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 230Th/234U 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 39Ar-40Ar 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-1 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.
All Science Journal Classification (ASJC) codes
- Earth and Planetary Sciences(all)