Fluid-rock interaction in eclogite-facies meta-peridotite (Erro-Tobbio Unit, Ligurian Alps)

Exhumed blueschist/eclogite-facies serpentinites, altered oceanic rocks and sediments can provide information on dehydration reactions, fluid activity and, possibly, seismicity occurring during subduction of oceanic lithosphere. Serpentinites, commonly formed through extensive hydration of mantle peridotites at the seafloor or at the forearc regions, provide a storage of water that is progressively released in subduction by breakdown reactions of hydrous minerals, leading to periodic fluid pressure build up that may eventually lead to brittle failure (dehydration embrittlement). This mechanism is likely responsible for triggering deep Episodic Tremor and Slow Slip Events (ETS), composed of correlated seismic and aseismic slip and occurring at 30-60 km depth along the subduction interface (Behr et al., 2021). Despite ETS are well documented by geodetic and seismic observations, the corresponding geological structures are still debated. Since ETS occur in a region with high fluid pressure and rheological heterogeneities, the occurrence of associated brittle and ductile structures formed at blueschist/eclogite-facies conditions within ultramafic rocks may be interpreted as a record of ETS. In the attempt of investigating deformation at depths consistent with ETS, this study focuses on the ultramafic Erro-Tobbio (E-T) Unit (Voltri Massif), which experienced metamorphic conditions of deformation mostly in the ETS depth range. The E-T Unit consists of metamorphosed spinel and plagioclase lherzolites, recording deformations related to the oceanic extension and the subsequent Eoalpine subduction and exhumation (Scambelluri et al., 1991, 1995). The uneven serpentinization experienced by the unit during the oceanic phase led to partitioning of the eclogite-facies deformation into high-strain domains of serpentinite mylonites, recording overprinting ductile and brittle deformation (interpreted as horizons of slow slip), and low strain domains of meta-peridotite, mainly affected by brittle deformation (as asperities reaching failure and triggering tremor). Both the low-strain and the high-strain domains are characterized by the presence of metamorphic olivine (Ol) + Ti-clinohumite (Ti-chu) veins/reaction bands developed after antigorite (Atg) dehydration. My study focuses on an outcrop of meta-peridotite characterized by a network of Ol+Ti-chu reaction bands/veins and enclosed by discrete mylonitic horizons. The field work, associated to on-ground and drone photogrammetric survey, evidenced the presence of a pervasive set of Ol+Ti-chu reaction bands (Olivine-Fabric-1), oriented at high angle to the mylonites, and of a set of veins/reaction bands (Olivine-fabric-2: OLF2), oriented parallel to the mylonites and progressively intensifying towards these, localized within a thinner low-strain domain. Two types of mylonitic horizons were recognized: (i) type 1 mylonites, made of plane-parallel foliation marked by olivine bands (OlF2), and (ii) type 2 mylonites, consisting of chaotic serpentinite mylonite. These relationships suggest a correlation between the mylonites and the reaction bands. Microstructural analysis on OlF1 has revealed the occurrence of two stages of Atg dehydration, reflecting the reaction Atg+BrcOl+H2O, and an intermediate stage of hydration. The first extensive dehydration following oceanic/forearc serpentinization led to the formation of the metamorphic olivine (Ol2) arranged along the reaction bands. The stage of hydration localized along Atg-bearing microcracks forming within the reaction bands, which was, in turn, affected by dehydration, resulting in the formation of granoblastic olivine (Ol3). LA-ICP-MS analysis evidenced an enrichment in fluid-mobile elements within the Ol2 and the microcracks-filling Atg, suggesting influx of slab-derived fluids during deformation, and the opening of the chemical system at eclogite-facies conditions. Serpentinite polymorphs were distinguished by micro-Raman spectroscopy.