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Disequilibirum metamorphism of stressed lithosphere

Periodic Reporting for period 3 - DIME (Disequilibirum metamorphism of stressed lithosphere)

Reporting period: 2018-09-01 to 2020-02-29

Most changes in mineralogy, density, and rheology of the Earth’s lithosphere take place by metamorphism, whereby rocks evolve through interactions between minerals and fluids. These changes are coupled with a large range of geodynamic processes and they have first order effects on the global geochemical cycles of a large number of elements.
In the presence of fluids, metamorphic reactions are fast compared to tectonically induced changes in pressure and temperature. Hence, during fluid-producing metamorphism, rocks evolve through near-equilibrium states. However, much of the Earth’s lower and middle crust, and a significant fraction of the upper mantle do not contain free fluids. These parts of the lithosphere exist in a metastable state and are mechanically strong. When subject to changing temperature and pressure conditions at plate boundaries or elsewhere, these rocks do not react until exposed to externally derived fluids.
Metamorphism of such rocks consumes fluids, and takes place far from equilibrium through a complex coupling between fluid migration, chemical reactions, and deformation processes. This disequilibrium metamorphism is characterized by fast reaction rates, release of large amounts of energy in the form of heat and work, and a strong coupling to far-field tectonic stress.
Our overarching goal is to provide physics-based models of disequilibrium metamorphism that properly connects fluid-rock interactions at the micro- and nanometer scale to lithosphere scale stresses. The main objectives of the research project ‘DIME’ are: 1) Characterizing the transport properties of systems during fluid-driven metamorphism; 2) Quantifying the effects of externally imposed differential stress on the progress of fluid-induced transformation processes; and 3) To quantify the rate of disequilibrium metamorphism and its tectonophysical effects by ICDP drilling in Oman.
"During the first 30 month of DIME, we have demonstrated that fluid-rock interactions far from thermodynamic equilibrium may lead to highly porous products, including a substantial amount of nanometer sized pores. In a paper published in Nature Geoscience (Plümper et al., 2017), we demonstrate that nanoscale mass transport by ‘diffusio-osmosis’, a mechanism that is driven by interactions between nano-pore walls and the pore filling fluids and solutes, is highly effective during replacement of feldspar minerals, the most abundant mineral group in the Earth crust. This is a transport mechanism not previously considered in the geoscience literature, and our results are achieved by a highly cross disciplinary team including a physicist, a chemical engineer, and four earth scientists.
Interface effects also seem to control, the progress of hydration reactions in systems subject to external stress. We have carried out novel microtriaxial experiments to study the effect of external stress on the hydration of periclase (MgO) to brucite (Mg(OH)2) with a purpose-designed rig. This rig is attached to beamline#19 of the European Synchrotron Research Facility in Grenoble to enable direct (4D-) observation of reaction progress and pore space evolution in situ by X-ray micro-tomography. In a recently resubmitted paper (Zheng et al., resubmitted) we report how the progress of this hydration reaction, which is associated with around 100% increase in solid volume, is halted if the mean stress exceeds ca. 30 MPa. We interpret this to indicate that at higher stress levels, the water layer between the reactant and product mineral needed for reaction to proceed is squeezed out. This is consistent with preliminary molecule-scale modelling of the same system carried out in our group, and indicates that external tectonic stress may be essential for the progress of hydration reactions in the Earth crust and upper mantle.
A major discovery of the ‘DIME’ project is the importance of dynamic rupture and seismic deformation (Earthquakes) for hydration processes in the Earth crust. Through detailed microstructural studies, we have been able to demonstrate that hydration processes of both mantle derived rocks (peridotites) and lower crustal lithologies (granulites) are initiated by seismic faults and associated fluid introduction. Papers in Science Advances (Austrheim et al, 2017), and Journal of Geophysical Research (Petley-Ragan et al., 2018; Aupart et al., submitted) describe novel microstructural evidence that wall rock damage and fluid introduction caused by earthquakes is a key factor in initiating shear zone development and subsequent large-scale transformation of initially fluid-free parts of the Earth lower crust. In a paper recently published in Nature (Jamtveit et al., 2018), we show how Earthquake activity in the normal seismogenic zone trigger aftershocks that affect a significant fraction of the lower crust and provide a novel top-down control on the structural and metamorphic evolution of the lower crust.
Finally, ICDP drilling in Oman relevant for this project commenced last autumn/winter (2017/18). A ‘DIME’ crew composed of the ‘DIME’ PI, two postdocs and a PhD student participated during the drilling in January and February 2018. Splitting of cores, preliminary analysis, selection of material for further studies etc will take place in the ODP drilling vessel Chikyu in Japan in the period 5August-5 September 2018.
• Our discovery that transport in nanometer-scale pores may occur through diffusio-osmosis introduces a new and effective mass transfer mechanism during metamorphism of rocks where porosity is produced during disequilibrium metamorphism. This may represent a major step forward in our understanding of the kinetics of metamorphic transformation processes in the Earth’s crust. We plan to pursue our studies of molecular scale modelling of both reactive transport and (rapid) deformation processes in order to progress towards a situation where modelling can be done at spatial and temporal scales that enable direct comparison between model output and nanoscale observations.

• Our studies of earthquake-triggered metamorphism demonstrate that initially strong and highly stressed lower crustal lithologies become weaker during the metamorphic transformation processes. Force-balance arguments suggest that this should lead to very significant pressure perturbations of lower crustal volumes at constant depth. Normally, metamorphic pressure is directly related to depth of burial, and this is the basis for the most dominant tectonic models of high pressure and ultrahigh pressure metamorphism. Major stress perturbations caused by mechanical weakening of highly stressed crustal lithologies may challenge some of the most highly cited work on high pressure metamorphism and associated tectonic scenarios. We expect to publish important contributions to this discussion within the next year.

• Many of the microstructures we observe in connection with deep crustal fault zones are indicative of extremely high stresses. Some of these structures have previously only been described from rocks that have been subject to meteorite impacts. The mechanisms of energy dissipation in the wall rocks during dynamic rupture and subsequent earthquakes in the highly confined environments of the Earth lower crust are not well understood and we expect to deliver important observational and experimental results on this within the remaining project period of DIME.