Evolving patterns of the fluids within the TAG hydrothermal field

Abstract

The mixing of seawater/hydrothermal fluid within the large seafloor hydrothermal sulfide deposits plays a key role in the formation processes of the sulfide deposits. Some issues attract considerable attentions in the study of seafloor hydrothermal system in recent years, such as the relationships among different types of vent fluids, the characteristics of chemical compositions and mineral assemblages of the hydrothermal deposits and their governing factors. Combined with the measured data of hydrothermal fluid in the TAG field, the thermodynamic model of mixing processes of the heated seawater at different temperatures and the hydrothermal fluid is calculated to understand the precipitation mechanism of anhydrite and the genetic relationships between the black and white smoker fluids within the TAG mound. The results indicate that the heating of seawater and the mixing of hydrothermal fluid/seawater are largely responsible for anhydrite precipitation and the temperature of the heated seawater is not higher than 150 and the temper ℃ ature of the end-member hydrothermal fluid is not lower than 400 . Based on the simulated r ℃ esults, the evolving patterns of fluids within the TAG deposit are discussed. The mixed fluid of the end-member hydrothermal fluid and the seawater heated by wall rock undergoes conductive cooling during upflowing within the deposit and forms "White Smoker" eventually. In addition, the end-member hydrothermal fluid without mixed with seawater, but undergoing conductive cooling, vents out of the deposit and forms "Black Smoker".seafloor hydrothermal system, mixing process, evolving patterns of fluids, TAG hydrothermal fieldThe formation of the large seafloor hydrothermal sulfide deposit is a complex dynamic process involving seawater entrainment, hydrothermal fluid/seawater interaction and minerals precipitation/dissolution [1] . It is very important for understanding the mineralization mechanisms and evolving processes of the deposit to study the geochemical characteristics of fluids and their circulation and mixing processes within the deposit.As the first discovery of active hydrothermal field in the Mid-Atlantic Ridge, detailed studies of the Trans-Atlantic Geo-Traverse (TAG) hydrothermal field allow clearly knowing about the internal structure, mineral assemblages and geochemical compositions of the TAG deposit [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] . This also provides a good opportunity for studying the genetic relationships among different vent fluids and the evolving patterns of the fluids within the deposit using numerical approach. Edmond et al. [21] compared the geochemical compositions of different vent fluids in the TAG field and ascribed the differences among the H 2 S, Fe, Cu, Zn concentrations and pH value of vent fluids to 15%-20% seawater entraining into the deposit and reacting with the black smoker fluid. Based on the relations of SO 4 2− and Ca 2+ concentrations in black and white smoker fluid, Tivey et al. [2] calculated the mixed fluid o...