ISOBAB

Isotope constraints on the contribution of metal-rich magmatic fluids to back-arc seafloor hydrothermal systems

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 Obiettivo del progetto (Objective)

'Chemical exchanges between the lithosphere-ocean-atmosphere are focused at the active plate boundaries and are important in the global geochemical balance. Eventually, they may influence climate and impact on human activities. Hydrothermal circulation at these boundaries is the engine that transfers chemicals between the lithosphere and ocean. However, the structure of lithospheric contribution (hydrothermal leaching of solid rocks vs. direct mantle degassing) to the ocean chemistry is poorly understood. Insight into this problem (sources of metals and location of ore deposits) becomes increasingly important with the current interest in mining seafloor deposits and has high societal relevance. Unique seafloor hydrothermal processes in back-arc basins (BAB) produce a wide range of vent fluid chemistry and mineral deposits, and are considered as critically important for the genesis of volcanogenic massive sulfide deposits. While it has been suggested that input of magmatic fluids plays a crucial role in the chemistry of the BAB hydrothermal systems, its contribution as source of both transition and precious metals, has been a matter of debate. We propose a research project aimed at providing new constraints to define the relative role of metal-rich magmatic fluids and sub-seafloor metal precipitation/remobilization in BAB mineralization. We will achieve this goal studying non-traditional isotope (Cd-Zn-Sb) systematic of seafloor hydrothermal systems (substrate rock-fluid-deposit) in a spectrum of seafloor hydrothermal fields (from basalt- to dacite-hosted) in 2 BAB (Manus, Lau). This project is backed-up by results from previous studies that suggest contribution of magmatic volatiles, and that the available sample set is ideal for testing our hypotheses. It is expected that insight gained from Cd-Zn-Sb isotope systematic has great potential to improve our understanding of fundamental sub-seafloor processes responsible for metallogenesis in BAB hydrothermal systems.'

Descrizione progetto (Article)

Growing interest in mining seafloor deposits of metals highlights the need for greater understanding of the chemical exchanges between the lithosphere (the Earth's outermost rocky shell) and the oceans covering much of it.

The global geochemical balance also has important implications for climate change.

EU-funded scientists working on the project ISOBAB studied unique hydrothermal systems associated with mineralisation to gain fundamental insight into sub-seafloor processes involved in metallogenesis.First discovered in 1977, hydrothermal vents are like geysers of super-heated mineral-rich water spewing from fissures along the ocean floor.

Hydrothermal processes in back-arc basins (BABs), a special type of sub-marine basin, generate a rich variety of vent chemistry and mineral deposits and therefore, chemical exchanges are important here.

Two sources likely contribute to BAB ore deposits: metal-rich magmatic fluids and sub-seafloor metal precipitation/remobilisation.

However, the contribution of each is not clear.ISOBAB chose non-traditional isotopes of cadmium, zinc and antimony coupled with sulphur isotopes in seafloor hydrothermal vent systems (fluids, deposits and substrate rocks) to gain insight.

They added iron and copper isotopes to the planned analyses early in the project for a comprehensive picture of mineralisation.

Through a combination of experiments and modelling, the team successfully described the main contributors to isotope variability in vent fluids as well as ruled out factors that do not contribute.Results are important to the EU's future in ocean exploration and ocean mining as well as to its excellence in non-traditional stable isotope chemistry.

A broader perspective includes a role for fundamental understanding of hydrothermal systems and their contribution to geochemical exchange in fostering better insight into implications for global climate change.