Serpentinization processes in modern and ancient oceanic environments

Hydration of the oceanic mantle directly influences geophysical properties in that serpentinization is accompanied by a decrease in bulk density, which directly affects the physical properties of the mantle, gravity signatures and seismic velocities, and may have important tectonic consequences.  Furthermore, the process of serpentinization may have major implications on the overall chemical budget of the oceans, for example by producing H2 and CH4 and providing Ca, Mg, Ni, B, etc. to phases formed during subsequent alteration of the oceanic crust.  We are investigating serpentinization processes in different tectonic environments along mid-ocean ridges; at oceanic fracture zones; and in arc-subduction zone environments, which includes field and laboratory studies of the Lost City Hydrothermal Field. This hydrothermal system, characterized by metal- and CO2-poor, high pH fluids with elevated hydrogen and methane contents provides a unique marine environment to study serpentinization processes at depth.

This project has focused on the links among fluid-rock interaction, hydrothermal deposits and deformation processes recorded in the Jurassic Bracco-Levanto and Val Graveglia ophiolite complexes in Liguria (Italy) and compared these processes to modern oceanic hydrothermal systems hosted in ultramafic and gabbroic rocks along the Mid-Atlantic Ridge (MAR). We conducted petrological, major element, trace element and isotopic (O, C, H, Sr) analyses of basement rocks and the hydrothermal deposits in Liguria to investigate fluid flow paths, mass transfer, and fluid fluxes during high and low temperature hydrothermal activity, with emphasis on processes leading to the formation of carbonate-vein systems in serpentinites, so-called ophicalcites which are characteristic of the Bracco-Levanto sequences. We compare these data to new and published data from the MAR and from the Lost City hydrothermal field in order determine if the present-day Lost City hydrothermal processes are a modern analogue for those that produced ancient ophicalcites. This comparative study provide a better understanding of evolving, subsurface processes in serpentinite-hosted hydrothermal systems and contribute to a comprehensive, integrated model of end-member hydrothermal systems in oceanic sequences formed at slow spreading ridge environments.

Processes similar to those operating at the Lost City hydrothermal system occur in the ophiolites of Liguria, Italy, where fluids originating from deep aquifers circulate through variably serpentinized peridotites, resulting in Mg-rich to Ca-rich, high pH (10 - 12) waters, with elevated hydrogen and methane concentrations. We linked inorganic reactions in the ultramafic basement rocks (i.e. serpentinization reactions) to microbial activity by characterizing the cycling of carbon and nitrogen in these high pH systems. Elucidating the sources of carbon to these systems is of particular interest as small organic compounds are hypothesized to form abiologically under conditions similar to those found in the subsurface of these two sites. Field and laboratory studies (1) indicated the high concentrations of the organic acid, formate, found at Lost City is abiologically formed and favorable under high hydrogen concentrations which are absent at the Ligurian site; (2) the primary carbon sources could be linked to microbial communities and these compounds are the result of serpentinization reactions (CH4, formate); (3) support a biological and abiogenic component to amino acid formation at Lost City;  (4) were studied to elucidate the role of nitrogen fixation at these locations; and (5) evaluate the metabolic reactions available for chemolithoautotrophy in these environments.

This project, and the continuation projects, formed a comparative bio-geochemical and isotopic study of the Lost City hydrothermal system (North Atlantic) with modern and ancient serpentinite-carbonate systems in Liguria (Italy) and the Iberian Margin. The Lost City hydrothermal system is distinctly different than all other known marine hydrothermal systems and represents an important analogue for ancient ophicalcite deposits. Our project focused on understanding the links between inorganic reactions that produce hydrogen and hydrocarbons, biogeochemical cycling of carbon and sulfur, and microbial activity in high pH systems associated with serpentinization. In addition to conducting follow-up geochemical analyses on samples from Lost City, we investigated high alkaline Ca-OH springs associated with present-day serpentinization and carbonate deposition in Liguria and Oman and compared these with studies of Jurassic ophicalcites. We quantified carbon and sulfur pools in active serpentinite-carbonate systems and modeled their changes over time. This investigation of the Lost City system and the comparative studies of similar systems on land provided information about the mass of CO₂ locked into serpentinite-dominated environments and the viability of variably serpentinized rocks to sequester anthropogenic CO₂.

Our research was part of a multidisciplinary investigation of the Lost City Hydrothermal Field (LCHF) that involved scientists from the ETH-Zürich, the University of Washington, Syracuse University, Woods Hole Oceanographic Institution, and Massachusetts Institute of Technology. A highly successful oceanographic mapping and sampling program was completed in April-May, 2003 and new imagery and samples were collected in 2005. The LCHF is distinct from all other known marine hydrothermal systems and is the product of reactions between seawater and ultramafic rocks that produce high alkaline (pH 10 to 11), 40 to 90°C fluids that form up to 60m tall carbonate-brucite towers. The fluids are enriched in H₂, CH₄ and other hydrocarbons, produced abiotically through Fischer-Tropsch type reactions, and support dense microbial communities that include anaerobic CH₄- and S-cycling thermophiles. Collectively, our multidisciplinary investigations have highlighted the complex interplay between deformation, fluid flow, mass transfer and microbial activity and indicate that high fluid fluxes and temperatures of ~100°-150°C have likely been sustained in the basement for many tens of thousands of years.