Mapping nano- and microporosity in ductile shear zones with X-ray ptychography

Petrological and geochemical evidence demonstrates that mylonitic ductile shear zones transport significant amounts of fluids through the lithosphere. Because of the lack of percolating pore networks in exhumed mylonitic rocks, it has been hypothesised that most of the fluid pathways in ductile shear zones consist of transient pores with micro- to nanoscale aperture. These transient pores typically include grain boundaries, creep cavities, and pores due to mineral reactions, all of which open and close cyclically during plastic deformation. However, it is not known how much each of these features contributes to crustal fluid flow. Moreover, nano-scale pores are notoriously difficult to map with non-destructive imaging methods. These are the key problems addressed by this research.

We recently used X-ray ptychography to image midcrustal quartzo-feldspathic mylonites at the X-ray Fluorescence Microprobe beamline of the Australian Synchrotron. This transmission small-angle scattering method maps X-ray phase contrast non-destructively with nanometre resolution. In addition, high-resolution trace-element maps are acquired coevally with X-ray Fluorescence Microscopy (XFM). We systematically sampled transects from mylonite to ultramylonite to capture the strain-time evolution of nano- and microvoids and to study how transient porosity and trace-element composition change with deformation intensity, composition, grain size, and deformation mechanism. This dataset will provide novel insights into mass transfer in ductile shear zones.