From seafloor spreading to subduction initiation: Oceanic Core Complex vs. forearc serpentinization in the Mirdita ophiolite (Albania)

The geochemical signatures of serpentinites inform on the provenance and composition of interacting fluids across various environments, from shallow seafloor to deep subduction zones. However, in obducted ophiolitic terranes, where multiple stages of fluid-rock interactions can take place, determining the timing of serpentinization and the different fluid sources remains challenging. The Mirdita ophiolite in Albania formed in a suprasubduction-zone setting during the lower- to middle-Jurassic and was then obducted onto the Adriatic Margin. It preserves oceanic lithosphere formed in an inferred Oceanic Core Complex (OCC) as well as a forearc setting with basal contacts that contain slices of the metamorphic sole.

We sampled serpentinized peridotites from the Puka OCC, that were largely unaffected by metamorphic overprinting during obduction, and two basal tectonic contacts with contrasting metamorphic soles (mafic amphibolite to the west, metasediment-rich to the east), allowing direct comparison of oceanic and forearc serpentinization within a single ophiolitic body. We combine oxygen and deuterium stable-isotope analyses within situ trace-element analyses of serpentine using LA-ICP-MS to constrain fluid sources and serpentinization conditions with the aim to infer the hydrothermal and geodynamic evolution of this ophiolite sequence.

Within the OCC of the Puka massif, the analyzed sample is a weakly serpentinized peridotite. The δ¹⁸O_serp composition is near primary mantle composition, whereas the δD_serp composition is comparatively lower than primary mantle. The OCC peridotite exhibits enrichment in Sr, Li, Sb, and slight B enrichment relative to primitive mantle, with no Cs enrichment, resulting in a Rb/Cs ratio greater than 1. These results suggest low degrees of serpentinization along the OCC shear zone that involved seawater. Serpentinite mineral separates at both basal contacts exhibit similar high δ¹⁸O_serp and low δD_serp signatures, within the range of ophiolitic serpentines and consistent with fluids derived from the downgoing plate. Both samples from the two contacts show high boron concentrations. Serpentinites from the eastern basal contact are enriched in Cs, Rb, and U, with a low Rb/Cs ratio (<1), suggesting Cs influx. In contrast, serpentinites from the western contact have higher Ba, Pb, and As concentrations, coupled with lower Nb, La, and Ce. These variations indicate fluids that equilibrate with different lithologies: the western contact reflects a mafic (AOC) source, whereas the eastern contact rather records a sedimentary influence with enrichment in LILE and low Rb/Cs.

This study demonstrates that serpentine δ¹⁸O compositions and trace-element signatures jointly provide ideal constraints on serpentinization conditions and fluid sources, thereby constraining their hydrothermal evolution and geodynamic setting.