Casting synchrotron light on remagnetized carbonates

We revisit the discussion on the geological processes able to remagnetize vast extensions of intracratonic basins. The main hypotheses for these processes involve: (1) external warm fluids, (2) hydrocarbon maturation, or (3) burial diagenesis and clay minerals transformation. Here we combine classical rock magnetic properties, with micro-to-nanoscale imaging/chemical analysis performed at the Brazilian Synchrotron Light Facility (SIRIUS). Highly sensitive X-ray fluorescence (XRF) and X-ray Absorption Near Edge Structure (XANES) were performed on a Coherent X-ray Nanoprobe Beamline, scanning microscopic regions on thin sections of remagnetized Neoproterozoic carbonate rocks of the São Francisco Craton. These rocks rarely yield any primary remanent magnetization. Instead, distinct geological formations separated by hundreds of kilometers bear an undistinguishable single-polarity characteristic direction carried by both monoclinic pyrrhotite and magnetite. Unmixing of susceptibility components of distorted magnetic hysteresis (potbellies and wasp-waisted) suggests two coercivity fractions and show a strong paramagnetic contribution that surpasses both ferromagnetic and diamagnetic (from calcite/dolomite) signatures. SEM-EDS analysis reveals iron oxides/sulfides embedded in a clay mineral matrix, while XRF data shows a strong spatial correlation of these nanometric remanence-bearing minerals (500-1000 nm) within regions enriched in potassium. XANES spectra of 1000 to 1200 nm particles indicate either stoichiometric euhedral magnetite, or spherical grains with a core-shell structure with magnetite rimmed by maghemite. The identification of these remanence-bearing phases within the pseudo-single domain (PSD) size range, systematically associated to clay minerals (responsible for the important paramagnetic contribution in magnetic hysteresis), might suggest that clay transformation, namely smectite to illite, is an important phenomenon controlling remagnetization of these carbonates