils varied from 12 to 30‰ in δ18O VSMOW values and -10 to 2‰ in δ13CVPDB values.
The δ13CVPDB values of calcite veins and cementations generally increased with depth between 400 to 1290 m, then decreased with depth below 1290 m. A mixture of dissolved inorganic carbon in groundwater and marine carbonate was suggested to be the carbon sources for these carbonate precipitation from 400-1290 m interval. The carbon isotopic compositions of calcite veins from 1290-1710 m interval might be controlled by the fault events or the hosted formations. The composition reflected the microbial processes involved during fault activity or strata deposition. Calculated δ18O values of fluids in chemical equilibrium with calcite veins were ranging from -14 to -4‰, which were in between that of meteoric water and seawater. These results inferred that the calcite veins were formed from fluids originating from meteoric water mixed with seawater or formation water and carrying bicarbonate dissolved from fossil or diagenetic carbonate.