Dehydration-driven deformation of eclogite: Interplay between fluid discharge and rheology
Metadata
Show full item recordAuthor
Bukała, Michał; Hidas, Károly; Klonowska, Iwona; Barnes, Christopher J.; Fassmer, Kathrin; Majka, JarosławEditorial
John Wiley & Sons
Materia
EBSD Microstructure Pressure–temperature
Date
2024-03-13Referencia bibliográfica
Bukała, M., Hidas, K., Klonowska, I., Barnes, C. J., Fassmer, K., & Majka, J. (2024). Dehydration-driven deformation of eclogite: Interplay between fluid discharge and rheology. Journal of Metamorphic Geology, 1–28. https://doi.org/10.1111/jmg.12765
Sponsorship
National Science Centre (Poland) research projects no. 2019/33/N/ST10/01479 and 2014/14/ST10/00321; “Juan de la Cierva” Fellowship no. JFJC2021-047505-I funded by MCIN/AEI/10.13039/501100011033, CSIC and EU Next-Generation EU/PRTR; Foundation for Polish Science (FNP); Agencia Estatal de Investigacion (MCIN/AEI/10.13039/501100011033) for funding under RUSTED project PID2022-136471N-B-C21&C22, and for grant no. PID2020-119651RB-I00; Support of the publication fee by the CSIC Open Access Publication Support Initiative through its Unit of Information Resources for Research (URICI)Abstract
Aqueous fluids released during dehydration of a subducting slab have a large
effect on the rheology of the subduction interface. While high-pressure experiments
and natural-case studies link deformation with critical dehydration
reactions during eclogitization, the exact interplay between these processes
remains ambiguous. To investigate fluid–rock interaction and associated deformation
at high-pressure, we studied a suite of eclogites from the Tsäkkok Lens
of the Scandinavian Caledonides that record prograde metamorphism within
an Early Palaeozoic cold subduction zone. Our results show that in-situ dehydration
during the blueschist to eclogite facies transition produces fluid fluxes
leading to rheological weakening and densification, consequently promoting
ductile-brittle deformation. Petrographic evidence, supported by thermodynamic
modelling and thermobarometry, attest to a prograde passage from
lawsonite-blueschist to peak eclogite facies of 2.5 GPa and 620 C.
Phengite-bearing eclogites imply interaction with an externally-derived fluid,
whereas rare phengite-free, kyanite-eclogites only record internally-derived
fluid production. Models predict that prograde breakdown of chlorite, lawsonite
and amphibole between 500 and 610 C lead to progressive dehydration
and release of up to 4.6 wt.% of aqueous fluid. Microstructural data reveal
elongated shapes of highly strained omphacite porphyroblasts, displaying
minor yet gradual changes in misorientation towards the grain boundaries.
Occasionally, these intragranular structures form subgrain cells that have similar
sizes to those of neoblasts in the rock matrix. These observations point to
the potential onset of dynamic recrystallization processes via dislocation creep.
Moreover, the omphacite neoblasts and rutile show non-random crystallographic
preferred orientations (CPOs), which are characterized by the subparallel
alignment of point-like maxima in rutile [001] and [100] axes to those of
[001] and (010) of omphacite neoblasts, respectively. Additionally, the [001]
axes of these minerals are also subparallel to the weak stretching mineral lineation,
and the (100) of rutile and the (010) of omphacite neoblasts are distributed
in the plane of the foliation. This suggests that the development of their CPOs was coeval and structurally controlled. Garnet microfractures normal to
the foliation are dilated and sealed predominantly by omphacite. The lack of
obliquity between CPO and foliation plane, as well as the systematic orientation
of garnet microfracture orientations, are consistent with coaxial deformation
at peak-pressure conditions. Unlike other studies, we show that neither
an external fluid source nor channelized fluid flow is needed to facilitate a
ductile-brittle deformation of eclogite in a subduction setting.