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Obducted ophiolites expose oceanic lithosphere slivers in suture zones. The few previous studies focusing specifically on the serpentinization of the ultramafic part of ophiolites mostly concluded that serpentinization was driven by seawater. Recent developments in B isotope geochemistry have enabled the tracking of fluid source(s) in serpentinites, suggesting δ11B values above +10‰ mostly reflect seawater hydration and δ11B values below +10‰ mostly reflect hydration by subducted crust-derived metamorphic fluids. In this study, we further investigate the fluid sources recorded by ophiolitic serpentinites by presenting B isotopic geochemistry for seventeen samples from well-characterized ophiolites in Guatemala, Iran, and Brazil. The samples from Guatemala and Iran are from fossil Supra Subduction Zones (SSZ), while samples from Brazil represent a fossil Ocean-Continent Transition (OCT). Most of the samples display the pseudomorphic mesh matrix and bastite microtexture typical of serpentinites. Nine of the samples are either totally (both microtextures) or partially (one microtexture only) out of isotopic equilibrium, with δ11B variations of up to 18‰ within individual samples/textures. Among the samples with a narrow δ11B range (i.e., at isotopic equilibrium), those from a fossil OCT ophiolite have δ11B ranging from +3.8 to +23.2‰, which is broadly consistent with serpentinization by seawater. In contrast, serpentinites from fossil SSZ ophiolite have δ11B straddling 0‰, ranging from −7.7 to +13.5‰. These data indicate that seawater alone cannot be responsible for serpentinization of these ultramafic rocks, but rather either subducted crust-derived metamorphic fluids, or a mixture of seawater and subducted crust-derived metamorphic fluids. This interpretation agrees with the recent suggestions that SSZ ophiolites possibly represent fossil forearc lithosphere. • Serpentinites in SSZ obducted ophiolites have discriminating B isotopic signatures. • Serpentinizing fluid is a mixture of seawater and subducted crust-derived fluids. • Mixing mostly occurred prior to serpentinization. [ABSTRACT FROM AUTHOR]