Search

Fluid-rock interactions play a critical role in Earth’s lithosphere and environmental subsurface systems. In the absence of chemical mass transport, mineral-hydration reactions would be accompanied by a solid-volume increase that may induce differential stresses and associated reaction-induced deformation processes, such as dilatant fracturing to increase fluid permeability. However, the magnitudes of stresses that manifest in natural systems remain poorly constrained. We used optical and electron microscopy to show that one of the simplest hydration reactions in nature [MgO + H2O = Mg(OH)2] can induce stresses of several hundred megapascals, with local stresses of as much as ∼1.5 GPa. We demonstrate that these stresses not only cause fracturing but also induce plastic deformation with dislocation densities (1015 m−2) exceeding those typical of tectonically deformed rocks. If these reaction-induced stresses can be transmitted across larger length scales, they may influence the bulk stress state of reacting regions. Moreover, the structural damage induced may be the first step toward catastrophic rock failure, triggering crustal seismicity.

International audience; Eclogitization reactions in mafic rocks involve large volume changes, porosity evolution and fluidtransfer. They impact many important geological processes such as the localization of deformationand fluid channeling at intermediate depth in subduction zone. The study of exhumed eclogiticbodies in orogens shows that eclogitization of the oceanic crust is heterogeneous from both astructural and metamorphic point of view. For example, in the European Alps, the Allalinmetagabbro shows high strain areas, consisting of hydrous metagabbros, fully equilibrated undereclogite-facies conditions during the Alpine orogeny. Conversely, large volumes of low strain, fluidundersaturatedgabbros remained largely unaffected by the high-pressure (HP) metamorphism,locally preserving igneous textures and even, occasionally, relics of their magmatic mineralogy.The intensity of deformation as well as the degree of eclogitization in the metagabbro have beenshown to be directly related to the extent of pre-Alpine hydration during high-temperaturehydrothermal alteration [1]. However, the influence of this degree of hydration on (1) reactionkinetics and/or (2) enhancing rheological contrasts leading to heterogeneous deformationpatterns and metamorphic conditions is still debated.In order to address this issue, we propose a multidisciplinary study involving petrographic andmicrotextural observations combined with 2D thermo-mechanical numerical models allowing todiscuss the role of pre-Alpine hydrothermal alteration on the development of HP metamorphicassemblages.We present petrographic and textural data from three different types of rocks from the Allalinmetagabbros: i) undeformed and mostly untransformed metagabbros, with relics of igneousaugite and plagioclase, ii) coronites, with olivine pseudomorphs showing different levels ofhydration, rimmed by a garnet corona, and iii) eclogitized metagabbros, with olivine andplagioclase sites fully replaced by high-pressure assemblages.The role of protolith hydration on the observed range in metamorphic facies is then tested byusing 2D thermo-mechanical models that allow to simulate the deformation of a strong and dryrock with several randomly oriented weak and hydrous zones. Our results show that the shearingof heterogeneous rock can lead to the formation of localized ductile shear zone within a matrixthat remains relatively undeformed but where plastic deformation can occur. The associated P field is also highly heterogeneous, with P ranging from 1 to 3 GPa. The deformation patterns and Pmodelled may suggest that locally hydrated portions of the gabbro acted as rheologicalperturbations sufficiently efficient in producing the structural and metamorphic record nowobserved in the field.[1] Barnicoat, A. C. & Cartwright, I. (1997) Journal of Metamorphic Geology 15, 93–104

We generated novel thermometric formulae for cation exchange reactions between mantle minerals involving species of different diffusivities (FeMg orthopyroxene-spinel; CrAl & CaMg clinopyroxene-orthopyroxene) and used them in conjunction with published expressions (FeMg olivine-spinel; CaMgFe clinopyroxene-orthopyroxene) to decipher the thermal history of peridotites from diverse tectonic settings. When tested on abyssal peridotites, TCrAlopx-cpx vs. TCaMgcpx-opx form tight trends with TCrAlopx-cpx ≥ TCaMgcpx-opx at T1060oC in accordance with crossing diffusivities of Cropx and CaMgcpx at about that temperature, with the latter becoming smaller at higher temperatures and blocking higher as the peridotites are quickly emplaced and cooling at the ridge. Both TCrAlopx-cpx and TCaMgcpx-opx are higher than the widely used TCaFeMgcpx-opx in abyssal peridotites by ~100°C on average. On a TCrAlopx-cpx vs. TFeMgopx-sp plot there is clear-cut distinction between peridotite massifs (major ocean basins, oceanic forearcs, oceanic backarcs, ophiolites, ocean-continent transitions, Alpine-type) from peridotite xenoliths (cratons, intracontinental rifts, continental backarcs, active continental margins, oceanic within-plate) with TCrAlopx-cpx being invariably larger than TFeMgopx-sp for the peridotite massifs reflecting the blocking of the slower-diffusing cations at higher temperatures upon cooling. The higher TFeMgopx-sp exhibited by the xenoliths are ascribed to some, probably magmatic, heating event at a time prior to their transport to the surface. The Alpine-type peridotites of Beni Bousera, Morocco, that were exhumed in the footwall of a lithosphere-scale extensional shear zone display increasing TCrAlopx-cpx and TCaMgcpx-opx towards the shear zone but flat TCaFeMgcpx-opx, TFeMgopx-sp and TFeMgol-sp profiles demonstrating the memory of the slower-diffusing cations.

Chromium (Cr) is considered to be one of the most economically important critical raw materials. The most economically significant mineral for Cr exploitation is Chromite which can be found in podiform Chromite deposits that are associated with ophiolite complexes from Supra-Subduction Zone (SSZ) settings. In Europe, such deposits are located in northwestern Greece and the Balkans in general. The precise nature of podiform chromitite generation is not fully understood and still under debates and could be the result of a complex sequence of enrichment processes.To improve our understanding of Cr extraction during mantle melting a series of experiments were performed on natural peridotite samples from the Pindos Ophiolite, Greece. Our experiments were conducted under pressure (1.0-1.5 GPa) and temperature (1350-1400°C) conditions that are relevant for the genesis of intra-oceanic arc magmas. To study the effect of hydrous conditions, the amount of water in the system was varied from 1 to 10 wt%. The samples had various modal proportions of olivine, ortho- and clinopyroxene and a different degree of alteration.All experiments produced Cr-enriched melts (up to 7000 µg/g Cr) with a positive dependence of Cr content on the degree of melting and on the concentration of water added to the system. Based on the obtained results we have developed forward numerical models to constrain the Cr-enrichment process in magmas during mantle melting. Our study demonstrates that previous estimates on the extraction efficiency of Cr during mantle melting in SSZ should be reconsidered.

Fluid-rock interactions play a critical role in Earth’s lithosphere and in engineered subsurface systems. In the absence of chemical mass transport, mineral-hydration reactions will be accompanied by a solid-volume increase that may induce differential stresses and associated reaction-induced deformation processes, such as dilatant fracturing to increase fluid permeability. However, the magnitudes of stresses that manifest in natural systems remain poorly constrained. Here we show that the simplest hydration reaction in nature MgO + H2O⇔ Mg(OH)2 can induce stresses of several hundred megapascals, with local stresses up to ∼1.5 GPa. We demonstrate that these stresses are dissipated not only by fracturing but also induce plastic deformationwith dislocation densities (10^15m−2) exceeding those typical of tectonically deformedrocks. If these reaction-induced stresses can be transmitted across larger length scales they may influence the bulk stress state of reacting regions. Moreover, the structural damage induced may be the first step towards catastrophic rock failure, triggering crustal seismicity.

The partially eclogitized crustal rocks on Holsnøy in the Bergen Arcs, Norway, indicate that eclogitization is caused by the interplay of brittle and ductile deformation promoted by fluid infiltration and fluid‐rock interaction. Eclogitization generated an interconnected network of millimeter‐to‐kilometer‐wide hydrous eclogite‐facies shear zones, which presumably caused transient weakening of the mechanically strong lower crust. To decipher the development of those networks, we combine detailed lithological and structural mapping of two key outcrops with numerical modeling. Both outcrops are largely composed of preserved granulite with minor eclogite‐facies shear zones, thus representing the beginning phases of eclogitization and ductile deformation. We suggest that deformation promoted fluid‐rock interaction and eclogitization, which gradually consumed the granulite until fluid‐induced reactions were no longer significant. The shear zones widen during progressive deformation. To identify the key parameters that impact shear zone widening, we generated scale‐independent numerical models, which focus on different processes affecting the shear zone evolution: (i) rotation of the shear zones caused by finite deformation, (ii) mechanical weakening due to a limited amount of available fluid, and (iii) weakening and further hydration of the shear zones as a result of continuous and unlimited fluid supply. A continuous diffusion‐type fluid infiltration, with an effective diffusion coefficient around D=10−16m2s, coupled with deformation is prone to develop structures similar to the ones mapped in field. Our results suggest that the shear zones formed under a continuous fluid supply, causing shear zone widening, rather than localization, during progressive deformation., Key Points: Continuous fluid supply causes shear zone widening. Shear zones widen during strain accumulation., Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659, Norges Forskningsråd http://dx.doi.org/10.13039/501100005416