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

Monica Vogel,Gretchen L. Früh‐Green,Esther M. Schwarzenbach,Chiara Boschi

doi:10.1029/2020jb020619

Abstract

AbstractFluid‐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 87Sr/86Sr values and δ13C values of 1.1–3.0‰ in carbonate veins of the ophicalcites. Bulk rock ophicalcites have low 87Sr/86Sr values of 0.70489–0.70599, elevated SiO2 contents, and talc druses filling calcite veins that record Si‐metasomatism. In contrast, underlying serpentinites have 87Sr/86Sr values above Jurassic seawater values. Bulk rock δD and δ18O 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.

Highlights

Ocean ridge spreading centers produce around 18 km3 of oceanic crust each year (Cogné & Humler, 2006)
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 fluid-rock interaction with overlying mafic lithologies leading to Si-metasomatism
This study summarizes the extensive alteration history of ophicalcites and serpentinites in the N

Summary

Introduction

Ocean ridge spreading centers produce around 18 km of oceanic crust each year (Cogné & Humler, 2006). The majority of newly formed oceanic crust is composed of mafic lithologies, approximately 9%– 20% of the crust formed along slow spreading ridges comprises serpentinized peridotite (Alt, Schwarzenbach, et al, 2013; Cannat et al, 1995, 2010) In these tectonic settings mantle rocks are exposed to seawater and altered via a complex interplay of magmatism, asymmetric extension, and detachment faulting, which is commonly linked to the formation of oceanic core complexes (Cannat, Sauter, et al, 2006; Früh-Green et al, 2018; John & Cheadle, 2010; Tucholke et al, 2008). Subduction of the altered oceanic lithosphere transports various elements into Earth’s interior (e.g., Alt, Schwarzenbach, et al, 2013; Scambelluri et al, 2004), affecting arc magmatism (e.g., Ulmer & Trommsdorff, 1995) and the chemical and isotopic evolution of Earth’s interior (e.g., Li et al, 2020)

Results

Discussion

Conclusion