Date of Award

Spring 2021

Document Type

Open Access Thesis

Department

Earth and Ocean Sciences

First Advisor

Susan Q. Lang

Abstract

Fluid residence times of deep-sea hydrothermal circulation influence chemical cycling in the ocean. While fluid residence time estimates exist for many basalt-hosted systems, no estimates exist for serpentinite hosted systems that emit fluids with inherently different chemistry. We measured short- and long-lived radium (Ra) nuclides in fluids from two hydrothermal systems on the Mid-Cayman Rise to constrain their residence times: the Von Damm Vent Field, a mixed ultramafic-mafic system, and the Piccard Vent Field, a mafic, neovolcanic 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 mirror results from the Lost City Hydrothermal Field, another ultramafic-hosted system. We hypothesize that the elevated 223Ra activity in the Von Damm field reflects alpha recoil from ultramafic rocks with elevated parent uranium acquired during seawater circulation and serpentinization. Another daughter of seawater-derived uranium, long-lived 226Ra (half-life = 1600 years), is used to normalize the 223Ra measurements. The fluid 223Ra/226Ra activity ratio is used to constrain the upper bound of residence time of fluids passing through the two fields. In contrast, fluids from the Piccard field have 223Ra activities that are less than the ultramafic-hosted fields but still larger than other mafic fields. A previously established model was run to constrain a residence time.

Available for download on Monday, November 15, 2021

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