Project

marine hydrothermal systems; multidisciplinary studies; serpentinization; ophicalcites; fluid fluxes; Lost City; Mid-Atlantic Ridge; carbonate precipitation; isotopes; strontium; neodymium; detachment faulting; oceanic core complexes

Lead
Lay summary
It is now recognized that ultramaficrocks - tectonically emplaced along major, large-scale offset normal faults(detachment faults) - comprise up to ~20%of the surface of the seafloor at slow spreading ridge environments. Oceanicdetachment faults lead to the formation of oceanic core complexes, characterizedby extensive exposure of gabbro and mantle rocks at the seafloor. Inaddition, detachment faults provide important conditions for fluid flow and maysustain hydrothermal systems that are often hosted in lithosphere dominated byultramafic rocks. The alteration of mantle rocks during serpentinization is afundamental process that has significant geophysical, geochemical andbiological importance for the global marine system and for subduction zoneprocesses. We will conduct a field and laboratory study of the petrology,geochemistry, and isotopic compositions (Sr, Nd, O, H, C and B) of deformationand alteration processes in two ultramafic-hosted seafloor systems: theJurassic Bracco-Levanto ophiolite of E. Liguria, Italy and the present-day LostCity hydrothermal system on the Atlantis Massif (30°N, Mid-Atlantic Ridge).Emphasis will be placed on the links between fluid flow and deformation, ondetermining fluid flow paths and fluid fluxes, and on processes leading tohydrothermal carbonate precipitation and the formation of ophicalcites. Ourproject aims to compare these two systems with the goals to: (1)  constrain hydrothermal alteration patterns and fluidflow paths in low to moderate temperature, serpentinite-hosted hydrothermalsystems; (2) examine the role of detachment faulting and changesfrom brittle to ductile deformation for hydrothermal activity during theevolution of OCCs; and (3) make estimates of fluid fluxes and mass transfer. This comparative study will ultimatelyprovide a better understanding of subsurface processes in serpentinite-hostedhydrothermal systems and will contribute to a comprehensive, integrated modelof end-member hydrothermal systems in crust formed at slow spreading ridgeenvironments.

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Last update: 21.02.2013

Active participation

Self-organised

Active participation

Number	Title	Start	Funding scheme

It is now recognized that ultramafic rocks - tectonically emplaced along major, large-scale offset normal faults (detachment faults) - comprise up to ~20% of the surface of the seafloor at slow spreading ridge environments. Oceanic detachment faults lead to the formation of oceanic core complexes, characterized by extensive exposure of gabbro and serpentinized mantle at the seafloor. In addition, detachment faults provide important conditions for fluid flow and may sustain hydrothermal systems that are often hosted in lithosphere dominated by ultramafic rocks. The alteration of mantle rocks during serpentinization is a fundamental process that has significant geophysical, geochemical and biological importance for the global marine system and for subduction zone processes. We propose to conduct a field and laboratory study of the petrology, geochemistry, and isotopic compositions (Sr, Nd, O, H, C and B) of deformation and alteration processes in two ultramafic-hosted seafloor systems: the Jurassic Bracco-Levanto ophiolite of E. Liguria, Italy and the present-day Lost City hydrothermal system on the Atlantis Massif (30°N, MAR). Emphasis will be placed on the links between fluid flow and deformation, on determining fluid flow paths and fluid fluxes, and on processes leading to hydrothermal carbonate precipitation and the formation of ophicalcites. Lost City is unlike all known marine hydrothermal systems: it hosts extensive carbonate-brucite structures that are deposited from 25-90°C, high pH (9-11) fluids emanating from fault zones that tap a region of active serpentinization in mantle peridotites that have been uplifted through detachment faulting during formation of the Atlantis Massif as an oceanic core complex. Ophiolite complexes in the Alps and Apennines often comprise large portions of serpentinized peridotite that are intruded by gabbroic rocks. These often contain domains of carbonate veins, carbonate-serpentine breccias and serpentine-hosted deposits, so-called ophicalcites, which show remarkable similarities with the serpentinite-hosted carbonate deposits recovered at Lost City. This leads to the hypothesis that the Lost City vent field is a modern analogue for the hydrothermal processes that produced ancient ophicalcites. Our project aims to compare these two systems with the goals to:•constrain hydrothermal alteration patterns and fluid flow paths in low to moderate temperature, serpentinite-hosted hydrothermal systems; •examine the role of detachment faulting and changes from brittle to ductile deformation for hydrothermal activity during the evolution of OCCs; •make estimates of fluid fluxes and mass transfer; and •determine the role of pyroxene alteration as well as brucite and serpentine formation in controlling the chemical and isotopic composition of venting fluids. This comparative study will ultimately provide a better understanding of subsurface processes in serpentinite-hosted hydrothermal systems and will contribute to a comprehensive, integrated model of end-member hydrothermal systems in crust formed at slow spreading ridge environments.