Nanopores enable fluid flux in mylonites and ultramylonites – novel insights from Scanning Small-angle X-ray Scattering and X-ray Fluorescence Microscopy

Mylonitic shear zones funnel significant amounts of fluids through the crust. However, the physical mechanisms and pathways for mass transfer remain debated. Grain boundaries, creep cavities, and pores formed by mineral reactions involving volume change and mass transport are considered the most important fluid conduits. So far, the imaging of these pores was either limited to µm-resolution, or in the case of nm-scale resolution, to very small areas (e.g., TEM investigates regions in the µm²-range) with limited statistical power. Moreover, many pores involved in ductile fluid flow only remain open intermittently before they are closed again by mineral growth and plastic deformation. To find traces of closed pores and assess pore production and consumption due to mineral reactions, high-resolution microchemical maps are needed.  

Here, we present a study that addresses these challenges through applying scanning small-angle X-ray scattering (SAXS) and X-ray Fluorescence Microscopy (XFM) to samples of a mylonite transitioning into ultramylonite, derived from a granitic protolith. SAXS delivers maps of nanopores with apertures between 1 and 280 nm while XFM enables spatially resolved mass balance computations, geochemical fluid fingerprinting, and the correlation of nanoporosity with mineral phase and trace-element composition. Importantly, both techniques are applied to an entire thin section, providing a sound observational statistical base from nm- to cm-scale.

Some key observations include:

1) A substantial amount of nanoporosity is discovered, with the same magnitude as microporosity measured previously in granitoid mylonites.

2) The ultramylonite contains twice as much nanoporosity as the mylonite.

3) Nanoporosity is strongly mineral-specific and highly elevated in regions enriched in epidote and mica.

4) Nanoporosity is highly anisotropic and usually aligned with, or at low angle to, the foliation, enabling mass flux along the shear zone.

5) Pore sheets from the mylonite connect with those of the ultramylonite, providing pathways for fluid exchange between high-strain shear zone and host rock.

These results highlight the importance of nanoscale fluid conduits and synkinematic mineral reactions for mass transfer in ductile shear zones. The implications for models of coupled fluid flow in the ductile crust will be discussed.