VARIATIONS IN HYDROTHERMAL ACTIVITY AT THE MARIANA ARC BACKARC SPREADING CENTER AT 18-DEGREES-13'-N BETWEEN 1987 AND 1990

An active hydrothermal vent field was discovered during the 1987 Atlan tis II expedition on the axial ridge of the Mariana Trough. Subsequent detailed surveys by R.V. SONNE in 1988 and 1990 failed to relocate th is field, and it was deduced that there has been a sharp decline in hy drothermal activity at this location over a period of about three year s, emphasizing the episodic nature of hydrothermalism. The decrease in hydrothermal activity was established by seawater analysis (hydrother mal indicators such as manganese, methane and helium), study of benthi c fauna, sediment porewater chemistry and hydrothermal mineralization. The subsequent relocation and observation of this hydrothermal ventin g during five dives of the submersible Shinkai 6500 in late 1992 casts doubt on the observed decline in hydrothermalism and shows the diffic ulty of identifying venting by conventional sampling and chemical tech niques. Hydrothermal minerals recovered from the Izu-Ogasawara and Mar iana Are are listed and a tentative sequence of mineralization propose d.