Crustal reworking and hydration: insights from element zoning and oxygen isotopes of garnet in high‑pressure rocks (Sesia Zone, Western Alps)

Vho, Alice; Rubatto, Daniela; Lanari, Pierre; Giuntoli, Francesco; Regis, Daniele; Hermann, Jörg (2020). Crustal reworking and hydration: insights from element zoning and oxygen isotopes of garnet in high‑pressure rocks (Sesia Zone, Western Alps). Contributions to mineralogy and petrology, 175(11), pp. 109-137. Springer-Verlag 10.1007/s00410-020-01745-6

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Subduction 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 δ18O values up to 5.3 ‰ 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 δ18O occurred before, or in correspondence to, the first Alpine garnet growth, while subsequent influxes of fluid had δ18O close to equilibrium. The observed shift in garnet δ18O 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.

Item Type:

Journal Article (Original Article)

Division/Institute:

08 Faculty of Science > Institute of Geological Sciences
08 Faculty of Science > Institute of Geological Sciences > Isotope Geology

UniBE Contributor:

Vho, Alice; Rubatto, Daniela; Lanari, Pierre and Hermann, Jörg

Subjects:

500 Science > 550 Earth sciences & geology

ISSN:

0010-7999

Publisher:

Springer-Verlag

Funders:

[4] Swiss National Science Foundation

Projects:

Projects 1 not found.

Language:

English

Submitter:

Daniela Rubatto

Date Deposited:

16 Nov 2020 15:23

Last Modified:

16 Nov 2020 15:30

Publisher DOI:

10.1007/s00410-020-01745-6

Related URLs:

BORIS DOI:

10.7892/boris.147754

URI:

https://boris.unibe.ch/id/eprint/147754

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