Carbonated ultramafic and mafic rock sequences from Point-Rousse Complex, Newfoundland: insights for carbon and sulfur mobility along plate-scale shear zones

Carbonation of ultramafic and mafic rocks is an important process that can occur in subduction zones, trapping and/or mobilizing carbon, sulfur, and fluid-mobile elements. Here we investigate two exceptional sequences of carbonated ultramafic and mafic rocks from the Point-Rousse Complex (Newfoundland, Canada) to understand the element mass-transfer, redox processes and physicochemical conditions triggered by fluid-flow along a major shear zone.

The complex comprises ophicarbonates (≤5.2 wt% CO2), soapstones (3.4–26.1 wt% CO2), carbonate-bearing greenschists (6.2–8.5 wt% CO2) and albite-dolomite rocks (29.1–35.4 wt% CO2). Petrography and chemical mapping reveal replacement and dissolution/precipitation textures of carbonation reactions. In ophicarbonate, serpentine is pseudomorphically replaced by magnesite and dolomite, preserving relict bastite and mesh textures. Foliated ophicarbonate exhibits the replacement of serpentine by coarse-grained talc and magnesite. Soapstone is characterized by corona-shaped reaction rims of quartz and talc surrounding dolomite crystals. Coarse-grained magnesite displays chemical zoning with increasing Fe content and core-to-rim decreasing of magnetite inclusions. Albite-dolomite rocks, composed primarily of dolomite, albite and muscovite are crosscut by centimetric albite veins. Carbonated greenschist, composed of albite, chlorite, dolomite, and pyrite, exhibits dissolution/precipitation structures associated with the formation of albite and carbonate veins. Pyrite mineralization is concentrated along albite vein margins in carbonated mafic rocks (bulk-rock S ≤3.3 wt%).

The reported carbonation occurred simultaneously in mafic and ultramafic rocks. Thus, this rock assemblage represents an ideal natural laboratory to investigate CO2 entrapment, sulfur mobility, and the sources and compositions of involved fluids.

Funding: FPI2022/PRE2023_IACT_059 linked to Grant PID2022-136471NB-C21 (RUSTED), granted by MCIN/AEI/10.13039/501100011033 and FSE+, Spain.