Serpentinization, Carbonation, and Metasomatism of Ultramafic Sequences in the Northern Apennine Ophiolite (NW Italy)

Fluid-rock interaction in ultramafic rocks considerably affects the chemical and isotopic composition of the oceanic lithosphere. We present a geochemical and petrological study of serpentinites and ophicalcites of the Northern Apennine ophiolite, Italy. This ophiolite sequence represents fragments of Jurassic oceanic lithosphere that have been denuded by low angle detachment faults, exposing peridotites on the ocean floor and triggering hydrothermal alteration. Seawater circulation is documented by (Jurassic) seawater-like Sr-87/Sr-86 values and delta C-13 values of 1.1-3.0 parts per thousand in carbonate veins of the ophicalcites. Bulk rock ophicalcites have low Sr-87/Sr-86 values of 0.70489-0.70599, elevated SiO2 contents, and talc druses filling calcite veins that record Si-metasomatism. In contrast, underlying serpentinites have Sr-87/Sr-86 values above Jurassic seawater values. Bulk rock delta D and delta O-18 values of ophicalcites and serpentinites suggest interaction with an evolved seawater-derived and/or magmatic fluid. These chemical signatures result from a complex history of serpentinization, carbonation, and metasomatism. Multiphase water-rock interaction includes infiltration of basement-derived fluids during initial mantle upwelling within an opening ocean basin, followed by localized high-temperature fluid infiltration, extensive seawater circulation resulting in carbonation, and oxidation near the seawater-exposed surface, and finally, fluid-rock interaction with overlying mafic lithologies leading to Si-metasomatism. The studied sequence represents an excellent example of the evolution from serpentinite to ophicalcite during continuous uplift and exposure of ultramafic rocks on the seafloor and documents the complex hydrothermal evolution of ultramafic rocks associated with this process. The extensive chemical transformation of mantle peridotites likely has an impact on geochemical cycles and subduction zone processes.

Automated summary

Fluid-rock interaction in ultramafic rocks plays a significant role in shaping the chemical and isotopic composition of the oceanic lithosphere. The study of serpentinites and ophicalcites in Italy's Northern Apennine ophiolite reveals a complex history of interactions, including seawater circulation, Si-metasomatism, and carbonation. These interactions likely have implications for geochemical cycles and subduction zone processes.