# Field observations uncover complex feedbacks between eclogitization kinetics and state of stress in reacting rocks

Marie Baïsset, Loïc Labrousse, Philippe Yamato, Anaïs Cochet

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Marie Baïsset, Loïc Labrousse, Philippe Yamato, Anaïs Cochet. Field observations uncover complex feedbacks between eclogitization kinetics and state of stress in reacting rocks. 29ème Réunion des Sciences de la Terre 2025, Société Géologique de France (SGF), Oct 2025, Montpellier, France. pp.657515. insu-05350440

HAL Id: insu-05350440

https://insu.hal.science/insu-05350440v1

Submitted on 6 Nov 2025

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Distributed under a Creative Commons Attribution 4.0 International License
Field observations uncover complex feedbacks
between eclogitization kinetics and state of stress in
reacting rocks

Marie Baïsset*†<fnref n="1" />,<fnref n="2" />, Loïc Labrousse*<fnref n="3" />, Philippe Yamato<fnref n="1" />, and Anaïs Cochet<fnref n="1" />

Résumé

Layered continental granulites from the island of Holsnøy (Norway) are partially eclogi-
tized along either shear zones or so-called " static " finger-shaped structures. While eclogite
shear zones suggest that reaction progress is mainly controlled by strain, a mechanism is
still required to explain the propagation of the still puzzling finger-shaped reaction fronts in
adjacent low-strain domains. Combining petrological observations with a detailed structural
analysis of the partially eclogitized Holsnøy massif, we discuss how the state of stress can
modify the mechanical and fluid transport properties of these rocks and in turn how transient
weakening induced by the reaction can affect the local stress field. We show that fingers and
shear zones are not randomly distributed. Finger-shaped reaction fronts indeed preferentially
propagate in specific directions depending on the relative orientation between the granulitic
foliation and the local maximal principal stress inferred from measured shear directions of
the adjacent eclogite shear zones. Dependance of reaction kinetics on the state of stress can
be described using the Damköhler number (ratio of reaction rate over diffusion coefficient).
Dependance of the local stress field on the reaction is suggested by evidence of weakening
processes linked to the eclogite paragenesis: mechanical twinning, subgrain individualiza-
tion and partial melting. We therefore propose that conjunction of anisotropic kinetics and
anisotropic state of stress state actually controls reaction patterns in metamorphic rocks, as
for instance the development of finger-shaped eclogitization reaction fronts studied here.

**Mots-Clés:** mechanical anisotropy, stress field, eclogitization, weakening, plagioclase mechanical twinning, grain size reduction, reaction propagation

<footnote n="1">¹Geosciences Rennes – Université de Rennes I, Observatoire des Sciences de l'Univers de Rennes, INSU, CNRS, CNRS : UMR6118 – France</footnote>

<footnote n="2">²Laboratoire de géologie de l'ENS – Institut National des Sciences de l'Univers, Centre National de la Recherche Scientifique, Département des Géosciences - ENS-PSL – France</footnote>

<footnote n="3">³Institut des Sciences de la Terre de Paris – Institut National des Sciences de l'Univers, Sorbonne Universite, Centre National de la Recherche Scientifique, CY Cergy Paris Université – France</footnote>

<footnote>* Intervenant</footnote>

<footnote>†Auteur correspondant: marie.baisset.pro@gmail.com</footnote>