OICC Press Iranian Journal of Earth Sciences 2228-785X 2026 07 14 Geochemical Evidence from Olivine for Mantle Metasomatism and Magma Evolution in Eocene Basalts, North Ardabil, NW Iran 10.57647/ijes.2027.04881 EN Solmaz Mohammadrezai Department of Earth Science, University of Tabriz, Tabriz, Iran Ahmad Jahangiri Department of Earth Science, University of Tabriz, Tabriz, Iran Luca Caricchi Department of Earth Sciences, University of Geneva, Geneva, Switzerland Mohsen Moayyed Department of Earth Science, University of Tabriz, Tabriz, Iran Journal Article 2026 07 14 The Eocene volcanic rocks of north Ardabil, NW Iran, provide valuable insight into the dynamic history of mantle processes and subduction-related magmatism along the Alborz-Azarbaijan magmatic belt. This study combines new petrographic observations, whole-rock geochemistry, and high-precision mineral chemistry, focusing on olivine phenocrysts from pyroxene-phyric basalt, basalt, and trachybasalt. These basalts exhibit porphyritic textures, characterized by a dominant presence of clinopyroxene, olivine, and plagioclase within a fine-grained groundmass. Four distinct olivine types were identified based on size, zoning patterns, and chemical composition, with forsterite (Fo) contents ranging from Fo59 to Fo92. Whole-rock geochemical analyses classify the host rocks as High-K subalkaline, characterized by LILEs and LREEs, along with depletion of HFSEs and HREEs, consistent with formation in a subduction-related arc setting. Elevated concentrations of Ni, Ca, and Mn in olivine phenocrysts and whole-rock compositions suggest a mantle source dominated by re-fertilized peridotite, with contributions from pyroxenite. Moreover, geochemical signatures indicate complex mantle metasomatism involving carbonate- and silicate-rich sediment melts, reflecting heterogeneous metasomatic processes beneath this volcanic field during the Eocene.