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Ages of sulfide and sulfate mineralized samples collected from active hydrothermal fields in the Southern Mariana Trough were determined. In addition to samples collected from active and inactive chimneys, and sulfide breccia during dive expeditions, massive sulfide ores obtained by shallow drilling were studied. We applied 230 Th/ 234 U radioactive disequilibrium dating technique to sulfide minerals, as the collected mineralized samples were dominated by marcasite, pyrite and sphalerite. In addition, electron spin resonance (ESR) dating was applied to a few barite-rich samples, for comparison purpose. A laser step heating 39 Ar dating of the basement volcanic rock samples was also attempted. Sulfide chimneys and ores collected from a hydrothermal mound located beside the spreading axis range in age from <100 to 3,520 years old, without notable hiatus. The growth rate of the massive sulfide ore body is calculated to be 0.12-1.5 mm year À1 based on results of the core samples. This age range is comparable for those previously reported for giant hydrothermal mounds of a few 100 m in diameter. These results suggest >1,000 years of continuous hydrothermal activity would be necessary for the formation of a massive sulfide deposit. Sulfide chimneys and breccia collected from two hydrothermal fields located on an offaxis knoll are up to 9,000 years old. Sulfide breccia collected from an active site on the spreading axis are 2,740 and 7,190 years old. Geophysical studies provided evidence for abundant magma supply in the Southern Mariana Trough, which would have fueled hydrothermal activities in this area for long duration. While geophysical evidence for crustal velocity anomaly below the off-axis knoll suggests mineralization at the off-axis sites is considered to be in the late-stage of the hydrothermal activity, the discrete ages from the on-axis site might reflect episodic hydrothermal activities related to diking events proposed by geophysical and geological studies.

Electron Spin Resonance (ESR) dating of marine barite in chimneys deposited from hydrothermal vents was attempted to determine the time since hydrothermal activity occurred. In this study, we used Barite (BaSO 4 ) precipitated in icroenvironments in the chimneys deposited from the hydrothermal vents at the Archaean site in South Mariana spreading centre (12°56.4'N, 143°37.9'E) and at Hakurei site in the Izena caldron (27°15'N, 127°04'E) for ESR measurements. ESR spectrum of marine barite is characterized by an electron-type centre with g values of 2.0034, 2.0022 and 1.9995 attributed to SO 3 -. The signal intensity increased with gamma ray dose. The dose rates of hydrothermal chimneys from the Archaean site and from the Hakurei site were calculated using a model that assumed a grain size and that incorporation of radionuclides after 226 Ra in U series into the chimney, and assuming the efficiency of the defect formation by alpha particles to be the same as that for OSL. The ESR ages were estimated to be 470 years old for barite from the Archaean site and 5670 years old for one from the Hakurei site, although there is a considerable difference between the present ESR ages and the 210 Pb/ 226 Ra disequilibrium ages previously reported.

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