Geochemistry of deep-sea hydrothermal vent fluids from the Mid-Cayman Rise, Caribbean Sea

Abstract

THESIS ABSTRACTThis thesis examines the controls on organic, inorganic, and volatile species distributions in hydrothermal fluids venting at Von Damm and Piccard, two recently discovered vent fields at the ultraslow spreading Mid-Cayman Rise, Earth's deepest mid-ocean ridge. A wide variety of possible temperatures and substrates for fluid/rock reaction exist at ultraslow spreading ridges. The flux of chemicals delivered to the ocean by circulating vent fluids exerts a major control on mass transfer into and out of the oceanic crust and supports chemosynthetic ecosystems. In Chapter 2, abiotic organic synthesis is shown to occur via two distinct mechanisms in the serpentinizing Von Damm system. Longstanding questions concerning the spatial, temporal, and mechanistic nature of carbon transformations in deep-sea hot springs are addressed. In contrast with the current paradigm, CH 4 is not actively forming during circulation of H 2 -rich vent fluids, but instead is derived from fluid inclusions in the host rocks. Chapters 3 and 4 present in-depth studies of the chemical and isotopic compositions of aqueous species in vent fluids at Von Damm and Piccard to elucidate the role of reaction temperature, pressure, substrate composition, and water/rock mass ratios during the chemical evolution of hydrothermal fluids at oceanic spreading centers. At Von Damm, sequential reaction of gabbroic and peridotite substrates at intermediate temperatures can explain generation of the observed fluids. Geochemical modeling shows that talc-quartz assemblage is expected to precipitate during fluid mixing with seawater at the seafloor. At Piccard, extremely high temperature subsurface water/rock reaction results in high temperature fluids that are richer in dissolved H 2 than any previously observed fluids worldwide. At both locations, high-H 2 conditions promote the abiotic reduction of ΣCO 2 to formate species, which may fuel a subsurface biosphere. In Chapter 5, multiple sulfur isotopes were measured on metal sulfide deposits, S 0 , and fluid H 2 S to constrain sulfur sources and the isotopic systematics of precipitation in a wide variety of seafloor hydrothermal vents. Areas studied include the eastern Manus Basin and Lau Basin back-arc spreading centers, the unsedimented basalt-hosted Southern East Pacific Rise, and the sediment-hosted Guaymas Basin mid-ocean ridge spreading centers. Limited isotope fractionation between fluid H 2 S and precipitating chalcopyrite implies that sulfur isotopes in a chimney lining may record past hydrothermal activity. ACKNOWLEDGEMENTSFirst, I would like to thank my co-advisor, Jeff Seewald, whose limitless scientific creavitity and sunny outlook have been a continual source of encouragement throughout the 5 years of my doctoral education and research. Jeff's gift for simultaneously challenging and motivating me has taught me many lasting lessons. His limitless optimism and imagination have left permanent impressions on my approach to scientific research, and will not be far from mind when I mentor oth...

The geochemistry of methane isotopologues

“…The modeled fluid is an ideal gas consisting of CO, CO 2 , H 2 , H 2 O, O 2 , ethane, and propane. The model is essentially that of French (1966), with the addition of C 2+ compounds (as also considered by Kawagucci et al (2013) and McDermott (2015), with different assumptions regarding redox, water activity, and total mass of carbon). To calculate the composition of the fluid, equilibrium constants were computed at various temperatures using CHNOSZ (Dick, 2008) from tabulated standard molal thermodynamic properties and equation of state parameters (Kelley, 1960;Helgeson et al, 1978;Wagman et al, 1982;Johnson et al, 1992;Shock, 1993;Helgeson et al, 1998), the fugacities of CO and CO 2 were calculated, and then the fugacities of all other gaseous species were solved iteratively under the constraint that ∑f = 1000 bar (a pressure typical of those indicated by fluid inclusion studies; Vanko, 1988).…”

“…Lower water-to-rock ratios are necessary if conversion efficiency is less than 100% (e.g., due to graphite precipitation) or if lower initial carbon contents are assumed. Constraints from mobile inorganic elements (e.g., Li, Rb, Sr) generally indicate that water/rock ratios are substantially lower than~10 in many mid-ocean ridge hydrothermal systems (Von Damm et al, 1985;Berndt et al, 1989) with values of 0.4 to 6 calculated for the subsurface at Von Damm (McDermott, 2015) and 2 to 4 at Lost City (Foustoukos et al, 2008) for example.…”

“…Constraining the sources of carbon (C) and hydrogen (H) for the production of CH 4 , as well as the depths and temperatures at which CH 4 is generated in these hydrothermal systems, is critical for understanding the origin of methane (Welhan, 1988b;Charlou et al, 2002;Proskurowski et al, 2008;McDermott et al, 2015). The abundance and isotopic composition of methane venting from submarine hydrothermal fields that are relatively free of sediment cover has been described at oceanic spreading centers characterized by a range of spreading rates (e.g., Welhan, 1988b;Charlou et al, 2002;McCollom and Seewald, 2007;Proskurowski et al, 2008;Cannat et al, 2010;Charlou et al, 2010;Proskurowski, 2010;McDermott et al, 2015;McDermott, 2015). In general, fluids that have interacted with ultramafic rocks are substantially enriched in CH 4 relative to fluids that have reacted with mafic rocks (Keir, 2010), although there are exceptions in which high-CH 4 fluids are associated with apparent mafic substrates (e.g., basalt) (Charlou et al, 2000).…”

“…Generation of small amounts of C 2+ hydrocarbons (~1 µM or less) from the thermal breakdown of dissolved organic matter carried in recharging seawater (~40 µM) may account for the excess propane and butanes relative to ethane and methane. Alternatively, the C 2+ hydrocarbons may not have equilibrated at a uniform temperature (McDermott, 2015), or may be formed via low-yield, kinetically-throttled reactions occurring in circulating fluids (Foustoukos and Seyfried, 2004). Regardless of their specific origins, similarities in the abundances and isotopic compositions of low 64 3.5.…”

“…Lower water-to-rock ratios are necessary if conversion efficiency is less than 100% (e.g., due to graphite precipitation) or if lower initial carbon contents are assumed. Constraints from mobile inorganic elements (e.g., Li, Rb, Sr) generally indicate that water/rock ratios are substantially lower than~10 in many mid-ocean ridge hydrothermal systems (Von Damm et al, 1985;Berndt et al, 1989) with values of 0.4 to 6 calculated for the subsurface at Von Damm (McDermott, 2015) and 2 to 4 at Lost City (Foustoukos et al, 2008) for example.While only slow-spreading environments were investigated in this study, we hypothesize that the same origin of methane applies at sites on the fast-spreading East Pacific Rise, particularly given the similar δ 13 C values of methane there (Welhan and Craig, 1983). The fact that these hydrothermal fluids contain low C 2+ along with low CH 4 concentrations (Welhan, 1988b;Keir, 2010) suggests a genetic link between CH 4 and the C 2+ hydrocarbons.…”