Detection of primary paleomagnetic remanence carriers in zircon crystals using high-resolution X-ray ptycho-tomography

We present the first application of X-ray ptycho-tomography to perform high-resolution, non-destructive, three-dimensional (3D) imaging of magnetic inclusions in paleomagnetically relevant materials (zircon crystals from the Bishop Tuff ignimbrite). Correlative microscopy using quantum diamond magnetic microscopy combined with X-ray fluorescence mapping was used to locate regions containing Fe-bearing magnetic remanence carriers. Ptycho-tomographic reconstructions with voxel sizes 85 nm and 21 nm were achievable across a field-of-view > 80 µm; voxel sizes as small as 5 nm were achievable over a limited field-of-view using local ptycho-tomography. Magnetite particles 300 nm in size were clearly resolved. We estimate that particles as small as 100 nm – approaching single-domain threshold for magnetite – could be resolvable using this “dual-mode” methodology. Our combined magnetic and tomography results support the presence of primary magnetic inclusions in relatively young and pristine zircon crystals, majority of which are in the optimal size range for carrying strong and stable paleomagnetic signals.