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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