Remarkably elevated activity of short-lived radium in an ultramafic hydrothermal system on the Mid-Cayman Rise

Abstract:

The residence time of fluids circulating through deep-sea hydrothermal systems influences the extent of water-rock reactions and the flux of major and minor elements to the ocean. While the fluid residence times in numerous basaltic and gabbroic systems have been determined, those of ultramafic systems are currently unknown. Fluids that pass through these ultramafic mantle rocks have fundamentally different chemistries and therefore have a unique influence on the ocean. We measured short- and long-lived radium (Ra) isotopes in fluids from two hydrothermal systems on the Mid-Cayman Rise to constrain their residence times: the Piccard Vent Field, a mafic, neovolcanic system, and the Von Damm Vent Field, a mixed ultramafic-mafic system. Von Damm fluids contain remarkably elevated ²²³Ra activities (half-life = 11.4 days), surpassing those from basalt-hosted hydrothermal systems by one to two orders of magnitude. These ²²³Ra activities are similar to results from the Lost City Hydrothermal Field, another ultramafic-hosted system. In contrast, fluids from Piccard have ²²³Ra activities that are similar to measurements from other mafic fields. We hypothesize that the elevated ²²³Ra activity in the Von Damm field reflects alpha recoil from rocks with elevated parent uranium acquired during serpentinization, and thus, high ²³¹Pa (protactinium) activities. Analyses of another daughter of uranium, long-lived ²²⁶Ra (half-life = 1600 years), are currently underway. The ²²³Ra/²²⁶Ra activity ratio will be used to determine the residence time of fluids passing through the two fields.