TECTONICS/Hydrothermal Activity 371 Figure Three dimensional perspective of the NaCl H2O phase diagram between 300 and 500 C Reproduced from Bischoff and Pitzer (1989) Liquid vapor relations for the system NACL H2O: Summary of the P T x surface from 300 to 500 C American Journal of Science 289: 217 248 Table Chlorinity of selected high temperature seafloor hydrothermal Ventsa Vent site East Pacific rise 10 N 21 N Juan de Fuca ridge North cleft South cleft Endeavour Axial volcano Mid Atlantic ridge TAG MARK Lau basin Year(s) sampled Value (mmol kg 1)b 1991 1979, 81, 85 32 860 489 579 1990 92 1984 1984 88 1986 88 730 896 253 176 1986 1986 1989 659 559 650 800 1245 1087 505 624 a Modified from Von Damm (1985) Normal seawater chlorinity 540 mmol kg b Phase separation also results in the formation of saline brines that, because of their high density, sink towards the base of the system Later mixing of these saline brines with seawater may result in vent chlorinity greater than seawater The formation of a brine layer at the base of a hydrothermal system may act as a thermal conductive barrier between the overlying hydrothermal circulation and the magma body (Figure 3A) and be a salinity source for saline vent fluids Future Directions Hydrothermal activity represents an exciting dynamic area for future research This is particularly true for submarine systems because of their links to studies of the origin of life, life in extreme environments, and the continued discovery of novel types of hydrothermal activity The detailed sampling and data analysis and continued exploration for serpentinization-driven hydrothermal activity will likely grow during the next decade At ridge crest and volcanic island arc systems, advances in ocean drilling technology, remote and autonomous sensing devices, long-term monitoring, integrated interdisciplinary experiments at various well-characterized seafloor