Serpentinization, Carbonation, and Metasomatism of Ultramafic Sequences in the Northern Apennine Ophiolite (NW Italy). Issue 5 (19th May 2021)

• Record Type: Journal Article
• Authors: Schwarzenbach, Esther M.; Vogel, Monica; Früh‐Green, Gretchen L.; Boschi, Chiara
• Is Part Of: Journal of geophysical research. Volume 126:Issue 5(2021)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2021-05-19
• Subjects: Carbonation -- isotope signatures -- Northern Apennine ophiolite -- peridotites -- serpentinization -- Si‐metasomatism; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2020JB020619
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: 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 87 Sr/ 86 Sr values and δ 13 C values of 1.1–3.0‰ in carbonate veins of the ophicalcites. Bulk rock ophicalcites have low 87 Sr/ 86 Sr values of 0.70489–0.70599, elevated SiO2 contents, and talc druses filling calcite veins that record Si‐metasomatism. In contrast, underlying serpentinites have 87 Sr/ 86 Sr values above Jurassic seawater values. Bulk rock δD and δ 18 O 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. Key Points: N. Apennine ophiolite records the evolution from serpentinite to ophicalcite during continuous uplift and exposure on the seafloor Continental basement‐derived fluids imprinted a chemical signature on serpentinites during initial stages of rifting and mantle upwelling Late stage mafic‐derived fluids induced pronounced Si‐metasomatism in ophicalcites