Volatiles, Fluid Flow and Geochemical Fluxes in the Oceanic Lithosphere

Characterizing the origin and evolution of volatiles such as CO₂, H₂ and CH₄ is crucial to studies of mid-ocean ridge systems because volatiles represent a common link between geological, hydrothermal and biological processes in ridge crest environments. Until recently little has been known about volatile evolution and fluids deep within the oceanic crust. A particular focus of our projects is to better understand the geological and geochemical processes that control volatile chemistry at depth in mid-ocean ridge systems and the impact of these in supporting microbial communities. By quantifying fluid compositions and characterizing the isotopic composition methane- and hydrogen-rich fluids in active vents on the seafloor or trapped as fluid inclusions in plutonic rocks from the oceanic layer 3, we aim to develop new models on the origin and spatial, temporal, and compositional variability of volatiles during the early evolution of the oceanic lithosphere and to constrain the significance of these fluids in mineral-fluid and microbial processes in submarine hydrothermal systems.

The interaction between fluids and the oceanic lithosphere is a first-order process that affects the cycling of elements and determines mass transfer between geochemical reservoirs. A major consequence of thermally driven migration of fluids through oceanic sediments and underlying oceanic crust is the mobilization, concentration and deposition of metals, locally to economically important levels. Our studies combine petrological, geochemical and stable isotope methods with microstructural investigations to characterize geochemical fluxes associated with fluid flow and hydrothermal alteration of the oceanic lithosphere. By examining dredged samples, drill cores from ODP and IODP and exhumed ocean floor sequences in orogenic belts we aim to quantify alteration on the ocean floor and distinguish this from metamorphism/metasomatism during collisional orogenesis.

This project was part of the European Science Foundation EUROCORES EuroMARC Programme, and consisted of six individual projects (IP) and one associated project. This interdisciplinary, international project aimed at studying geodynamic and hydrothermal processes and their links to the deep biosphere along the Southern Knipovich Ridge (SKR) and along the Arctic Mohn’s Ridge, one of the slowest spreading segments of the global ridge system. Comparative studies were made with the Northeast Lau Basin, a volcanic arc environment.

Our project combined petrological, geochemical, light stable isotope and organic geochemical studies on sediments obtained by gravity coring and on pore and vent fluids to constrain the nature of the hydrothermal system(s) two contrasting geotectonic settings: a mid-ocean ridge and a volcanic arc setting. Specific questions were addressed: What are the dominant mineral-fluid reactions and conditions (temperature, fluid flux, and relations to deformation) of alteration and hydrothermal activity at the SKR? Are the Jan Mayen Vent fields, a bare rock hydrothermal system at the Mohn’s Ridge, a sediment free analogue to the SKR? How does seawater-rock interaction change spatially and temporally along the ridge segment and influence the chemical compositions of the fluids and rocks in these systems? In which species do carbon, hydrogen and sulfur occur in these systems and what processes control their speciation and the evolution of volatiles (CH₄, H₂, CO₂, and H₂S) in these environments? How are volatiles linked to microbial activity at hydrothermal vent sites, in the subsurface, and in submarine volcanic eruptions characterized by event plumes?

We conducted boron concentration, NMR, and boron isotope studies of the Lost City hydrothermal fluids and hydrothermal carbonates to better understand fluid-rock interaction and mixing processes during fluid discharge at seafloor. Our studies also aimed at better understanding the (paleo)hydrology of the Lost City system and the role of early precipitates of brucite in controlling the chemical and isotopic composition of the venting fluids. These studies have provided important constraints for understanding the fate of Mg in peridotite-hosted hydydrothermal systems and the role of brucite as a sink for Mg and B.