Radiometric Dating of Volcanogenic Massive Sulfides and Associated Iron Oxide Crusts with an Emphasis on 226Ra/Ba and 228Ra/226Ra in Volcanic and Hydrothermal Processes at Intraoceanic Arcs

Radiometric dating methods using U-238 and Th-232 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, Th-228/Ra-228, Ra-228/Ra-226 (Bq.Bq(-1)), and Ra-226/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, Pb-210/Ra-226, derived from Pb-210/[Pb] versus Ra-226/[Pb] isochron plots. Iron oxide crusts are dated using Pb-210 in combination with either As or Be-10. The Ra-228/Ra-226 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 Ra-228/Ra-226 values for VMS are most likely due to excess Ra-228 (the daughter of Th-232) 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 similar to 35 years, considering the half-life of Ra-228 (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.