Dating of Hydrothermal Mineralization in Active Hydrothermal Fields in the Southern Mariana Trough

Abstract

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.

High‐resolution magnetic signature of active hydrothermal systems in the back‐arc spreading region of the southern Mariana Trough

“…On the other hand, the Archean site is located 2 km from the NVZ, and although there is no direct information regarding the heat source for this site, the existence of a large hydrothermal mound indicates a stronger hydrothermal flux or a longer duration of hydrothermal circulation. Radiometric dating showed that the maximum age of the sulfide deposits from the Archean site is at least ~3500 years [ Takamasa et al ., ; Ishibashi et al ., ], substantiating the presence of long‐lived hydrothermal activity.…”

“…[] showed that the seismicity around the off‐axis volcano was very low during the observation period of 3 months, suggesting that volcanism might have ceased. Sulfide chimneys and debris/breccia collected from these sites also indicates an age of up to ~9000 years [ Ishibashi et al ., ]. The growth of the Pika and Urashima hydrothermal sites might therefore be in their late stage.…”

230Th/238U dating of hydrothermal sulfides from Duanqiao hydrothermal field, Southwest Indian Ridge

“…The growth rate of the barite-containing sulfide crust from South Mariana Trough is less than 0.1 mm/yr. Then Ishibashi et al (2015) dated core samples from South Mariana Trough and estimated that the growth rate of the massive sulfide were 0.2-1.5 mm/yr. More than 1000 years of continuous hydrothermal activity would be necessary for the formation of a massive sulfide deposit.…”

Potential for biogeochemical cycling of sulfur, iron and carbon within massive sulfide deposits below the seafloor

“…The global mass of SMS deposits is estimated to be in the order of 10 8 tons (Hannington et al ., ). In some cases, the SMS deposits are distributed at depths of tens to hundreds of metres below the seafloor (mbsf) (Humphris et al ., ; Zierenberg et al ., ) and persist for several thousand years or more (Lalou et al ., ; You and Bickle, ; Takamasa et al ., ; Ishibashi et al ., in press).…”

“…In the present study, we carried out shallow sub‐seafloor drilling investigations using the Benthic Multi‐Coring System (BMS), and collected two sulfide core samples, BMS9 at the Pika site and BMS3 at another hydrothermal site (Archaean) in the SMT (http://onlinelibrary.wiley.com/doi/10.1111/1462-2920.12648/suppinfo). The age of the sulfide samples at the two sites ranges from 300 to 9000 years old depending on the depth from the seafloor (Ishibashi et al ., in press). Here, we report the abundance, composition and potential biogeochemical roles of microbial communities in the sub‐seafloor sulfide samples as characterized by culture‐independent molecular analyses targeting functional genes involved in sulfur cycling and carbon fixation, in addition to 16S rRNA genes, and geochemical and thermodynamic analyses.…”