Biogeochemical processes in the oceanic lithosphere
Hydrothermal alteration of ultramafic rocks has a major impact on the global biogeochemical cycles. In our group we study the links between mineral reactions and biogenic activity and how these chemical interactions impact the global geochemical cycles. Using mineralogical and petrological techniques (e.g., EMP, SEM, micro-XANES) combined with bulk rock and in-situ (isotope) geochemistry we study mafic und ultramafic rocks from ophiolite sequences and ocean drill cores to reveal the complex interplay between magmatic, hydrothermal, and biogenic processes taking place in the oceanic lithosphere.
Alteration history of peridotite-hosted hydrothermal systems
Ultramafic rocks exposed along slow- to ultraslow-spreading oceanic ridges can undergo a complex sequence of alteration reactions including serpentinization, silica metasomatism, carbonation, and oxidation. In our research, we use geochemical tracers such as sulfur and carbon stable isotope and Sr isotope compositions to investigate the origin of the fluids that altered these rocks. Especially during early stages of the opening of an ocean basin, there is evidence that basement-derived fluids impact the chemistry of upwelling mantle rocks. In contrast, during later oceanic core complex formation and exposure of upper mantle rocks to seawater, gabbroic (micro)intrusions can have an impact on the chemistry of circulating fluids and the temperature regime of the subsurface of active hydrothermal systems. Exploring the complex alteration history of peridotite-hosted hydrothermal systems is key to infer the episodes of water-rock interaction that may facilitate a deep biosphere and, on the other hand, to constrain the impact of this extensive chemical transformation on the global geochemical cycles and subduction zone processes.
For this we study rocks from numerous ophiolite sequences and drill cores recovered by the Ocean Drilling Program (ODP) and the International Ocean Discovery Program (IODP). Current study locations include the Northern Apennine ophiolite in Italy, the Mirdita ophiolite in Albania, the Santa Elena ophiolite in Costa Rica, and the Atlantis Massif at 30°N along the Mid-Atlantic Ridge that hosts the Lost City hydrothermal field.
Figure caption: Talc-amphibole-chlorite alteration zone in a serpentinite drill core from Hole M0072B from IODP Exp. 357 to the Atlantis Massif. See also Früh-Green et al. (2018). Click on the image for a larger view!