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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]

Serpentinization occurs both within the seafloor, when seawater percolates through fractures to alter underlying peridotite, and in subduction zones, when aqueous fluids from the dehydration of the slab are released within the subduction channel. The main difference between these two environments is, therefore, the source of serpentinizing fluid. However, distinguishing between the various types of serpentinites encountered in suture zones is difficult, essentially because of the lack of significant differences in their mineralogy and major and trace element chemistry. Eight samples from Corsica and the Western Alps representing slab serpentinites (i.e., serpentinites embedded within the subducting slab, which are fragments of former abyssal serpentinites) and seven samples from Guatemala and Nicaragua representing mantle wedge serpentinites (i.e., serpentinites that form in situ above the subducting slab, by hydration of the overlying mantle wedge) have been selected to study their Fluid-Mobile Elements, REE concentrations, and boron (B) isotopic signatures. The trace element results do not allow a conclusion about the source of fluid(s), but the B isotopic signatures do permit identification of the different fluid sources responsible for the serpentinization. The δ11B values of slab serpentinites are significantly positive, ranging from +12 to +34‰, in the same range as present-day abyssal serpentinites (+5.5 to +40.5‰), which are altered by seawater. Therefore, these positive signatures indicate serpentinization by seawater-derived fluid. The B isotopic signature acquired during oceanic residency is therefore preserved even after subduction processes subjected the serpentinites to eclogite facies conditions. By contrast, δ11B in samples from the mantle wedge range from −14 to +10‰, supporting the interpretation of interactions with fluids derived from the progressive dehydration of the slab, which releases fluids with slightly positive and negative δ11B. It indicates that mantle wedge serpentinite isotopic signature is acquired via the infiltration of subducted crust-derived metamorphic fluids during the subduction processes. [ABSTRACT FROM AUTHOR]