Laboratory and field-based investigations of subsurface geochemical processes in seafloor hydrothermal systems

Abstract

This thesis presents the results of four discrete investigations into processes governing the organic and inorganic chemical composition of seafloor hydrothermal fluids in a variety of geologic settings. Though Chapters 2 through 5 of this thesis are disparate in focus, each represents a novel investigation aimed at furthering our understanding of subsurface geochemical processes affecting hydrothermal fluid compositions. Chapters 2 and 3 concern the abiotic (nonbiological) formation of organic compounds in high temperature vent fluids, a process which has direct implications for the emergence of life in early Earth settings and sustainment of present day microbial populations in hydrothermal environments. Chapter 2 represents an experimental investigation of methane (CH 4 ) formation under hydrothermal conditions. The overall reduction of carbon dioxide (CO 2 ) to CH 4 , previously assumed to be kinetically inhibited in the absence of mineral catalysts, is shown to proceed on timescales pertinent to crustal residence times of hydrothermal fluids. In Chapter 3, the abundance of methanethiol (CH 3 SH), considered to be a crucial precursor for the emergence of primitive chemoautotrophic life, is characterized in vent fluids from ultramafic-, basalt-and sediment-hosted hydrothermal systems. Previous assumptions that CH 3 SH forms by reduction of CO 2 are not supported by the observed distribution in natural systems. Chapter 4 investigates factors regulating the hydrogen isotope composition of hydrocarbons under hydrothermal conditions. Isotopic exchange between low molecular weight n-alkanes and water is shown to be facilitated by metastable equilibrium reactions between alkanes and their corresponding alkenes, which are feasible in natural systems. In Chapter 5, the controls on vent fluid composition in a backarc hydrothermal system are investigated. A comprehensive survey of the inorganic geochemistry of fluids from sites of hydrothermal activity in the eastern Manus Basin indicates that fluids there are influenced by input of acidic magmatic solutions at depth, and subsequently modified by variable extents of seawater entrainment and mixing-related secondary acidity production. CHAPTER 1 IntroductionSeafloor hydrothermal systems associated with the generation of oceanic crust represent a dramatic expression of heat and mass transfer from the interior of the Earth. In addition to profound effects on ocean chemistry, convective circulation of seawater through volcanically active oceanic crust leads to the formation of metal sulfide deposits. In addition, seafloor hot springs are often invoked as ideal settings from which early microbial life could have emerged.Since the initial discovery of low temperature venting at the Galápagos Spreading Center in 1977 (CORLISS et al., 1979), our understanding of the composition and diversity of hydrothermal solutions has expanded greatly. During convection through oceanic crust of basaltic composition, O 2 -replete seawater reacts with crustal rock, typically producin...

The geochemistry of methane isotopologues

“…Carbonate precipitation occurs rapidly during alteration of peridotite (Grozeva et al, 2017). In contrast, laboratory studies consistently find sluggish reaction kinetics for the reduction of ∑CO2 to CH 4 in the presence and absence of powdered peridotite or mafic mineral phases (McCollom and Seewald, 2001;McCollom and Seewald, 2003;Seewald et al, 2006;Reeves, 2010;McCollom, 2016;Grozeva et al, 2017). Certain transition metal catalysts can enhance rates of CH 4 production (Horita and Berndt, 1999;Foustoukos and Seyfried, 2004), but H 2 concentrations several orders of magnitude higher than those found in vent fluids are required to render native Fe-Ni alloys stable (Frost, 1985;Charlou et al, 2002;Sleep et al, 2004;McCollom and Bach, 2009).…”

Insights to magmatic–hydrothermal processes in the Manus back-arc basin as recorded by anhydrite

“…Fluids discharging from black-and gray-smoker chimneys have measured temperatures between 270 and 341°C and pH (25°C) in the range of 2.3-2.6 (Seewald et al, 2006;Craddock et al, 2010;Reeves, 2010). Diffuse fluids, with temperatures ranging from 54 to 106°C, discharge from fissures within the volcanic basement and through chimney and volcanic talus in areas surrounding sulfide deposits.…”

“…The pH (25°C) of sampled vent fluids ranges from 3.4 to 4.6 (Seewald et al, 2006;Craddock et al, 2010;Reeves, 2010). The sulfide composition of chimneys venting low temperature fluids is predominantly bornite (±chalco-pyrite) and tennantite with minor sphalerite (Craddock, 2009).…”

“…The summit of the mound at a water depth $1705 m is composed of an extensive black smoker complex with fluids venting vigorously at temperatures up to 358°C (boiling temperature at ambient pressures) and pH (25°C) $2.3-2.7 (Seewald et al, 2006;Craddock et al, 2010;Reeves, 2010). Rock sampling of the black smoker complex recovered chalcopyrite-rich open conduit chimneys (Craddock, 2009).…”

“…Seafloor rocks are composed of vesicular dacite flows and hyaloclastite, overlain by hydrothermal talus and sediment, native sulfur flows and locally extensive sulfide deposits (Binns et al, 1997;Tivey et al, 2007). At North Su, hydrothermal activity on the flanks of the volcano ($1260 m depth) is dominated by vigorous discharge of yellow-white fluids (Seewald et al, 2006;Craddock et al, 2010;Reeves, 2010). In the vicinity of active venting, native sulfur oozes through volcanic and hydrothermal sediments.…”