Chemical and Isotopic Exchanges in Serpentinites during Hydration and Dehydration Events in the Erro-Tobbio Massif (Italy): from Ocean to Subduction

Abstract Subducted serpentinites are important for recycling water and fluid mobile elements into the deep Earth. Here we present a texturally controlled in situ oxygen and boron isotope and trace element study of the Erro-Tobbio ultramafic rocks (Western Alps, Italy) that have experienced exhumation to the seafloor, hydration and subsequent dehydration upon subduction. High variability in δ18O from +0.2‰ to +12.5‰ in the low-temperature serpentine polymorph lizardite coupled to strong enrichment in fluid mobile elements are associated to variable serpentinisation conditions at the ocean floor during the opening of the Piemonte-Liguria Ocean. Incipient oceanic serpentinisation, i.e. mesh formation after mantle olivine, occurred at approximately 160–325°C ± 20–60°C. Subsequently, mantle pyroxene was transformed into serpentine bastite with decreasing temperatures down to 75°C to 100°C. The transition metals V, Sc, Co, Zn and Mn, and the Ni/Cr in lizardite are indicative of precursor olivine or orthopyroxene. Remarkably, some lizardite in partially serpentinised and undeformed mantle peridotites remained metastable during subsequent subduction. During early subduction, with increasing temperature, the serpentine polymorph antigorite replaced lizardite in highly serpentinised ultramafic rocks. Antigorite shows a narrower δ18O range from +5.3‰ to +7.5‰, indicating isotopic homogenisation at the regional scale. A redistribution of the transition metals is observed at the sample scale, and the fluid mobile elements B, Cl and Li are partly lost. The isotopic homogenisation and the element redistribution are likely the results of (i) sample internal equilibration during antigorite crystallisation and (ii) mobility of serpentinite-derived fluids during the lizardite-to-antigorite transition. During the peak subduction stage at 1.8 to 2.5 GPa and 550°C to 650°C, metamorphic olivine with δ18O of ~+4‰ to +5‰ is formed from the dehydration of brucite + antigorite. This olivine is enriched in fluid mobile elements such as Li and B compared to primitive mantle and is in oxygen isotopic equilibrium with the co-existing antigorite. Based on this equilibrium and the coupled trace element systematics, there is no evidence of large influxes of external slab fluids from other lithologies. Recrystallised mantle olivine has lower B and Li contents compared to metamorphic olivine formed during brucite dehydration, and has similar δ18O values, XMg and Ni/Mn as mantle olivine, and is not in isotopic equilibrium with the antigorite. Such metamorphic olivine produced by recrystallisation, as well as the metastable lizardite during subduction, cannot be used as indicators of fluid production from serpentine dehydration.