Ductile deformation during carbonation of serpentinized peridotite

ARTICLE https://doi.org/10.1038/s41467-022-31049-1 OPEN Ductile deformation during carbonation of serpentinized peridotite 1234567890():,; Manuel D. Menzel 1,2 ✉, Janos L. Urai1, Estibalitz Ukar3, Greg Hirth4, Alexander Schwedt5, András Kovács Lidia Kibkalo6 & Peter B. Kelemen 7 6, Carbonated serpentinites (listvenites) in the Samail Ophiolite, Oman, record mineralization of 1–2 Gt of CO2, but the mechanisms providing permeability for continued reactive fluid flow are unclear. Based on samples of the Oman Drilling Project, here we show that listvenites with a penetrative foliation have abundant microstructures indicating that the carbonation reaction occurred during deformation. Folded magnesite veins mark the onset of carbonation, followed by deformation during carbonate growth. Undeformed magnesite and quartz overgrowths indicate that deformation stopped before the reaction was completed. We propose deformation by dilatant granular flow and dissolution-precipitation assisted the reaction, while deformation in turn was localized in the weak reacting mass. Lithostatic pore pressures promoted this process, creating dilatant porosity for CO2 transport and solid volume increase. This feedback mechanism may be common in serpentinite-bearing fault zones and the mantle wedge overlying subduction zones, allowing massive carbonation of mantle rocks. 1 Tectonics and Geodynamics, RWTH Aachen University, Lochnerstrasse 4-20, D-52056 Aachen, Germany. 2 now at: Instituto Andaluz de Ciencias de la Tierra (IACT) (CSIC-Universidad de Granada), Avenida de las Palmeras 4, 18100 Armilla, Granada, Spain. 3 The University of Texas at Austin, Bureau of Economic Geology, Austin, TX, USA. 4 Brown University, Department of Earth, Environmental and Planetary Sciences, Providence, RI, USA. 5 RWTH Aachen University, Central Facility for Electron Microscopy, Aachen, Germany. 6 Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Jülich, Jülich, Germany. 7 Lamont–Doherty Earth Observatory, Columbia University, New York, NY, USA. ✉email: NATURE COMMUNICATIONS | (2022)13:3478 | https://doi.org/10.1038/s41467-022-31049-1 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-022-31049-1 L istvenites–fully carbonated peridotites mostly composed of Mg-rich carbonate minerals and quartz1–can be a natural analogue for carbon storage applications2,3, and may offer an opportunity to study mass transfer and deformation processes acting at the leading edge of the mantle wedge4. Listvenites commonly occur along major shear zones or faults that can act as fluid conduits (e.g.5–9), but the extent to which deformation plays a role in carbonation reactions is poorly constrained. Deformation can enhance fluid-rock interactions, and altered rocks are far more abundant in tectonically active zones. Thermohydro-mechanical-chemical processes of coupled fluid flow, metasomatism, and deformation are common in extensional detachments, oceanic transform faults, and the plate interface of subduction zones10–15. The presence of aqueous fluids can enhance deformation by lowering effective stress, allowing dilatancy and activation of dislocation creep as well as pressure solution at high water activities16–19. At multiple scales, this involves nonlinear coupling of deformation, fluid flow and chemical reactions, over time scales that are difficult to investigate in the laboratory. The formation of listvenite requires prolonged fluid flow that adds about 30 wt% CO2 to the rock (e.g.6,20,). Even for unusually high CO2 contents in aqueous fluids (on the order of 1 wt%), this requires time-integrated fluid rock ratios > 30. Serpentinization and carbonation converting peridotite to listvenite involve a combined increase in solid volume by up to 68%21–24, potentially clogging pore space and decreasing permeability with reaction progress. This effect, in combination with the formation of reaction rims that can inhibit diffusion and reaction, is the main reason why carbonation is inferred to be self-limiting in many experiments25–27. In natural settings such as the Samail ophiolite, where massive carbonation of peridotite to listvenite went to completion in volumes of 1–2 km³8 (equivalent to 1–2 Gt CO2), tectonic stress and related deformation may play a key role in maintaining sufficiently high permeability for complete carbonation. Positive feedback mechanisms of deformation on permeability and reactivity include: (i) grain size reduction and related increase in reactive surface area18, (ii) creep cavitation during viscous grain boundary sliding and pressure solution28, (iii) dilatancy during granular flow29, and (iv) fluid transport along fractures30,31. Listvenites contain a rich variety of (micro)structures, such as preserved reaction fronts, a multitude of veins, and growth zoning in magnesite due to variable redox conditions during reaction progress6,20,32. Their textural evolution over time can shed light on the relationship between carbonation and deformation. The Samail ophiolite exposes large-scale CO2-fluid-rock interactions in mantle rocks8,9. The Oman Drilling Project (OmanDP) was an international endeavor to systematically sample key sections of the oceanic lithosphere from crust to the basal thrust. OmanDP Hole BT1B32 provided a unique sample of fully carbonated mantle rocks (listvenites) unaffected by surface weathering for targeted analysis. In this study we analyze the temporal evolution of microstructures in listvenite of Hole BT1B to understand the interaction between changing rheology, effective stress, and fluid flow during progressive carbonation of peridotite. We use multiscale optical and electron microscopic imaging and analysis to study the temporal relationships between phase changes and rock fabrics. Results indicate syn-carbonation, brittle and ductile deformation, and creation of inter- and intra-granular porosity, which, together with the abundant veins present in these rocks, contributed to the permeability network. Results Geological background. Listvenites crop out as thick bands in serpentinite at or close to the base of the Samail ophiolite (Fig. 1). 2 Hole BT1B recovered 196 m of listvenite and serpentinite, and, separated by a fault, 104 m of underlying metamorphic sole (Fig. 1d)32. The presence of quartz-serpentine intergrowths, together with recrystallized quartz and chalcedony after opal suggest temperatures of 80–150 °C during listvenite formation8. Low temperatures are supported by a nearly complete lack of talc, and by intergrown hematite and graphite or amorphous carbon, which require <200 °C if formed in equilibrium33. Clumped isotope thermometry points to temperatures from 45 ± 5 to 247 ± 52 °C for carbonate precipitation in listvenite and serpentinite8,34, consistent with the inferences from mineral parageneses. The pressure of listvenite formation is poorly constrained, with a possible range from ~0.3 GPa (con (...truncated)