HR: 09:35h
AN: OS41C-06 INVITED    [Abstracts]
TI: From Mantle to Microbes: The Cycling of Volatiles in Slow- and Ultraslow Ridge Environments
AU: * Früh-Green, G L
EM: frueh@erdw.ethz.ch
AF: ETH Zurich, Institute for Mineralogy and Petrology, Zurich, CH-8092, Switzerland
AB: Olivine-rich gabbros and mantle peridotites are significant components of the seafloor in slow- and ultraslow- spreading ridge environments. The alteration of these rocks has fundamental geophysical and biogeochemical consequences for the global marine system and is critical in cycling volatiles from the mantle to the biosphere. Thus, it is important to understand the extent and conditions of alteration (e.g., temperature, fluid flux, oxidation states) and the impact of these processes for the formation and speciation of C-O-H-S fluids as nutrients for biological activity. This presentation is intended to provide an overview of magmatic and hydrothermal processes that control hydrothermal fluid compositions and the formation of hydrocarbons in submarine environments at the lower end of the spreading rate spectrum and with variable tectonic histories. Emphasis will be placed on how fluid chemistries in peridotite-hosted hydrothermal systems (Rainbow, Logatchev, Lost City and Saldanha along the MAR) differ from those in black smoker systems, and on the role of serpentinization in generating highly reduced, volatile-rich hydrothermal fluids and in the biological communities they may support. In addition to directly influencing the geophysical properties of the oceanic lithosphere, serpentinization processes provide an important sink for many major and minor elements from seawater (e.g., H₂O, Mg, B, U, SO₄, DIC, and DOC) and result in major changes in the isotopic compositions of O, H, S, C, B, Sr, and Nd in the rocks. Serpentinization also produces elevated concentrations of primarily abiogenic methane (up to 2.5 mmol/kg) and hydrogen (up to 16 mmol/kg) that have important consequences for vent microbial communities and for the existence of a deep H₂-based biosphere. Varying fluid fluxes influence redox conditions and the stabilities of oxides, sulfides, and FeNi alloys, which together with microbial activity, can significantly affect C-H-S cycles in these systems. In addition, recent experimental studies and vent fluid data provide evidence for abiotic synthesis of organic compounds through Fischer-Tropsch type reactions during serpentinization and provide constraints on alternative pathways for the formation of early membranes and the origin of life. At moderate temperatures, such as at Lost City, serpentinization produces high pH fluids that promote carbonate precipitation and produce large, porous hydrothermal structures. The production of reduced volatiles and variable mixing with ambient seawater in the subsurface and in near-vent environments is an important process in creating strong chemical gradients that provide micro-niches for distinct communities of H-, S- and CH ₄-utilizing archaea, bacteria as well as eubacteria, and which results in high organic carbon contents (up to 1 wt%) in the hydrothermal structures. The ultraslow-spreading Arctic ridges are characterized by a variable degree of volcanism, a lack of transform faults, and the juxtaposition of crustal and mantle rocks along magmatic and amagmatic ridge segments. These ridges are potential sites in which moderate-temperature, peridotite-hosted hydrothermal systems, similar to Lost City, may occur in close proximity to basalt-hosted, black smoker systems - and are thus areas with potentially highly variable volatile contents and fluid compositions which could support a large range of micro-organisms in the subsurface and in near-vent environments.
UR: http://www.oceanexplorer.noaa.gov/explorations/05lostcity/welcome.html
DE: 1030 Geochemical cycles (0330)
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1065 Major and trace element geochemistry
DE: 3017 Hydrothermal systems (0450, 1034, 3616, 4832, 8135, 8424)
DE: 4806 Carbon cycling (0428)
SC: Ocean Sciences [OS]
MN: 2007 Fall Meeting