The Rainbow hydrothermal system at 36°N on the Mid-Atlantic Ridge vents >360°C high temperature, acidic H2, CH4, Ca, Cl and Fe-enriched fluids that cannot be explained by the interaction of seawater derived fluids with either a basalt or ultramafic substrate. Since most ultramafic-bearing environments also contain significant proportions of gabbro, greater quantities of chlorite and tremolite form than serpentinites at upper greenschist facies conditions. To test whether the alteration of a gabbro + ultramafic rock is capable of producing a fluid rich in H2 without forming serpentine, a suite of experiments were conducted at a fluid:rock ratio of 4 at 420°C and 500bars. Two experiments were conducted with a natural troctolitic rock, three experiments were conducted with plagioclase + either olivine, enstatite, or serpentine to better determine mass transfer processes in more compositionally restricted systems, and the final experiment was conducted with talc + chlorite + tremolite + magnetite + plagioclase to examine how alteration phases might affect fluid chemistry. The only minerals to form in the experiments were large quantities of chlorite and smectite and a little talc, and still the troctolite experiments produced >22mmol/kg H2 by incorporating ~14% and <40% Fe3+/ΣFetotal in chlorite and smectite, respectively. While the pH formed in all of the experimental solutions was close to neutral, computer models containing Rainbow-type non-zero Mg concentrations matched the acidity of vent fluids and released high major element concentrations to solution while forming chlorite and tremolite. Computer modelling further illustrated that higher fluid:rock ratios than used in the experiments were necessary to continue altering the rock while releasing high concentrations of Fe, Ca and SiO2 to solution, which indicates that Rainbow fluid chemistry likely formed in a high permeability zone created by gabbroic intrusions in the subsurface and then flowed for short distances along a fault surface. The experiments conducted between plagioclase + serpentine and the mixture of alteration phases all released the trace elements B and Li into solution which contrasts with trace element concentrations of Rainbow fluids, so it is unlikely that fluid chemistry is controlled by alteration along a fault surface. Therefore, this study indicates that non-serpentine producing alteration processes involving the flow of large quantities of fluid near a magmatic intrusion should be considered to explain the high H2 and major element concentrations measured at the Rainbow vent field.
University of Minnesota M.S. thesis. July 2011. Major: Geology. Advisor: William E. Seyfried, Jr. 1 computer file (PDF); x, 105 pages.
Rough, Mikaella Elaina Macander.
H2 and chlorite production from an olivine-rich Gabbroic Rock assemblage:a modeling and experimental study at 420C, 500 bars.
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