Davidson, Donald M., Jr.
M.S. (Master of Science)
Department of Geology
Rare earth metal compounds; Marine sediments--North Pacific Ocean; Sulphur deposits--North Pacific Ocean
This study investigates the Rare Earth Element (REE) systematics of sea-floor ocean ridge hydrothermal sulfide- sulfate deposits at the sediment-starved southern segment of Juan de Fuca Ridge and the sediment-covered southern segment of the Gorda Ridge (Escanaba Trough). REE abundances and patterns were determined in each deposit type for sulfides, sulfates, oxides, mixtures of sulfides and sulfates, basalt, and sediment and compared to the REE signature of hydrothermal fluid and seawater to determine the relative effects that seafloor substrate, seawater, and hydrothermal fluid have on the REE systematics of the deposits. A suite of altered basalt samples from the Turner- Albright massive sulfide deposit in the Josephine Ophiolite Complex, Oregon, were analyzed to compare with unaltered midocean ridge basalt (MORB). Carbonate-rich sulfide samples from the Turner-Albright and Island Mountain, California deposits were analyzed before and after carbonate removal to determine the effect of small amounts of carbonate on REE abundances in bulk sulfide-rich material. Chondrite-normalized REE patterns of unaltered basalt glass from both ocean ridges are characteristic of normal MORB. Altered basalt from the Turner-Albright deposit also exhibit MORB-like REE patterns indicating relative REE immobility during subsea-floor alteration. All sediment samples, altered and unaltered, from Escanaba Trough have shale-like REE patterns indicating limited mobility of REE during sub-sea-floor Mg metasomatism. Nearly pure sulfate samples have higher ?REE contents than massive sulfide samples and all hydrothermal samples from both ridges exhibit LREE enrichment patterns. Sulfate samples have large positive Eu anomalies similar to patterns for high-temperature hydrothermal fluids. Sulfide samples have very low REE contents with no Ce or Eu anomalies. Variably mixed sulfide and sulfate samples from Escanaba Trough have positive Eu anomalies. Fe-oxide crusts from weathered surfaces of sulfide chimneys have small positive Eu anomalies and negative Ce anomalies. Silica-rich sulfide samples have negative Ce and small positive Eu anomalies. The data suggests that compositional differences of sea-floor substrates at the two spreading centers have a minor effect on the REE distribution in sea-floor hydrothermal deposits in comparison to the hydrothermal fluid-seawater mixing processes that control sulfate deposition and that secondary minerals may have a contamination effect. The implications from the results of this study for previous studies is that the effects of minute amounts of hydrothermal barite and anhydrite in sulfide interstices and secondary carbonate have been overlooked resulting in misinterpretation of the REE patterns associated with hydrothermal sulfides.
Murnane, John E., "REE systematics of sulfide-sulfate deposits and sea-floor substrates at sediment-starved and sediment-covered spreading centers in the Northeast Pacific Ocean" (1994). Graduate Research Theses & Dissertations. 4669.
Northern Illinois University
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