Date of Award

Spring 2009

Project Type


Program or Major

Geochemical Systems

Degree Name

Master of Science

First Advisor

Linda Kalnejais

Second Advisor

Julia G Bryce


Low-temperature hydrothermal fluids, long considered to be simple binary mixtures of high-temperature fluids and seawater (Edmond et al, 1979ab), impact ocean chemistry while providing energy to thriving ecosystems at mid-ocean ridges (MORs). This thesis investigates whether processes other than simple mixing may influence the evolution of low temperature hydrothermal fluids. In particular, the role of anhydrite dissolution is addressed.

9°50'N EPR contains high-temperature vents (Ty and lo) that have been associated with low-temperature venting (BM82) for over a decade and remained active following an eruption in 2005-2006. The Sr isotope systematics of adjacent high- and low-temperature fluids will support either a binary seawater-hydrothermal mixture, or will support additional components. The Sr isotopic composition of fluids sampled in 1994 to 2007, determined via thermal ionization mass spectrometry, is reported. 87Sr/ 86Sr ranges are 0.70902 to 0.70917 in BM82 fluids. Pre-eruption Ty and lo end member 87Sr/86Sr range is 0.70394 to 0.70419, which aligns with previous EPR studies (0.7030-0.7040, (Ravizza et al., 2001)). However, immediate post-eruption end member 87Sr/ 86Sr ranges from 0.70511 to 0.70654, closer to the seawater value. These results imply that immediate post-eruption water-rock reactions do not achieve the same degree of isotopic equilibrium with basalt as reactions during more stable periods (Butterfield et al., 1997; Von Damm, 2000). Additionally, anhydrite dissolution following an eruption may contribute 2-7% of low-temperature fluid Ca. Our findings demonstrate that MOR hydrothermal anhydrite deposits may be a temporary sink for Ca, Sr, and SO42-, and that the hydrothermal flux component of these elemental budgets must be reinterpreted. Additional findings include evidence for halite precipitation and albitization in the high-temperature fluid time series, as well as indication of seawater infiltration into the discharge zone in March 2004 and in the post-eruptive 2006-2007 sampling interval.