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Understanding peridotite-hosted hydrothermal systems on the seafloor: Insights from Lost City

English title Understanding peridotite-hosted hydrothermal systems on the seafloor: Insights from Lost City
Applicant Bernasconi-Green Gretchen
Number 107620
Funding scheme Project funding (Div. I-III)
Research institution Institut für Mineralogie und Petrographie ETH Zürich
Institution of higher education ETH Zurich - ETHZ
Main discipline Other disciplines of Earth Sciences
Start/End 01.04.2005 - 31.03.2007
Approved amount 139'903.00
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Keywords (12)

Lost City; Mid-Atlantic Ridge; marine hydrothermal systems; serpentinization; carbonate precipitation; microbial activity; hydrothermal carbonate; geochemistry; isotopes; volatiles; carbon; sulfur

Lay Summary (English)

Lay summary
The discovery of the Lost City Hydrothermal Field (LCHF) at the top of the Atlantis Massif at 30N near the Mid-Atlantic Ridge provided an unprecedented opportunity to study geological, biological and hydrothermal processes in a system controlled by moderate temperature serpentinization reactions. Our project is a continuation of SNF Project 2100-068055, both of which supported the PhD thesis work of Adélie Delacour. Our study is part of an extensive multidisciplinary, international project as part of the Ridge2000 Program, funded by the US-NSF and involves scientists from ETH-Zurich, Univ of Washington, Duke Univ, WHOI, Harvard Univ and MIT. Lost City is unlike all other known marine hydrothermal systems: it is off-axis, peridotite-hosted and consists of carbonate-brucite structures deposited from 40-90°C, high pH (9-11) fluids emanating from fault zones that tap a region of active serpentinization. The diffusely venting fluids are rich in CH4 and H2 and support dense microbial communities that include anaerobic anaerobic CH4- and S-cycling thermophiles. Our project combines petrological, geochemical, isotopic, and organic geochemical studies to address the questions: What mineral-fluid reactions and conditions of serpentinization are documented at the Atlantis Massif? How do these influence chemical compositions during seawater-rock interaction? What are the physical, chemical and biological controls on carbonate precipitation? What processes control C-H-S speciation and the evolution of volatiles (CH4, H2, and H2S) in these environments, and how are these linked to microbial activity at the vent sites and in the subsurface?
Our studies show that serpentinites underlying Lost City are harzburgitic in composition and reflect formation and uplift of a heterogeneous lithosphere in a magma-starved spreading environment, with progressive serpentinization, talc-amphibole metasomatism and veining. Seawater-peridotite interaction at 150-250°C and high fluid-rock ratios (>100) produced enrichments in B, U and light REE, systematic changes in Sr- and Nd-isotope ratios towards seawater values, and highly depleted bulk rock O-, H-, and B-isotopic compositions. B-isotope analyses indicate that brucite is a significant, temporally variable, reservoir for Mg and B. High fluid fluxes have important implications for S- and C-cycles: a loss of primary sulfide, an uptake of seawater sulfate, and local microbial remediated sulfate reduction and sulfide oxidation. Our studies suggest that serpentinites may represent an as yet unidentified reservoir for dissolved organic carbon from seawater. We conclude that the interaction of seawater with peridotites as well as with pervasively serpentinized peridotites is crucial to the formation of Lost City-type systems and that transform-related normal faulting and mass wasting facilitates seawater penetration necessary to sustain hydrothermal activity over tens of thousands of years.
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Direct link to Lay Summary Last update: 21.02.2013

Responsible applicant and co-applicants


Associated projects

Number Title Start Funding scheme
68055 Return to Lost City: Understanding peridotite-hosted hydrothermal systems on the seafloor 01.04.2003 Project funding (Div. I-III)
116226 Serpentinization, Fluids and Life: Comparing Carbon and Sulflur Cycles in Modern and Ancient Environments 01.05.2007 Project funding (Div. I-III)