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Disequilibirum metamorphism of stressed lithosphere

Periodic Reporting for period 4 - DIME (Disequilibirum metamorphism of stressed lithosphere)

Reporting period: 2020-03-01 to 2022-02-28

The physical properties of the Earth’s lithosphere evolve through metamorphism. This effects a large range of geodynamic processes, including mountain building processes. In the presence of fluids, metamorphic reactions are fast compared to tectonically induced changes in pressure and temperature. Hence, rocks evolve through near-equilibrium states. However, much of the Earth’s lower 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, rocks do not react until exposed to externally derived fluids. Fluid introduction to the originally dry lower crust or upper mantle drive disequilibrium metamorphism 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.
My overarching goal is to understand how fluid-rock interactions at the micro- and nanometer scale is coupled to mechanical processes and lithosphere scale stresses. The main objectives of ‘DIME’ are: 1) To characterize the transport properties of systems during fluid-driven metamorphism; 2) To quantify the effects of both internally produced and externally imposed differential stress on the progress of fluid-induced transformation processes; and 3) To understand the mechanisms of hydration of oceanic lithosphere through the Oman Drilling Project.

Conclusions of the action
DIME shows that transformation processes within lower crust and upper mantle is initiated by earthquakes. The required stress levels may be achieved by stress pulses originated by shallow level earthquakes, providing a top-down controls on deep crustal/upper mantle dynamics. Seismic slip may be facilitated by overpressured frictional melts. Fluid introduction following brittle failure and wall rock damage drive metamorphic reactions and causes mechanical weakening of the wall rocks. Wall rock metamorphism is made effective by fast transport through nanometer scale pores. Mechanical weakening lead to the onset of ductile deformation and shear zone formation. Continued tectonic loading of rock volumes with variable strengths leads to a heterogeneous distribution of pressures at any given depth, and locally to high pressure-gradients. DIME shows that lower crustal evolution is much more dynamic than previously thought; that deformation, fluid migration, and metamorphism are tightly coupled and that the interpretation of metamorphic rocks in terms of an underlying tectonic process is far more complex than traditionally assumed.
Papers in Science Advances, Journal of Geophysical Research and Earth and Planetary Science Letters show that fluid introduction caused by earthquakes is key factor in initiating large-scale transformation of the Earth lower crust and upper mantle.
In a Nature paper, we show how shallow level Earthquake activity may trigger aftershocks that affect the lower crust and provide a top-down control on the structural and metamorphic evolution of the lower crust. Overpressured frictional melt may represent the strain weakening mechanism needed for deep earthquakes (Nature Geoscience, 2021).
Fluid introduction to dry lower crust of upper mantle follow in the wake of initial brittle deformation and trigger reactions with the wall rocks. Metamorphic reactions may lead to highly porous products, including a substantial amount of nanometer sized pores. In a Nature Geoscience paper, we demonstrate that nanoscale mass transport by ‘diffusio-osmosis’, is highly effective during replacement of feldspar minerals, the most abundant mineral group in the Earth crust. This transport mechanism was not previously considered in the geoscience literature.
Pressures exceeding the lithostatic may arise in any weak rock confined in a strong and highly stressed matrix. In a Scientific Reports paper, we demonstrate that this may explain the presence of high-pressure rocks at normal crustal depths. A new visco-elasto-plastic model was developed that describes the evolution of the lower crust from initial brittle fracturing, via subsequent fluid infiltration, to mechanical weakening and shear zone development (Communications Earth and Environment, 2022). I believe this is the most coherent, self-consistent, and accurate description of the mechanical and metamorphic evolution of the lower crust during orogeny described so far.

Interface effects control the progress of hydration reactions in systems subject to external stress. Novel experiments in a purpose-designed rig were carried out to study the effect of external stress on the hydration of periclase to brucite. This rig is attached to beamline#19 of the European Synchrotron Research Facility in Grenoble to enable direct observation of reaction progress by X-ray micro-tomography. In a G-cubed paper we report how the progress of this hydration reaction is halted if the mean stress exceeds ca. 30 MPa. At higher stress levels, the water layer between the reactant and product mineral is squeezed out. This is also demonstrated by a molecule-scale modelling study (Geochim Cosmochim Acta, 2021), and indicates that external tectonic stress may be essential for the progress of hydration reactions in the Earth crust and upper mantle. An atomic force microscopy (AFM) study indicates that surface forces may be very sensitive to fluid composition. Hence, it may be hard to draw general conclusions about interface forces for complex fluids (ACS Earth and Space Chemistry 2021).
ICDP drilling in Oman commenced in 2017/18. A DIME crew participated during the drilling in January and February 2018. Sampling and further studies took place on board the ODP drilling vessel Chikyu. Our studies demonstrate that the initial hydration of peridotites was associated with seismic deformation and introduction of sea-water derived fluids (Earth and Planetary Science Letters, 2021). Oxygen isotope and trace element data show that serpentinization occured at several kilometers depths in the oceanic lithosphere at temperatures around 200-250C.
• Diffusio-osmosis represents a new and effective mechanism of mass transfer during metamorphism.

• Strong and highly stressed lower crustal lithologies locally become weaker during metamorphic transformation. This leads to significant pressure perturbations of lower crustal volumes at constant depth and challenge some of the most highly cited work on high pressure metamorphism and associated tectonic scenarios. We present a model that describes a dynamic evolution of the lower crust where the pressure varies significantly and dynamically at any given depth and where pressure gradients locally become far higher than lithostatic.

• Observed microstructures locally indicate extremely high stresses. Similar structures have previously been described from rocks that have been subject to meteorite impacts. The wall rock damage observed during deep earthquakes is due to such short-lived dynamic stress.

• Our study of quartz inclusions in garnets grown from frictional melts formed during deep crustal earthquakes show that such melt are severely overpressured. This may allow earthquakes to take place way below the ‘seismogenic’ zone in the Earth’s crust.

• Early stage serpentinization of oceanic upper mantle is triggered by off-axis earthquakes and fluid introduction near 200-250C.
Continental drilling taking place in the Batin dunite, a 10 km long and 2.5 km wide tabular dunitic