Abstract

Radiometric dating methods using 238U and 232Th decay chain isotopes have been developed and applied to volcanogenic massive sulfide (VMS) samples, most of which were collected from hydrothermal sites at volcanoes along the Kermadec and Mariana intraoceanic arcs. The activity ratios, 228Th/228Ra, 228Ra/226Ra (Bq.Bq−1), and 226Ra/Ba values (Bq.g−1), are used for dating VMS in the age ranges 0.3 to 12, 3 to 35, and 500 to 15,000 years, respectively. Black smoker chimneys 35 to 100 years old are dated using the activity ratio, 210Pb/226Ra, derived from 210Pb/[Pb] versus 226Ra/[Pb] isochron plots. Iron oxide crusts are dated using 210Pb in combination with either As or 10Be.

The 228Ra/226Ra values for recent VMS mineralization from Brothers and East Diamante volcanoes are reasonably constant but greater than the theoretical ratios calculated from the Th and U contents of volcanic rock from the same area. The high initial 228Ra/226Ra values for VMS are most likely due to excess 228Ra (the daughter of 232Th) that is contained within sediment on the subducted plate. This implies that the residence time for Ba and Ra in the volcano would be less than ∼35 years, considering the half-life of 228Ra (5.75 years). These Ra isotopes are used as a proxy for better understanding the transfer of Ba from magmas to the sea floor, via circulating fluids of the hydrothermal system.

Radiometric dating shows VMS chimneys have been forming at Brothers volcano for at least 1,000 years. The supply of Ba and Ra to the chimneys and associated VMS deposits comes from magmas rich in these elements continuously emplaced via dike intrusion into a zone of hydrothermally altered rock. There, the dikes interact with evolved seawater to produce the metal-rich hydrothermal fluid discharging on the sea floor, forming the chimneys.

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