O-H isotope ratios of high pressure ultramafic rocks: implications for fluid sources and mobility in the subducted hydrous mantle

We examine the O-H isotope signatures of Alpine ultramafic rocks and eclogi tic metagabbros of the Erro-Tobbio peridotite Unit (western Italian Alps), which record a subduction and exhumation cycle. Localization of subduction- related deformation along serpentinite mylonite shear zones favoured preser vation of pre-subduction mantle and low temperature (oceanic) alteration as semblages within undeformed (meta)peridotite that underwent partial static recrystallization to high-pressure metamorphic parageneses. Bulk rock and m ineral separate (clinopyroxene and serpentine) oxygen isotope ratios of the serpentinized mantle peridotites (5-8 parts per thousand) are slightly enr iched in O-18 compared with those of the high-pressure metaperidotites and the serpentinite mylonites (4.4-7.6 parts per thousand). The lowest values occur in high-pressure veins (3.5-5.7 parts per thousand) and eclogitic met agabbros (3.1-5.3 parts per thousand). These variations are comparable to v ariations observed in modern oceanic rocks and in non-subducted ophiolites. Preservation of pre-eclogitic delta O-18 signatures of the Erro-Tobbio roc ks and a lack of oxygen isotope re-equilibration between different shear zo nes imply local-scale fluid flow at low water/rock ratios and closed system behaviour during high-pressure metamorphism. Different serpentine generati ons show a bimodal distribution in deltaD values: pre-eclogitic lizardite a nd chrysotile range from -102 to -77 parts per thousand; high-pressure anti gorite in the mylonites and in low strain metaperidotites range from -71 to -57 parts per thousand and -83 to -60 parts per thousand, respectively. Co mparable ranges occur in antigorite in the associated high-pressure veins, suggesting that the hydrogen signatures were acquired prior to veining. We propose that the isotopic variations reflect multiple events of fluid uptak e in different geodynamic environments. The H- and O-isotope ratios in the eclogitic mylonites suggest that initial hydration occurred over a range of temperatures during local interaction with altered seawater along oceanic shear zones. The O-18-enriched and H-depleted compositions of chrysotile an d lizardite in the mantle peridotites suggest that a second hydration event may have occurred as a result of interaction with metamorphic fluids at th e early stages of burial in a forearc setting, where slabs undergo large-sc ale, low-temperature fluid fluxing. The oceanic mantle is thus a candidate for continuous hydration during its oceanic and early subduction history. T he Erro-Tobbio unit thus represents an example of cycling of internally-der ived fluids, whereby the different structural and textural domains behaved as relatively closed systems to fluid circulation during high-pressure meta morphism.