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TitleCrustal reworking and hydration: insights from element zoning and oxygen isotopes of garnet in high-pressure rocks (Sesia Zone, Western Alps)
AuthorAlice, V; Daniela, R; Pierre, L; Francesco, G; Daniele, R; Jörg, H
SourceContributions To Mineralogy and Petrology vol. 175, issue 11, 109, 2020 p. 1-28, Open Access
logo Open Access
Alt SeriesNatural Resources Canada, Contribution Series 20200531
PublisherSpringer Science and Business Media Deutschland GmbH
Mediapaper; on-line; digital
File formatpdf; html
AreaAlps; Switzerland; Italy
Lat/Long WENS 7.2208 8.3311 45.8669 45.0192
Subjectstectonics; Science and Technology; oxygen isotopes; subduction; garnet; Sesia Zone
Illustrationslocation maps; tables; graphs; diagrams
Released2020 10 30
AbstractSubduction zones represent one of the most critical settings for fluid recycling as a consequence of dehydration of the subducting lithosphere. A better understanding of fluid flows within and out of the subducting slab is fundamental to unravel the role of fluids during burial. In this study, major and trace element geochemistry combined with oxygen isotopes were used to investigate metasediments and eclogites from the Sesia Zone in order to reconstruct the effect of internal and external fluid pulses in a subducted continental margin. Garnet shows a variety of textures requiring dissolution-precipitation processes in presence of fluids. In polycyclic metasediments, garnet preserves a partly resorbed core, related to pre-Alpine high-temperature/low-pressure metamorphism, and one or multiple rim generations, associated with Alpine subduction metamorphism. In eclogites, garnet chemical zoning indicates monocyclic growth with no shift in oxygen isotopes from core to rim. In metasediments, pre-Alpine garnet relics show d18O values up to 5.3 per-mil higher than the Alpine rims, while no significant variation is observed among different Alpine garnet generations within each sample. This suggests that an extensive re-equilibration with an externally-derived fluid of distinct lower d18O occurred before, or in correspondence to, the first Alpine garnet growth, while subsequent influxes of fluid had d18O close to equilibrium. The observed shift in garnet d18O is attributed to a possible combination of (1) interaction with sea-water derived fluids during pre-Alpine crustal extension and (2) fluids from dehydration reactions occurring during subduction of previously hydrated rocks, such as the serpentinised lithospheric mantle or hydrated portions of the basement.

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