Geochemical characteristics and source of pore fluids of the mud volcanoes in Shin-yan-ny-hu, Taiwan, China

The sources and geochemical evolution of pore fluids of terrestrial mud volcanoes are critical to the understanding of the features and migrating paths of the fluids in subduction accretionary prisms. They are also important to oil and gas exploration. In order to get an answer to the problems mentioned above, we analyzed the ionic composition, and the hydrogen and oxygen isotopes of mud volcanic fluids as well as the mineral composition of the sediments of mud volcanos taking the Shin-yan-ny-hu (SYNH) mud volcano, Taiwan, China as a case. The analysis results suggest that the erupted minerals by the mud volcano are dominated by quartz, feldspar, illite and chlorite, with a small amount of calcite and dolomite, while in the fluids, Na⁺ and Cl⁻ dominate and there is a significant positive correlation between Na⁺ and Cl⁻. Such a correlation suggests that the fluids are derived mainly from pore water of marine sediments. The Cl⁻ concentration of the fluid is about one-fifth of the seawater, characterized by enriched oxygen isotopes (δ¹⁸O: 6.24‰～6.59‰) and depleted hydrogen isotopes (δD: −23.72‰～−12.9‰), indicating the dehydration of smectite. The hydrogen and oxygen isotopes are obviously deviated from meteoric precipitation, the results support the conclusion that the influences of meteoric precipitation on this area are limited. Comparing to seawater, the fluids are rich in Na⁺ and severely depleted in K⁺, owing to the increase in smectite by illite/smectite interaction. The geochemical signatures of the pore fluids further suggest that the fluids sourced from the deep with temperatures ranging between 79 ℃−181 ℃ responding to a source region 2.6 km to 6.0 km in depth, where, as we know, occur the Kai-tzu-liao shales of Pliocene, and the interbedded shale and sandstone sequence of Miocene Chang-chi-keng and Wu-shan Formations. Based on the tectonic background, mineral composition and fluid sources of the SYNH mud volcanos, we established a theoretical model in this paper: The pore fluids of marine sediments trapped by the plastic shales caused the uneven ejection of pore water. With the compaction by the overlying sediments and lateral extrusions, the pressures of pore fluids increased sharply. Finally, the fluids carrying sediments erupted out, when the pressures are greater than that of overlying rocks if there are high-permeability channels.